DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
MECKLENBURG COUNTY WATER AND LAND RESOURCES DIVISION
FINAL REPORT
April 2003
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
MECKLENBURG COUNTY WATER AND LAND RESOURCES DIVISION
FINAL REPORT
April 2003
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
700 North Tryon Street
Charlotte, NC 28202
ABS Project No. 251045.01
Contents
Page
1. Executive Summary.............................................................................................................................. 1-1
1.1
Project Background ....................................................................................................................... 1-1
1.2
Report Organization ...................................................................................................................... 1-3
1.3
Analysis Synopsis ......................................................................................................................... 1-4
2. Data Analysis ........................................................................................................................................ 2-1
2.1
Data Collection and Processing..................................................................................................... 2-1
2.1.1
Input Data .............................................................................................................................. 2-1
2.1.2
Data Processing Summary ..................................................................................................... 2-3
2.2
Building Damage Analysis............................................................................................................ 2-6
2.3
Building Valuation ........................................................................................................................ 2-7
2.4
Damage Curve Selection ............................................................................................................... 2-8
2.4.1
FIA Depth-Damage Curves.................................................................................................... 2-8
2.4.2
USACE Wilmington District Depth-Damage Curves ........................................................... 2-10
2.4.3
USACE Galveston District Curves....................................................................................... 2-11
2.5
Occupancy Mapping & Damage Assessment ............................................................................. 2-11
3. Flood Hazard Assessments .................................................................................................................. 3-1
3.1
McAlpine Creek Analysis ............................................................................................................. 3-7
3.1.1
Results Summary for the McAlpine Creek Basin.................................................................... 3-7
3.1.2
Data Analysis ......................................................................................................................... 3-9
3.1.2.1
Parcel and Building Data Review ................................................................................................ 3-9
3.1.2.2
Parcels Without a Surveyed Structure ........................................................................................ 3-11
3.1.2.3
Surveyed Structures Intersecting a Parcel .................................................................................. 3-13
3.1.2.4
Surveyed Structures Not Intersecting a Parcel ........................................................................... 3-14
3.1.3
3.2
Detailed Results ................................................................................................................... 3-14
Sugar Creek Analysis .................................................................................................................. 3-21
3.2.1
Results Summary for the Sugar Creek Basin (below Irwin Creek) ...................................... 3-21
3.2.2
Data Analysis ....................................................................................................................... 3-24
3.2.2.1
Sugar Creek Basin Characteristics and Study Data .................................................................... 3-24
3.2.2.2
Sugar Creek Watershed Depth Grid ........................................................................................... 3-25
3.2.2.3
Sugar Creek Parcel and Building Data Review .......................................................................... 3-30
iii
3.2.3
3.3
Detailed Results for the Sugar Creek Basin ......................................................................... 3-31
Long Creek Analysis ................................................................................................................... 3-38
3.3.1
Results Summary for the Long Creek Basin ......................................................................... 3-38
3.3.2
Data Analysis ....................................................................................................................... 3-41
3.3.2.1
3.3.3
3.4
Long Creek Basin Characteristics and Study Data ..................................................................... 3-41
Detailed Results for the Long Creek Basin .......................................................................... 3-48
Little Sugar and Briar Creek Analysis ........................................................................................ 3-55
3.4.1
Results Summary for the Little Sugar and Briar Creek Basin.............................................. 3-55
3.4.2
Data Analysis ....................................................................................................................... 3-58
3.4.2.1
3.4.3
3.5
Little Sugar & Briar Creeks Basin Characteristics and Study Data ............................................ 3-58
Detailed Results for the Little Sugar & Briar Creek Basins ................................................ 3-64
Four Mile Creek and Rocky Branch Analysis ............................................................................. 3-72
3.5.1
Results Summary for the Four Mile Creek and Rocky Branch Basins ................................. 3-72
3.5.2
Data Analysis ....................................................................................................................... 3-74
3.5.2.1
3.5.3
3.6
Four Mile Creek and the Rocky Branch Basin Characteristics and Study Data ......................... 3-74
Detailed Results Four Mile Creek and Rocky Branch Basins .............................................. 3-81
McDowell Creek Analysis .......................................................................................................... 3-88
3.6.1
Results Summary for the McDowell Creek Basin ................................................................ 3-88
3.6.2
Data Analysis ....................................................................................................................... 3-91
3.6.2.1
3.6.3
3.7
McDowell Creek Basin Characteristics and Study Data ............................................................ 3-91
Detailed Results for the McDowell Creek Basin .................................................................. 3-97
McMullen Creek Analysis ........................................................................................................ 3-104
3.7.1
Results Summary for the McMullen Creek Basin ............................................................... 3-104
3.7.2
Data Analysis ..................................................................................................................... 3-107
3.7.2.1
3.7.3
3.8
McMullen Creek Basin Characteristics and Study Data .......................................................... 3-107
Detailed Results for the McMullen Creek Basin ................................................................ 3-114
McKee Creek Analysis ............................................................................................................. 3-121
3.8.1
Results Summary for the McKee Creek Basin .................................................................... 3-121
3.8.2
Data Analysis ..................................................................................................................... 3-124
3.8.2.1
3.8.3
3.9
McKee Creek Basin Characteristics and Study Data ............................................................... 3-124
Detailed Results for the McKee Creek Basin ..................................................................... 3-130
Back Creek Analysis ................................................................................................................. 3-136
3.9.1
Results Summary for the Back Creek Basin ....................................................................... 3-136
3.9.2
Data Analysis ..................................................................................................................... 3-139
3.9.2.1
3.9.3
3.10
Back Creek and Back Tributary Basin Characteristics and Study Data ................................... 3-139
Detailed Results for the Back Creek Basin ........................................................................ 3-145
Six Mile Creek Analysis .................................................................................................... 3-151
3.10.1 Results Summary for the Six Mile Creek Basin .................................................................. 3-151
iv
3.10.2 Data Analysis ..................................................................................................................... 3-153
3.10.2.1 Six Mile Creek and Flat Branch Basin Characteristics and Study Data ........................................ 3-153
3.10.3 Detailed Results for the Six Mile Creek Basin ................................................................... 3-160
3.11
Clems Branch Analysis ...................................................................................................... 3-166
3.11.1 Results Summary for the Clems Branch Basin ................................................................... 3-166
3.11.2 Data Analysis ..................................................................................................................... 3-167
3.11.2.1 Clems Branch Basin Characteristics and Study Data.................................................................... 3-167
3.11.3 Detailed Results for the Clems Branch Basin .................................................................... 3-173
3.12
Reedy Creek Analysis ........................................................................................................ 3-175
3.12.1 Results Summary for the Reedy Creek Basin ..................................................................... 3-175
3.12.2 Data Analysis ..................................................................................................................... 3-177
3.12.2.1 Reedy Creek Basin Characteristics and Study Data...................................................................... 3-177
3.12.3 Detailed Results for the Reedy Creek Basin....................................................................... 3-184
3.13
Clarke Creek Analysis ....................................................................................................... 3-190
3.13.1 Results Summary for the Clarke Creek Basin .................................................................... 3-190
3.13.2 Data Analysis ..................................................................................................................... 3-192
3.13.2.1 Clarke Creek Basin Characteristics and Study Data ..................................................................... 3-192
3.13.3 Detailed Results for the Clarke Creek Basin ..................................................................... 3-199
3.14
Mallard Creek Analysis ..................................................................................................... 3-205
3.14.1 Results Summary for the Mallard Creek Basin .................................................................. 3-205
3.14.2 Data Analysis ..................................................................................................................... 3-208
3.14.2.1 Mallard Creek Basin Characteristics and Study Data ................................................................... 3-208
3.14.3 Detailed Results for the Mallard Creek Basin ................................................................... 3-214
3.15
Irwin Creek Analysis ......................................................................................................... 3-222
3.15.1 Results Summary for the Irwin Creek Basin ...................................................................... 3-222
3.15.2 Data Analysis ..................................................................................................................... 3-224
3.15.2.1 Irwin Creek Basin Characteristics and Study Data ....................................................................... 3-224
3.15.3 Detailed Results for the Irwin Creek Basin ........................................................................ 3-231
3.16
Rocky River Analysis ........................................................................................................ 3-239
3.16.1 Results Summary for the Rocky River Basin ...................................................................... 3-239
3.16.2 Data Analysis ..................................................................................................................... 3-241
3.16.2.1 Rocky River Basin Characteristics and Study Data ...................................................................... 3-241
3.16.3 Detailed Results for the Rocky River Basin ....................................................................... 3-249
3.17
Goose Creek Analysis ........................................................................................................ 3-256
3.17.1 Results Summary for the Goose Creek Basin ..................................................................... 3-256
3.17.2 Data Analysis ..................................................................................................................... 3-258
3.17.2.1 Goose Creek Basin Characteristics and Study Data...................................................................... 3-258
3.17.3 Detailed Results for the Goose Creek Basin ...................................................................... 3-266
v
3.18
Lower Clarke Creek Analysis ............................................................................................ 3-272
3.18.1 Results Summary for the Lower Clarke Creek Tributary Basin ......................................... 3-272
3.18.2 Data Analysis ..................................................................................................................... 3-274
3.18.2.1 Clarke Creek Tributary Basin Characteristics and Study Data ..................................................... 3-274
3.18.3 Detailed Results for the Lower Clarke Creek Tributary Basin .......................................... 3-281
3.19
Beaverdam Creek Analysis ................................................................................................ 3-286
3.19.1 Results Summary for the Beaverdam Basin ....................................................................... 3-286
3.20
Clear Creek Analysis ......................................................................................................... 3-287
3.20.1 Results Summary for the Clear Creek Basin ...................................................................... 3-287
3.21
Gar Creek Analysis ............................................................................................................ 3-288
3.21.1 Results Summary for the Gar Creek Basin......................................................................... 3-288
3.22
Paw Creek Analysis ........................................................................................................... 3-289
3.22.1 Results Summary for the Paw Creek Basin ........................................................................ 3-289
3.23
Steele Creek Analysis ........................................................................................................ 3-290
3.23.1 Results Summary for the Steele Creek Basin ..................................................................... 3-290
4. Recommendations for Data Enhancements ....................................................................................... 4-1
4.1
County Tax Assessor Data Enhancement...................................................................................... 4-1
4.2
Integration of Building Footprints and Parcel Data....................................................................... 4-2
4.3
Merging Survey Data and Assignment Data to Building Footprints ............................................. 4-2
4.4
Revisit Parcel Boundary Inconsistencies ...................................................................................... 4-3
4.5
Building Valuation ........................................................................................................................ 4-4
Appendices
A. Reference Tables ................................................................................................................................. A-1
vi
Tables
1-1. Total Estimated Flood Losses for Mecklenburg County ..................................................................... 1-8
1-2. Total Parcels and Total Vacant Parcels By Watershed ....................................................................... 1-9
1-3. Total Number of Flooded Structures by General Occupancy for all Watersheds Analyzed in This
Project to Date ............................................................................................................................. 1-11
1-4. Total Buildings and “Ultimate Build out” ......................................................................................... 1-12
3-1. Estimated Structure Losses for Mecklenburg County ......................................................................... 3-2
3-2. Estimated Contents Losses for Mecklenburg County ......................................................................... 3-6
3.1-1. Number of Flooded Structures by General Occupancy: McAlpine Creek Watershed ..................... 3-7
3.1-2. Summary of Building and Content Damage by General Occupancy: McAlpine Creek
Watershed.................................................................................................................................... 3-10
3.1-3. Foundation Type Distribution by Zone: McAlpine Creek Watershed ........................................... 3-13
3.1-4. Number of Flooded Buildings by HAZUS®MH Occupancy Type: McAlpine Creek Watershed .... 3-15
3.1-5. Structure Damage Percent by HAZUS®MH Occupancy Type: McAlpine Creek Watershed .......... 3-17
3.1-6. Structure Dollar Damage by HAZUS®MH Occupancy Type: McAlpine Creek Watershed ............ 3-18
3.1-7. Contents Damage Percent by HAZUS®MH Occupancy Type: McAlpine Creek Watershed .......... 3-19
3.1-8. Contents Dollar Damage by HAZUS®MH Occupancy Type: McAlpine Creek Watershed............. 3-20
3.2-1. Number of Flooded Structures by General Occupancy: Sugar Creek Watershed .......................... 3-21
3.2-2. Summary of Building and Content Damage by General Occupancy: Sugar Creek Watershed ..... 3-23
3.2-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Sugar Creek Watershed ........... 3-32
3.2-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Sugar Creek Watershed .. 3-34
3.2-5. Structure Dollar Damage by HAZUS®MH Occupancy Type: Sugar Creek Watershed................... 3-35
3.2-6. Contents Damage Percent by HAZUS®MH Occupancy Type: Sugar Creek Watershed ................. 3-36
3.2-7. Contents Dollar Damage by HAZUS®MH Occupancy Type: Sugar Creek Watershed ................... 3-37
3.3-1. Number of Flooded Structures by General Occupancy: Long Creek Watershed ........................... 3-38
3.3-2. Summary of Building and Content Damage by General Occupancy: Long Creek Watershed ...... 3-40
3.3-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Long Creek Watershed ............ 3-49
3.3-4. Structure Damage Percent by HAZUS®MH Occupancy Type: Long Creek Watershed .................. 3-51
3.3-5. Structure Dollar Damage by HAZUS®MH Occupancy Type: Long Creek Watershed ................... 3-52
3.3-6. Contents Damage Percent by HAZUS®MH Occupancy Type: Long Creek Watershed .................. 3-53
3.3-7. Contents Dollar Damage by HAZUS®MH Occupancy Type: Long Creek Watershed .................... 3-54
3.4-1. Number of Flooded Structures by General Occupancy: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-55
3.4-2. Summary of Building and Content Damage by General Occupancy: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-57
3.4-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-66
vii
3.4-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-68
3.4-5. Structure Dollar Damage by HAZUS®MH Occupancy Type: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-69
3.4-6. Contents Damage Percent by HAZUS®MH Occupancy Type: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-70
3.4-7. Contents Dollar Damage by HAZUS®MH Occupancy Type: Little Sugar and Briar Creek
Watershed.................................................................................................................................... 3-71
3.5-1. Number of Flooded Structures by General Occupancy: Four Mile Creek and Rocky Branch
Watersheds .................................................................................................................................. 3-72
3.5-2. Summary of Building and Content Damage by General Occupancy Four Mile Creek and Rocky
Branch Watersheds...................................................................................................................... 3-74
3.5-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Four Mile Creek and Rocky Branch
Watersheds .................................................................................................................................. 3-83
3.5-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Four Mile Creek and Rocky
Branch Watersheds...................................................................................................................... 3-85
3.5-5. Structure Dollar Damage by HAZUS®MH Occupancy Type: Four Mile Creek and Rocky Branch
Watersheds .................................................................................................................................. 3-86
3.5-6. Contents Damage Percent by HAZUS®MH Occupancy Type: Four Mile Creek and Rocky Branch
Watersheds .................................................................................................................................. 3-86
3.5-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Four Mile Creek and Rocky Branch
Watersheds .................................................................................................................................. 3-87
3.6-1. Number of Flooded Structures by General Occupancy: McDowell Creek Watershed ................. 3-88
3.6-2. Summary of Building and Content Damage by General Occupancy: McDowell Creek
Watershed.................................................................................................................................... 3-90
3.6-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: McDowell Creek Watershed ... 3-98
3.6-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: McDowell Creek
Watershed.................................................................................................................................. 3-100
3.6-5. Structure Dollar Damage by HAZUS®MH Occupancy Type McDowell Creek Watershed .......... 3-101
3.6-6. Contents Damage Percent by HAZUS®MH Occupancy Type McDowell Creek Watershed ......... 3-101
3.6-7. Contents Dollar Damage by HAZUS®MH Occupancy Type McDowell Creek Watershed .......... 3-103
3.7-1. Number of Flooded Structures by General Occupancy: McMullen Creek Watershed................ 3-104
3.7-2. Summary of Building and Content Damage by General Occupancy: McMullen Creek
Watershed.................................................................................................................................. 3-106
3.7-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: McMullen Creek
Watershed.................................................................................................................................. 3-115
viii
3.7-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: McMullen Creek
Watershed.................................................................................................................................. 3-117
3.7-5. Structure Dollar Damage by HAZUS®MH Occupancy Type McMullen Creek Watershed .......... 3-118
3.7-6. Contents Damage Percent by HAZUS®MH Occupancy Type McMullen Creek Watershed ......... 3-119
3.7-7. Contents Dollar Damage by HAZUS®MH Occupancy Type McMullen Creek Watershed ........... 3-120
3.8-1. Number of Flooded Structures by General Occupancy: McKee Creek Watershed..................... 3-121
3.8-2. Summary of Building and Content Damage by General Occupancy: McKee Creek
Watershed.................................................................................................................................. 3-123
3.8-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: McKee Creek Watershed ...... 3-131
3.8-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: McKee Creek
Watershed.................................................................................................................................. 3-133
3.8-5. Structure Dollar Damage by HAZUS®MH Occupancy Type McKee Creek Watershed ............... 3-134
3.8-6. Contents Damage Percent by HAZUS®MH Occupancy Type McKee Creek Watershed .............. 3-134
3.8-7. Contents Dollar Damage by HAZUS®MH Occupancy Type McKee Creek Watershed ................ 3-135
3.9-1. Number of Flooded Structures by General Occupancy: Back Creek Watershed ........................ 3-136
3.9-2. Summary of Building and Content Damage by General Occupancy: Back Creek Watershed .... 3-138
3.9-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Back Creek Watershed ......... 3-146
3.9-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Back Creek Watershed . 3-148
3.9-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Back Creek Watershed ................... 3-149
3.9-6. Contents Damage Percent by HAZUS®MH Occupancy Type Back Creek Watershed ................. 3-149
3.9-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Back Creek Watershed ................... 3-150
3.10-1. Number of Flooded Structures by General Occupancy: Six Mile Creek Watershed................. 3-151
3.10-2. Summary of Building and Content Damage by General Occupancy: Six Mile Creek
Watershed.................................................................................................................................. 3-153
3.10-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Six Mile Creek
Watershed.................................................................................................................................. 3-161
3.10-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Six Mile Creek
Watershed.................................................................................................................................. 3-163
3.10-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Six Mile Creek Watershed ........... 3-164
3.10-6. Contents Damage Percent by HAZUS®MH Occupancy Type Six Mile Creek Watershed .......... 3-164
3.10-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Six Mile Creek Watershed ............ 3-165
3.11-1. Number of Flooded Structures by General Occupancy: Clems Branch Watershed .................. 3-166
3.11-2. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Clems Branch Watershed .... 3-174
3.12-1. Number of Flooded Structures by General Occupancy: Reedy Creek Watershed .................... 3-175
3.12-2. Summary of Building and Content Damage by General Occupancy: Reedy Creek
Watershed.................................................................................................................................. 3-177
3.12-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Reedy Creek Watershed ...... 3-185
ix
3.12-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Reedy Creek
Watershed.................................................................................................................................. 3-187
3.12-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Reedy Creek Watershed ............... 3-188
3.12-6. Contents Damage Percent by HAZUS®MH Occupancy Type Reedy Creek Watershed ............. 3-188
3.12-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Reedy Creek Watershed ............... 3-189
3.13-1. Number of Flooded Structures by General Occupancy: Clarke Creek Watershed .................... 3-190
3.13-2. Summary of Building and Content Damage by General Occupancy: Clarke Creek
Watershed.................................................................................................................................. 3-192
3.13-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Clarke Creek Watershed .... 3-200
3.13-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Clarke Creek
Watershed.................................................................................................................................. 3-202
3.13-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Clarke Creek Watershed .............. 3-203
3.13-6. Contents Damage Percent by HAZUS®MH Occupancy Type Clarke Creek Watershed ............. 3-203
3.13-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Clarke Creek Watershed ............... 3-204
3.14-1. Number of Flooded Structures by General Occupancy: Mallard Creek Watershed .................. 3-205
3.14-2. Summary of Building and Content Damage by General Occupancy: Mallard Creek
Watershed.................................................................................................................................. 3-207
3.14-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Mallard Creek Watershed ... 3-216
3.14-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Mallard Creek
Watershed.................................................................................................................................. 3-218
3.14-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Mallard Creek Watershed ............ 3-219
3.14-6. Contents Damage Percent by HAZUS®MH Occupancy Type Mallard Creek Watershed ........... 3-220
3.14-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Mallard Creek Watershed ............. 3-221
3.15-1. Number of Flooded Structures by General Occupancy: Irwin Creek Watershed ...................... 3-222
3.15-2. Summary of Building and Content Damage by General Occupancy: Irwin Creek
Watershed.................................................................................................................................. 3-224
3.15-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Irwin Creek Watershed ....... 3-233
3.15-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Irwin Creek
Watershed.................................................................................................................................. 3-235
3.15-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Irwin Creek Watershed ................ 3-236
3.15-6. Contents Damage Percent by HAZUS®MH Occupancy Type: Irwin Creek Watershed .............. 3-237
3.15-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Irwin Creek Watershed ................. 3-238
3.16-1. Number of Flooded Structures by General Occupancy: Rocky River Basin............................. 3-239
3.16-2. Summary of Building and Content Damage by General Occupancy: Rocky River Basin ........ 3-241
3.16-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Rocky River Basin ............. 3-251
3.16-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Rocky River Basin ...... 3-253
3.16-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Rocky River Basin ....................... 3-253
x
3.16-6. Contents Damage Percent by HAZUS®MH Occupancy Type Rocky River Basin ...................... 3-254
3.16-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Rocky River Basin ........................ 3-255
3.17-1. Number of Flooded Structures by General Occupancy: Goose Creek Watershed .................... 3-256
3.17-2. Summary of Building and Content Damage by General Occupancy: Goose Creek
Watershed.................................................................................................................................. 3-258
3.17-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Goose Creek Watershed ...... 3-267
3.17-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Goose Creek
Watershed.................................................................................................................................. 3-269
3.17-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Goose Creek Watershed ............... 3-269
3.17-6. Contents Damage Percent by HAZUS®MH Occupancy Type Goose Creek Watershed.............. 3-270
3.17-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Goose Creek Watershed ............... 3-271
3.18-1. Number of Flooded Structures by General Occupancy: Lower Clarke Creek Watershed ........ 3-272
3.18-2. Summary of Building and Content Damage by General Occupancy: Lower Clarke Creek
Watershed.................................................................................................................................. 3-274
3.18-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type: Lower Clarke Creek
Watershed.................................................................................................................................. 3-282
3.18-4. Average Structure Damage Percent by HAZUS®MH Occupancy Type: Lower Clarke Creek
Watershed.................................................................................................................................. 3-283
3.18-5. Structure Dollar Damage by HAZUS®MH Occupancy Type Lower Clarke Creek Watershed ... 3-284
3.18-6. Contents Damage Percent by HAZUS®MH Occupancy Type Lower Clarke Creek Watershed .. 3-285
3.18-7. Contents Dollar Damage by HAZUS®MH Occupancy Type Lower Clarke Creek Watershed ... 3-285
A-1. Mapping of Mecklenburg Use Codes into HAZUS®MH Occupancy Codes....................................... A-1
A-2. Estimated Typical Floor Area and Cost per Square Foot by Mecklenburg Occupancy Code ........... A-4
A-3. HAZUS®MH Content Value as a Percentage of Structure Value........................................................ A-6
A-4. HAZUS®MH Foundation Distribution (default) and Adjusted Foundation Distribution Used in
Mecklenburg County .................................................................................................................... A-7
Figures
1-1. Watersheds Analyzed For Mecklenburg County ................................................................................ 1-3
1-2. Total Estimated Losses for Mecklenburg County by Analysis Run.................................................... 1-5
1-3. Total Estimated Building and Content Losses for Mecklenburg County by Analysis Run ................ 1-6
2-1. Overview of Data Provided by Mecklenburg County (Image is from the Goose Creek Watershed) .. 2-4
2-2. FIA Credibility-Weighted Building Depth-Damage Curves as of 12/31/1998 ................................ 2-10
3-1. Total Estimated Losses by Watershed for Run 2 ................................................................................ 3-5
3.1-1. McAlpine Creek Watershed Aggregate Damage Estimation Results .............................................. 3-8
3.1-2. Floor Height Distribution Zones .................................................................................................... 3-12
xi
3.1-3. McAlpine Creek Watershed Aggregated Damage Estimation Results .......................................... 3-16
3.2-1. Sugar Creek Watershed Aggregate Damage Estimation Results ................................................... 3-22
3.2-2. Sugar Creek Basin Stream Network ............................................................................................... 3-24
3.2-3. Existing Conditions Flood Depth Grid ........................................................................................... 3-26
3.2-4. Distribution of Existing Condition Flood Depths........................................................................... 3-26
3.2-5. Future Conditions Flood Depth Grid.............................................................................................. 3-27
3.2-6. Distribution of Future Condition Flood Depths ............................................................................. 3-28
3.2-7. Increases in 100-year Flood Depths ............................................................................................... 3-29
3.2-8. Differences in 100-year Flood Depths ........................................................................................... 3-29
3.2-9. Sugar Creek Watershed Aggregate Damage Estimation Results ................................................... 3-33
3.3-1. Long Creek Watershed Aggregate Damage Estimation Results .................................................... 3-39
3.3-2. Long Creek Basin Stream Network................................................................................................ 3-41
3.3-3. Existing Conditions Flood Depth Grid ........................................................................................... 3-43
3.3-4. Distribution of Existing Condition Flood Depths........................................................................... 3-43
3.3-5. Future Conditions Flood Depth Grid.............................................................................................. 3-44
3.3-6. Distribution of Future Condition Flood Depths ............................................................................. 3-45
3.3-7. Increases in 100-year Flood Depths ............................................................................................... 3-46
3.3-8. Differences in 100-year Flood Depths ........................................................................................... 3-46
3.3-9. Long Creek Watershed Aggregate Damage Estimation Results .................................................... 3-50
3.4-1. Little Sugar & Briar Creeks Aggregate Damage Estimation Results ............................................. 3-56
3.4-2. Little Sugar Creek Basin Stream Network ..................................................................................... 3-58
3.4-3. Existing Conditions Flood Depth Grid ........................................................................................... 3-59
3.4-4. Distribution of Existing Condition Flood Depths........................................................................... 3-60
3.4-5. Future Conditions Flood Depth Grid.............................................................................................. 3-61
3.4-6. Distribution of Future Condition Flood Depths ............................................................................. 3-61
3.4-7. Increases in 100-year Flood Depths ............................................................................................... 3-62
3.4-8. Differences in 100-year Flood Depths ........................................................................................... 3-63
3.4-9. Little Sugar & Briar Creek Aggregate Damage Estimation Results .............................................. 3-67
3.5-1. Four Mile Creek and Rocky Branch Watersheds Aggregate Damage Estimation Results ............ 3-73
3.5-2. McAlpine Creek Basin Stream Network ........................................................................................ 3-75
3.5-3. Existing Conditions Flood Depth Grid: Four Mile Creek and Rocky Branch ................................ 3-76
3.5-4. Distribution of Existing Condition Flood Depths: Four Mile Creek and Rocky Branch ............... 3-77
3.5-5. Future Conditions Flood Depth Grid Four Mile Creek and Rocky Branch.................................... 3-78
3.5-6. Distribution of Future Condition Flood Depths Four Mile Creek and Rocky Branch ................... 3-78
3.5-7. Increases in 100-year Flood Depths ............................................................................................... 3-79
3.5-8. Differences in 100-year Flood Depths: Four Mile Creek and Rocky Branch ................................ 3-80
3.5-9. Four Mile Creek And Rocky Branch Aggregate Damage Estimation Results ............................... 3-84
xii
3.6-1. McDowell Creek Watershed Aggregate Damage Estimation Results............................................ 3-89
3.6-2. McDowell Creek Basin Stream Network ....................................................................................... 3-91
3.6-3. Existing Conditions Flood Depth Grid McDowell Creek Watershed ............................................ 3-92
3.6-4. Distribution of Existing Condition Flood Depths McDowell Creek Watershed ............................ 3-93
3.6-5. Future Conditions Flood Depth Grid McDowell Creek Watershed ............................................... 3-94
3.6-6. Distribution of Future Condition Flood Depths McDowell Creek Watershed ............................... 3-94
3.6-7. Increases in 100-year Flood Depths McDowell Creek Watershed ................................................. 3-95
3.6-8. Differences in 100-year Flood Depths McDowell Creek Watershed ............................................. 3-96
3.6-9. McDowell Creek Watershed Aggregate Damage Estimation Results............................................ 3-99
3.7-1. McMullen Creek Watershed Aggregate Damage Estimation Results .......................................... 3-105
3.7-2. McAlpine Creek Basin Stream Network McMullen Creek Watershed ........................................ 3-107
3.7-3. Existing Conditions Flood Depth Grid McMullen Creek Watershed........................................... 3-109
3.7-4. Distribution of Existing Condition Flood Depths McMullen Creek Watershed .......................... 3-109
3.7-5. Future Conditions Flood Depth Grid McMullen Creek Watershed ............................................. 3-110
3.7-6. Distribution of Future Condition Flood Depths McMullen Creek Watershed ............................. 3-111
3.7-7. Increases in 100-year Flood Depths McMullen Creek Watershed ............................................... 3-112
3.7-8. Differences in 100-year Flood Depths McMullen Creek Watershed ........................................... 3-112
3.7-9. McMullen Creek Watershed Aggregate Damage Estimation Results .......................................... 3-116
3.8-1. McKee Creek Watershed Aggregate Damage Estimation Results ............................................... 3-122
3.8-2. McKee Creek and Nearby Stream Systems.................................................................................. 3-124
3.8-3. Existing Conditions Flood Depth Grid ......................................................................................... 3-125
3.8-4. Distribution of Existing Condition Flood Depths......................................................................... 3-126
3.8-5. Future Conditions Flood Depth Grid............................................................................................ 3-127
3.8-6. Distribution of Future Condition Flood Depths ........................................................................... 3-127
3.8-7. Increases in 100-year Flood Depths ............................................................................................. 3-128
3.8-8. Differences in 100-year Flood Depths ......................................................................................... 3-129
3.8-9. McKee Creek Watershed Aggregate Damage Estimation Results ............................................... 3-132
3.9-1. Back Creek Watershed Aggregate Damage Estimation Results .................................................. 3-137
3.9-2. Back Creek and Back Creek Tributary ......................................................................................... 3-139
3.9-3. Existing Conditions Flood Depth Grid ......................................................................................... 3-140
3.9-4. Distribution of Existing Condition Flood Depths......................................................................... 3-141
3.9-5. Future Conditions Flood Depth Grid............................................................................................ 3-142
3.9-6. Distribution of Future Condition Flood Depths ........................................................................... 3-142
3.9-7. Increases in 100-year Flood Depths ............................................................................................. 3-143
3.9-8. Differences in 100-year Flood Depths ......................................................................................... 3-144
3.9-9. Back Creek Watershed Aggregate Damage Estimation Results .................................................. 3-147
3.10-1. Six Mile Creek Watershed Aggregate Damage Estimation Results ........................................... 3-152
xiii
3.10-2. Six Mile Creek and Flat Branch ................................................................................................. 3-154
3.10-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-155
3.10-4. Distribution of Existing Condition Flood Depths....................................................................... 3-156
3.10-5. Future Conditions Flood Depth Grid.......................................................................................... 3-157
3.10-6. Distribution of Future Condition Flood Depths ......................................................................... 3-157
3.10-7. Increases in 100-year Flood Depths ........................................................................................... 3-158
3.10-8. Differences in 100-year Flood Depths ....................................................................................... 3-159
3.10-9. Six Mile Creek Watershed Aggregate Damage Estimation Results ........................................... 3-162
3.11-1. Clems Branch and Nearby Stream Systems ............................................................................... 3-167
3.11-2. Existing Conditions Flood Depth Grid ....................................................................................... 3-168
3.11-3. Distribution of Existing Condition Flood Depths....................................................................... 3-169
3.11-4. Future Conditions Flood Depth Grid.......................................................................................... 3-170
3.11-5. Distribution of Future Condition Flood Depths ......................................................................... 3-170
3.11-6. Increases in 100-year Flood Depths ........................................................................................... 3-171
3.11-7. Differences in 100-year Flood Depths ....................................................................................... 3-172
3.12-1. Reedy Creek Watershed Aggregate Damage Estimation Results .............................................. 3-176
3.12-2. Reedy Creek Basin Stream Network .......................................................................................... 3-178
3.12-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-179
3.12-4. Distribution of Existing Condition Flood Depths....................................................................... 3-180
3.12-5. Future Conditions Flood Depth Grid.......................................................................................... 3-181
3.12-6. Distribution of Future Condition Flood Depths ......................................................................... 3-181
3.12-7. Increases in 100-year Flood Depths ........................................................................................... 3-182
3.12-8. Differences in 100-year Flood Depths ....................................................................................... 3-183
3.12-9. Reedy Creek Watershed Aggregate Damage Estimation Results .............................................. 3-186
3.13-1. Clarke Creek Watershed Aggregate Damage Estimation Results .............................................. 3-191
3.13-2. Clarke Creek Basin Stream Network ......................................................................................... 3-193
3.13-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-194
3.13-4. Distribution of Existing Condition Flood Depths....................................................................... 3-195
3.13-5. Future Conditions Flood Depth Grid.......................................................................................... 3-196
3.13-6. Distribution of Future Condition Flood Depths ......................................................................... 3-196
3.13-7. Increases in 100-year Flood Depths ........................................................................................... 3-197
3.13-8. Differences in 100-year Flood Depths ....................................................................................... 3-198
3.13-9. Clarke Creek Watershed Aggregate Damage Estimation Results .............................................. 3-201
3.14-1. Mallard Creek Watershed Aggregate Damage Estimation Results ............................................ 3-206
3.14-2. Mallard Creek Basin Stream Network ....................................................................................... 3-208
3.14-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-209
3.14-4. Distribution of Existing Condition Flood Depths....................................................................... 3-210
xiv
3.14-5. Future Conditions Flood Depth Grid.......................................................................................... 3-211
3.14-6. Distribution of Future Condition Flood Depths ......................................................................... 3-211
3.14-7. Increases in 100-year Flood Depths ........................................................................................... 3-212
3.14-8. Differences in 100-year Flood Depths ....................................................................................... 3-213
3.14-9. Mallard Creek Watershed Aggregate Damage Estimation Results ............................................ 3-217
3.15-1. Irwin Creek Watershed Aggregate Damage Estimation Results ................................................ 3-223
3.15-2. Irwin Creek Stream System........................................................................................................ 3-225
3.15-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-226
3.15-4. Distribution of Existing Condition Flood Depths....................................................................... 3-227
3.15-5. Future Conditions Flood Depth Grid.......................................................................................... 3-228
3.15-6. Distribution of Future Condition Flood Depths ......................................................................... 3-228
3.15-7. Increases in 100-year Flood Depths ........................................................................................... 3-229
3.15-8. Differences in 100-year Flood Depths ....................................................................................... 3-230
3.15-9. Irwin Creek Watershed Aggregate Damage Estimation Results ................................................ 3-234
3.16-1. Rocky River Basin Aggregate Damage Estimation Results ....................................................... 3-240
3.16-2. Rocky River Stream System ...................................................................................................... 3-242
3.16-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-243
3.16-4. Distribution of Existing Condition Flood Depths....................................................................... 3-244
3.16-5. Future Conditions Flood Depth Grid.......................................................................................... 3-245
3.16-6. Distribution of Future Condition Flood Depths ......................................................................... 3-246
3.16-7. Increases in 100-year Flood Depths ........................................................................................... 3-247
3.16-8. Differences in 100-year Flood Depths ....................................................................................... 3-248
3.16-9. Rocky River Basin Aggregate Damage Estimation Results ....................................................... 3-252
3.17-1. Goose Creek Watershed Aggregate Damage Estimation Results .............................................. 3-257
3.17-2. Goose Creek Basin Stream Network .......................................................................................... 3-259
3.17-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-260
3.17-4. Distribution of Existing Condition Flood Depths....................................................................... 3-261
3.17-5. Future Conditions Flood Depth Grid.......................................................................................... 3-262
3.17-6. Distribution of Future Condition Flood Depths ......................................................................... 3-263
3.17-7. Increases in 100-year Flood Depths ........................................................................................... 3-264
3.17-8. Differences in 100-year Flood Depths ....................................................................................... 3-264
3.17-9. Goose Creek Watershed Aggregate Damage Estimation Results .............................................. 3-268
3.18-1. Clarke Creek Tributary Aggregate Damage Estimation Results ................................................ 3-273
3.18-2. Clarke Creek Tributary and Adjacent Basins ............................................................................. 3-275
3.18-3. Existing Conditions Flood Depth Grid ....................................................................................... 3-276
3.18-4. Distribution of Existing Condition Flood Depths....................................................................... 3-277
3.18-5. Future Conditions Flood Depth Grid.......................................................................................... 3-278
xv
3.18-6. Distribution of Future Condition Flood Depths ......................................................................... 3-278
3.18-7. Increases in 100-year Flood Depths ........................................................................................... 3-279
3.18-8. Differences in 100-year Flood Depths ....................................................................................... 3-280
3.18-9. Clarke Creek Tributary Aggregate Damage Estimation Results ................................................ 3-283
4-1. ArcView Image of Parcel Data and Survey Points.............................................................................. 4-4
xvi
1. Executive Summary
1.1
Project Background
The National Institute of Building Sciences (NIBS) and the Federal Emergency
Management Agency (FEMA) have contracted with ABSG Consulting (ABS), formerly
EQE International, and Michael Baker Jr. Corporation (MBJ) to develop a nationally
applicable flood loss estimation methodology that will be incorporated into the HazardsUS (HAZUS) software. The software development process requires a significant amount
of time, and software release is scheduled for summer 2003. An oversight committee
that includes a representative from Mecklenburg County guides the flood model
development.
It was through this effort that officials from Mecklenburg County
envisioned the following study.
Mecklenburg County was seeking a new approach to determine the impacts of changes
to floodplain mapping, to drive mitigation planning initiatives and improve public
In January 2000, the HAZUS®MH Flood Project Team
information dissemination.
presented a status report to the flood model oversight committee regarding the
methodology for inventory development and flood loss estimation.
At that time, it
became evident that portions of the HAZUS®MH methodology could be utilized to address
the
County’s
concerns
regarding
mapping
revisions
and
public
information
dissemination. It was felt that providing estimates of potential flood losses would help
policy makers and the general public better understand the effects of map revisions,
thereby increasing support for future mitigation planning decisions.
Mecklenburg
requested that the HAZUS®MH flood model contractors submit a proposal to study a
single watershed to evaluate the potential benefits of using loss estimates to evaluate
the impacts of map revisions.
1-1
In April 2000, ABS and MBJ undertook a project to estimate flood losses1 for a pilot
watershed in Mecklenburg County – the McAlpine Creek watershed2. The purpose of
the effort was to utilize the preliminary NIBS/FEMA HAZUS®MH Flood Loss Estimation
Methodology to estimate potential flood damage based on Mecklenburg’s original and
revised floodplain maps. Upon completion of the project, Mecklenburg County, ABS,
and MBJ entered into discussions about performing similar analyses on the remaining
watersheds within the county.
ABS and MBJ developed a multi-task approach that resulted in this binder of analysis
results for 17 watersheds. The original project was scoped with all 24 watersheds, but
as the project progressed, it became apparent that some watersheds could be combined
into a single report and some watersheds were deemed minor issues that could be
addressed at a later time. Therefore, this binder, while providing 22 sections, contains
reports for only 17 watersheds.
Figure 1-1 below shows the analyzed watersheds
included in this report.
1
Flood depth data for the pilot watershed were generated by MBJ, while inventory development
and damage estimation were performed by ABS Consulting (formerly EQE International).
2
EQE International, “Determination of Impacts From Flood Study Modifications: McAlpine Creek
Watershed (Final),” Prepared for Mecklenburg County Engineering and Building Standards, June
2000.
1-2
Figure 1-1. Watersheds Analyzed For Mecklenburg County
1.2
Report Organization
The organization of the report binder is as follows:
1. Executive Summary – this section provides an overview of the project and a
synopsis of results with a comparison of all the watersheds and a brief discussion on
mitigation or floodplain regulation opportunities for the County.
2. Data Analysis – this section provides a detailed discussion related to the
development of the data, the application of the HAZUS®MH methodology including the
application of the default building parameters, modified to support local trends, the
selection of building damage functions, and finally the analysis methods.
1-3
3. Flood Hazard Assessments – this section is further subdivided into sections
containing the individual watershed results reported to the County during the course
of the project. Each watershed has been assigned a section and the results are
presented as independent reports allowing the County to remove the sections and
utilize, reprint, or update without having to revise the entire report.
4. Suggestions for Geographic Information Systems (GIS) Data enhancement and
future data collection – this section is provided to the County to assist their own
internal planning and data development. The recommendations presented in this
section are intended to advance the County’s ability to continue to lead the floodplain
management community through the coordinated application of data developed by
the various departments within the County.
1.3
Analysis Synopsis
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed for each watershed, as follows:
1. Estimated losses based on the current building inventory within the watershed
(developed from surveyed data and parcel data) and the original 100-year floodplain
boundary, based on the 1975 flood study (Run 1 – current inventory, original
floodplain).
2. Estimated losses based on the current building inventory, expanded to include
structures within the new floodplain that were previously outside the floodplain, and
the newly revised floodplain boundary (Run 2 – current inventory, new floodplain).
3. Estimated losses to a projected future inventory that incorporates projected growth
(“build out”), relative to the newly revised floodplain boundary (Run 3 – future
inventory, new floodplain).
The following tables provide a combined view of the results from the 17 reports prepared
for the County and represents 19 of the County’s 24 watersheds. The remaining 5
watersheds are smaller watersheds that are not considered significant problem areas at
this time.
1-4
Figure 1-2 shows the total estimated losses (structure and contents) for Mecklenburg
County based on the watersheds analyzed to date. The figure shows how the new flood
study methodology, including improved H&H models and digital terrain modes, and
increased impervious soils from future development, indicates that a 100-year flood will
result in greater estimated damages than those estimated using the older 1975 flood
study. This is indicated by the increase in estimated damage from Run 1 to Run 2.
Finally, Run 3 shows the increased estimated damage resulting from development within
the floodplain that could occur without the increased floodplain regulations currently
approved by the County.
$700,000,000
$600,000,000
Estimated Loss ($)
$500,000,000
$400,000,000
$300,000,000
$200,000,000
$100,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 1-2. Total Estimated Losses for Mecklenburg County by Analysis Run
Assuming effective floodplain management, Figure 1-2 shows that the County has the
potential to prevent over $300 million in structure and content losses throughout the
county. The County has been very progressive in its mitigation planning and floodplain
management and therefore has a very good opportunity to maximize the net reduction in
losses.
1-5
To break these losses, Figure 1-3 shows the distribution of the structure and content
losses for the county as a whole. It is interesting to note that the estimated content
losses are greater than the estimated building damage. This is discussed in more detail
later in this report, but in general that is an indication that losses to non-residential
properties are contributing a significant amount to the total losses and typically the
contents of non-residential properties is higher than the value of the building itself.
$400,000,000
Structure
Content
$350,000,000
Estimated Loss (Dollars)
$300,000,000
$250,000,000
$200,000,000
$150,000,000
$100,000,000
$50,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 1-3. Total Estimated Building and Content Losses for Mecklenburg
County by Analysis Run
Table 1-1 provides a quick look at the total estimated losses for all of the watersheds
analyzed in this project. The total losses are a combination of structure (i.e., frame,
drywall, insulation, etc.) and contents (i.e., furnishings, equipment, carpet, etc.) losses
and represent the total impact of the flooding on the building. It is important to note that
the occupancy type of the buildings can have a significant impact on the ratio of
structure value to contents value. For example, in residential homes the contents values
are typically estimated to be 50% of the value of the structure, whereas in commercial
1-6
buildings, the contents may be estimated to be as much as 150% of the value of the
structure.
Table 1-1 shows the total losses for all three runs as discussed above. The Table also
demonstrates the increased damage between Run 2 and Run 3 where the 100-year
flood elevations from the 2000 flood study were used with the current inventory (Run 2)
and future or ultimate build-out (Run 3). The differences reflect new structures that may
fall within areas of flooding.
For that reason, the difference between the two runs
(tabulated as “Future development Damage”) can be seen as potential damage that can
be avoided with proper floodplain management. The differences are shown as both the
net difference and the percent increase between Run 2 and Run 3.
Finally a column is provided showing the increase in damage between Run 2 and Run 3
for each watershed as a function of the total increase in damage for all the watersheds in
the County. The purpose of this column is to allow the County to rank each watershed
as potential candidates for focused mitigation strategies. As seen in Table 1-1 three
watersheds comprise 66% of the total potential increased damage throughout the
County, and the top five watersheds account for 86% of the total potential increased
damage. For clarity, these watersheds are shown in bold.
1-7
Table 1-1. Total Estimated Flood Losses for Mecklenburg County
Watershed (Report Section)
McAlpine1 (3.1)
Sugar Creek (3.2)
Long Creek (3.3)
Little Sugar & Briar Creek (3-4)
Four Mile Creek (3.5)
McDowell Creek (3.6)
McMullen Creek (3.7)
McKee Creek (3.8)
Back Creek (3.9)
Six Mile Creek (3.10)
Clems Branch (3.11)
Reedy Creek (3.12)
Clarke Creek (3.13)
Mallard Creek (3.14)
Irwin Creek (3.15)
Rocky River (3.16)
Goose Creek (3.17)
Lower Clarke Creek (3.18)
Total
Total Estimated Losses
(Structure and Content Losses)
Run 1
Run 2
Run 3
$8,832,391
$6,665,418
$473,400
$150,033,099
$689,203
$566,154
$3,373,730
$11,904
$80,646
$718,663
$0
$285,267
$290,869
$3,893,488
$37,062,512
$3,485
$62,947
$0
$213,043,176
$14,078,086
$13,241,821
$860,879
$214,156,486
$1,318,264
$4,167,603
$5,729,832
$329,093
$105,480
$1,139,041
$0
$558,995
$358,509
$3,948,353
$57,320,865
$158,173
$199,602
$0
$317,671,082
$47,163,181
$67,593,742
$8,439,091
$326,867,763
$5,617,336
$16,894,746
$12,703,814
$2,872,244
$1,141,915
$4,899,923
$0
$3,153,789
$1,671,756
$38,442,556
$110,203,790
$2,270,997
$512,974
$202,407
$650,652,024
Future Development
Damage (Run 2- Run 3)
$33,085,095
$54,351,921
$7,578,212
$112,711,277
$4,299,072
$12,727,143
$6,973,982
$2,543,151
$1,036,435
$3,760,882
$0
$2,594,794
$1,313,247
$34,494,203
$52,882,925
$2,112,824
$313,372
$202,407
$332,980,942
Increase in
Damage Between
Run 2 & Run 3 (%)
235.0
410.5
880.3
52.6
326.1
305.4
121.7
772.8
982.6
330.2
0.0
464.2
366.3
873.6
92.3
1,335.8
157.0
---
Future
Development
Damage (% of
Total)
9.9
16.3
2.3
33.8
1.3
3.8
2.1
0.8
0.3
1.1
0.0
0.8
0.4
10.4
15.9
0.6
0.1
0.1
1. The McAlpine watershed analysis was performed on a parcel basis as the building footprint coverage was not available when the analysis for this watershed was
performed.
1-8
Table 1-2 provides insight into the mitigation or regulation possibilities for the County.
Each watershed contains a number of developed and undeveloped or vacant parcels.
For analysis purposes, the project team took the 100-year floodplain boundary from the
2000 flood studies and added a 100-foot buffer. This buffer was added to ensure that
the project was conservative and that all parcels very close to and within the floodplain
boundary were selected. While this is conservative in estimating the total number of
parcels potentially impacted, it has no affect on the final estimated losses for the county.
Table 1-2 lists the watersheds and the number of parcels within the floodplain boundary,
and indicates how many of those parcels were vacant and subject to future
development. The top five watersheds from Table 1-1 are also highlighted in this table
with bold letters so the reader may quickly crosscheck the tables.
Table 1-2. Total Parcels and Total Vacant Parcels By Watershed
Watershed (Report Section)
McAlpine1 (3.1)
Sugar Creek (3.2)
Long Creek (3.3)
Little Sugar & Briar Creek (3.4)
Four Mile Creek (3.5)
McDowell Creek (3.6)
McMullen Creek (3.7)
McKee Creek (3.8)
Back Creek (3.9)
Six Mile Creek (3.10)
Clems Branch (3.11)
Reedy Creek (3.12)
Clarke Creek (3.13)
Mallard Creek (3.14)
Irwin Creek (3.15)
Rocky River (3.16)
Goose Creek (3.17)
Lower Clarke Creek (3.18)
Total
Parcels in
Floodplain2
Vacant
Parcels
Percent
Vacant
2,516
1,115
1,228
4,657
1,098
1,657
1,427
345
204
533
15
464
249
1,394
2,005
263
135
89
19,394
418
416
330
715
182
454
95
136
75
154
10
135
126
474
638
113
23
42
4,536
17
37
35
15
17
27
7
39
37
29
67
29
51
34
32
43
17
47
Rank by
Vacant
Parcels (%)
14
7
8
17
15
13
18
5
6
11
1
12
2
9
10
4
16
3
1. The McAlpine watershed analysis was performed on a parcel basis as the building footprint coverage was not
available when the analysis for this watershed was performed.
2. Includes parcels that intersect the 100-foot buffer surrounding the floodplain.
1-9
When used together, Table 1-1 and Table 1-2 provide a clear picture for the County by
identifying watersheds where the current inventory is the most vulnerable (those
watersheds with large losses and large numbers of developed parcels) and identifying
those areas where the potential for “at risk” development exists (those watersheds with
large losses and a high percentage of vacant parcels). For example, Sugar Creek is a
watershed with a substantial current exposure and still has a large potential for
development with over a third of the parcels in the watershed being identified as vacant.
Interestingly, in general the five watersheds with the largest damage differential between
Run 2 and Run 3 (increased damage due to future development) rank in the lower
2/3rds among the watersheds in terms of vacant parcels as a percentage of built out
parcels. Several possible inferences can be drawn from this apparent paradox. The first
inference is that while the number of vacant parcels is limited, potential structures added
to vacant parcels could be concentrated in the 100-year floodplain. In other words, it is
possible that these vacant parcels are close to or within the 100-year floodplain and
structures developed at or near the centroid of the parcel are subject to flooding. The
second possibility is that the occupancies zoned for these parcels are either high value
structures or high content value facilities thereby skewing the estimated damages when
flooded. For example, the parcel may be zoned for large retail stores, which have a high
content value to structure ratio. These possibilities will be discussed in further detail in
Chapter 3 of this binder.
Since the actual loss estimation for all studies, except the McAlpine Creek, used the
building footprint data layer, it is also possible to provide a tabular view of the total
number of buildings within the 100-year floodplain including the 100-foot buffer. Table 13 shows the number of structures within the 100-year floodplain (including the 100-foot
buffer) in the current inventory and estimated to be within the floodplain at build-out. The
table shows that overall, Mecklenburg County could see nearly 24% growth in the
number of structures within the floodplain at build out.
This percentage of growth, however, does not equate to growth in flooded structures.
Table 1-3 shows that in some cases a modest increase in the number of buildings at
build-out greatly increases the number of structures that will potentially flood.
For
example, build-out in the Back Creek watershed is estimated to increase the number of
1-10
structures by nearly 37% but this increase will translate into a 250% increase in the
number of structures incurring flood damage.
The reader is reminded that the
methodology only assigns a single building to the centroid of every vacant parcel within
the 100-year floodplain (plus the 100-foot buffer) and therefore this may not be a realistic
estimation of the growth, but it is conservative.
As previously mentioned, the HAZUS®MH specific occupancies (e.g. RES1) were mapped
to the County Assessor occupancies for the parcels. The occupancy mapping can be
seen in Table A-1 of Appendix A.
Table 1-3 shows the total number of flooded
structures for the County by general occupancy. Table 1-3 shows that the large majority
of flooded structures are single or multi-family homes.
This means that the
implementation of standard mitigation strategies, such as a voluntary buy-out, has the
potential to greatly reduce potential damages to existing homes.
Non-residential
structures create an interesting dilemma as most current mitigation programs are aimed
at and most effective for residential structures. This analysis may help identify those
areas where new approaches to non-residential mitigation efforts may need to be
identified.
Table 1-3. Total Number of Flooded Structures by General Occupancy for all
Watersheds Analyzed in This Project to Date
General
Occupancy
NonResidential
Residential
Total
Description
Commercial,
Industrial,
Religious,
Education
Single and
Multi-Family
Residential
Run 1
334
Run 2
363
Run 3
699
1,438
1,965
3,158
1,772
2,328
3,857
1-11
Table 1-4. Total Buildings and “Ultimate Build out”
Watershed (Report Section)
McAlpine1 (3.1)
Sugar Creek (3.2)
Long Creek (3.3)
Little Sugar & Briar Creek (3.4)
Four Mile Creek (3.5)
McDowell Creek (3.6)
McMullen Creek (3.7)
McKee Creek (3.8)
Back Creek (3.9)
Six Mile Creek (3.10)
Clems Branch (3.11)
Reedy Creek (3.12)
Clarke Creek (3.13)
Mallard Creek (3.14)
Irwin Creek (3.15)
Rocky River (3.16)
Goose Creek (3.17)
Lower Clarke Creek (3.18)
Total
Current
Inventory
Buildings in
Floodplain
N/A
1,240
1,248
5,314
902
988
1,694
223
308
412
9
397
291
1,578
1,917
259
176
48
17,004
Built-out
Inventory
Buildings in
2
floodplain
N/A
1,617
1,578
5,961
1,084
1,442
1,789
359
383
566
19
532
413
2,052
2,555
372
198
90
21,010
Net
Growth
(%)
N/A
30.4
26.4
12.2
21.3
46.0
5.6
61.0
24.4
37.4
111.1
34.0
41.9
30.0
33.3
43.6
12.5
87.5
23.6
No. of Flooded
Buildings
Run 1
N/A
91
27
972
7
25
115
2
4
23
0
13
1
27
222
1
6
0
1,536
Run 2
N/A
130
38
1,187
23
66
159
23
4
33
0
24
1
31
288
5
11
0
2,023
Percent Change
in No. of Flooded
Buildings
(Run 1 to Run 2)
N/A
42.9
40.7
22.1
228.6
164.0
38.3
1,050.0
0.0
43.5
0.0
84.6
0.0
14.8
29.7
400.0
83.3
0.0
31.7
Flooded
Buildings
Run 3
N/A
253
117
1,565
71
159
205
62
14
90
0
62
20
125
540
28
17
2
3,330
Percent Change in
No. of Flooded
Buildings
(Run 2 to Run 3)
N/A
94.6
207.9
31.9
208.7
140.9
28.9
169.6
250.0
172.7
0.0
158.3
1,900.0
303.2
87.5
460.0
54.5
200.0
64.6
1. The McAlpine watershed analysis was performed on a parcel basis as the building footprint coverage was not available when the analysis for this watershed was
performed.
2. Includes buildings within parcels that intersect the 100-foot buffer surrounding the floodplain. Build-out assumes 1 building per vacant parcel.
1-12
2. Data Analysis
The loss estimation analysis included the phases of data collection and processing,
inventory development, flood hazard assessment, and damage evaluation.
These
processes are described in this section.
2.1
Data Collection and Processing
As expected, this project utilized a large number of Geographic Information System
(GIS) and database files containing building inventory information. As with every data
intensive project, issues such as variations in the data and minor inconsistencies
between tables require that a series of assumptions be put in place before the analysis
proceeds. These issues and assumptions are discussed in detail below. Additionally,
as the project progressed and the data sophistication increased, the scope of the data
analysis also increased and the quality of the end results improved.
As will be
highlighted in Section 4 of this report, there is always an opportunity to improve the GIS
data and subsequently the results of these hazard-based analyses.
2.1.1 Input Data
Mecklenburg County provided a variety of flood hazard and building inventory data to
ABS and MBJ.
With the exception of the flood study data, these data were used
throughout the entire project.
These data served as the starting point for the loss
estimation analysis, and included:
1. Digital terrain data:
Mecklenburg County provided terrain data in an ESRI grid
format with 20-foot postings developed by Watershed Concepts. Metadata indicated
that the terrain data was developed in 1997.
Examination by the county has
validated the accuracy of the terrain data.
2. Adopted FEMA flood maps: Existing FEMA flood map surface elevations that are
based on the 1975 flood study of the watersheds in Mecklenburg County.
2-1
3. Local flood map surface elevations (proposed):
The Mecklenburg County
watersheds were resurveyed and new floodplain maps were developed consisting of
floodplain areas identified based on current, 2000, land use as well as future
floodplain areas based on ultimate build-out areas in the watershed
4. County assessor data: The 2000 County Assessor data were provided to ABS at the
start of this project. These data were used throughout the duration of all tasks. The
assessor building data included such information as year of construction, effective
floor area, occupancy type, and assessed valuation. The database consists of a
total of 255,940 records for the entire county
5. Surveyed building information: Mecklenburg County has a comprehensive program
for surveying structures within or near the floodplain boundary. Mecklenburg County
provided four separate files containing survey data, where each file provides results
from different survey methods and vintages. The surveyed building data includes
structure information not contained in the assessor database, such as low floor
elevation, or lowest adjacent grade elevation. The four files include:

Flood 3B: This file contains data developed for new construction through “old
fashioned” transit surveys. This file was developed several years ago and was
deemed the lowest quality by Mecklenburg County.

Flood 3C:
This dataset is more current than the Flood 3B and therefore
considered more accurate when duplicate sites were identified. This file contains
data developed through “old fashioned” transit surveys.

GPSBFE:
This is a newer dataset created between 1997 and 1998 and is
therefore considered more accurate than either the Flood 3B and Flood 3C
datasets when duplicates were identified.
This file contains information
developed through the use of GPS units.

Master Elevation Certificates:
This dataset is the most recent survey data.
Mecklenburg County identified this dataset as the most recent and accurate.
Therefore, whenever duplicates with any of the other dataset exist, this dataset
took precedence.
2-2
6. County Parcels: This GIS coverage contained polygon boundaries of all land parcels
for the entire county. Only parcels that were within a 100-foot buffer surrounding the
floodplain were used in the analyses for each watershed.
7. Building Footprints: This layer was used to refine the location of structures within the
floodplain. The dataset is countywide and comprehensive. Note: This GIS coverage
was not available when ABS performed the McAlpine Creek Watershed analysis and
therefore the results in that watershed have slightly higher uncertainty.
8. Digital orthophotography:
Mecklenburg provided digital orthoimagery that greatly
assisted in the development of the inventory data and understanding potential issues
within the GIS parcel coverage and associated attribute data.
2.1.2
Data Processing Summary
While MBJ performed the data processing on the flood hazard data (see the discussion
in each watersheds section), ABS performed processing on the countywide data for the
building inventory. Generally, the inventory development process required attributing the
building footprint polygons with the data necessary to perform the loss estimation
analysis. This approach differs from the McAlpine effort in that the McAlpine study was
performed at the parcel level, unless the parcel had a building that had been surveyed.
In other words, during the McAlpine study the flood depth was attributed to the centroid
of the parcel, because the parcel footprint data was not available and there was no way
of knowing where the building was located within the parcel. In the remainder of studies,
the flood depth has been applied to the centroid of the building footprint, or footprints if
there were multiple buildings on the parcel. It is our belief that this results in the most
accurate representation of the potential damages.
Figure 2-1 shows a GIS map displaying a sample of the inventory data provided to ABS.
This figure shows the parcels that intersect with the Goose Creek Floodplain, the
building footprints, master-elevation-certificate, and “gpsbfe” data. With the exception of
the McAlpine Creek study, this figure is good representation of the data utilized
throughout the project. As the figure shows, the building footprint data represents a
significant improvement over utilizing the centroid of the parcel for assessing the
potential impact from inundation.
For example, Figure 2.1 shows that a number of
2-3
parcels contain more than one structure and therefore we can appropriately apply the
flood depths to individual footprints. Unfortunately, these parcels also turn out to require
additional data manipulation because the assessor data typically had the data as a sum
of all structures within the parcel. In general the value of the parcel was distributed over
the structures in the parcel based on the floor area as discussed below.
Figure 2-1. Overview of Data Provided by Mecklenburg County
(Image is from the Goose Creek Watershed)
Generally, the building inventory data processing procedure was as follows:
1. The building footprint polygons were intersected with the parcel polygons to
assign a parcel number to all structures in the footprint file. All building footprints
smaller than 500 square feet were omitted from this process because they were
typically hidden in the assessor data, and more often than not these were small
structures behind residential buildings, or “out-buildings”. Overall, it is assumed
2-4
that the damages to these structures would be dwarfed by damages to the larger
buildings.
2. The master-elevation-certificate and “gpsbfe” survey data (both “point” GIS
layers) were then assigned to building footprints. Typically the master-elevation
certificate and “gpsbfe’ points intersected the building footprint polygons
simplifying this task, but the addition of a small buffer around the data points
ensured that the point layer and the footprint polygon had an intersection.
3. The county assessor data was then joined with the building footprint data,
thereby providing information on floor area, year built, existence of a basement,
valuation, number of floors, and occupancy. This join was accomplished using
the parcel number assigned through the process in Step 1.
4. This joined dataset was reviewed to determine whether the parcel data reflected
the building footprint or the entire parcel. In those cases where the parcel data
appeared to reflect a sum of all structures on the parcel, the data was parsed to
the various footprints based on the relative area (ratio) of the footprint polygons.
This is discussed in greater detail in Section 2.3 below.
5. The HAZUS®MH occupancy classifications were assigned to the footprints based
on the assessor occupancy classification.
The assignments of the County
Assessor occupancy classifications to the HAZUS®MH occupancy classes is
provided in Table A-1 of Appendix A.
6. The HAZUS®MH building area (typical values) and valuation methodology were
utilized to complete missing inventory data and to estimate future development
on vacant parcels. In other words, using the parcel’s occupancy it was possible
to assign a typical building area and valuation (see Table A-1 in Appendix A).
7. With the assignment of the HAZUS®MH occupancy classification and the typical
building areas, it is possible to assign content value ratios, and building and
content damage functions (see Table A-3 in Appendix A).
8. The HAZUS®MH foundation type and heights were assigned randomly to the
footprints (see Table A-4 in Appendix A) unless a basement was specified in the
Assessor’s dataset, in which case the first floor height was assigned the height
2-5
for the HAZUS®MH basement default.
The foundation type and height
distributions were based on the HAZUS®MH default distributions for North
Carolina. These distributions were further modified to reflect distributions seen in
the County Assessor and Survey data when performing the McAlpine Creek
study.
9. Losses were estimated for each of the three runs.
o
Run 1: Original flood study and current inventory
o
Run 2: New flood study and current inventory
o
Run 3: New flood study and ‘built-out” inventory
With the exception of the additional data and locational accuracy provided by the
building footprint polygons, the process is the same as that used in the McAlpine
analysis. In other words, the only difference between the McAlpine Creek analysis and
the analysis performed for all other watersheds was the attribution of the flood depth at
the centroid of the parcel rather than the centroid of the building footprint.
2.2 Building Damage Analysis
To develop an estimate of the flood hazard losses, it is necessary to determine the flood
depth of each structure within the inundated area. MBJ provided ABS with a flood depth
grid representing the current (1975 flood study) and ultimate (new 2000 flood study) 100year flood scenarios. The flood grid included both positive (flooded) and negative (flood
waters below grade) flood depths. The structures and vacant parcels were intersected
with the depth grids to determine the depth of flooding at grade for each structure
(building footprint) or proposed structure (centroid of the parcel). For those buildings
where a survey point was available, the latitude and longitude of the survey point was
used rather than the centroid of the building footprint.
Upon assignment of the depth of flooding at grade for each building or vacant parcel, the
project team adjusted the flood depth in order to account for the height of the first floor
assigned to the building. For those building footprints with a survey point, the actual first
2-6
floor height from the survey was used. Those building footprints without floor height
from survey data, had the HAZUS®MH floor height randomly assigned per Step 8 in the
data process. This provided the project team with the flood depth within the structure.
With this value and the known occupancy, the project team was able to apply the
assigned depth-damage relationship for the structure and the building contents (these
depth damage relationships are discussed in more detail in Section 2.4). From these
depth-damage relationships, the estimated damage, in percent of value, was determined
and subsequently the estimated loss, in dollars, was also determined.
2.3
Building Valuation
Building valuations were provided as part of the County Tax Assessor files and were
applied to structures on the parcels. For the McAlpine study, this was relatively straight
forward as the analysis occurred at the parcel level and the valuation included all
structures on that parcel. When the building footprint layer was added into the process,
it became apparent that some data issues and limitations would need to be overcome.
Examples of these issues include:

A parcel with multiple building footprints, but only one recorded floor area and
value for the parcel. In this case, the project team assigned the valuation from
the assessor file according to a ratio of the footprint area for an individual building
to the total area of the footprints.

Parcels with multiple records for improvements (buildings) corresponding to
multiple footprints shown in the footprint file, but the valuation and floor area in
the Assessor file is constant for all of the improvements and appears to be the
aggregated total for the improvements. As with the previous case, the project
team took the total valuation and distributed it based on the relative areas of the
footprints.

Parcels with multiple improvements recorded and only a single footprint in the
footprint file. In this case, the project team assumed the footprint file to be the
most accurate and assigned the total value of the parcel to the footprint.
2-7

Parcels identified as vacant, but with footprints in the footprint file. The project
team assigned a valuation based on the occupancy using the HAZUS®MH
defaults. These values can be seen in Appendix A-2.

Contents values were determined as a percentage of the assigned structure
value. The HAZUS®MH content value ratios for each occupancy were applied
throughout the project. Some records in the County Assessor file contained an
estimate of the building contents value.
As this data was not consistent
throughout the database, these values were not used. The HAZUS®MH default
contents value expressed as a percentage of structure values can be seen in
Table A-4 of Appendix A.
2.4 Damage Curve Selection
Once the depth of water relative to the lowest floor was determined, occupancy-specific
depth-damage functions were applied to determine the damage percentage for each
structure.
For single-family residential buildings, credibility-weighted depth-damage
curves from the Federal Insurance Administration (FIA) were applied.
For non-
residential buildings, depth-damage curves from the U.S. Army Corps of Engineers
(USACE) Wilmington and Galveston Districts were applied.
2.4.1
FIA Depth-Damage Curves
The FIA has developed national depth-damage curves that are used in the actuarial rate
setting process. The original FIA depth-damage functions, developed in 1970 and 1973,
are referred to as “theoretical base tables”. Some of the information used to develop the
initial curves came from post-flood surveys conducted by the Corps of Engineers. With
time, a wealth of damage and loss data has been collected as part of the flood insurance
claims process. Losses include both structure and contents losses, and are determined
relative to actual cash value (depreciated replacement cost). The majority of claims are
for residential structures.
The FIA damage functions are updated annually based on this damage data, as part of
the flood insurance rate review process. A statistical “credibility” analysis is used to
2-8
combine the “theoretical base tables” with the “rate review” results. When sufficient
claims exist to provide statistical confidence in the results, the depth-damage
relationship is based exclusively on the claims data. When claims data are insufficient,
the claims data and base tables are combined using a weighting process. The result is
two sets of curves: pure summaries of claims data, and credibility analyses combining
available claims data into weighted curves. According to the NFIP Actuarial Information
System “Credibility and Weighting” report (1998), credibility analyses and the resulting
weighted curves are available for six structure categories:
1. One floor, no basement
2. Two or more floors, no basement
3. Two or more floors, with basement
4. Split-level, no basement
5. Split-level, with basement
6. Mobile home
Figure 2-2 presents the six FIA credibility-weighted damage functions. It should be
noted that several of the curves are not continuous. Because claims data are often
sparse, damage values are not provided for all depths.
For use in the HAZUS®MH
software, missing damage values (e.g., damage at 6.0 feet for structures with 2 floors,
no basement) was interpolated between known water depths to facilitate damage
function application.
2-9
Figure 2-2. FIA Credibility-Weighted Building Depth-Damage Curves
as of 12/31/1998
2.4.2
USACE Wilmington District Depth-Damage Curves
The Wilmington District provided 13 residential structure and contents damage functions,
and 49 non-residential structure functions that may be applied to contents using a
contents-to-structure value ratio.
The non-residential classes include: apartments,
appliances, auto dealership, auto junk yard, auto parts, bait stand, bank, barber shop,
beauty shop, boat stalls, book store, bowling alley, business, church, cleaners, clinic
(medical), clothing, dentist office, department store, doctor’s office, drug/super, funeral
home, furniture, garage, halls, hardware, hotel, jewelry, laundry, liquor, lumber,
market/super, market/drive, motel, newspaper, office building, post office, private club,
restaurant, rest home, school, service station, theater, theater (drive-in), TV station,
tavern, variety store, wash-a-teria, and warehouse.
2-10
2.4.3
USACE Galveston District Curves
The Galveston District has numerous damage functions, including residential, and more
than 145 different non-residential flood damage functions. The non-residential damage
functions include damage to the structure, as well as to inventory and equipment. The
damage functions are based on flood damage records, as well as post-event surveys,
including surveys following Hurricane Claudette in 1979.
The damage curves are
currently used by Galveston and other Districts, including Tulsa and Fort Worth, and are
applicable to fresh-water flooding, under slow-rise, slow-recession conditions, with little
velocity.
The functions are based on damage to structures without basements, as
structures along the Texas Coastal Plain are built without basements because of the
high water table.
2.5
Occupancy Mapping & Damage Assessment
Each depth-damage curve is associated with a specific building occupancy type. ABS
has developed a damage curve library that includes the FIA, Wilmington, and Galveston
depth-damage curves, for use in the HAZUS®MH loss estimation software. These curves
are identified by the HAZUS®MH occupancy code. In order to apply the curves to the
Mecklenburg data, it was necessary to associate each Mecklenburg occupancy code
with a HAZUS®MH occupancy code, and select an appropriate depth damage curve for
that occupancy. The results of this effort are shown in Table A-1 in Appendix A.
There are a few notable items regarding the assignment of depth-damage curves and
computation of flood damage:

FIA curves were applied for all single-family residential buildings. The style code
from the Mecklenburg County database was used to determine the number of
stories. The presence of a basement was assigned based on the building foundation
distributions shown in Table A-3 in Appendix A.

For non-residential buildings, an effort was made to associate the building with a
depth-damage curve from the USACE Wilmington District. If an appropriate match
could not be made, a curve from the USACE Galveston District was applied.
2-11

The USACE damage curves are based on specific building types, so matching a
particular curve to the Mecklenburg County use codes of “Commercial” and
“Industrial” was not possible.
For buildings described as “Commercial”, damage
curves for the categories of Retail Trade and Business/Professional/Technical
Services were combined to develop a single depth-damage curve. It is assumed that
these categories are representative of commercial buildings in Mecklenburg County.
Likewise for “Industrial” buildings, curves for the categories of Light Industrial and
Construction were combined.

For the calculation of contents damage, the contents curves collected from the FIA,
Galveston District or Wilmington District curves were applied and the value of the
contents within the structure was determined using the values seen in Table A-4 of
Appendix A.
2-12
3. Flood Hazard Assessments
The following tabs identify the individual damage assessment results for each watershed
within Mecklenburg County.
This report is designed to allow the County to update
watersheds over time as the flood studies and inventory data are updated and
enhanced. Since the methodology is consistent with HAZUS®MH and the HAZUS®MH
Flood Information Tool (FIT), the County can perform these updates internally.
Table 3-1 presents the overall structure losses estimated for each watershed. As can be
seen in the table, nearly all the watersheds showed increasing estimated losses from
Run 1 through to Run 3.
It is important to note that since the inventory was held
constant, the difference between Run 1 and Run 2 are the changes between the 1975
flood study flood depths and the 2000 flood study depths, where the BFE includes the
increased runoff from future or ultimate build-out. Conversely, since the flood depths
were held constant, the difference between Run 2 and Run 3 are revisions to the
building inventory estimating the future or ultimate development on the vacant parcels
within the floodplains. As discussed later, a vacant parcel was assumed to have a single
building developed on the parcel and the size of the structure was based on the known
zoning of the parcel and the HAZUS®MH “average” structure for that type of occupancy.
3-1
Table 3-1. Estimated Structure Losses for Mecklenburg County
Estimated Structure Losses
Watershed (Report Section)
1
McAlpine (3.1)
Sugar Creek (3.2)
Long Creek (3.3)
Little Sugar & Briar Creek (3.4)
Four Mile Creek (3.5)
McDowell Creek (3.6)
McMullen Creek (3.7)
McKee Creek (3.8)
Back Creek (3.9)
Six Mile Creek (3.10)
Clems Branch (3.11)
Reedy Creek (3.12)
Clarke Creek (3.13)
Mallard Creek (3.14)
Irwin Creek (3.15)
Rocky River (3.16)
Goose Creek (3.17)
Lower Clarke Creek (3.18)
Total
Run 1
Run 2
Run 3
$5,635,741
$3,167,644
$316,227
$56,546,435
$442,926
$385,125
$2,141,966
$8,200
$51,615
$473,083
$0
$185,520
$173,044
$1,665,160
$11,169,985
$2,446
$45,644
$0
$82,410,761
$8,477,724
$6,117,840
$558,456
$82,124,414
$847,470
$2,607,031
$3,644,863
$217,095
$63,339
$728,967
$0
$351,145
$216,453
$1,693,843
$18,064,510
$112,446
$135,712
$0
$125,961,308
$25,216,387
$32,827,578
$5,228,264
$137,170,007
$3,225,844
$8,813,209
$7,491,171
$1,822,089
$563,791
$3,008,766
$0
$2,048,533
$1,120,419
$17,228,457
$46,986,475
$1,265,911
$348,193
$125,000
$294,490,094
Future Development
Damage
(Run 2- Run 3)
$16,738,663
$26,709,738
$4,669,808
$55,045,593
$2,378,374
$6,206,178
$3,846,308
$1,604,994
$500,452
$2,279,799
$0
$1,697,388
$903,966
$15,534,614
$28,921,965
$1,153,465
$212,481
$125,000
$168,528,786
Increase in Damage
Between Run 2 and
Run 3 (%)
197.4
436.6
836.2
67.0
280.6
238.1
105.5
755.9
790.1
312.7
--483.4
417.6
917.1
160.1
1,025.8
156.2
---
1. The McAlpine watershed analysis was performed on a parcel basis as the building footprint coverage was not available when the project started.
3-2
Future Development
Damage
(% of Total)
9.9
15.8
2.8
32.7
1.4
3.7
2.3
1.0
0.3
1.4
0.0
1.0
0.5
9.2
17.2
0.7
0.1
0.1
Table 3-1 is designed to allow the reader to view the losses from two perspectives. The
first is on a watershed basis. The reader can see the estimated increase in damage
between Run 2 and Run 3, representing the potential for increasing damage as the
watershed develops but conditions (floodplain regulation) remains constant. The reader
can also view the watershed as part of the overall increased potential for losses
throughout the County.
As Table 3-1 shows, there are three major groupings of the watersheds that have been
analyzed. There are three watersheds (Sugar Creek, Little Sugar & Briar Creeks, and
Irwin Creek) where increased floodplain regulation could greatly reduce the potential
loss relative to the total losses for all of the county’s watersheds. The three watersheds
represent nearly 66% of the total loss differential between Run 2 and Run 3. The next
two watersheds (McAlpine Creek and Mallard Creek) represent another 19% of the total
loss differential between Run 2 and Run 3. This means that nearly 85% of the total
potential increased losses due to future development is within 5 watersheds with the
remaining 15 percent of the losses scattered among the remaining 13 watersheds that
were analyzed.
Table 3-1 shows the overall impact of flooding on buildings or the overall structural
damage that can be expected when a structure is inundated. Typically in flooding where
there is little or no warning, the contents within the structure are also subjected to
damage.
As previously discussed, it is possible to estimate the total value of the
contents within each structure based on the occupancy and value of the structure. Table
A-4 in Appendix A shows the estimated value of contents relative to the value of the
building as a percentage.
Applying depth-damage relationships for contents it is
possible to develop a similar table for the damage to building contents. Table 3-2 shows
the results of this effort for each watershed. As expected, the same five watersheds
represents over 87% of the total contents damage for every watershed in the County.
As with the structure losses, this indicates opportunities to reduce or eliminate some of
these damages through effective floodplain management.
A review of the reports for the top five watersheds (McAlpine, Mallard, Irwin, Sugar and
Little Sugar Briar) shows that while there is a significant residential contribution to the
increased damage, a large part of the increase is due to commercial and industrial
3-3
development. This may complicate the County’s efforts to manage the development to
prevent future damage as commercial and industrial structures are harder to mitigate
against flooding during the time of development. For example, a property owner can
build a home on piles above the BFE to prevent damage to the structure and also limit
the impact of the development on structures downstream. With commercial structures,
especially large retail or manufacturing facilities, it is significantly more difficult to build
the structure in a fashion that has no adverse impact in the floodway or to structures
downstream (due to the large asphalt requirements for parking).
Reviewing the GIS data, it is evident that there are some vacant parcels that are
completely subjected to flooding with the new flood study depth grid and significant effort
will need to be put forward to guide development such that losses are minimized without
adversely impacting other structures within the watershed. Conversely, there are clearly
some parcels where there are areas on the parcel that can be developed without
subjecting any structures to flooding. As the natural tendency is to develop available
land, it is recommended that the County overlay the food depth grids onto the parcel
boundary layer and identify such areas.
Obviously, Mecklenburg County is well
positioned and has a clear policy regarding the prevention of flood losses and future
development that is supported by the results of this study.
One of the direct benefits of this analysis is that the County can map the losses by
watershed and identify those with the greatest estimated damage. This would indicate
areas where mitigation and floodplain regulatory measures may have the greatest
impact. For Example, Figure 3-1 graphically shows the total estimated losses (structure
and content) for each watershed based on analysis Run 2.
3-4
Figure 3-1. Total Estimated Losses by Watershed for Run 2
3-5
Table 3-2. Estimated Contents Losses for Mecklenburg County
Estimated Content Losses
Watershed (Report Section)
1
McAlpine (3.1)
Sugar Creek (3.2)
Long Creek (3.3)
Little Sugar & Briar Creek (3.4)
Four Mile Creek (3.5)
McDowell Creek (3.6)
McMullen Creek (3.7)
McKee Creek (3.8)
Back Creek (3.9)
Six Mile Creek (3.10)
Clems Branch (3.11)
Reedy Creek (3.12)
Clarke Creek (3.13)
Mallard Creek (3.14)
Irwin Creek (3.15)
Rocky River (3.16)
Goose Creek (3.17)
Lower Clarke Creek (3.18)
Total
Run 1
Run 2
Run 3
$3,196,650
$3,497,774
$157,173
$93,486,664
$246,277
$181,029
$1,231,764
$3,704
$29,031
$245,580
0
$99,747
$117,825
$2,228,328
$25,892,527
$1,039
$17,303
$0
$130,632,415
$5,600,362
$7,123,981
$302,423
$132,032,072
$470,794
$1,560,572
$2,084,969
$111,998
$42,141
$410,074
0
$207,850
$142,056
$2,254,510
$39,256,355
$45,727
$63,890
$0
$191,709,774
$21,946,794
$34,766,164
$3,210,827
$189,697,756
$2,391,492
$8,081,537
$5,212,643
$1,050,155
$578,124
$1,891,157
0
$1,105,256
$551,337
$21,214,099
$63,217,315
$1,005,086
$164,781
$77,407
$356,161,930
Future Development
Damage
(Run 2- Run 3)
$16,346,432
$27,642,183
$2,908,404
$57,665,684
$1,920,698
$6,520,965
$3,127,674
$938,157
$535,983
$1,481,083
0
$897,406
$409,281
$18,959,589
$23,960,960
$959,359
$100,891
$77,407
$164,452,156
Increase in Damage
Between Run 2 and
Run 3 (%)
291.9
388.0
961.7
43.7
408.0
417.9
150.0
837.7
1,271.9
361.2
0.0
431.8
288.1
841.0
61.0
2,098.0
157.9
---
1. The McAlpine watershed analysis was performed on a parcel basis, as the building footprint coverage was not available when the project started.
3-6
Future Development
Damage
(% of Total)
9.9
16.8
1.8
35.1
1.2
4.0
1.9
0.6
0.3
0.9
0.0
0.5
0.2
11.5
14.6
0.6
0.1
0.0
DETERMINATION OF IMPACTS FROM FLOOD STUDY MODIFICATIONS
McALPINE CREEK WATERSHED (Report Delivered June 2000)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.1
McAlpine Creek Analysis
3.1.1
Results Summary for the McAlpine Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.1-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 236, 330, and 527 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.1-1. Number of Flooded Structures by General Occupancy:
McAlpine Creek Watershed
General
Occupancy
NonResidential
Residential
Total
Description
Commercial,
Industrial,
Religious,
Education
Single and
Multi-Family
Residential
Run 1
7
Run 2
6
Run 3
35
229
328
492
236
330
527
The dollar damage resulting from the analysis is shown on Figure 3.1-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $5.6
million, contents $3.2 million) and Run 2 (structure $8.5 million, contents $5.6 million).
This corresponds to an increase of $2.9 million (52%) for structures and $2.4 million
(75%) for contents. The increase tracks the effect of changes in the floodplain extent and
depth between the 1975 flood study and newly revised floodplain map. Based on the
assumption that the revised floodplain map is more accurate than the 1975 flood study,
the estimated damage results from Run 2 are representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 signifies the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were developed based on
3-7
their zoning. The result is a sharp increase in estimated structure damage from $8.5
million to $25.2 million and a corresponding increase in estimated contents damage from
$5.6 million to approximately $22 million. This corresponds to an increase of $16.7 million
(196%) for structures and $16.4 million (293%) for contents. Although the exact value of
the estimated damage (Run 3) was not know to Mecklenburg County, this significant
escalation of potential damages based on future development was predicted by county
staff and has led to efforts requiring new development to be 1-foot above the one-hundred
base flood elevation. Figure 3.1-1 supports this effort, as the 1-foot requirement would
reduce the results of Run 3 to levels more closely resembling the results of the existing
development results presented by Run 2.
$30,000,000
$25,000,000
Structure
Contents
$20,000,000
$15,000,000
$10,000,000
$5,000,000
$0
Run 1
Run 2
Run 3
Building Inventory and Floodplain Scenario
Figure 3.1-1. McAlpine Creek Watershed Aggregate Damage Estimation Results
3-8
Table 3.1-2 shows a summary of the building and content damage results by general
occupancy classification.
In general residential buildings represent the bulk of the
damages for all three runs. However, increased development in non-residential buildings
leads to a large difference in damages between Run 2 and Run 3. The contents damages
by general occupancy generally reflect the building damages with the exception of Run 3
where the increase in industrial and retail development causes a large increase in nonresidential content damages.
The results are discussed in greater detail later in this
report.
3.1.2 Data Analysis
Please note that the data analysis for McAlpine Creek is different from the remaining
sections within this report since the building footprint data was not available when the
McAlpine Creek analysis was performed. For that reason, a synopsis of the data analysis
is included for the reader’s convenience.
3.1.2.1 Parcel and Building Data Review
The data for the parcel polygons, the vacant parcel polygons, and the surveyed structures
were matched and processed to produce a consistent database representative of the
existing and future built environment. Data were extracted that assisted in developing
critical information such as distributions of foundation type and floor area. If information
was not available, the HAZUS methodology was applied, and in some areas modified to
reflect local conditions based on other information available within the datasets.
The first dataset reviewed contained descriptive information for all parcels within the
McAlpine Creek floodplain. These data were provided in the form of a GIS data layer with
an associated dBase file.
Initial review of the 2,525 records indicated that duplicate
records existed for 9 parcels. In these cases, examination showed that all data were
duplicated with the exception of the perimeter and area of the polygon boundary. These
parcels were examined in ArcView® and it became apparent that the duplicate records
were actually slivers that may have been an easement or a correction to the parcel layer.
As such, the duplicate records were eliminated using the dissolve functionality in GIS,
reducing the number of parcel records to 2,516.
3-9
Mecklenburg also provided a dBase file containing the parcels designated as vacant
within the floodplain. Review of the 440 records indicated that the vacant parcels were a
subset in the file of parcels discussed above, and that 19 of the vacant parcels were in
fact developed (the fields for effective area, year built, and building value were non-zero).
These 19 records were removed from the vacant parcel database but left in the total
parcel database. In addition, 3 of the vacant parcels were duplicates, and were therefore
dissolved as described in the preceding paragraph. After these minor modifications were
performed, the number of vacant parcels was reduced from 440 records to 418.
Table 3.1-2. Summary of Building and Content Damage by General Occupancy:
McAlpine Creek Watershed
General
Occupancy
Description
Run 1
Run 2
Run 3
Structural Damage
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$374,663
Residential
Single and
Multi-Family
Residential
$5,261,080 $22,974
$53,523
Total
$523,437
Average
$52,344
$7,954,288 $24,857
Total
$9,068,717
Average
$259,106
$16,147,671 $32,820
Content Damage
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$504,491
Residential
Single and
Multi-Family
Residential
$2,692,158 $12,699
$56,084
Total
$701,084
Average
$70,825
$4,899,188 $16,330
Total
Average
$11,881,830 $325,986
$10,064,964 $21,880
The 325 surveyed structures were intersected with the parcel polygons and 20 structures
were determined to be unassociated with a parcel polygon.
The 20 structures were
overlaid upon the parcel polygons and digital orthoimagery to determine the cause. It was
found that in some of these cases, the surveyed structure was located within a parcel
polygon, but the building footprints were clipped from the parcel boundaries (i.e., a hole
3-10
existed within the parcel at the boundary of the building location). In other cases, the
surveyed structure was located outside of the extent of the parcel polygons.
For the 305 surveyed structures that did intersect a parcel polygon, a comparison of the
surveyed floor area data with the assessor data indicates general agreement. However,
the valuation data were at times significantly different. Another inconsistency included
some cases in which the parcel layer did not contain any information regarding the
structure. This occurrence could lead to an overestimation of the future growth condition
because the parcel itself is described as vacant although there are structures on the
parcel.
3.1.2.2 Parcels Without a Surveyed Structure
The majority of attributes required for loss estimation were contained in the parcel polygon
coverage. However, the parcel data did not contain any first floor elevation information.
To estimate the height of the first floor above grade, the following methodology was used:
The locations of the surveyed structures were reviewed in ArcView® GIS and three
clusters were noted, designated as Zone 1, Zone 2, and Zone 3 (Figure 3.1-2). Within
each zone, the distribution of foundation types was determined. For parcels outside of the
zones, the surveyed data points were used to develop an average distribution of
foundation types for the entire McAlpine Creek basin. The resulting distribution is shown
in Table 3.1-1.
The distribution was compared to the HAZUS®MH default inventory
distribution (Table 6-9 in the Technical Document) and found to be reasonably consistent.
For this project, no effort was made to ensure that pile foundations are near the creek or
slab-on-grade further away as might normally be assumed.
3-11
Figure 3.1-2. Floor Height Distribution Zones
3-12
Table 3.1-3. Foundation Type Distribution by Zone:
McAlpine Creek Watershed
Foundation
Type
Slab on
Grade
Crawlspace
Basement
Pier
Solid Wall
Pile
Assumed Floor
Height (where
survey data not
available)
1 ft. above grade
Zone 1
(%)
74
Zone 2
(%)
55
Zone 3
(%)
37
Avg. Distn
Applied to
Remaining
Parcels (%)
52
3 ft. above grade
4 ft. above grade
5 ft. above grade
7 ft. above grade
7 ft. above grade
13
6
5
0
2
17
17
9
0
2
18
16
22
3
4
16
13
14
2
3
The vacant parcel data were used to estimate the future development of the McAlpine
floodplain.
However, the vacant parcels lacked a key data attribute:
valuation.
To
overcome this deficiency, the following procedure was employed:
1. The parcel occupancy type was used to determine the future use of each vacant
parcel.
2. The county assessor data were used to estimate the typical floor area for each
occupancy type. The query was restricted to post-1980 construction in order to reflect
the growth of structures observed in recent years. For example, the average floor
area for a home in Mecklenburg County is 1,624 square feet. For the post-1980
structures, the average floor area is 2,290 square feet. It is believed that the recent
development will be more representative of future construction practices.
3. In a similar fashion, the average valuation for each occupancy type per square foot
was calculated using the average values of the assessor’s data of post-1980
structures.
The results of this analysis are presented in Table A-1 in Appendix A. Using this table, an
estimated valuation was computed for each vacant parcel, based on its occupancy type.
3.1.2.3 Surveyed Structures Intersecting a Parcel
Where available, the surveyed structure data were used to identify the structures in the
parcel. The surveyed data had accurate assessments of the floor elevation and lowest
adjacent grade, which are used to estimate the floor height above grade (required to
estimate water depth in the structure). The attributes taken from the surveyed structure
coverage included floor area, occupancy type, number of stories, first floor elevation, and
3-13
lowest adjacent grade. The valuation was assigned based on the characteristics of the
intersecting parcel polygon.
3.1.2.4 Surveyed Structures Not Intersecting a Parcel
For surveyed structures unassociated with a parcel polygon, the required building
inventory attributes were culled from the surveyed structure coverage.
Because the
buildings did not match to a parcel record, the valuation data was unavailable. Hence,
Table A-1 was used to compute a valuation based on the floor area of each structure.
3.1.3 Detailed Results
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the McAlpine Creek watershed (developed from surveyed
data and parcel data) and the original 100-year floodplain boundary, based on the
1975 flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the newly revised
floodplain boundary.
Table 3.1-4 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the new map revision.
The
increase between Run 2 and Run 3 is attributable to the projected rise in floodplain
development.
3-14
The values shown in Table 3.1-4 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
For example, there are relatively few flooded buildings in the commercial category. It is
therefore difficult to assert that the damage computed for these buildings is representative
of other commercial buildings commercial buildings constructed within the watershed. On
the other hand, there are quite a few flooded single-family residential buildings,
suggesting that these results are more representative of the category as a whole.
Table 3.1-4. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
McAlpine Creek Watershed
HAZUS®MH
Occupancy Code
Description
COM1/COM4
Mix of Retail Trade & Business/
Professional/Technical Services
COM3
Personal & Repair Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment & Recreation
EDU1
Schools/Libraries
IND2/IND6
Mix of Light Industrial &
Construction
REL1
Church/Membership Organizations
RES1A
Single Family Dwelling, 1 Story, No
Basement
RES1B
Single Family Dwelling, 2 Story, No
Basement
RES1D
Single Family Dwelling, Split Level,
No Basement
RES1E
Single Family Dwelling, Split Level,
with Basement
RES2
Mobile Home
RES3
Multi-Family Dwelling
Total No. of flooded buildings
Total No. of unflooded buildings
Run 1
0
Run 2
0
Run 3
8
1
1
1
3
1
9
3
1
0
3
1
0
4
1
10
1
49
2
72
2
200
46
76
76
27
32
32
78
107
137
0
29
0
33
1
46
236
1,923
330
1,829
527
2,050
The dollar damage resulting from the analysis is shown on Figure 3.1-2. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2 to
Run 3. Tables 3.1-5 through 3.1-8 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the significant
escalation of estimated damage based on future development suggests the need for the
3-15
mitigation measures such as requiring future development to be 1-foot above the future
one-hundred year flood elevation (as currently proposed by county staff).
$30,000,000
$25,000,000
Structure
Contents
$20,000,000
$15,000,000
$10,000,000
$5,000,000
$0
Run 1
Run 2
Run 3
Building Inventory and Floodplain Scenario
Figure 3.1-3. McAlpine Creek Watershed Aggregated Damage Estimation
Results
Table 3.1-5 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Although there are changes in the building inventory and floodplain depth and extent
between the three scenarios, the differences between the resulting structure damage
percentages are relatively small. In general, the structure damage ranges from 10 to 25
percent. The exceptions to this case tend to occur for building types with a small number
of flooded structures. Between Runs 1 and 2, there are a few building occupancy types,
for which the computed damage decreased.
3-16
This is because the changes in the
floodplain between the 1975 flood study and the recent map revision were not consistent
in all places. Although there was an overall increase in floodplain extent & depth, there
were some areas in which the floodplain actually decreased.
Table 3.1-5. Structure Damage Percent by HAZUS®MH Occupancy Type:
McAlpine Creek Watershed
HAZUS®MH
Occupancy Code
Description
COM1/COM4
Mix of Retail Trade & Business/
Professional/Technical Services
COM3
Personal & Repair Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment & Recreation
IND2/IND6
Mix of Light Industrial &
Construction
EDU1
Schools/Libraries
REL1
Church/Membership Organizations
RES1A
Single Family Dwelling, 1 Story, No
Basement
RES1B
Single Family Dwelling, 2 Story, No
Basement
RES1D
Single Family Dwelling, Split Level,
No Basement
RES1E
Single Family Dwelling, Split Level,
w/Basement
RES2
Mobile Home
RES3
Multi-Family Dwelling
Run 1
---
Run 2
---
Run 3
37.5
12.0
19.7
15.1
13.2
15.1
39.4
11.0
---
29.2
---
29.2
25.6
3.6
45.2
19.6
3.3
32.3
20.5
3.3
32.3
24.8
10.8
12.3
12.3
11.5
13.2
13.2
19.1
20.1
25.0
--15.2
--18.8
64.0
21.8
Table 3.1-6 shows the dollar damage to building structures that is computed when the
damage percentages are multiplied by the structure/parcel valuation. In contrast to the
small relative difference between scenarios shown in the previous table, there are
significant differences in the dollar damage between the three scenarios. Between Run 1
and Run 2, there is an increase in total structure damage from $5.6 million to $8.5 million.
This corresponds to an increase of $2.9 million, or 52%. As the building inventory used
for these scenarios are identical, the difference in damage is attributable to differences in
the floodplain between the 1975 flood study and the recent revision of the 100-yr
floodplain map.
Proceeding from Run 2 to Run 3, there is a significant increase in
estimated damage from $8.5 million to $25.2 million caused by projected development in
the floodplain. This corresponds to an increase of $16.7 million, or 196%. A notable
3-17
finding from the results of Run 3 is a spike in both the total and average damage to
commercial and industrial properties.
Table 3.1-6. Structure Dollar Damage by HAZUS®MH Occupancy Type:
McAlpine Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM3
COM4
COM8
EDU1
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES2
RES3
Run 1
Total
--$1,783
$15,877
$28,036
$152,124
--$176,843
$1,039,298
$735,383
$327,155
$2,092,447
--$1,066,797
Total $5,635,741
Run 2
Avg.
--$1,783
$15,877
$9,345
$152,124
--$176,843
$21,210
$15,987
$12,117
$26,826
--$36,786
Total
--$2,242
$28,143
$104,209
$138,602
--$250,241
$1,548,293
$1,325,143
$466,153
$2,952,764
--$1,661,935
$8,477,724
Run 3
Avg.
--$2,242
$9,381
$34,736
$138,602
--$125,120
$21,504
$17,436
$14,567
$27,596
--$50,362
Total
$1,254,703
$2,242
$3,841,280
$215,397
$138,602
$3,366,252
$250,241
$6,624,889
$1,325,143
$466,153
$4,770,281
$24,467
$2,936,738
Avg.
$156,838
$2,242
$426,809
$53,849
$138,602
$336,625
$125,120
$33,124
$17,436
$14,567
$34,820
$24,467
$63,842
$25,216,387
Table 3.1-7 presents the overall average damage percentage calculated for building
contents.
Similar to the structure damage percent results shown in Table 3-2, the
difference in contents damage percentages between runs is relatively small. In general,
the contents damage ranges from 15 to 40 percent. Again, the outliers primarily occur for
building types with a small number of flooded structures. Between Runs 1 and 2, there
are a few building occupancy types, for which the computed damage decreased. This is
because the changes in the floodplain between the 1975 flood study and the recent map
revision were not consistent in all places. Although there was an overall increase in
floodplain extent & depth, there were some areas in which the floodplain actually
decreased.
3-18
Table 3.1-7. Contents Damage Percent by HAZUS®MH Occupancy Type:
McAlpine Creek Watershed
HAZUS®MH
Occupancy Code
Description
COM1/COM4
Mix of Retail Trade & Business/
Professional/Technical Services
COM3
Personal & Repair Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment & Recreation
IND2/IND6
Mix of Light Industrial &
Construction
EDU1
Schools/Libraries
REL1
Church/Membership Organizations
RES1A
Single Family Dwelling, 1 Story, No
Basement
RES1B
Single Family Dwelling, 2 Story, No
Basement
RES1D
Single Family Dwelling, Split Level,
No Basement
RES1E
Single Family Dwelling, Split Level,
w/Basement
RES2
Mobile Home
RES3
Multi-Family Dwelling
Run 1
---
Run 2
---
Run 3
37.5
49.6
17.4
83.6
8.9
83.6
38.0
11.0
---
29.2
---
34.4
35.3
3.6
45.2
24.3
3.3
36.3
25.0
3.3
36.3
29.7
12.9
15.3
15.3
13.8
15.7
15.7
19.3
20.4
23.0
--9.3
--33.6
51.1
41.2
Table 3.1-8 shows the estimated dollar damage to building contents.
Similar to the
structure dollar damage shown in Table 3-3, there are significant differences in the dollar
damage between scenarios. Between Run 1 and Run 2, there is an increase in total
estimated contents damage from $3.2 million to $5.6 million. This corresponds to an
increase of $2.4 million, or 75%.
The increase in damage is due to changes in the
floodplain depth and extent between the 1975 flood study and the recent revision of the
100-yr floodplain map.
Between Run 2 to Run 3, there is a significant increase in
estimated damage from $5.6 million to nearly $22 million attributable to projected growth.
This corresponds to an increase of $16.4 million, or 293%. Again, there is a notable spike
in both the total and average damage to commercial and industrial properties between
Runs 2 and 3. The significant escalation of estimated structure and contents damage
based on future development, suggests the need for mitigation measures within the
McAlpine Creek floodplain.
3-19
Table 3.1-8. Contents Dollar Damage by HAZUS®MH Occupancy Type:
McAlpine Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM3
COM4
COM8
EDU1
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES2
RES3
Run 1
Total
Run 2
Avg.
Total
--$3,687
$6,990
$37,007
$196,848
--$259,959
$652,284
$439,920
$195,209
$1,068,416
--$336,329
----$3,687
$6,213
$6,990
$10,200
$12,336
$137,556
$196,848
$179,351
----$259,959
$367,854
$13,312
$946,337
$9,563
$837,537
$7,230
$280,172
$13,698 $1,502,898
----$11,598 $1,332,244
Total $3,196,650
$5,600,362
3-20
Run 3
Avg.
--$6,213
$3,400
$45,852
$179,351
--$183,927
$13,144
$11,020
$8,755
$14,046
--$40,371
Total
$2,525,717
$6,213
$6,065,462
$410,988
$179,351
$2,326,245
$367,854
$3,964,776
$837,537
$280,172
$2,187,394
$9,764
$2,785,321
$21,946,794
Avg.
$315,715
$6,213
$673,940
$102,747
$179,351
$232,625
$183,927
$19,824
$11,020
$8,755
$15,966
$9,764
$60,550
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
SUGAR CREEK WATERSHED (Report Delivered November 2001)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.2
3.2.1
Sugar Creek Analysis
Results Summary for the Sugar Creek Basin (below Irwin Creek)
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.2-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 91, 130, and 253 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.2-1. Number of Flooded Structures by General Occupancy:
Sugar Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
21
25
82
Residential
Single and MultiFamily Residential
70
105
171
91
130
253
Total
The dollar damage resulting from the analysis is shown on Figure 3.2-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $3.2
million, contents $3.5 million) and Run 2 (structure $6.1 million, contents $7.1 million).
This corresponds to an increase of $2.95 million (93%) for structures and $3.6 million
(103%) for contents. The increase demonstrates the effect of changes in the floodplain
extent and depth between the 1975 flood study and newly revised (2000) floodplain map.
Based on the assumption that the revised floodplain map is more accurate than the 1975
flood study, the estimated damage results from Run 2 are representative of current
conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-21
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $6.1 million to $32.8 million and a corresponding increase in estimated
contents damage from $7.1 million to approximately $34.8 million. This corresponds to an
increase of $26.7 million (438%) for structures and $27.6 million (388%) for contents. Prior
to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from future or ultimate build-out. These results support
that effort as the 1-foot requirement would reduce the results of Run 3 to levels more
closely resembling the results of the existing development results presented by Run 2
(see Figure 3.2-1).
$40,000,000.00
$35,000,000.00
Estimated Damage (Dollars)
$30,000,000.00
$25,000,000.00
Structure
Contents
$20,000,000.00
$15,000,000.00
$10,000,000.00
$5,000,000.00
$Run 1
Run 2
Run 3
Building Inventory and Floodplain Scenario
Figure 3.2-1. Sugar Creek Watershed Aggregate Damage Estimation Results
3-22
Table 3.2-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 3.2-2 shows that the damages are fairly
evenly split between residential and non-residential structures in Runs 1 and 2. However,
increased development of non-residential buildings results in that occupancy group
suffering the greatest proportion of damages in Run 3.
The contents damage by
occupancy generally increases with increasing building damage, again with the increase
in non-residential development in Run 3 causing a large increase in content damage to
that general occupancy classification. The results are discussed in greater detail later in
this report.
Table 3.2-2. Summary of Building and Content Damage by General Occupancy:
Sugar Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory,
New Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$1,515,237
$72,154
$2,733,629
$109,345 $26,146,868 $318,864
Residential
Single and
Multi-Family
Residential
$1,652,407
$23,606
$3,384,211
$32,231
$6,680,709
$39,068
Average
Total
Average
Content Damage
Total
NonResidential
Commercial,
Industrial,
Religious,
Education
Residential
Single and
Multi-Family
Residential
Average
Total
$2,661,309 $126,729 $5,040,033
$836.466
$11,950
3-23
$2,083,948
$201,601 $30,685,583 $374,214
$19,847
$4,080,581
$23,863
3.2.2 Data Analysis
3.2.2.1 Sugar Creek Basin Characteristics and Study Data
Sugar Creek is formed by the confluence of Irwin and Taggart Creeks. Depth grids were
created for Sugar Creek, Taggart Creek, and three tributaries contributing flow to Sugar
Creek. The grids depict the depth of flooding that will be exceeded with a 1-percent
chance in any given year (100-year flood) under both existing and future development
conditions. The network of streams analyzed is shown in Figure 3.2-2. The Irwin Creek
basin was studied separately and, therefore, was not included in the Sugar Creek Basin.
Figure 3.2-2. Sugar Creek Basin Stream Network
3-24
The studied reaches include approximately 12 miles of Sugar Creek and approximately 16
miles of tributary reaches, including Taggart Creek
The following sections outline the specific analysis performed on Sugar Creek.
3.2.2.2 Sugar Creek Watershed Depth Grid
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 419 cross
sections were used: 166 on Sugar Creek and 253 on the tributaries, including 66 on
Taggart Creek. The flood elevation grids were extended at the upstream end of each
stream, at the confluence with each tributary, and at relatively large areas where
floodwaters are not conveyed but, rather, pond at the elevation on the main stream.
Where two stream flood elevation grids overlap the final flood elevation grid uses the
greater of the two elevations. The flood elevations at the confluence of Irwin and Taggart
Creeks were extended up into the Irwin Creek Basin.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids. There are, apparently, gravel-mining operations in
and near the floodplain along Sugar Creek. Flood depths in those areas were eliminated
from the grids.
The resulting existing conditions flood depth grid is shown in Figure 3.2-3. Approximately
1,758 acres are within the 100-year floodplain.
Figure 3.2-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. Except in the mining areas, the maximum depth is approximately 23 feet.
The average (mean) depth is 6.84 feet. Approximately 434 acres is inundated at a depth
greater than 10 feet.
3-25
Figure 3.2-3. Existing Conditions Flood Depth Grid
Sugar Creek Basin
180
160
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22
Flood Depth (feet)
Figure 3.2-4. Distribution of Existing Condition Flood Depths
3-26
The future conditions flood depth grid is shown in Figure 3.2-5. Approximately 1,992
acres are within the 100-year floodplain, an increase of 234 acres or a little more than 13
percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.2-6.
The maximum depth is 25.32 feet (more than 2 feet greater than existing conditions). The
mean depth is 7.76 feet (about 0.9 foot greater than existing conditions).
The area
inundated by more than 10 feet is approximately 647 acres (213 acres more than the
existing conditions).
Figure 3.2-5. Future Conditions Flood Depth Grid
3-27
Sugar Creek Basin
180
Area Inundated (acres)
160
140
120
100
80
60
40
20
0
1
3
5
7
9
11
13
15
17
19
21
23
25
Flood Depth (feet)
Figure 3.2-6. Distribution of Future Condition Flood Depths
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.2-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.2-8. Note that
most (approximately 97.5 percent) of the differences are less than three feet. The 100year flood depth increases are expected to exceed 3 feet in approximately 50 acres within
the Coffey Creek floodplain. Those increases occur in approximately 40 acres in the
vicinity of the upstream of the York Street crossing and approximately 10 acres upstream
of Piney Top Road.
3-28
Figure 3.2-7. Increases in 100-year Flood Depths
Sugar Creek Basin
1400
Area Affected (acres)
1200
1000
800
600
400
200
0
1
2
3
4
5
Difference in Flood Depths (feet)
Figure 3.2-8. Differences in 100-year Flood Depths
3-29
3.2.2.3 Sugar Creek Parcel and Building Data Review
To determine the parcels and building footprints impacted by Sugar Creek, the floodplain
boundaries from both the 1975 flood study and the 2000 flood study were combined and a
buffer of 100-feet added. The buffer was added to ensure that not only those structures
and parcels intersecting the parcels were selected, but those that are fairly close are also
selected to provide Mecklenburg County with a more complete perspective on the
development surrounding Sugar Creek.
There are 1,115 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding Sugar Creek. Some duplicate parcels were identified and reviewed.
In all cases, these turned out to be slivers in the GIS data, most likely right-of-ways that
have been either returned to the parcel, or offers of dedication that were never acted
upon. In all cases, the slivers were merged back into the main parcel. Of the 1,115
parcels, 416 (37%) are identified as “vacant” of these parcels, 39 parcels had structures
on them, typically large structures that covered more than one parcel, and an additional 6
parcels had information in the “effective area” field in the county assessor data indicating
that a structure was on the parcel. The high percentage of vacant parcels (33% based on
377 parcels) provides Mecklenburg County opportunity to manage future development
and reduce future damage.
There are 1,234 building footprints greater than 500 square feet that intersect the
floodplain boundary.
Of these, 123 were surveyed as part of the master-elevation-
certificate surveys and another 79 were surveyed as part of the “gpsbfe” survey data.
Unfortunately of the 123 data points from the master-elevation-certificate surveys, 20 were
missing data in either the first floor elevation or the lowest adjacent grade fields rendering
the data unusable, all of the “gpsbfe” records contained the necessary data. An additional
34 parcels matched with survey data from the Flood 3C or Flood 3B survey data.
Therefore, a total of 216 of the 1,234 could be assigned a first floor elevation and lowest
adjacent grade from the survey data available. For the current inventory analysis, the 6
parcels without a building footprint that were identified as non-vacant based on the square
footage in the assessors data will be carried as single structures at the centroid of the
parcel for a total of 1,240 “buildings” in the floodplain (including the 100-foot buffer).
3-30
For the future conditions or “build-out”, a single structure was placed at the centroid of the
377 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 1,617.
3.2.3
Detailed Results for the Sugar Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Sugar Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new (2000)
floodplain boundary.
Table 3.2-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2 a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.2-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-31
Table 3.2-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Sugar Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM3
Personal & Repair
Services
COM4
Business/Professional/
Technical Services
COM7
Medical Office/Medical
Clinic
COM8
Entertainment &
Recreation
GOV1
General Government
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
RES4
Temporary
Lodging/Hotel/Motel
Current
Inventory,
Original
Floodplain
(Run 1)
1
Current
Inventory,
New
Floodplain
(Run 2)
1
Future
Inventory,
New
Floodplain
(Run 3)
14
4
3
5
4
6
4
9
10
15
0
1
1
0
0
1
1
2
1
2
1
39
42
69
127
14
15
15
4
4
4
6
5
10
4
1
12
1
15
1
91
130
253
1,149
1,110
1,364
Total No. of flooded
buildings
Total No. of unflooded
buildings
The aggregate dollar damage across all building occupancy types is shown in Figure 3.29. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.2-3 through 3.2-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the significant
escalation of estimated damage based on future development suggests the need for
3-32
mitigation measures, such as requiring future development to be 1-foot above the future
one hundred year flood elevation (as currently proposed by county staff).
$40,000,000.00
$35,000,000.00
Estimated Damage (Dollars)
$30,000,000.00
$25,000,000.00
Structure
Contents
$20,000,000.00
$15,000,000.00
$10,000,000.00
$5,000,000.00
$Run 1
Run 2
Run 3
Building Inventory and Floodplain Scenario
Figure 3.2-9. Sugar Creek Watershed Aggregate Damage Estimation Results
Table 3.2-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.2-4 clearly shows that changes in inventory development greatly affects the
average percent damage in Run 3. Viewing the GIS data indicates that many of the
currently vacant parcels assumed to be developed in the “build out” are either largely or
completely within the future floodplain boundary. In Runs 1 and 2, most of the average
structure damage levels are less than 20 percent. In Run 3, most of the occupancies
have structure damage ranges between 20 and 45 percent. In many cases, the larger
percent damage can be attributed to occupancies with a small number of damaged
3-33
structures, most notable being the occupancy class GOV1 where a single small structure
drives the damage percentage.
Table 3.2-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Sugar Creek Watershed
HAZUS®MH
Occupancy
Code
Description
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM3
Personal & Repair
Services
COM4
Business/Professional/
Technical Services
COM7
Medical Office/Medical
Clinic
COM8
Entertainment &
Recreation
GOV1
General Government
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
RES4
Temporary
Lodging/Hotel/Motel
Current
Inventory,
Original
Floodplain
(Run 1)
4.4
Current
Inventory,
New
Floodplain
(Run 2)
4.4
Future
Inventory,
New
Floodplain
(Run 3)
40.8
7.5
10.4
20.8
12.7
33.8
12.7
30.3
45.7
52.0
---
5.6
5.6
---
---
42.5
1.0
16.2
1.6
26.7
1.6
32.9
17.8
19.9
25.3
12.6
15.6
15.6
14.7
21.1
21.1
43.5
46.5
40.2
13.5
34.1
18.7
43.0
20.7
43.0
Table 3.2-5 shows the dollar damage to building structures that is computed when the
damage percentages are multiplied by the structure/parcel valuation. As with the previous
table, there are significant differences in the dollar damage between the three scenarios.
Between Run 1 and Run 2, there is an increase in total structure damage from $3.2 million
to $6.1 million. This corresponds to an increase of $2.95 million, or 93%. As the building
inventories used for these scenarios are identical, the difference in damage is attributable
3-34
to differences in the floodplain between the 1975 flood study and the 2000 revision of the
100-yr floodplain map. Proceeding from Run 2 to Run 3, there is a significant increase in
estimated damage from $6.1 million to $32.8 million, suffered by projected development in
the floodplain. This corresponds to an increase of $26.7 million, or 438%. A notable
finding from the results of Run 3 is a spike in both the total and average damage to
commercial and industrial properties and a significant increase in the single-family losses
Table 3.2-5. Structure Dollar Damage by HAZUS®MH Occupancy Type:
Sugar Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM2
COM3
COM4
COM7
COM8
GOV1
IND2/IND6
RES1A
RES1B
RES1D
RES1E
RES3
RES4
Current Inventory,
Original Floodplain
(Run 1)
Total
Avg.
$17,869
$17,869
$78,085
$19,521
$10,031
$3,344
$1,317,996
$146,444
--------$1,492
$1,492
$4,491
$2,246
$708,186
$16,862
$309,012
$22,072
$48,455
$12,114
$342,341
$57,057
$244,414
$61,103
$85,273
$85,273
Total $3,167,644
Current Inventory, New
Floodplain
(Run 2)
Total
Avg.
$17,869
$17,869
$299,022
$59,804
$16,488
$4,122
$2,272,909
$227,291
$9,193
$9,193
----$2,375
$2,375
$8,247
$4,124
$1,295,099
$18,770
$400,183
$26,679
$69,048
$17,262
$353,753
$70,751
$1,266,129
$105,511
$107,526
$107,526
$34,809 $6,117,840
Future Inventory, New
floodplain
(Run 3)
Total
Avg.
$2,383,879
$170,277
$1,108,184
$184,697
$16,488
$4,122
$6,187,158
$412,477
$9,193
$9,193
$162,225
$162,225
$2,375
$2,375
$16,169,840
$414,611
$3,967,198
$31,238
$400,183
$26,679
$69,048
$17,262
$600,265
$60,027
$1,644,016
$109,601
$107,526
$107,526
$47,060 $32,827,578
$129,753
Table 3.2-6 presents the overall average damage percentage calculated for building
contents by occupancy type. Similar to the structure damage percent results shown in
Table 3.2-4, the difference in contents damage percentages between Run 1 and Run 2 is
relatively small, while Run 3 exhibits a large increase over Run 2. In general, the contents
damage ranges from 15 to 40 percent. Again, many of the outliers occur primarily for
building types with a small number of flooded structures.
3-35
Table 3.2-6. Contents Damage Percent by HAZUS®MH Occupancy Type:
Sugar Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM3
Personal & Repair
Services
COM4
Business/Professional/
Technical Services
COM7
Medical Office/Medical
Clinic
COM8
Entertainment &
Recreation
GOV1
General Government
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
RES4
Temporary
Lodging/Hotel/Motel
Current
Inventory,
Original
Floodplain
(Run 1)
4.4
Current
Inventory,
New
Floodplain
(Run 2)
4.4
Future
Inventory,
New
Floodplain
(Run 3)
40.8
7.5
42.1
20.8
50.7
33.8
50.7
30.3
45.7
52.0
---
5.6
5.6
---
---
42.5
75.3
25.2
90.9
37.1
90.9
46.7
21.0
24.3
28.6
15.8
20.9
20.9
18.3
24.4
24.4
19.5
26.3
31.3
12.3
34.1
24.7
43.0
33.2
43.0
Table 3.2-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.2-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $3.5 million to $7.1 million. This
corresponds to an increase of $3.6 million, or 103%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $7.1 million to nearly $34.8 million attributable to
projected growth. This corresponds to an increase of $27.6 million, or 388%. Again,
3-36
there is a notable spike in both the total and average damage to commercial and industrial
properties between Runs 2 and 3.
residential contents damage.
Also notable is the increase in the single-family
The significant escalation of estimated structure and
contents damage based on future development, suggests the need for mitigation
measures within the Sugar Creek floodplain.
Table 3.2-7. Contents Dollar Damage by HAZUS®MH Occupancy Type:
Sugar Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM2
COM3
COM4
COM7
COM8
GOV1
IND2/IND6
RES1A
RES1B
RES1D
RES1E
RES3
RES4
Current Inventory,
Current Inventory, New
Current Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
$35,970
$35,970
$35,970
$35,970
$270,954
$67,739 $1,037,608
$207,522
$21,613
$7,204
$33,626
$8,406
$2,092,977
$232,553 $3,609,380
$360,938
----$22,982
$22,982
--------$56,200
$56,200
$67,841
$67,841
$3,498
$1,749
$5,532
$2,766
$414,294
$9,864
$782,318
$11,338
$197,261
$14,090
$271,847
$18,123
$$30,272
$7,568
$40,167
$10,042
$78,504
$13,084
$85,758
$17,152
$116,134
$29,033
$903,858
$75,321
$180,097
$180,097
$227,095
$227,095
Total $3,497,774
$38,437 $7,123,981
3-37
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$4,798,750
$342,768
$3,845,399
$640,900
$33,626
$8,406
$9,825,207
$655,014
$22,982
$22,982
$360,139
$360,139
$67,841
$67,841
$11,504,545
$294,988
$2,208,445
$17,389
$271,847
$18,123
$40,167
$10,042
$218,376
$21,838
$1,341,746
$89,450
$227,095
$227,095
$54,800 $34,766,164
$137,416
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
LONG WATERSHED (Report Delivered November 2001)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.3
3.3.1
Long Creek Analysis
Results Summary for the Long Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.3-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 27, 38, and 117 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.3-1. Number of Flooded Structures by General Occupancy:
Long Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial,
Religious,
Education
2
2
9
Residential
Single and
Multi-Family
Residential
25
36
108
27
38
117
Total
The dollar damage resulting from the analysis is shown on Figure 3.3-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $316
thousand, contents $157 thousand) and Run 2 (structure $558 thousand, contents $302
thousand). This corresponds to an increase of $242 thousand (77%) for structures and
$145 thousand (92%) for contents. The increase tracks the effect of changes in the
floodplain extent and depth between the 1975 flood study and newly 2000 revised
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
3-38
A comparison of the aggregate damage from Run 2 and Run 3 signifies the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were developed based on
their zoning. The result is a sharp increase in estimated structure damage from $558
thousand to $5.2 million and a corresponding increase in estimated contents damage from
$302 thousand to approximately $3.2 million. This corresponds to an increase of $4.7
million (836%) for structures and $2.9 million (962%) for contents. Prior to this analysis,
this significant escalation of potential damages based on future development was
predicted by county staff and has led to efforts requiring new development to be 1-foot
above the one-hundred year base flood elevation, where the BFE includes the increased
runoff from the future or ultimate build-out. These results support that effort as the 1-foot
requirement would reduce the results of Run 3 to levels more closely resembling the
results of the existing development results presented by Run 2 (see Figure 3.3-1).
$6,000,000.00
$5,000,000.00
Estimated Damage (Dollars)
$4,000,000.00
Structure
Content
$3,000,000.00
$2,000,000.00
$1,000,000.00
$Run 1
Run 2
Run 3
Building Inventory and Floodplain Scenario
Figure 3.3-1. Long Creek Watershed Aggregate Damage Estimation Results
3-39
Table 3.3-2 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 3.3-2 shows that the damages are dominated
by damage and losses to residential structures in Runs 1 and 2. However, increased
development to both sectors cause the significant differences between Run 2 and Run 3.
The contents damages by occupancy generally reflect the building damages, although the
typically higher content to structure ratios involved with non-residential development in
Run 3 causing a large increase in content damages to that general occupancy
classification. The results are discussed in greater detail in Section 3 of this report.
Table 3.3-2. Summary of Building and Content Damage by General Occupancy:
Long Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory,
New Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$11,913
$5,957
$21,106
$10,553
$1,845,766
$205,085
Residential
Single and
Multi-Family
Residential
$304,314
$12,173
$537,350
$14,926
$3,382,499
$31,319
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$15,529
$7,764
$25,915
$12,957
$1,332,420
$148,047
Residential
Single and
Multi-Family
Residential
$141,645
$5,666
$276,508
$7,681
$1,878,407
$17,393
3-40
3.3.2 Data Analysis
3.3.2.1 Long Creek Basin Characteristics and Study Data
Depth grids were created for Long Creek and nine tributaries contributing flow to Long
Creek. The grids depict the depth of flooding that will be exceeded with a 1-percent
chance in any given year (100-year flood) under both existing and future development
conditions. The network of streams analyzed is shown in Figure 3.3-2.
Figure 3.3-2. Long Creek Basin Stream Network
The studied reaches include approximately 15 miles of Long Creek and approximately
14.5 miles of tributary reaches.
3-41
3.3.2.1.1 Long Creek Watershed Depth Grid
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 427 cross
sections were used: 196 on Long Creek and 231 on the tributaries. The flood elevation
grids were extended at the upstream end of each stream, at the confluence with each
tributary, and at relatively large areas where floodwaters are not conveyed but, rather,
pond at the elevation on the main stream. Where two stream flood elevation grids overlap
the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.3-3. Approximately
1423 acres are within the 100-year floodplain.
Figure 3.3-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 17.36 feet and the average (mean) depth is 4.37 feet.
Approximately 79 acres, mostly channel area, is inundated at a depth greater than 10 feet.
Note that a 10-foot buffer on either side of the channels studied has an area of
approximately 70 acres.
3-42
Figure 3.3-3. Existing Conditions Flood Depth Grid
Long Creek Basin
250
200
150
100
50
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
Flood Depth (feet)
Figure 3.3-4. Distribution of Existing Condition Flood Depths
3-43
The future conditions flood depth grid is shown in Figure 3.3-5. Approximately 1560 acres
are within the 100-year floodplain, an increase of 137 acres or almost 10 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.3-6.
The maximum depth is 19.18 feet (about 1.6 greater than existing conditions). The mean
depth is 5.06 feet (about 0.7 foot greater than existing conditions). The area inundated by
more than 10 feet is approximately 140 acres (61 acres or 77 percent more than the
existing conditions).
Figure 3.3-5. Future Conditions Flood Depth Grid
3-44
Long Creek Basin
250
200
150
100
50
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
Flood Depth (feet)
Figure 3.3-6. Distribution of Future Condition Flood Depths
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.3-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.3-8. Note that
most (approximately 97.5 percent) of the differences are less than two feet. The 100-year
flood depth increases are expected to be between 2 and 3 feet in approximately 8 acres
just upstream of the Valleydale Road crossing of Gum Branch.
The increases in
approximately 30 acres in the vicinity of the Interstate 77 and U.S. Highway 21 crossings
of Dixon Branch Tributary #1 are expected to be between 2 and 5 feet.
3-45
Figure 3.3-7. Increases in 100-year Flood Depths
Long Creek Basin
900
800
700
600
500
400
300
200
100
0
1
2
3
4
5
Difference in Flood Depths (feet)
Figure 3.3-8. Differences in 100-year Flood Depths
3-46
3.3.2.1.2 Long Creek Parcel and Building Data Review
To determine the parcels and building footprints impacted by Long Creek, the floodplain
boundaries from both the 1975 flood study and the 2000 flood study were combined and a
buffer of 100-feet added. The buffer was added to ensure that not only those structures
and parcels intersecting the parcels were selected, but those that are fairly close are also
selected to provide Mecklenburg County with a more complete perspective on the
development surrounding Long Creek.
There are 1,228 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding Long Creek. Some duplicate parcels were identified and reviewed. In
all cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 1,228 parcels, 330
(35%) are identified as “vacant” and an additional 23 parcels had information in the
“effective area” field in the county assessor data indicating that a structure was on the
parcel although there was no building footprint in the footprint layer. The high percentage
of vacant parcels (35% based on 1,228 parcels) provides Mecklenburg County opportunity
to manage future development and reduce future damage.
There are 1,225 building footprints greater than 500 square feet that intersect the
floodplain boundary.
Of these, 87 were surveyed as part of the master-elevation-
certificate surveys and none were surveyed as part of the “gpsbfe” survey data.
Unfortunately of the 87 data points from the master-elevation-certificate surveys, 1 was
missing data in either the first floor elevation or the lowest adjacent grade fields rendering
the data unusable. An additional 95 parcels matched with survey data from the Flood 3C
or Flood 3B survey data. Therefore, a total of 182 of the 1,225 could be assigned a first
floor elevation and lowest adjacent grade from the survey data available. For the current
inventory analysis, the 23 parcels without a building footprint that were identified as nonvacant based on the square footage in the assessors data will be carried as single
structures at the centroid of the parcel for a total of 1,248 “buildings” in the floodplain
(including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
330 vacant parcels identified earlier.
This structure was established to have the
3-47
HAZUS®MH default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 1,578.
3.3.3
Detailed Results for the Long Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Long Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the 2000
floodplain boundary.
Table 3.3-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2 a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected rise in
floodplain development.
The values shown in Table 3.2-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-48
Table 3.3-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Long Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES2
Manufactured Housing
RES3
Multi-Family Dwelling
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
1
1
1
1
1
1
7
15
20
71
6
11
11
3
4
21
--1
--1
1
4
27
38
117
1,221
1,210
1,461
The aggregate dollar damage across all building occupancy types is shown in Figure 3.39. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.3-3 through 3.3-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the significant
escalation of estimated damage based on future development suggests the need for
mitigation measures, such as requiring future development to be 1-foot above the future
one hundred year flood elevation (as currently proposed by county staff).
3-49
$6,000,000.00
$5,000,000.00
Estimated Damage (Dollars)
$4,000,000.00
Structure
Content
$3,000,000.00
$2,000,000.00
$1,000,000.00
$Run 1
Run 2
Run 3
Building Inventory and Floodplain Scenario
Figure 3.3-9. Long Creek Watershed Aggregate Damage Estimation Results
Table 3.3-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.3-4 clearly shows that changes in inventory development greatly affects the
average percent damage in Run 3. In Runs 1 and 2, most of the average structure
damage levels are less than 20 percent.
In Run 3, most of the occupancies have
structure damage ranges between 20 and 45 percent. In many cases, the larger percent
damage can be attributed to occupancies with a small number of damaged structures,
most notable being the occupancy class RES2 where two vacant parcels with land use
codes of manufactured housing are the source of the estimated damage. Obviously, the
single structure assumption is not valid here and Mecklenburg County should review the
land use of the parcels.
3-50
Table 3.3-4. Structure Damage Percent by HAZUS®MH Occupancy Type:
Long Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Manufactured Housing
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
1.5
9.8
15.8
15.8
0.4
5.3
20.5
11.3
14.4
21.3
9.0
9.0
9.0
47.4
46.9
40.9
--7.2
--21.5
49.5
28.8
Table 3.3-5 shows the dollar damage to building structures that is computed when the
damage percentages are multiplied by the structure/parcel valuation. As with the previous
table, there are significant differences in the dollar damage between the three scenarios.
Between Run 1 and Run 2, there is an increase in total structure damage from $316
thousand to $558 thousand. This corresponds to an increase of $242 thousand, or 77%.
As the building inventory used for these scenarios are identical, the difference in damage
is attributable to differences in the floodplain between the 1975 flood study and the 2000
revision of the 100-yr floodplain map.
Proceeding from Run 2 to Run 3, there is a
significant increase in estimated damage from $558 thousand to $5.2 million caused by
projected development in the floodplain. This corresponds to an increase of $4.7 million,
or 836%. A notable finding from the results of Run 3 is a spike in both the total and
average damage to industrial properties and a significant increase in the single-family
losses.
3-51
Table 3.3-5. Structure Dollar Damage by HAZUS®MH Occupancy Type:
Long Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM8
IND2/IND6
RES1A
RES1B
RES1E
RES2
RES3
Total
Current Inventory,
Current Inventory, New
Original Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
--------$11,749
$11,749
$18,946
$18,946
$163
$163
$2,160
$2,160
$156,452
$10,430
$270,927
$13,546
$65,203
$10,867
$119,334
$10,849
$65,475
$21,825
$96,016
$24,004
--------$17,184
$17,184
$51,073
$51,073
$316,227
$11,712
$558,456
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$6,081
$6,081
$18,946
$18,946
$1,820,738
$260,106
$2,052,027
$28,902
$119,334
$10,849
$1,074,047
$51,145
$18,937
$18,937
$118,153
$29,538
$14,696 $5,228,264
$44,686
Table 3.3-6 presents the overall average damage percentage calculated for building
contents by occupancy type. Similar to the structure damage percent results shown in
Table 3.3-4, the difference in contents damage percentages between Run 1 and Run 2 is
relatively small and Run 3 exhibits a large increase. In general, the contents damage
ranges from 10 to 35 percent. Again, many of the outliers primarily occur for building
types with a small number of flooded structures.
3-52
Table 3.3-6. Contents Damage Percent by HAZUS®MH Occupancy Type:
Long Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES2
Manufactured Housing
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
1.5
9.8
15.8
15.8
0.1
4.4
28.7
10.8
16.5
25.7
9.8
10.6
10.6
35.8
27.2
35.7
--5.9
--18.34
33.3
33.5
Table 3.3-7 shows the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.3-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $157 thousand to $302 thousand.
This corresponds to an increase of $145 thousand, or 92%. The increase in damage is
due to changes in the floodplain depth and extent between the 1975 flood study and the
recent 2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a
significant increase in estimated damage from $302 thousand to $3.2 million attributable
to projected growth. This corresponds to an increase of $2.9 million, or 962%. Again,
there is a notable spike in both the total and average damage to industrial properties
between Runs 2 and 3.
Also notable is the increase in the single-family residential
contents damage. The significant escalation of estimated structure and contents damage
based on future development, suggests the need for mitigation measures within the Long
Creek floodplain.
3-53
Table 3.3-7. Contents Dollar Damage by HAZUS®MH Occupancy Type:
Long Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM8
IND2/IND6
RES1A
RES1B
RES1E
RES2
RES3
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$12,242
$12,242
$15,509
$15,509
$25,008
$25,008
$25,008
$25,008
$20
$20
$907
$906 $1,295,170
$185,024
$72,002
$4,800
$149,823
$7,491 $1,237,010
$17,423
$34,331
$5,722
$71,278
$6,480
$71,278
$6,480
$28,254
$9,418
$33,615
$8,404
$500,533
$23,835
--------$6,377
$6,377
$7,058
7,058
$21,792
$21,792
$63,210
$15,802
$157,173
$5,821
$302,423
3-54
$7,958 $3,210,827
$21,443
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
LITTLE SUGAR & BRIAR CREEK WATERSHEDS (Report
Delivered November 2001)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.4
3.4.1
Little Sugar and Briar Creek Analysis
Results Summary for the Little Sugar and Briar Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.4-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 972, 1,187, and 1,565 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.4-1. Number of Flooded Structures by General Occupancy:
Little Sugar and Briar Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
224
255
363
Residential
Single and MultiFamily Residential
748
932
1,202
972
1,187
1,565
Total
The dollar damage resulting from the analysis is shown on Figure 3.4-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $56.5
million, contents $93.5 million) and Run 2 (structure $82.1 million, contents $132.0
million). This corresponds to an increase of $25.6 million (45%) for structures and $38.5
million (41%) for contents.
The increase demonstrates the effect of changes in the
floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-55
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $82.1 million to $137.1 million and a corresponding increase in estimated
contents damage from $132.0 million to approximately $189.7 million. This corresponds
to an increase of $26.7 million (67%) for structures and $27.6 million (44%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.4-1).
$200,000,000
$180,000,000
Structure
Content
$160,000,000
Estimate Damage (Dollar)
$140,000,000
$120,000,000
$100,000,000
$80,000,000
$60,000,000
$40,000,000
$20,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.4-1. Little Sugar & Briar Creeks Aggregate Damage Estimation Results
3-56
Table 3.4-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 3.4-2 shows that the damages are fairly
evenly split between residential and non-residential structures, with slightly more nonresidential damage, in Runs 1 and 2. This trend continues in Run 3 with only a moderate
increase in the non-residential occupancies.
The contents damage by occupancy
generally increases with increasing building damage, again with the moderate increase in
non-residential development in Run 3 causing a large increase in content damage to that
general occupancy classification. The results are discussed in greater detail later in this
report.
Table 3.4-2. Summary of Building and Content Damage by General Occupancy:
Little Sugar and Briar Creek Watershed
General
Occupancy Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial
, Industrial,
Religious,
Education
$31,712,768
$141,575
$49,726,939
$195,006
$87,186,782
$240,184
Residential
Single and
Multi-Family
Residential
$24,833,667
$33,200
$32,397,774
$34,762
$49,983,225
$41,583
Content Damage
Total
Average
Total
Average
NonResidential
Commercial
, Industrial,
Religious,
Education
$78,165,102
$348,951
$109,683,821
$430,133
$157,323,971
$433,399
Residential
Single and
Multi-Family
Residential
$15,321,562
$20,483
$22,348,252
$23,979
$32,373,785
$26,933
3-57
Total
Average
3.4.2 Data Analysis
3.4.2.1 Little Sugar & Briar Creeks Basin Characteristics and Study Data
Depth grids were created for Little Sugar Creek and seven tributaries, including Briar
Creek, contributing flow to Little Sugar Creek. The grids depict the depth of flooding that
will be exceeded with a 1-percent chance in any given year (100-year flood) under both
existing and future development conditions. The network of streams analyzed is shown in
Figure 3.4-2.
Figure 3.4-2. Little Sugar Creek Basin Stream Network
The studied reaches include approximately 19 miles of Little Sugar Creek and
approximately 17 miles of tributary reaches, including 9.7 miles of Briar Creek.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 684 cross
sections were used: 335 on Little Sugar Creek and 349 on the tributaries, including 209 on
Briar Creek.
The flood elevation grids were extended at the upstream end of each
stream, at the confluence with each tributary, and at relatively large areas where
3-58
floodwaters are not conveyed but, rather, pond at the elevation on the main stream.
Where two stream flood elevation grids overlap the final flood elevation grid uses the
greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.4-3. Approximately
2300 acres are within the 100-year floodplain.
Figure 3.4-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 30.37 feet and the average (mean) depth is 6.84 feet.
Approximately 555 acres are inundated at a depth greater than 10 feet.
Figure 3.4-3. Existing Conditions Flood Depth Grid
3-59
Little Sugar Creek Basin
250
200
150
100
50
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Flood Depth (feet)
Figure 3.4-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.4-5. Approximately 2500 acres
are within the 100-year floodplain, an increase of 200 acres or almost 9 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.4-6.
The maximum depth is 30.60 feet (only 0.23 feet greater than existing conditions). The
mean depth is 7.48 feet (0.64 foot greater than existing conditions). The area inundated
by more than 10 feet is approximately 709 acres (154 acres or 28 percent more than the
existing conditions).
3-60
Figure 3.4-5. Future Conditions Flood Depth Grid
Little Sugar Creek Basin
Area Inundated (acres)
250
200
150
100
50
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Flood Depth (feet)
Figure 3.4-6. Distribution of Future Condition Flood Depths
3-61
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.4-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.4-8. Note that
most (approximately 86 percent) of the differences are less than two feet. Except for a
small area in the Little Sugar Creek floodplain just above the confluence with Dairy
Branch, the increases exceeding 2 feet occur within the Briar Creek Floodplain. The
largest area with increases exceeding 2 feet is approximately 210 acres of Briar Creek
floodplain upstream of the Seaboard Coast Line Railroad crossing (just downstream of the
confluence with Edwards Branch).
Figure 3.4-7. Increases in 100-year Flood Depths
3-62
Little Sugar Creek Basin
1400
1200
1000
800
600
400
200
0
1
2
3
4
5
Difference in Flood Depths (feet)
Figure 3.4-8. Differences in 100-year Flood Depths
3.4.2.1.1 Little Sugar & Briar Creek Parcel and Building Data Review
To determine the parcels and building footprints impacted by Little Sugar and Briar
Creeks, the floodplain boundaries from both the 1975 flood study and the 2000 flood study
were combined and a buffer of 100-feet added. The buffer was added to ensure that not
only those structures and parcels intersecting the floodplain were selected, but those that
are fairly close are also selected to provide Mecklenburg County with a more complete
perspective on the development surrounding Little Sugar and Briar Creek.
There are 4,657 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding Little Sugar and Briar Creek. Some duplicate parcels were identified
and reviewed. In all cases, these turned out to be slivers in the GIS data, most likely rightof-ways that have been either returned to the parcel, or offers of dedication that were
never acted upon. In all cases, the slivers were merged back into the main parcel. Of the
4,657 parcels, 715 are identified as “vacant” of these parcels of which, 68 parcels had
structures on them, typically large structures that covered multiple parcels precluding
development on the “vacant” parcels, and an additional 36 parcels had information in the
“effective area” field in the county assessor data indicating that a structure was on the
parcel. Therefore of the 4,657 parcels, 647 are defined as vacant for this study. Although
the percentage of vacant parcels (14% based on 4,657 parcels) does not provide
3-63
Mecklenburg County the opportunities to manage future development and reduce future
damage seen in other watersheds, the County should consider reviewing future
development conditions.
There are 5,278 building footprints greater than 500 square feet that intersect the
floodplain boundary.
Of these, 568 were surveyed as part of the master-elevation-
certificate surveys and another 1,174 were surveyed as part of the “gpsbfe” survey data.
Unfortunately of the 568 data points from the master-elevation-certificate surveys, 33 were
missing data in either the first floor elevation or the lowest adjacent grade fields rendering
the data unusable, of the 1,174 ‘gpsbfe” survey data points, 3 were missing data in either
the lowest adjacent grade or the first floor elevation. An additional 31 parcels matched
with survey data from the Flood 3C or Flood 3B survey data. Therefore, a total of 1,737 of
the 5,278 could be assigned a first floor elevation and lowest adjacent grade from the
survey data available. For the analysis, the 36 parcels without a building footprint that
were identified as non-vacant based on the square footage in the assessors data will be
carried as single structures at the centroid of the parcel for a total of 5,314 “buildings” in
the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
647 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 5,961.
3.4.3
Detailed Results for the Little Sugar & Briar Creek Basins
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Little Sugar & Briar Creek watershed (developed from
surveyed data and parcel data) and the original 100-year floodplain boundary, based
on the 1975 flood study.
3-64
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new (2000)
floodplain boundary.
Table 3.4-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2 a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.4-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-65
Table 3.4-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Little Sugar and Briar Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1
Retail Trade
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM3
Personal & Repair
Services
COM4
Business/Professional/
Technical Services
COM5
Banking
COM6
Hospital
COM7
Medical Office/Clinic
COM8
Entertainment &
Recreation
COM9
Theater
EDU2
College, University
GOV1
General Government
IND2/IND6
Mix of Light Industrial &
Construction
REL1
Religious
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
RES4
Temporary
Lodging/Hotel/Motel
RES5
Institutional, Dormitory
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
13
30
Current
Inventory,
New
Floodplain
(Run 2)
15
33
Future
Inventory,
New
Floodplain
(Run 3)
15
80
86
10
93
14
93
15
30
35
46
2
3
5
28
2
5
5
33
2
5
5
36
3
2
4
2
3
2
4
2
3
2
5
46
3
433
3
535
4
687
56
66
66
42
63
63
97
120
198
120
3
147
6
187
6
--972
1
1,187
1
1,565
4,342
4,127
4,396
3-66
The aggregate dollar damage across all building occupancy types is shown in Figure 3.49. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.4-3 through 3.4-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development suggests the need for mitigation
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
$200,000,000
$180,000,000
Structure
Content
$160,000,000
Estimate Damage (Dollar)
$140,000,000
$120,000,000
$100,000,000
$80,000,000
$60,000,000
$40,000,000
$20,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.4-9. Little Sugar & Briar Creek Aggregate Damage Estimation Results
Table 3.4-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.4-4 shows that changes in inventory development have relatively minor affects to
the average percent damage in Run 3. Slight reductions in the average percent damage
may indicate that many of the parcels are on the edges of the floodplain. In Runs 1 and 2,
most of the average structure damage levels are between 15 and 40 percent. In Run 3,
3-67
most of the occupancies have structure damage ranges between 15 and 55 percent. It is
notable that structure damage over 50% is typically deemed destroyed by the Federal
Insurance and Mitigation Administration.
Structures receiving over 50% estimated
damage are likely to be excellent acquisition candidates.
Table 3.4-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Little Sugar and Briar Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1
Retail Trade
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM3
Personal & Repair
Services
COM4
Business/Professional/
Technical Services
COM5
Banking
COM6
Hospital
COM7
Medical Office/Medical
Clinic
COM8
Entertainment &
Recreation
COM9
Theater
EDU2
College, University
GOV1
General Government
IND2/IND6
Mix of Light Industrial &
Construction
REL1
Religious
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES3
Multi-Family Dwelling
RES4
Temporary
Lodging/Hotel/Motel
RES5
Institutional, Dormitory
Current
Inventory,
Original
Floodplain
(Run 1)
19.6
37.5
Current
Inventory,
New
Floodplain
(Run 2)
23.7
43.9
Future
Inventory,
New
Floodplain
(Run 3)
23.7
47.4
48.9
12.1
50.0
13.4
50.0
14.6
50.6
49.6
50.6
2.1
33.8
32.5
5.1
31.0
36.4
5.1
31.0
36.4
35.6
40.3
37.9
24.5
50.9
2.3
9.6
24.8
56.2
2.8
12.9
24.8
56.2
2.4
33.3
26.6
21.8
29.7
23.3
27.3
26.0
13.8
12.8
12.8
13.1
14.4
14.4
35.4
39.2
38.9
23.4
23.5
25.2
16.9
26.8
16.9
---
14.6
14.6
3-68
Table 3.4-5 shows the dollar damage to building structures that is computed when the
damage percentages are multiplied by the structure/parcel valuation. As with the previous
table, there are significant differences in the dollar damage between the three scenarios.
Between Run 1 and Run 2, there is an increase in total structure damage from $56.4
million to $82.1 million. This corresponds to an increase of $25.7 million, or 46%. As the
building inventories used for these scenarios are identical, the difference in damage is
attributable to differences in the floodplain between the 1975 flood study and the 2000
revision of the 100-yr floodplain map.
Proceeding from Run 2 to Run 3, there is a
significant increase in estimated damage from $82.1 million to $137.2 million, suffered by
projected development in the floodplain. This corresponds to an increase of $55 million,
or 67%.
It is notable that Run 3 has large increases in all general occupancies
(commercial, industrial, and residential) with the greatest increase occurring in the
commercial classification.
Table 3.4-5. Structure Dollar Damage by HAZUS®MH Occupancy Type:
Little Sugar and Briar Creek Watershed
HAZUS®MH
Occupancy
Code
COM1
COM1/COM4
COM2
COM3
COM4
COM5
COM6
COM7
COM8
COM9
EDU2
GOV1
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES3
RES4
RES5
Total
Current Inventory,
Original Floodplain
(Run 1)
Total
Avg.
$663,289
$51,022
$2,836,14
$94,539
$12,829,022
$149,175
$265,707
$26,571
$5,005,997
$166,867
$20,413
$10,206
$626,919
$208,973
$2,395,638
$479,128
$2,471,302
$88,260
$914,618
$304,873
$2,834,044 $1,417,022
$22,618
$5,654
$5,506
$2,753
$139,722
$46,574
$5,709,561
$13,186
$2,963,594
$52,921
$528,275
$12,578
$8,341,520
$85,995
$7,290,716
$60,756
$681,811
$227,270
----$56,546,435
$58,175
Current Inventory, New Future Inventory, New
Floodplain
floodplain
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
$1,049,513
$69,968
$1,049,513
$69,968
$3,512,784
$106,448 $13,320,542 $166,507
$13,704,382
$147,359 $13,704,382 $147,359
$352,158
$25,154
$606,288
$40,419
$18,055,517
$515,872 $25,117,738 $546,038
$47,105
$23,553
$47,105
$23,553
$973,964
$194,793
$973,964 $194,793
$2,807,263
$561,453
$2,807263 $561,453
$3,910,239
$118,492
$4,310,940 $119,748
$921,767
$307,256
$921,767 $307,256
$2,970,575 $1,485,287
$2,970,575 $1,485,287
$27,704
$6,926
$41,541
$8,308
$7,490
$3,745 $19,849,047 $431,501
$152,655
$50,885
$232,594
$58,149
$7,569,859
$14,149 $15,476,071
$22,527
$2,131,449
$32,295
$2,131,449
$32,295
$992,481
$15,754
$992,481
$15,754
$11,176,158
$93,135 $15,549,887
$78,535
$10,236,720
$69,638 $15,542,229
$83,114
$1,233,523
$205,587
$1,233,523 $205,587
$291,107
$291,107
$291,107 $291,107
$82,124,414
$69,187 $137,170,007
$87,649
3-69
Table 3.4-6 presents the overall average damage percentage calculated for building
contents by occupancy type. Similar to the structure damage percent results shown in
Table 3.4-4, the difference in the range of contents damage percentages between Run 1
and Run 2 is relatively small, while Run 3 exhibits a large increase over Run 2.
general, the contents damage ranges from 15 to 55 percent.
Table 3.4-6. Contents Damage Percent by HAZUS®MH Occupancy Type:
Little Sugar and Briar Creek Watershed
HAZUS®MH
Occupancy
Code
COM1
COM1/COM4
COM2
COM3
COM4
COM5
COM6
COM7
COM8
COM9
EDU2
GOV1
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES3
RES4
RES5
Description
Retail Trade
Mix of Retail Trade &
Professional/Technical
Services
Wholesale Trade
Personal & Repair
Services
Business/Professional/
Technical Services
Banking
Hospitals
Medical Office/Medical
Clinic
Entertainment &
Recreation
Theater
College, University
General Government
Mix of Light Industrial &
Construction
Religious
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling,
Split Level, No Basement
Single Family Dwelling,
Split Level, with Basement
Multi-Family Dwelling
Temporary
Lodging/Hotel/Motel
Institutional, Dormitory
Current
Inventory,
Original
Floodplain
(Run 1)
19.6
37.5
Current
Inventory,
New
Floodplain
(Run 2)
23.7
43.9
Future
Inventory,
New
Floodplain
(Run 3)
23.7
46.8
49.4
49.1
50.5
50.2
50.5
50.5
50.4
49.3
50.3
2.1
52.3
32.5
5.1
40.7
36.4
5.1
40.7
36.4
47.7
53.3
54.1
24.5
50.9
76.4
11.5
24.8
56.2
88.1
17.5
24.8
56.2
86.9
49.2
26.6
25.9
29.7
27.4
27.3
28.6
18.2
16.5
16.5
15.8
17.2
17.2
33.2
36.5
33.4
39.8
23.5
46.4
16.9
48.9
16.9
----
14.6
14.6
3-70
In
Table 3.4-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.4-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $93.5 million to $132 million. This
corresponds to an increase of $38.5million, or 41%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $132 million to $189.7 million attributable to projected
growth. This corresponds to an increase of $57.6 million, or 44%. While the overall
increase in contents damage is relatively small (40 percentile), the total estimated
damages (over $100 million) warrant the review of mitigation measures.
Table 3.4-7. Contents Dollar Damage by HAZUS®MH Occupancy Type:
Little Sugar and Briar Creek Watershed
HAZUS®MH
Occupancy
Code
COM1
COM1/COM4
COM2
COM3
COM4
COM5
COM6
COM7
COM8
COM9
EDU2
GOV1
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES3
RES4
RES5
Current Inventory,
Current Floodplain
(Run 1)
Total
Avg.
$1,405,096
$108,084
$5,709,197
$190,307
$44,370,422
$515,935
$620,701
$62,070
$7,787,242
$259,575
$27,782
$13,891
$226,823
$75,608
$5,989,095 $1,197,819
$4,676,604
$167,022
$1,642,654
$547,551
$3,667,253 $1,833,626
$393,589
$98,397
$3,269
$1,635
$205,391
$68,464
$3,375774
$7,796
$2,117,439
$37,811
$325,111
$7,741
$2,875,908
$29,649
$6,627,329
$55,228
$1,439,985
$479,995
-----
Total $93,486,664
Current Inventory, New Future Inventory, New
Floodplain
Floodplain
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
$2,241,986 $149,466
$2,241,986 $149,466
$7,071,234 $214,280 $26,814,251 $335,178
$47,375,736 $509,417 $47,375,736 $509,417
$760,341
$54,310
$1,059,197
$70,613
$28,478,770 $813,679 $39,683,716 $862,689
$64,110
$32,055
$64,110
$32,055
$475,264
$95,053
$475,264
$95,053
$7,018,157 $1,403,631
$7,018,157 $1,403,631
$7,413,281 $224,645
$8,440,504 $234,458
$1,655,493 $551,831
$1,655,493 $551,831
$3,843,924 $1,921,962
$3,843,924 $1,921,962
$450,887 $112,722
$1,019,744 $203,949
$5,034
$2,517 $14,684,775 $319,234
$224,403
$74,801
$341,914
$85,478
$4,488,817
$8,390
$8,099,175
$11,789
$1,340,818
$20,315
$1,340,718
$20,315
$602,298
$9,560
$602,298
$9,560
$5,031,146
$41,926
$6,654,119
$33,607
$10,270,354
$69,866 $15,062,556
$80,548
$2,605,200 $434,200
$2,605,200 $434,200
$614,818 $614,818
$614,818 $614,818
$96,180 $132,032,072
3-71
$111,232 $189,697,756
$121,213
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
FOUR MILE CREEK AND ROCKY BRANCH WATERSHEDS
(Report Delivered December 2001)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.5 Four Mile Creek and Rocky Branch Analysis
3.5.1
Results Summary for the Four Mile Creek and Rocky Branch Basins
Two building inventories and two floodplain boundary maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.5-1 shows a summary of
the number of flooded buildings by general occupancy classification. The simulations
predicted 7, 23, and 71 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.5-1. Number of Flooded Structures by General Occupancy:
Four Mile Creek and Rocky Branch Watersheds
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
1
1
4
Residential
Single and MultiFamily Residential
6
22
67
7
23
71
Total
The dollar damage resulting from the analysis is shown on Figure 3.5-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $443
thousand, contents $246 thousand) and Run 2 (structure $847 thousand, contents $471
thousand). This corresponds to an increase of $404 thousand (91%) for structures and
$225 thousand (91%) for contents. The increase demonstrates the effect of changes in
the floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-72
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $847 thousand to $3.2 million and a corresponding increase in estimated
contents damage from $471 thousand to approximately $2.4 million. This corresponds to
an increase of $2.4 million (281%) for structures and $1.9 million (407%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.5-1).
$3,500,000
$3,000,000
Structure
Content
Estimated Damage (Dollars)
$2,500,000
$2,000,000
$1,500,000
$1,000,000
$500,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.5-1. Four Mile Creek and Rocky Branch Watersheds Aggregate
Damage Estimation Results
3-73
Table 3.5-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 3.5-2 shows that the damages are
predominantly residential, with only one non-residential structure being impacted in Run 1
and Run 2. While more non-residential structures are impacted in Run 3, the residential
damages clearly dominate the total losses.
The contents damage by occupancy is
dominated by the residential contents damage, reflecting the structure damages above.
The results are discussed in greater detail later in this report.
Table 3.5-2. Summary of Building and Content Damage by General Occupancy
Four Mile Creek and Rocky Branch Watersheds
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
$20,491
$30,388
$30,388
$369,756
$92,439
$70,406
$817,082
$37,140
$2,856,089
$42,628
NonResidential
Commercial,
Industrial,
Religious,
Education
$20,491
Residential
Single and
Multi-Family
Residential
$422,435
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$7,119
$7,119
$14,371
$14,371
$818,610
$204,652
Residential
Single and
Multi-Family
Residential
$239,108
$39,851
$456,423
$20,746
$1,572,883
$23,476
3.5.2 Data Analysis
3.5.2.1 Four Mile Creek and the Rocky Branch Basin Characteristics and Study Data
Four Mile Creek is a major tributary to McAlpine Creek. Depth grids were created for Four
Mile Creek and its tributary, Rocky Branch. The grids depict the depth of flooding that will
3-74
be exceeded with a 1-percent chance in any given year (100-year flood) under both
existing and future development conditions. The streams analyzed and the McAlpine
Creek basin are shown in Figure 3.5-2.
Figure 3.5-2. McAlpine Creek Basin Stream Network
The studied reaches include approximately 9.8 miles of Four Mile Creek and
approximately 1.6 miles of Rocky Branch.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 164 cross
sections were used: 129 on Four Mile Creek and 35 on Rocky Branch.
The flood
elevation grids were extended at the upstream end of each stream, at the confluence of
Four Mile Creek and Rocky Branch, and at relatively large areas where floodwaters are
not conveyed but, rather, pond at the elevation on the main stream. Where the two
stream flood elevation grids overlap the final flood elevation grid uses the greater of the
two elevations. Although McAlpine Creek was not included in this study, the elevation
expected at the confluence of Four Mile and McAlpine Creeks was used for the reach of
Four Mile Creek where that elevation was greater than the flood elevation grid for Four
Mile Creek.
3-75
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.5-3. Approximately
475 acres are within the 100-year floodplain.
Figure 3.5-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 17.31 feet and the average (mean) depth is 5.27 feet.
Approximately 64 acres are inundated at a depth greater than 10 feet.
Figure 3.5-3. Existing Conditions Flood Depth Grid:
Four Mile Creek and Rocky Branch
3-76
Four Mile Creek Basin
80
Area Inundated (acres)
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Flood Depth (feet)
Figure 3.5-4. Distribution of Existing Condition Flood Depths:
Four Mile Creek and Rocky Branch
The future conditions flood depth grid is shown in Figure 3.5-5. Approximately 516 acres
are within the 100-year floodplain, an increase of 41 acres or approximately 9 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.5-6.
The maximum depth is 21.62 feet (4.31 feet greater than existing conditions). The mean
depth is 6.13 feet (0.86 feet greater than existing conditions). The area inundated by
more than 10 feet is approximately 95 acres (31 acres or almost 50 percent more than
under existing conditions).
3-77
Figure 3.5-5. Future Conditions Flood Depth Grid
Four Mile Creek and Rocky Branch
Four Mile Creek Basin
70
Area Inundated (acres)
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
Flood Depth (feet)
Figure 3.5-6. Distribution of Future Condition Flood Depths
Four Mile Creek and Rocky Branch
3-78
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.5-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.5-8. Note that
most (more than 98 percent) of the differences are less than two feet.
Most of the
increases exceeding 2 feet occur within the Four Mile Creek floodplain upstream of the
Old Monroe Road crossing.
Figure 3.5-7. Increases in 100-year Flood Depths
3-79
Four Mile Creek Basin
600
Area Affected (acres)
500
400
300
200
100
0
1
2
3
4
5
Difference in Flood Depths (feet)
Figure 3.5-8. Differences in 100-year Flood Depths:
Four Mile Creek and Rocky Branch
3.5.2.1.1 Four Mile Creek and Rocky Branch Parcel and Building Data Review
To identify the parcels and building footprints impacted by Four Mile Creek and the Rocky
Branch, the floodplain boundaries from both the 1975 flood study and the 2000 flood study
were combined and a buffer of 100-feet added. The buffer was added to ensure that not
only those structures and parcels intersecting the parcels were selected, but those that
are fairly close are also selected to provide Mecklenburg County with a more complete
perspective on the development surrounding Four Mile Creek.
There are 1,098 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding Four Mile Creek and the Rocky Branch. Some duplicate parcels were
identified and reviewed. In all cases, these turned out to be either slivers in the GIS data,
most likely right-of-ways that have been either returned to the parcel, or offers of
dedication that were never acted upon. In all cases, the slivers were merged back into the
main parcel. Of the 1,098 parcels, 182 (17%) are identified as “vacant”. An additional 38
parcels had information in the “effective area” field in the county assessor data indicating
that a structure was on the parcel, but had no footprint in the footprint database. Although
the percentage of vacant parcels (17% based on 1,098 parcels) does not provide
Mecklenburg County the opportunities to manage future development and reduce future
3-80
damage seen in other watersheds, the County should consider reviewing future
development conditions.
There are 864 building footprints greater than 500 square feet that intersect the floodplain
boundary. Of these, 70 were surveyed as part of the master-elevation-certificate surveys,
but none were surveyed as part of the “gpsbfe” survey data. Unfortunately, of the 70 data
points from the master-elevation-certificate surveys, 5 were missing data in either the first
floor elevation or the lowest adjacent grade fields, rendering the data unusable.
No
additional parcels were matched with survey data from the Flood 3C or Flood 3B survey
data. Therefore, a total of 65 of the 864 could be assigned a first floor elevation and
lowest adjacent grade from the survey data available. For the analysis, the 38 parcels
without a building footprint that were identified as non-vacant based on the square footage
in the assessors data, were carried as single structures at the centroid of the parcel for a
total of 902 “buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
182 vacant parcels identified earlier.
This structure was established to have the
HAZUS®MH default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 1,084.
3.5.3
Detailed Results Four Mile Creek and Rocky Branch Basins
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Four Mile Creek and Rocky Branch watersheds
(developed from surveyed data and parcel data) and the original 100-year floodplain
boundary, based on the 1975 flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3-81
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new (2000)
floodplain boundary.
Table 3.5-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs.
The increase in the number of flooded buildings
between Run 1 and Run 2 is due to differences in the floodplain boundary between the
1975 flood study and the 2000 flood study map revisions. The increase between Run 2
and Run 3 is attributable to the projected increase in floodplain development.
The values shown in Table 3.5-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-82
Table 3.5-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Four Mile Creek and Rocky Branch Watersheds
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment &
Recreation
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory,
New
Floodplain
(Run 2)
0
Future
Inventory,
New
Floodplain
(Run 3)
1
1
1
1
0
0
2
1
6
43
2
9
9
0
2
2
3
5
13
7
23
71
895
879
1,013
The aggregate dollar damage across all building occupancy types is shown in Figure 3.59. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.5-4 through 3.5-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development suggests the need for mitigation
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
3-83
$3,500,000
$3,000,000
Structure
Content
Estimated Damage (Dollars)
$2,500,000
$2,000,000
$1,500,000
$1,000,000
$500,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.5-9. Four Mile Creek And Rocky Branch Aggregate Damage Estimation
Results
Table 3.5-4 presents the overall average damage percentage calculated for damaged
building structures by occupancy, through the application of the various depth-damage
curves. Table 3.5-4 shows that changes in inventory development have relatively minor
effects on the average percent damage in Run 3 with the exception of RES1A and
RES1E. Slight reductions in the average percent damage may indicate that many of the
additional impacted parcels are on the edges of the floodplain. In Runs 1 and 2, most of
the average structure damage levels are between 10 and 20 percent. In Run 3, most of
the occupancies have structure damage ranges between 10 and 30 percent.
3-84
Table 3.5-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Four Mile Creek and Rocky Branch Watersheds
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment &
Recreation
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
21.4
15.3
22.7
22.7
---
---
11.4
16.9
12.7
26.3
9.5
8.0
8.0
---
2.0
2.0
35.0
36.3
40.5
Table 3.5-5 shows the dollar damage to building structures that is computed when the
damage percentages are multiplied by the structure/parcel valuation. As with the previous
table, there are significant differences in the dollar damage between the three scenarios.
Between Run 1 and Run 2, there is an increase in total structure damage from $443
thousand to $847 thousand. This corresponds to an increase of $405 thousand, or 91%.
As the building inventories used for these scenarios are identical, the difference in
damage is attributable to differences in the floodplain between the 1975 flood study and
the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run 3, there is
a significant increase in estimated damage from $847 thousand to $3.2 million, suffered
by projected development in the floodplain. This corresponds to an increase of $2.4
million, or 280%.
It is notable that the primary increases occur in the residential
occupancies (RES1A through RES1E).
3-85
Table 3.5-5. Structure Dollar Damage by HAZUS®MH Occupancy Type:
Four Mile Creek and Rocky Branch Watersheds
HAZUS®MH
Occupancy
Code
COM1/COM4
COM4
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$89,552
$89,552
$20,490
COM8
$20,490
$30,388
$30,388
$30,388
$30,388
---
---
---
---
$249,816
$124,908
RES1A
$19,069
$19,069
$73,414
$12,236
$1,610,742
$37,459
RES1B
$23,678
$11,839
$95,139
$10,571
$95,139
$10,571
RES1D
---
---
$5,422
$2,711
$5,422
$2,711
RES1E
$379,689
$126,563
$643,108
$128,622
$1,144,786
$88,060
$442,926
$63,275
$847,470
$36,847
$3,225,844
$45,434
Total
Table 3.5-6 presents the overall average damage percentage calculated for damaged
building contents by occupancy type. Similar to the structure damage percent results
shown in Table 3.5-4, the difference in the range of contents damage percentages
between Run 1 and Run 2 is relatively small, while Run 3 exhibits a large increase over
Run 2. In general, the contents damage ranges from 10 to 55 percent.
Table 3.5-6. Contents Damage Percent by HAZUS®MH Occupancy Type:
Four Mile Creek and Rocky Branch Watersheds
HAZUS®MH
Occupancy
Code
COM1/COM4
COM4
COM8
RES1A
RES1B
RES1D
RES1E
Description
Mix of Retail Trade &
Professional/Technical
Services
Business/Professional/
Technical Services
Entertainment & Recreation
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling, Split
Level, No Basement
Single Family Dwelling, Split
Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory, New
Floodplain
(Run 3)
21.4
10.6
21.5
21.5
--18.9
--13.2
57.1
30.8
9.2
9.3
9.3
---
4.7
4.7
40.0
40.3
37.6
3-86
Table 3.5-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.5-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $246 thousand to $471 thousand.
This corresponds to an increase of $225 thousand, or 91%. The increase in damage is
due to changes in the floodplain depth and extent between the 1975 flood study and the
recent 2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a
significant increase in estimated damage from $471 thousand to $2.4 million attributable
to projected growth. This corresponds to an increase of $1.9 million, or 408%. The total
increase in estimated damages (nearly $2 million) warrant the review of mitigation
measures.
Table 3.5-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Four Mile Creek and Rocky Branch Watersheds
HAZUS®MH
Occupancy
Code
COM1/COM4
COM4
COM8
RES1A
RES1B
RES1D
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$180,261 $180,268
$7,119
$7,119
$14,371
$14,371
$14,371
$14,371
--------$623,970 $311,985
$10,675
$10,675
$37,988
$6,331
$994,998
$21,977
$11,511
$5,756
$55,169
$6,130
$55,169
$6,130
----$6,333
$3,166
$6,333
$3,166
$216,922
$72,307
$356,933
$71,387
$566,384
$43,568
$246,277
$35,175
$470,794
3-87
$20,469
$2,391,492
$33,683
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
McDOWELL CREEK WATERSHED (Report Delivered December 2001)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.6 McDowell Creek Analysis
3.6.1
Results Summary for the McDowell Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.6-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 25, 66, and 159 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.6-1. Number of Flooded Structures by General Occupancy:
McDowell Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
15
Residential
Single and MultiFamily Residential
25
66
144
25
66
159
Total
The dollar damage resulting from the analysis is shown on Figure 3.6-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $385
thousand, contents $181 thousand) and Run 2 (structure $2.6 million, contents $1.6
million). This corresponds to an increase of $2.2 million (577%) for structures and $1.4
million (762%) for contents. The increase demonstrates the effect of changes in the
floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-88
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $2.6 million to $8.8 million and a corresponding increase in estimated
contents damage from $1.6 million to approximately $8.1 million. This corresponds to an
increase of $6.2 million (238%) for structures and $6.5 million (418%) for contents. Prior
to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.6-1).
$10,000,000
$9,000,000
Structure
Content
$8,000,000
Estimated Damage (Dollars)
$7,000,000
$6,000,000
$5,000,000
$4,000,000
$3,000,000
$2,000,000
$1,000,000
$0
Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.6-1. McDowell Creek Watershed Aggregate Damage Estimation
Results
3-89
Table 3.6-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 3.6-2 shows that initially damages are
concentrated in the residential occupancy classification in Runs 1 and 2. While there is a
significant increase in the losses to residential occupancies in Run 3 there is also a
substantial increase in losses to non-residential occupancies due to “development” of the
vacant parcels. As expected for Run 1 and Run 2, the contents damage by occupancy
generally increases with increasing building damage. With Run 3, however, the nonresidential content damages exceed the damages resulting from the residential
occupancy classification. The results are discussed in greater detail later in this report.
Table 3.6-2. Summary of Building and Content Damage by General Occupancy:
McDowell Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$2,766,921
$276,921
Residential
Single and
Multi-Family
Residential
$385,125
$15,405
$2,607,031
$39,500
$6,046,288
$40,579
Content Damage
Total
Average
Total
Average
Total
Average
$0
$0
$0
$4,705,929
$470,593
$7241
$1,560,572
$23,645
$3,375,608
$22,655
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
Residential
Single and
Multi-Family
Residential
$181,029
3-90
3.6.2 Data Analysis
3.6.2.1 McDowell Creek Basin Characteristics and Study Data
Depth grids were created for McDowell Creek and six tributaries, including Torrence
Creek, contributing flow to McDowell Creek. The grids depict the depth of flooding that
will be exceeded with a 1-percent chance in any given year (100-year flood) under both
existing and future development conditions. The network of streams analyzed is shown in
Figure 3.6-2.
Figure 3.6-2. McDowell Creek Basin Stream Network
The studied reaches include approximately 9.2 miles of McDowell Creek and
approximately 10.9 miles of tributary reaches.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 266 cross
sections were used: 119 on McDowell Creek and 147 on the tributaries.
The flood
elevation grids were extended at the upstream end of each stream, at the confluence with
each tributary, and at relatively large areas where floodwaters are not conveyed but,
3-91
rather, pond at the elevation on the main stream. Where two stream flood elevation grids
overlap, the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.6-3. Approximately
1270 acres are within the 100-year floodplain.
Figure 3.6-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 22.25 feet and the average (mean) depth is 5.06 feet.
Approximately 160 acres are inundated at a depth greater than 10 feet.
The future
conditions flood depth grid is shown in Figure 3.6-5. Approximately 1440 acres are within
the 100-year floodplain, an increase of 170 acres or approximately 13 percent.
Figure 3.6-3. Existing Conditions Flood Depth Grid
McDowell Creek Watershed
3-92
McDowell Creek Basin
200
180
160
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Flood Depth (feet)
Figure 3.6-4. Distribution of Existing Condition Flood Depths
McDowell Creek Watershed
The distribution of floodplain area inundated at various depths is shown in Figure 3.6-6.
The maximum depth is 27.71 feet (5.46 feet greater than existing conditions). The mean
depth is 6.31 feet (1.25 feet greater than existing conditions). The area inundated by
more than 10 feet is approximately 340 acres (180 acres or more than double the existing
conditions).
3-93
Figure 3.6-5. Future Conditions Flood Depth Grid
McDowell Creek Watershed
McDowell Creek Basin
200
Area Inundated (acres)
180
160
140
120
100
80
60
40
20
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
Flood Depth (feet)
Figure 3.6-6. Distribution of Future Condition Flood Depths
McDowell Creek Watershed
3-94
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.6-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.6-8. Note that
most (approximately 95 percent) of the differences are less than three feet, and two-thirds
of those are less than two feet. Most of the increases exceeding 2 feet occur within the
McDowell Creek floodplain downstream of the confluence with Torrence Creek.
The
areas with increases exceeding 2 and 3 feet are within the floodplains of the two
tributaries to Torrence Creek upstream of the Interstate 77 crossings.
Figure 3.6-7. Increases in 100-year Flood Depths
McDowell Creek Watershed
3-95
McDowell Creek Basin
600
500
400
300
200
100
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Difference in Flood Depths (feet)
Figure 3.6-8. Differences in 100-year Flood Depths
McDowell Creek Watershed
3.6.2.1.1 McDowell Creek Parcel and Building Data Review
To determine the parcels and building footprints impacted by inundation from McDowell
Creek, the floodplain boundaries from both the 1975 flood study and the 2000 flood study
were combined and a buffer of 100-feet added. The buffer was added to ensure that not
only those structures and parcels intersecting the parcels were selected, but those that
are fairly close were also selected to provide Mecklenburg County with a more complete
perspective on the development surrounding McDowell Creek.
There are 1,657 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding McDowell Creek.
Some duplicate parcels were identified and
reviewed. In all cases, these turned out to be slivers in the GIS data, most likely right-ofways that have been either returned to the parcel, or offers of dedication that were never
acted upon. In all cases, the slivers were merged back into the main parcel. Of the 1,657
parcels, 454 (27%) are identified as “vacant”, and an additional 17 parcels had information
in the “effective area” field in the county assessor data indicating that a structure was on
the parcel, although no building footprint was available. The percentage of vacant parcels
(27% based on 1,657 parcels) provides Mecklenburg County with significant opportunities
to manage future development and reduce future damage within the watershed.
3-96
There are 971 building footprints greater than 500 square feet that intersect the floodplain
boundary. Of these, 59 were surveyed as part of the master-elevation-certificate surveys,
but none were surveyed as part of the “gpsbfe” survey data. Unfortunately, of the 59 data
points from the master-elevation-certificate surveys, 5 were missing data in either the first
floor elevation or the lowest adjacent grade fields rendering the data unusable. Therefore,
a total of 54 of the 971 were assigned a first floor elevation and lowest adjacent grade
from the survey data available.
For the analysis, the 17 parcels without a building
footprint that were identified as non-vacant based on the square footage in the assessors
data were carried as single structures at the centroid of the parcel, for a total of 988
“buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
454 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 1,442.
3.6.3
Detailed Results for the McDowell Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the McDowell Creek watershed (developed from surveyed
data and parcel data) and the original 100-year floodplain boundary, based on the
1975 flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new (2000)
floodplain boundary.
3-97
Table 3.6-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.6-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.6-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
McDowell Creek Watershed
HAZUS®MH
Occupancy
Code
Description
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
7
---
---
1
---
---
2
6
14
79
13
31
31
2
2
2
4
14
32
--25
5
66
5
159
963
922
1,283
3-98
The aggregate dollar damage across all building occupancy types is shown in Figure 3.69. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.6-4 through 3.6-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development suggests the need for mitigation
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
$10,000,000
$9,000,000
Structure
Content
$8,000,000
Estimated Damage (Dollars)
$7,000,000
$6,000,000
$5,000,000
$4,000,000
$3,000,000
$2,000,000
$1,000,000
$0
Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.6-9. McDowell Creek Watershed Aggregate Damage Estimation
Results
Table 3.6-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.6-4 shows that changes in inventory development have minor impacts on the
average percent damage for most residential occupancy classifications, but development
greatly increases the average damage percentage in the non-residential classification
3-99
(Run 3). Slight reductions in the average percent damage may indicate increased impact
on parcels that are on the edges of the floodplain. In Runs 1 and 2, most of the average
structure damage levels are between 8 and 30 percent.
In Run 3, most of the
occupancies have structure damage ranges between 12 and 70 percent. It is notable that
structure damage over 50% of market value is considered “substantial damage”3 by the
Federal Insurance and Mitigation Administration.
Structures receiving over 50%
estimated damage are likely to be excellent acquisition candidates.
Table 3.6-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: McDowell Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
68.1
---
---
8.0
---
---
26.1
15.5
22.9
24.5
8.4
13.5
13.5
11.4
14.8
14.8
31.9
34.3
38.4
---
18.7
18.7
Table 3.6-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $385 thousand to $2.6 million. This corresponds to an increase of $2.2
million, or 577%. As the building inventories used for these scenarios are identical, the
3
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
3-100
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is a significant increase in estimated damage from $2.6 million to $8.8 million,
suffered by projected development in the floodplain. This corresponds to an increase of
$6.2 million, or 762%. It is notable that Run 3 has large increases in the losses to nonresidential occupancies attributable to future development, additionally, there is a
significant increase in the single-family residential occupancy classification.
Table 3.6-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
McDowell Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM8
IND2/IND6
RES1A
RES1B
RES1D
RES1E
RES3
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$1,991,394 $284,485
--------$87,463
$87,463
--------$688,064 $344,032
$89,272
$14,879
$328,632
$23,474
$2,678,708
$33,908
$116,914
$8,993
$669,171
$21,586
$669,171
$21,586
$29,184
$14,592
$38,038
$19,019
$38,038
$19,019
$149,755
$37,439 $1,030,274
$73,591
$2,199,456
$66,233
----$540,916
$108,183
$540,916 $108,183
$385,125
$15,405 $2,607,031
$39,500
$8,813,209
$55,429
Table 3.6-6 presents the overall average damage percentage calculated for building
contents by occupancy type. Similar to the structure damage percent results shown in
Table 3.6-5, the difference in the range of contents damage percentages between Run 1
and Run 2 is relatively small, ranging from 10 to 30 percent. Run 3 exhibits a large
increase over Run 2, notably in the non-residential occupancies.
Damages in the
residential occupancies do not vary much between Run 2 and Run 3.
Overall, the
estimated damage percent ranges between 15 and 40 percent.
Table 3.6-6. Contents Damage Percent by HAZUS®MH Occupancy Type
McDowell Creek Watershed
Current
Inventory,
Current
Inventory,
Future
Inventory,
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-101
HAZUS®MH
Occupancy
Code
Description
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
Original
Floodplain
(Run 1)
---
New
Floodplain
(Run 2)
---
New
Floodplain
(Run 3)
42.3
---
---
35.5
---
---
38.2
17.2
27.1
28.3
9.7
17.4
17.4
14.7
16.1
16.1
21.3
32.9
34.3
---
29.7
29.7
Table 3.6-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.6-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $181 thousand to $1.6 million. This
corresponds to an increase of $1.4 million, or 762%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $1.6 million to $8.1 million attributable to projected
growth. This corresponds to an increase of $6.5 million, or 418%. The overall increase in
contents damage is relatively large (400 percentile), and the total estimated damages
(over $8.1 million) warrant the review of mitigation measures.
3-102
Table 3.6-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
McDowell Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM8
IND2/IND6
RES1A
RES1B
RES1D
RES1E
RES3
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$4,008,677 $572,668
--------$194,186 $194,186
--------$503,066 $251.533
$48,936
$8,156
$200,247
$14,303
$1,551,313
$19,637
$64,795
$4,984
$440,781
$14,219
$440,781
$14,219
$18,874
$9,437
$20,790
$10,395
$20,790
$10,395
$48,424
$12,106
$525,008
$37,501
$988,978
$30,906
----$373,746
$74,749
$373,746
$74,749
$181,029
$7,241
$1,560,572
3-103
$23,645
$8,081,537
$50,827
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
McMULLEN CREEK WATERSHED (Report Delivered December 2001)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.7
3.7.1
McMullen Creek Analysis
Results Summary for the McMullen Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.7-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 115, 159, and 205 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.7-1. Number of Flooded Structures by General Occupancy:
McMullen Creek Watershed
General
Occupancy
Description
NonResidential
Commercial,
Industrial, Religious,
Education
Residential
Single and MultiFamily Residential
Total
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
3
7
10
112
152
195
115
159
205
The dollar damage resulting from the analysis is shown on Figure 3.7-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $2.1
million, contents $1.2 million) and Run 2 (structure $3.6 million, contents $2.1 million).
This corresponds to an increase of $1.5 million (70%) for structures and $852 thousand
(69%) for contents. The increase demonstrates the effect of changes in the floodplain
extent and depth between the 1975 flood study and newly revised (2000) floodplain map.
Based on the assumption that the revised floodplain map is more accurate than the 1975
flood study, the estimated damage results from Run 2 are representative of current
conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-104
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $3.6 million to $7.5 million and a corresponding increase in estimated
contents damage from $2.1 million to approximately $5.2 million. This corresponds to an
increase of $3.8 million (106%) for structures and $3.1 million (150%) for contents. Prior
to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.7-1).
$8,000,000
$7,000,000
Structure
Content
Estimated Damage (Dollars)
$6,000,000
$5,000,000
$4,000,000
$3,000,000
$2,000,000
$1,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.7-1. McMullen Creek Watershed Aggregate Damage Estimation
Results
3-105
Table 3.7-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 1-2 shows that initially damages are
concentrated in the residential occupancy classification in Runs 1 and 2. While there is a
significant increase in the losses to residential occupancies in Run 3, there is substantial
“development” of vacant parcels zoned for non-residential occupancies increasing losses
in this general occupancy class to nearly one-third of the total losses (structure and
content). As expected for Run 1 and Run 2, the contents damage by occupancy generally
increases with increasing building damage. With Run 3, however, the non-residential
content damages become a significant percentage of the overall contents damage. The
results are discussed in greater detail later in this report.
Table 3.7-2. Summary of Building and Content Damage by General Occupancy:
McMullen Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$105,083
$35,028
$153,089
$21,870
$1,759,865
$175,986
Residential
Single and
Multi-Family
Residential
$2,036,883
$18,186
$3,491,773
$22,972
$5,731,306
$29,391
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$106,928
$35,643
$176,381
$25,197
$2,201,389
$220,139
Residential
Single and
Multi-Family
Residential
$1,124,836
$10,043
$1,908,588
$12,557
$3,011,254
$15,442
3-106
3.7.2 Data Analysis
3.7.2.1 McMullen Creek Basin Characteristics and Study Data
McMullen Creek is a major tributary to McAlpine Creek. Depth grids were created for
McMullen Creek and McMullen Creek Tributary. The grids depict the depth of flooding
that will be exceeded with a 1-percent chance in any given year (100-year flood) under
both existing and future development conditions. The streams analyzed and the McAlpine
Creek basin are shown in Figure 3.7-2.
Figure 3.7-2. McAlpine Creek Basin Stream Network
McMullen Creek Watershed
The studied reaches include approximately 10.9 miles of McMullen Creek and
approximately 0.7 mile of McMullen Creek Tributary.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 180 cross
sections were used: 163 on McMullen Creek and 17 on McMullen Creek Tributary. The
flood elevation grids were extended at the upstream end of each stream, at the
confluence of McMullen Creek and the tributary, and at relatively large areas where
3-107
floodwaters are not conveyed but, rather, pond at the elevation on the main stream.
Where the two stream flood elevation grids overlap the final flood elevation grid uses the
greater of the two elevations. Although McAlpine Creek was not included in this study, the
elevation expected at the confluence of McMullen and McAlpine Creeks was used for the
reach of McMullen Creek where that elevation was greater than the flood elevation grid for
McMullen Creek.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.7-4. Approximately
650 acres are within the 100-year floodplain.
Figure 3.7-5 shows the distribution of floodplain area inundated at 1-foot depth
increments.
The maximum depth is 26.68 feet occurring near the confluence with
McAlpine Creek The average (mean) depth is 5.56 feet. Approximately 75 acres are
inundated at a depth greater than 10 feet.
3-108
Figure 3.7-3. Existing Conditions Flood Depth Grid
McMullen Creek Watershed
McMullen Creek Basin
80
70
60
50
40
30
20
10
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
Flood Depth (feet)
Figure 3.7-4. Distribution of Existing Condition Flood Depths
McMullen Creek Watershed
3-109
The future conditions flood depth grid is shown in Figure 3.7-5. Approximately 685 acres
are within the 100-year floodplain, an increase of 35 acres or approximately 5 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.7-6.
The maximum depth is 28.28 feet (1.6 feet greater than existing conditions). The mean
depth is 6.01 feet (0.45 foot greater than existing conditions). The area inundated by
more than 10 feet is approximately 108 acres (33 acres or 44 percent more than under
existing conditions).
Figure 3.7-5. Future Conditions Flood Depth Grid
McMullen Creek Watershed
3-110
McMullen Creek Basin
80
70
60
50
40
30
20
10
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
Flood Depth (feet)
Figure 3.7-6. Distribution of Future Condition Flood Depths
McMullen Creek Watershed
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.7-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.7-8. Note that,
as indicated in Figure 3.7-7, the differences are less than two feet. Most of the increases
exceeding 1 foot occur near the confluence with McAlpine Creek. Those increases result
from the increase in the backwater elevations from McAlpine Creek.
3-111
Figure 3.7-7. Increases in 100-year Flood Depths
McMullen Creek Watershed
McMullenCreek Basin
600
500
400
300
200
100
0
1
2
Difference in Flood Depths (feet)
Figure 3.7-8. Differences in 100-year Flood Depths
McMullen Creek Watershed
3-112
3.7.2.1.1 McMullen Creek Parcel and Building Data Review
To determine the parcels and building footprints impacted by inundation from McMullen
Creek, the floodplain boundaries from both the 1975 flood study and the 2000 flood study
were combined and a buffer of 100-feet added. The buffer was added to ensure that not
only those structures and parcels intersecting the parcels were selected, but those that
are fairly close were also selected to provide Mecklenburg County with a more complete
perspective on the development surrounding McMullen Creek.
There are 1,427 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding McMullen Creek.
Some duplicate parcels were identified and
reviewed. In all cases, these turned out to be slivers in the GIS data, most likely right-ofways that have been either returned to the parcel, or offers of dedication that were never
acted upon. In all cases, the slivers were merged back into the main parcel. Of the 1,427
parcels, 95 (6.7%) are identified as “vacant”, and an additional 102 parcels had
information in the “effective area” field in the county assessor data indicating that a
structure was on the parcel, although no building footprint was available. The percentage
of vacant parcels (6.7% based on 1,427 parcels) shows that Mecklenburg County has
only a few opportunities left to manage future development and reduce future damage
within the watershed.
There are 1,592 building footprints greater than 500 square feet that intersect the
floodplain boundary.
Of these, 125 were surveyed as part of the master-elevation-
certificate surveys, and an additional 192 were surveyed as part of the “gpsbfe” survey
data. Unlike other watersheds, only one (1) of the 125 data points from the masterelevation-certificate surveys was missing data in either the first floor elevation or the
lowest adjacent grade fields rendering the data unusable. Of the “gpsbfe” survey data
points, all contained sufficient data for utilization in the study. For the analysis, the 102
parcels without a building footprint that were identified as non-vacant based on the square
footage in the assessors data were carried as single structures at the centroid of the
parcel, for a total of 1,694 “buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
95 vacant parcels identified earlier. This structure was established to have the HAZUS®MH
default floor area and value based on the occupancy class assigned to the approved
3-113
zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 1,789.
3.7.3
Detailed Results for the McMullen Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the McMullen Creek watershed (developed from surveyed
data and parcel data) and the original 100-year floodplain boundary, based on the
1975 flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new (2000)
floodplain boundary.
Table 3.7-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.7-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-114
Table 3.7-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
McMullen Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM4
Business Professional
Technical
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
REL1
Religious
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory,
New
Floodplain
(Run 2)
1
Future
Inventory,
New
Floodplain
(Run 3)
1
0
0
1
0
1
2
2
2
2
1
2
3
0
35
1
44
1
75
26
35
35
19
27
27
10
16
27
22
115
30
159
31
205
1,579
1,535
1,584
The aggregate dollar damage across all building occupancy types is shown in Figure 3.79. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.7-4 through 3.7-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development suggests the need for mitigation
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
3-115
$8,000,000
$7,000,000
Structure
Content
Estimated Damage (Dollars)
$6,000,000
$5,000,000
$4,000,000
$3,000,000
$2,000,000
$1,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.7-9. McMullen Creek Watershed Aggregate Damage Estimation
Results
Table 3.7-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.7-4 shows that changes in inventory development have minor impacts on the
average percent damage for most residential occupancy classifications (except RES1A),
but development greatly increases the average damage percentage in the non-residential
classification (Run 3). In Runs 1 and 2, most of the average structure damage levels
range widely between 4 and 50 percent. This trend is continued in Run 3, where most of
the occupancies have structure damage ranges between 4 and 50 percent, but
development has increased the number of structures impacted. It is notable that structure
damage over 50% of market value is considered “substantial damage”4 by the Federal
4
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-116
Insurance and Mitigation Administration.
Structures receiving over 50% estimated
damage are likely to be excellent acquisition candidates.
Table 3.7-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: McMullen Creek Watershed
HAZUS®MH
Occupancy
Code
Description
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM4
Business Professional
Technical
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
REL1
Religious
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
2.5
Future
Inventory,
New
Floodplain
(Run 3)
2.5
-----
0.2
---
0.2
45.3
50.1
52.4
52.3
21.9
17.6
27.4
--14.8
1.1
18.5
1.1
24.0
9.1
9.9
9.9
4.8
6.8
6.8
30.4
30.2
37.6
11.8
16.5
17.0
Table 3.7-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $2.1 million to $3.6 million. This corresponds to an increase of $1.5 million,
or 70%. As the building inventories used for these scenarios are identical, the difference
in damage is attributable to differences in the floodplain between the 1975 flood study and
the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run 3, there is
a significant increase in estimated damage from $2.6 million to $8.8 million, suffered by
projected development in the floodplain. This corresponds to an increase of $3.8 million,
3-117
or 106%. It is notable that Run 3 has large increases in damages across nearly every
occupancy classification.
Table 3.7-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
McMullen Creek Watershed
Current Inventory,
Current Inventory, New Future Inventory, New
HAZUS®MH
Original Floodplain
Floodplain
floodplain
Occupancy
(Run 1)
(Run 2)
(Run 3)
Code
Total
Avg.
Total
Avg.
Total
Avg.
COM1/COM4
----$19,812
$19,812
$19,812
$19,812
COM2
----$30
$30
$30
$30
COM4
----$57,975
$28,988
$988,293 $494,146
COM8
$55,501
$27,750
$71,121
$35,560
$57,975
$28,988
IND2/IND6
$49,582
$49,582
$71,121
$35,560
$689,604 $229,868
REL1
----$4,152
$4,152
$4,152
$4,152
RES1A
$491,070
$14,031
$786,575
$17,877
$2,228586
$29,714
RES1B
$383,836
$14,763
$538,353
$15,382
$538,353
$15,382
RES1D
$110,595
$5,821
$217,491
$8,055
$217,491
$8,055
RES1E
$349,003
$34,900
$542,873
$33,930
$1,316,824
$48,771
RES3
$702,379
$31,926 $1,406.480
$46,883
$1,430,051
$46,131
Total $2,141,966
$18,626 $3,644,863
$22,924
$7,491,171
$36,542
Table 3.7-6 presents the overall average damage percentage calculated for building
contents by occupancy type. Similar to the structure damage percent results shown in
Table 3.7-4, the difference in the range of contents damage percentages between Run 1
and Run 2 is varies over a large range from 7 to 50 percent. Run 3 exhibits similarities to
Run 2 with slight increases on many of the residential occupancies, but with large
increases in the non-residential occupancies. Overall, the average estimated contents
damage percent ranges between 1 and 50 percent.
3-118
Table 3.7-6. Contents Damage Percent by HAZUS®MH Occupancy Type
McMullen Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
COM2
Wholesale Trade
COM4
Business Professional
Technical
COM8
Entertainment &
Recreation
IND2/IND6
Mix of Light Industrial &
Construction
REL1
Religious
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
2.5
Future
Inventory,
New
Floodplain
(Run 3)
2.5
-----
0.2
---
0.2
41.7
50.1
52.4
52.4
29.7
25.3
40.5
--16.3
1.1
21.5
1.1
28.0
10.5
11.4
11.4
7.4
10.3
10.3
27.2
28.0
29.0
11.7
16.1
17.0
Table 3.7-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.7-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $1.2 million to $2.1 million. This
corresponds to an increase of $852 thousand, or 69%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $2.1 million to $5.2 million attributable to projected
growth. This corresponds to an increase of $3.1 million, or 150%. The overall increase in
contents damage is relatively large (150 percentile), and the total estimated damages
(over $5.2 million) warrant the review of mitigation measures.
3-119
Table 3.7-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
McMullen Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM2
COM4
COM8
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES3
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
----$39,881
$39,881
$39,881
$39,881
----$103
$103
$103
$103
--------$1,558,131 $779,065
$73,261
$36,630
$76,527
$38,264
$76,527
$38,264
$33,668
$33,668
$53,766
$26,883
$520,643 $173,578
----$6,103
$6,103
$6,103
$6,103
$264,890
$7,568
$459,203
$10,436
$1,302,487
$17,366
$217,653
$8,371
$309,772
$8,851
$309,722
$8,851
$83,846
$4,413
$165,500
$6,130
$165,500
$6,130
$178,497
$17,850
$271,367
$16,960
$515,466
$19,091
$379,949
$17,270
$702,746
$23,425
$718,030
$23,162
Total $1,231,764
$10,711
$2,084,969
3-120
$13,113
$5,212,643
$25,428
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
MCKEE CREEK WATERSHED (Report Delivered January 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.8
3.8.1
McKee Creek Analysis
Results Summary for the McKee Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.8-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 2, 23, and 62 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.8-1. Number of Flooded Structures by General Occupancy:
McKee Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
0
Residential
Single and MultiFamily Residential
2
23
62
2
23
62
Total
The dollar damage resulting from the analysis is shown on Figure 3.8-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $8.2
thousand, contents $3.7 thousand) and Run 2 (structure $217 thousand, contents $112
thousand). This corresponds to an increase of $209 thousand (2,548%) for structures and
$108 thousand (2,928%) for contents. The increase demonstrates the effect of changes
in the floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-121
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $217 thousand to $1.8 million and a corresponding increase in estimated
contents damage from $112 thousand to approximately $1.1 million. This corresponds to
an increase of $1.6 million (739%) for structures and $938 thousand (838%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.8-1).
$2,000,000
$1,800,000
Structure
Content
$1,600,000
Estimated Damage (Dollars)
$1,400,000
$1,200,000
$1,000,000
$800,000
$600,000
$400,000
$200,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.8-1. McKee Creek Watershed Aggregate Damage Estimation Results
3-122
Table 3.8-2 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 1-2 shows all damages are concentrated in
the residential occupancy classification in Runs 1, Run 2, and Run 3. The differences
between Run 1 and Run 2 can be attributed to changes in the flood depths resulting from
differences in the 1975 and 2000 flood studies. While the average loss is still low ($9,000)
the addition of 21 structures being flooded greatly increases the total loss. The flood
study is the same for Run 2 and Run 3, however new “development” in Run 3 causes
significant increases in building and contents damages. The results are discussed in
greater detail later in this report.
Table 3.8-2. Summary of Building and Content Damage by General Occupancy:
McKee Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$8,200
$4,100
$217,095
$9,439
$1,822,089
$29,389
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$3,699
$1,850
$111,998
$4,869
$1,050,155
$16,938
3-123
3.8.2 Data Analysis
3.8.2.1 McKee Creek Basin Characteristics and Study Data
Depth grids were created for McKee Creek. The grids depict the depth of flooding that will
be exceeded with a 1-percent chance in any given year (100-year flood) under both
existing and future development conditions. The location of McKee Creek relative to
nearby stream systems is shown in Figure 3.8-2. Approximately 3.14 miles of McKee
Creek were studied.
Figure 3.8-2. McKee Creek and Nearby Stream Systems
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches. In all, 50 cross sections
were used to study McKee Creek.
The flood elevation grids were extended at the
upstream end of the reach and at relatively large areas where floodwaters are not
conveyed but, rather, pond at the elevation on the main stream.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
3-124
The resulting existing conditions flood depth grid is shown in Figure 3.8-3. Approximately
168 acres are within the 100-year floodplain.
Figure 3.8-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is approximately 11.39 feet. The average (mean) depth
is 2.98 feet. Less than 0.15 acre is inundated at a depth greater than 10 feet.
Figure 3.8-3. Existing Conditions Flood Depth Grid
3-125
McKee Creek
45
Area Inundated (acres)
40
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
Flood Depth (feet)
Figure 3.8-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.8-5. Approximately 202 acres
are within the 100-year floodplain, an increase of 34 acres or a little more than 20 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.8-6.
The maximum depth is 14.58 feet (more than 3 feet greater than existing conditions). The
mean depth is 4.77 feet (about 1.8 feet greater than existing conditions).
The area
inundated by more than 10 feet is approximately 4.43 acres (about 4.3 acres or 30 times
more than the existing conditions).
3-126
Figure 3.8-5. Future Conditions Flood Depth Grid
Mckee Creek
40
Area Inundated (acres)
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Flood Depth (feet)
Figure 3.8-6. Distribution of Future Condition Flood Depths
3-127
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.8-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.8-8. Note that
most (over 70 percent) of the differences are more than two feet; more than 15 percent of
the differences exceed three feet (the mean depth under existing conditions).
Figure 3.8-7. Increases in 100-year Flood Depths
3-128
McKee Creek
120
Area Affected (acres)
100
80
60
40
20
0
1
2
3
4
Difference in Flood Depths (feet)
Figure 3.8-8. Differences in 100-year Flood Depths
3.8.2.1.1 McKee Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from McKee Creek, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 345 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding McKee Creek. Some duplicate parcels were identified and reviewed. In all
cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 345 parcels, 136
(39%) are identified as “vacant”, no parcels had information in the “effective area” field in
the county assessor data indicating that a structure was on the parcel, although no
3-129
building footprint was available. The percentage of vacant parcels (39% based on 345
parcels) shows that Mecklenburg County has significant opportunities to manage future
development and reduce future damage within the watershed.
Within the 354 parcels, there are 223 building footprints greater than 500 square feet that
are near the buffered floodplain boundary. Of these, none were surveyed as part of the
master-elevation-certificate surveys or the “gpsbfe” survey data. Since there were no
parcels identified as non-vacant based on square footage information within the assessors
data, the final count of buildings for this analysis is 223.
For the future conditions or “build-out”, a single structure was placed at the centroid of the
136 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 359.
3.8.3
Detailed Results for the McKee Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the McKee Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.8-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
3-130
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.8-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.8-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
McKee Creek Watershed
HAZUS®MH
Occupancy
Description
Code
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
1
Current
Inventory,
New
Floodplain
(Run 2)
9
Future
Inventory,
New
Floodplain
(Run 3)
39
1
14
14
0
0
9
2
23
62
221
200
297
The aggregate dollar damage across all building occupancy types is shown in Figure 3.89. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.8-4 through 3.8-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development validates the need for additional
mitigation measures, such as requiring future development to be 1-foot above the future
one hundred year flood elevation (as currently proposed by county staff).
3-131
$2,000,000
$1,800,000
Structure
Content
$1,600,000
Estimated Damage (Dollars)
$1,400,000
$1,200,000
$1,000,000
$800,000
$600,000
$400,000
$200,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.8-9. McKee Creek Watershed Aggregate Damage Estimation Results
Table 3.8-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.8-4 shows that the differences between the 2000 flood study and the 1975 flood
study had a significant impact on the average damage (%) to the existing structures. In
Runs 1 has an average damage level between 3 and 10 percent. Run 2 increases the
lowest values for the average structure damage levels closing the range for all
occupancies to 9 and 11 percent. Table 3.8-4 also shows that new “development” also
greatly impacts the average damage (%) with the structure damage ranging between 9
and 40 percent. Run 3 show additional damage in the RES1A occupancy and adds the
RES1E occupancy to increase the average damage level range between 10 and 40
percent. It is notable that structure damage over 50% of market value is considered
3-132
“substantial damage”5 by the Federal Insurance and Mitigation Administration. Structures
receiving over 50% estimated damage are likely to be excellent acquisition candidates.
Table 3.8-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: McKee Creek Watershed
HAZUS®MH
Occupancy
Code
Description
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
3.9
Current
Inventory,
New
Floodplain
(Run 2)
10.3
Future
Inventory,
New
Floodplain
(Run 3)
21.4
9.4
9.4
9.4
---
---
39.8
Table 3.8-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $8.2 thousand to $217 thousand. This corresponds to an increase of $209
thousand, or 2,548%. As the building inventories used for these scenarios are identical,
the difference in damage is attributable to differences in the floodplain between the 1975
flood study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to
Run 3, there is a significant increase in estimated damage from $217 thousand to $1.8
million, suffered by projected development in the floodplain.
This corresponds to an
increase of $1.6 million, or 739%. All losses are related to single-family dwellings (RES1
classification).
5
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-133
Table 3.8-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
McKee Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$1,405
$1,405
$75,430
$8,381
$1,158,830
$29,714
$75,430
$8,381
$141,665
$10,119
$141,665
$10,119
--------$521,594
$57,955
$8,200
$4,100
$217,095
$9,439
$1,822,089
$29,389
Table 3.8-6 presents the overall average damage percentage calculated for building
contents by occupancy type.
Run 1 has an average damage percent for contents
between 2 and 9 percent. Increases in the content damage for Run 2 narrows the range
to between 10 and 11 percent.
Run 3 exhibits large increases over the damage
percentages seen in Run 2. Overall, the average estimated contents damage percent
ranges between 10 to 40 percent.
Table 3.8-6. Contents Damage Percent by HAZUS®MH Occupancy Type
McKee Creek Watershed
HAZUS®MH
Occupancy
Description
Code
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Current
Inventory,
Original
Floodplain
(Run 1)
2.7
Current
Inventory,
New
Floodplain
(Run 2)
10.7
Future
Inventory,
New
Floodplain
(Run 3)
26.8
8.9
10.0
10.0
---
---
37.8
Table 3.8-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.8-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $3.7 thousand to $112 thousand.
This corresponds to an increase of $108 thousand, or 2,928%. The increase in damage is
due to changes in the floodplain depth and extent between the 1975 flood study and the
recent 2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a
3-134
significant increase in estimated damage from $112 thousand to $1.1 million attributable
to projected growth. This corresponds to an increase of $938 thousand, or 838 %.
Table 3.8-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
McKee Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$500
$500
$36,949
$4,105
$727,621
$18,657
$3,204
$3,204
$75,049
$5,361
$75,049
$5,361
--------$247,485
$27,498
$3,704
$3,704
$111,998
3-135
$4,869
$1,050,155
$16,938
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
BACK CREEK WATERSHED (Report Delivered January 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.9
3.9.1
Back Creek Analysis
Results Summary for the Back Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.9-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 4, 4, and 14 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.9-1. Number of Flooded Structures by General Occupancy:
Back Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
1
Residential
Single and MultiFamily Residential
4
4
13
4
4
14
Total
The dollar damage resulting from the analysis is shown on Figure 3.9-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $51
thousand, contents $29 thousand) and Run 2 (structure $63 thousand, contents $42
thousand). This corresponds to an increase of $12 thousand (23%) for structures and $13
thousand (45%) for contents. The increase demonstrates the effect of changes in the
floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-136
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $63 thousand to $564 thousand and a corresponding increase in estimated
contents damage from $42 thousand to approximately $578 thousand. This corresponds
to an increase of $500 thousand (790%) for structures and $536 thousand (1,200%) for
contents. Prior to this analysis, this significant escalation of potential damages based on
future development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.9-1).
$700,000
$600,000
Structure
Content
Estimated Damage (Dollars)
$500,000
$400,000
$300,000
$200,000
$100,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.9-1. Back Creek Watershed Aggregate Damage Estimation Results
3-137
Table 3.9-1 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 3.9-2 shows all damages are concentrated in
the residential occupancy classification in Runs 1 and 2. While there is a significant
increase in the losses to residential potential occupancies in Run 3, “development” of
vacant parcels zoned for non-residential occupancies adds a single structure that
accounts for slightly less than half of the total losses (structure and content). As expected
for Run 1 and Run 2, the contents damage by occupancy generally increases with
increasing building damage. With Run 3, however, the non-residential content damages
from the aforementioned structure accounts for over half of the contents damage. The
results are discussed in greater detail later in this report.
Table 3.9-2. Summary of Building and Content Damage by General Occupancy:
Back Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$163,812
$163,812
Residential
Single and
Multi-Family
Residential
$51,615
$29,031
$63,339
$15,835
$399,978
$30,768
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$329,754
$329,754
Residential
Single and
Multi-Family
Residential
$29,031
$7,258
$42,141
$10,535
$248,370
$19,105
3-138
3.9.2 Data Analysis
3.9.2.1 Back Creek and Back Tributary Basin Characteristics and Study Data
Back Creek is located in east-central Mecklenburg County, just south of the Mallard Creek
Basin. Depth grids were created for Back Creek and Back Creek Tributary. The grids
depict the depth of flooding that will be exceeded with a 1-percent chance in any given
year (100-year flood) under both existing and future development conditions.
The
streams analyzed and the location relative to the Mallard Creek basin are shown in Figure
3.9-2.
Figure 3.9-2. Back Creek and Back Creek Tributary
The studied reaches include approximately 4.6 miles of Back Creek and approximately
2.3 miles of Back Creek Tributary.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 100 cross
sections were used: 71 on Back Creek and 29 on Back Creek Tributary.
The flood
elevation grids were extended at the upstream end of each stream, at the confluence with
Back Creek Tributary, and at relatively large areas where floodwaters are not conveyed
3-139
but, rather, pond at the elevation on the main stream. Where two stream flood elevation
grids overlap the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions (1975 flood study) flood depth grid is shown in Figure
3.9-3. Approximately 210 acres are within the 100-year floodplain.
Figure 3.9-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is approximately 10.30 feet. The average (mean) depth
is 2.98 feet. Approximately 24 acres are inundated at a depth greater than 5 feet.
Figure 3.9-3. Existing Conditions Flood Depth Grid
3-140
Back Creek and Back Creek Tributary
60
Area Inundated (acres)
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
Flood Depth (feet)
Figure 3.9-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.9-5. Approximately 234 acres
are within the 100-year floodplain, an increase of 24 acres or a little more than 10 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.9-6.
The maximum depth is 10.88 feet (about 0.5 foot greater than existing conditions). The
mean depth is 3.70 feet (about 0.7 foot greater than existing conditions).
The area
inundated by more than 5 feet is approximately 60 acres (36 acres or 150 percent more
than the existing conditions).
3-141
Figure 3.9-5. Future Conditions Flood Depth Grid
Back Creek and Back Creek Tributary
50
Area Inundated (acres)
45
40
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
Flood Depth (feet)
Figure 3.9-6. Distribution of Future Condition Flood Depths
3-142
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.9-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.9-8. Note that
most (approximately 95 percent) of the differences are less than two feet. The 100-year
flood depth increases are expected to exceed 2 feet in approximately 11.6 acres within
the Back Creek floodplain.
Figure 3.9-7. Increases in 100-year Flood Depths
3-143
Back Creek and Back Creek Tributary
140
Area Affected (acres)
120
100
80
60
40
20
0
1
2
3
4
Difference in Flood Depths (feet)
Figure 3.9-8. Differences in 100-year Flood Depths
3.9.2.1.1 Back Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Back Creek, the floodplain boundaries from both the 1975 flood study and
the 2000 flood study were combined and a buffer of 100-feet added.
The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 204 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding Back Creek. Some duplicate parcels were identified and reviewed. In all
cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 204 parcels, 75
(37%) are identified as “vacant”, an additional parcel had information in the “effective
area” field in the county assessor data indicating that a structure was on the parcel,
although no building footprint was available. The percentage of vacant parcels (37%
3-144
based on 204 parcels) shows that Mecklenburg County has significant opportunities to
manage future development and reduce future damage within the watershed.
Within these 204 parcels, there are 307 building footprints greater than 500 square feet
that are near the buffered floodplain boundary. Of these, none were surveyed as part of
the master-elevation-certificate surveys or the “gpsbfe” survey data. For the analysis, the
parcel without a building footprint that was identified as non-vacant based on the square
footage in the assessors data was carried as single structure at the centroid of the parcel,
for a total of 308 “buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
75 vacant parcels identified earlier. This structure was established to have the HAZUS®MH
default floor area and value based on the occupancy class assigned to the approved
zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 383.
3.9.3
Detailed Results for the Back Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Back Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.9-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
3-145
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.9-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.9-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Back Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory,
New
Floodplain
(Run 2)
0
Future
Inventory,
New
Floodplain
(Run 3)
1
2
2
10
2
2
2
0
0
1
4
4
14
304
304
369
The aggregate dollar damage across all building occupancy types is shown in Figure 3.99. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from Run 2
to Run 3. Tables 3.9-4 through 3.9-7 and the associated discussion go into greater detail
about the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development validates the need for mitigation
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
3-146
$700,000
$600,000
Structure
Content
Estimated Damage (Dollars)
$500,000
$400,000
$300,000
$200,000
$100,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.9-9. Back Creek Watershed Aggregate Damage Estimation Results
Table 3.9-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.9-4 shows that changes in inventory development have minor impacts on the
average percent damage for most residential occupancy classifications, but development
increases the average damage percentage in the non-residential classification and the
RES1E classification (Run 3). In Runs 1 and 2, the average structure damage levels
range between 10 and 20 percent. In Run 3, the structure damage ranges expand to
between 10 and 50 percent. It is notable that structure damage over 50% of market value
is considered “substantial damage”6 by the Federal Insurance and Mitigation
6
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-147
Administration.
Structures receiving over 50% estimated damage are likely to be
excellent acquisition candidates.
Table 3.9-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Back Creek Watershed
HAZUS®MH
Occupancy
Code
Description
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
39.2
15.9
20.4
22.4
12.3
14.7
14.7
---
---
48.2
Table 3.9-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $51 thousand to $63 thousand. This corresponds to an increase of $12
thousand, or 23%. As the building inventories used for these scenarios are identical, the
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is a significant increase in estimated damage from $63 thousand to $564
thousand, suffered by projected development in the floodplain. This corresponds to an
increase of $500 thousand, or 790%. It is notable that Run 3 has increased damages
across nearly every occupancy classification.
3-148
Table 3.9-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Back Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
RES1A
RES1B
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$163,812 $163,812
$17,431
$8,716
$22,384
$11,192
$288,892
$28,889
$34,184
$17,092
$40,955
$20,478
$40,955
$20,478
--------$70,131
$70,131
$51,615
$29,031
$63,339
$15,835
$563,791
$40,271
Table 3.9-6 presents the overall average damage percentage calculated for building
contents by occupancy type. Similar to the structure damage percent results shown in
Table 3.9-4, the difference in the range of contents damage percentages between Run 1
and Run 2 is varies from 15 to 30 percent. Run 3 has an increased range of damage due
to development in the COM1/COM4 and RES1E occupancy classifications. Overall, the
average estimated contents damage percent ranges between 15 and 55 percent.
Table 3.9-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Back Creek Watershed
HAZUS®MH
Occupancy
Code
Description
COM1/COM4 Mix of Retail Trade &
Professional/Technical
Services
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Current
Inventory,
Original
Floodplain
(Run 1)
---
Current
Inventory,
New
Floodplain
(Run 2)
---
Future
Inventory,
New
Floodplain
(Run 3)
39.2
17.1
29.0
28.7
14.2
18.9
18.9
---
---
54.3
3-149
Table 3.9-7 provides the estimated dollar damage to building contents by occupancy type.
Similar to the structure dollar damage shown in Table 3.9-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $29 thousand to $42 thousand. This
corresponds to an increase of $13 thousand, or 45%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $42 thousand to $578 thousand attributable to
projected growth.
This corresponds to an increase of $536 thousand, or 1,272%.
Interestingly, while overall increase in contents damage is relatively large (1,270
percentile), the total estimated damages ($536 thousand) is relatively low when compared
to other watersheds in Mecklenburg County.
Table 3.9-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Back Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
RES1A
RES1B
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
--------$329,754 $329,754
$9,343
$4,671
$15,873
$7,936
$182,597
$18,260
$19,689
$9,844
$26,269 $13,1334
$26,269
$13,134
--------$39,504
$39,504
$29,031
$7,258
$42,141
3-150
$10,535
$578,124
$41,295
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
SIX MILE CREEK WATERSHED (Report Delivered January 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.10 Six Mile Creek Analysis
3.10.1 Results Summary for the Six Mile Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.10-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 23, 33, and 90 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.10-1. Number of Flooded Structures by General Occupancy:
Six Mile Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
1
2
2
Residential
Single and MultiFamily Residential
22
31
88
23
33
90
Total
The dollar damage resulting from the analysis is shown on Figure 3-10-1. The damage
for each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $473
thousand, contents $246 thousand) and Run 2 (structure $729 thousand, contents $410
thousand). This corresponds to an increase of $256 thousand (54%) for structures and
$164 thousand (67%) for contents. The increase demonstrates the effect of changes in
the floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-151
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $729 thousand to $3.0 million and a corresponding increase in estimated
contents damage from $410 thousand to approximately $1.9 million. This corresponds to
an increase of $2.3 million (313%) for structures and $1.5 million (361%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3-10-1).
$3,500,000
$3,000,000
Structure
Content
Estimated Damage (Dollars)
$2,500,000
$2,000,000
$1,500,000
$1,000,000
$500,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.10-1. Six Mile Creek Watershed Aggregate Damage Estimation Results
3-152
Table 3.10-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 1-2 shows the majority of damages are
concentrated in the residential occupancy classification in Runs 1, 2 and 3. Run 2 shows
that differences between the 1975 and 2000 flood study nearly double the damages
(structure and contents). As expected, there is a significant increase in the losses to
residential occupancies in Run 3, because of “development” of vacant parcels.
The
results are discussed in greater detail later in this report.
Table 3.10-2. Summary of Building and Content Damage by General
Occupancy: Six Mile Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
$6,778
$20,584
$10,292
$20,584
$10,292
$21,196
$708,383
$22,851
$2,988,182
$33,957
NonResidential
Commercial,
Industrial,
Religious,
Education
$6,778
Residential
Single and
Multi-Family
Residential
$466,305
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$8,947
$8,947
$27,170
$13,585
$27,170
$13,585
Residential
Single and
Multi-Family
Residential
$236,637
$10,756
$382,903
$12,352
$1,863,987
$21,182
3.10.2 Data Analysis
3.10.2.1 Six Mile Creek and Flat Branch Basin Characteristics and Study Data
Depth grids were created for Six Mile Creek and Flat Branch. The grids depict the depth
of flooding that will be exceeded with a 1-percent chance in any given year (100-year
3-153
flood) under both existing and future development conditions. The streams analyzed and
the location relative to the McAlpine Creek basin are shown in Figure 3.10-2.
Figure 3.10-2. Six Mile Creek and Flat Branch
The studied reaches include approximately 8.9 miles of Six Mile Creek and approximately
3.0 miles of Flat Branch.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 148 cross
sections were used: 109 on Six Mile Creek and 39 on Flat Branch. The flood elevation
grids were extended at the upstream end of each stream, at the confluence with Flat
Branch, and at relatively large areas where floodwaters are not conveyed but, rather,
pond at the elevation on the main stream. Where two stream flood elevation grids overlap
the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
3-154
The resulting existing conditions flood depth grid is shown in Figure 3.10-3.
Approximately 895 acres are within the 100-year floodplain.
Figure 3.10-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. There are two small pits, totaling less than 0.5 acre, in the floodplain on the
right side of the lower reach of Six Mile Creek. The maximum depth (not within the pits) is
approximately 16.57 feet. The average (mean) depth is 4.55 feet. Approximately 69 acres
are inundated at a depth greater than 10 feet.
Figure 3.10-3. Existing Conditions Flood Depth Grid
3-155
Six Mile Creek and Flat Branch
160
Area Inundated (acres)
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14
15 16
17
Flood Depth (feet)
Figure 3.10-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.10-5. Approximately 970 acres
are within the 100-year floodplain, an increase of 75 acres or a little more than 8 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.10-6.
The maximum depth (not within the pits) is 17.89 feet (about 1.3 feet greater than existing
conditions). The mean depth is 5.08 feet (about 0.5 foot greater than existing conditions).
The area inundated by more than 10 feet is approximately 102 acres (33 acres or
approximately 48 percent more than the existing conditions).
3-156
Figure 3.10-5. Future Conditions Flood Depth Grid
Six Mile Creek and Flat Branch
140
Area Inundated (acres)
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18
Flood Depth (feet)
Figure 3.10-6. Distribution of Future Condition Flood Depths
3-157
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.10-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.10-8. Note that
the differences are less than two feet. Approximately two-thirds of the differences are less
than 1 foot.
Figure 3.10-7. Increases in 100-year Flood Depths
3-158
Six Mile Creek and Flat Branch
700
Area Affected (acres)
600
500
400
300
200
100
0
1
2
Difference in Flood Depths (feet)
Figure 3.10-8. Differences in 100-year Flood Depths
3.10.2.1.1 Six Mile Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Six Mile Creek, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 533 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding Six Mile Creek. Some duplicate parcels were identified and reviewed. In all
cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 533 parcels, 154
(29%) are identified as “vacant”, an additional parcel had information in the “effective
area” field in the county assessor data indicating that a structure was on the parcel,
although no building footprint was available. The percentage of vacant parcels (29%
3-159
based on 533 parcels) shows that Mecklenburg County has significant opportunities to
manage future development and reduce future damage within the watershed.
Within these 533 parcels, there are 411 building footprints greater than 500 square feet
that are near the buffered floodplain boundary. Of these, none were surveyed as part of
the master-elevation-certificate surveys or the “gpsbfe” survey data. For the analysis, the
parcel without a building footprint that was identified as non-vacant based on the square
footage in the assessors data was carried as single structure at the centroid of the parcel,
for a total of 412 “buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
154 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 566.
3.10.3 Detailed Results for the Six Mile Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Six Mile Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.10-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
3-160
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.10-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.10-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Six Mile Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM8
Entertainment and
Recreation
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
1
Current
Inventory,
New
Floodplain
(Run 2)
2
Future
Inventory,
New
Floodplain
(Run 3)
2
4
8
49
17
21
21
1
1
1
0
1
16
0
23
0
33
1
90
389
379
476
The aggregate dollar damage across all building occupancy types is shown in Figure
3.10-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3. The majority of damage are to residential structures with only some
development occurring in the non-residential occupancies. Tables 3.10-4 through 3.10-7
and the associated discussion go into greater detail about the results of the loss
3-161
estimation. However, it should be noted that the escalation of estimated damage based
on future development validates the need for mitigation measures, such as requiring
future development to be 1-foot above the future one hundred year flood elevation (as
currently proposed by county staff).
$3,500,000
$3,000,000
Structure
Content
Estimated Damage (Dollars)
$2,500,000
$2,000,000
$1,500,000
$1,000,000
$500,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.10-9. Six Mile Creek Watershed Aggregate Damage Estimation Results
Table 3.10-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.10-4 shows that the damage (shown in percent) does not change significantly for
the existing occupancies within the floodplain. New development, shown in Run 3, shows
the impact of flooding on new development. In Run 1 the average structure damage
levels range between 1 and 20 percent. Run 2 adds the RES1E occupancy to the flooded
structures and therefore the average structure damage levels range from 2 to 30 percent.
In Run 3, most of the occupancies have structure damage ranges between 2 and 40
percent. It is notable that structure damage over 50% of market value is considered
3-162
“substantial damage”7 by the Federal Insurance and Mitigation Administration. Structures
receiving over 50% estimated damage are likely to be excellent acquisition candidates.
Table 3.10-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Six Mile Creek Watershed
HAZUS®MH
Occupancy
Code
Description
COM8
Entertainment and
Recreation
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
8.7
Current
Inventory,
New
Floodplain
(Run 2)
13.1
Future
Inventory,
New
Floodplain
(Run 3)
13.1
19.0
14.9
19.7
9.5
10.6
10.6
1.0
2.7
2.7
---
28.2
39.7
---
---
37.5
Table 3.10-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $473 thousand to $729 thousand. This corresponds to an increase of $256
thousand, or 54%. As the building inventories used for these scenarios are identical, the
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is a significant increase in estimated damage from $729 thousand to $3 million,
suffered by projected development in the floodplain. This corresponds to an increase of
$2.3 million, or 313%. It is notable that Run 3 has large increases in damages across
nearly every occupancy classification intersecting the floodplain.
7
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-163
Table 3.10-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Six Mile Creek Watershed
HAZUS®MH
Occupancy
Code
COM8
RES1A
RES1B
RES1D
RES1E
RES3
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$6,778
$6,778
$20,584
$10,292
$20,584
$10,292
$131,457
$32,864
$180,017
$27,502
$1,413,367
$28,844
$334,125
$19,654
$475,044
$22,621
$475,044
$22,621
$724
$724
$2,712
$2,712
$2,712
$2,712
----$50,610
$50,610
$934,596
$58,412
--------$162,464 $162,464
$473,083
$20,569
$728,967
$22,090
$3,008,766
$33,431
Table 3.10-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.10-4. Run 1 has an estimated damage percentage
ranging from 1 to 20 percent. This range shifts and increases from 5 to 30 percent for
Run 2 indicating the increased loss due to changes in the flood study. Run 3, shows the
impact from development with the expected range of content damage increasing from 5 to
nearly 100 percent.
The most significant increase is in the multi-family dwelling
occupancy however, the estimated development occurs within a single parcel and
therefore could be easily managed.
Table 3.10-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Six Mile Creek Watershed
HAZUS®MH
Occupancy
Code
COM8
RES1A
RES1B
RES1D
RES1E
RES3
Description
Entertainment and
Recreation
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling, Split
Level, No Basement
Single Family Dwelling, Split
Level, with Basement
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
8.7
Current
Inventory,
New
Floodplain
(Run 2)
13.1
Future
Inventory, New
Floodplain
(Run 3)
19.6
14.7
23.8
9
11.6
11.6
7
6.6
6.6
1.8
29.5
44.8
---
---
99.5
3-164
13.1
Table 3.10-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.10-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $246 thousand to $410 thousand.
This corresponds to an increase of $164 thousand, or 67%. The increase in damage is
due to changes in the floodplain depth and extent between the 1975 flood study and the
recent 2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a
significant increase in estimated damage from $410 thousand to $1.9 million attributable
to projected growth. This corresponds to an increase of $1.5 million, or 361%.
Table 3.10-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Six Mile Creek Watershed
HAZUS®MH
Occupancy
Code
COM8
RES1A
RES1B
RES1D
RES1E
RES3
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$8,947
$8,947
$27,170
$13,585
$27,170
$13,585
$72,589
$18,147
$93,932
$11,741
$858,803
$17,527
$163,151
$9,597
$259,113
$12,339
$259,113
$12,339
$893
$893
$3,347
$3,347
$3,347
$3,347
----$26,512
$26,512
$526,960
$32,935
--------$215,764 $215,764
$245,580
$10,677
$410,074
3-165
$12,426
$1,891,157
$21,013
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
CLEMS BRANCH WATERSHED (Report Delivered January 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.11 Clems Branch Analysis
3.11.1 Results Summary for the Clems Branch Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.111-1 shows a summary
of the number of flooded buildings by general occupancy classification. The simulation
predicted no damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.11-1. Number of Flooded Structures by General Occupancy:
Clems Branch Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
0
Residential
Single and MultiFamily Residential
0
0
0
0
0
0
Total
The resulting analysis shows that no structures were impacted in any of the runs. This
means the differences between Run 1 and Run 2, essentially the difference between the
1975 flood study and the 2000 flood study applied to the existing inventory, did not add
any structures or increase the flooding within any structures.
The differences between Run 2 and Run 3, essentially new development within the
floodplain, also did not add any structures within the floodplain. It should be cautioned
that the methodology applies new development to the centroid of the vacant parcel. Of
the 10 vacant parcels adjacent to the floodplain, Mecklenburg County can ensure the new
development does not encroach into areas impacted by the 100-year boundary.
3-166
3.11.2 Data Analysis
3.11.2.1 Clems Branch Basin Characteristics and Study Data
Depth grids were created for Clems Branch. The grids depict the depth of flooding that
will be exceeded with a 1-percent chance in any given year (100-year flood) under both
existing and future development conditions. The location of Clems Branch relative to
nearby stream systems is shown in Figure 3.11-1.
Approximately 0.6 mile of Clems
Branch was studied.
Figure 3.11-1. Clems Branch and Nearby Stream Systems
Flood elevation grids were created interpolating between water surface elevations
determined at 11 cross sections along Clems Branch. The flood elevation grids were
extended at the upstream end of the study reach and at relatively large areas where
floodwaters are not conveyed but, rather, pond at the elevation on the main stream.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
3-167
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.11-2.
Approximately 23.5 acres are within the 100-year floodplain.
Figure 3.11-3 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is approximately 9.57 feet. The average (mean) depth is
2.51 feet. Less than 3 acres are inundated at a depth greater than 5 feet.
Figure 3.11-2. Existing Conditions Flood Depth Grid
3-168
Clems Branch
7
Area Inundated (acres)
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
Flood Depth (feet)
Figure 3.11-3. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.11-4. Approximately 28.9
acres are within the 100-year floodplain, an increase of 5.4 acres or more than 20 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.11-5.
The maximum depth is 11.61 feet (more than 2 feet greater than existing conditions). The
mean depth is 3.15 feet (about 0.65 foot greater than existing conditions). The area
inundated by more than 5 feet is approximately 5.6 acres (about 2.6 acres or
approximately 85 percent more than the existing conditions).
3-169
Figure 3.11-4. Future Conditions Flood Depth Grid
Clems Branch
7
Area Inundated (acres)
6
5
4
3
2
1
0
1
2
3
4
5
6
7
8
9
10
11
12
Flood Depth (feet)
Figure 3.11-5. Distribution of Future Condition Flood Depths
3-170
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.11-6.
The distribution of changes in 100-year flood depths is shown in Figure 3.11-7. Note that
most (over 70 percent) of the differences are less than one foot. The differences are
greater than one foot in approximately 8.3 acres in the vicinity of the Lancaster Highway
crossing.
Figure 3.11-6. Increases in 100-year Flood Depths
3-171
Clems Branch
25
Area Affected (acres)
20
15
10
5
0
1
2
3
Difference in Flood Depths (feet)
Figure 3.11-7. Differences in 100-year Flood Depths
3.11.2.1.1 Clems Branch Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from the Clems Branch, the floodplain boundaries from both the 1975 flood
study and the 2000 flood study were combined and a buffer of 100-feet added. The
buffered boundary was used to select intersecting parcel and every building footprint
contained within those parcels. This process was selected because it allows Mecklenburg
County to identify all structures that are considered close to the floodplain. Structures
outside the flood depth grids do not impact the results of this analysis because they will
have no net damage, but knowing what structures are nearby was deemed advantageous
to Mecklenburg County.
There are 15 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding the Clems Branch. Some duplicate parcels were identified and reviewed. In
all cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 15 parcels, 10 (67%)
are identified as “vacant”, an additional parcel had information in the “effective area” field
in the county assessor data indicating that a structure was on the parcel, although no
building footprint was available. The percentage of vacant parcels (67% based on 15
3-172
parcels) shows that Mecklenburg County has significant opportunities to ensure that
damages can be maintained at a zero impact level.
Within these 15 parcels, there are 8 building footprints greater than 500 square feet that
are near the buffered floodplain boundary. Of these, none were surveyed as part of the
master-elevation-certificate surveys or the “gpsbfe” survey data. For the analysis, the
parcel without a building footprint that was identified as non-vacant based on the square
footage in the assessors data was carried as single structure at the centroid of the parcel,
for a total of 9 “buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
10 vacant parcels identified earlier. This structure was established to have the HAZUS®MH
default floor area and value based on the occupancy class assigned to the approved
zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 19.
3.11.3 Detailed Results for the Clems Branch Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Clems Branch watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
3-173
Of the nine buildings that lie near the floodplain, none experience damaging flooding (i.e.
non-zero dollar damage). This remains true throughout all three runs. For run three, this
means that the centroid of the vacant developable parcels lies outside of the floodplain.
The values shown in Table 3.11-2 are presented to give the reader perspective on the
number of buildings within the floodplain, and the potential increase due to development in
Run 3.
Table 3.11-2. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Clems Branch Watershed
HAZUS®MH
Occupancy
Code
Description
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory,
New
Floodplain
(Run 2)
0
Future
Inventory,
New
Floodplain
(Run 3)
0
9
9
19
Run 1 and Run 2 shows that increases in the depth of flooding between the 1975 study
and the 2000 study did not cause any damage to the existing inventory. Run 3, however,
applies the future development of the 10 vacant parcels at the centroid of the parcels.
While the analysis itself shows no damage, it is incumbent on Mecklenburg County to
utilize the depth grids produced in this analysis to ensure that future development is either
outside of the flood boundary or higher than the expected flood depths from the 2000
flood study.
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DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
REEDY CREEK WATERSHED (Report Delivered February 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.12 Reedy Creek Analysis
3.12.1 Results Summary for the Reedy Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.12-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 13, 24, and 62 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.12-1. Number of Flooded Structures by General Occupancy:
Reedy Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
0
Residential
Single and MultiFamily Residential
13
24
62
13
24
62
Total
The dollar damage resulting from the analysis is shown on Figure 3.12-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $186
thousand, contents $100 thousand) and Run 2 (structure $351 thousand, contents $208
thousand). This corresponds to an increase of $166 thousand (89%) for structures and
$108 thousand (108%) for contents. The increase demonstrates the effect of changes in
the floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-175
developed based on their zoning. The result is a increase in estimated structure damage
from $351 thousand to $2.0 million and a corresponding increase in estimated contents
damage from $208 thousand to approximately $1.1 million.
This corresponds to an
increase of $1.6 million (483%) for structures and $897 thousand (432%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.12-1).
$2,500,000
$2,000,000
Structure
Estimated Loss (Dollars)
Content
$1,500,000
$1,000,000
$500,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.12-1. Reedy Creek Watershed Aggregate Damage Estimation Results
Table 3.12-2 shows a summary of the building and content damage results by general
occupancy classification.
Table 3.12-2 shows that damages are concentrated in the
3-176
residential occupancy classification in Runs 1, 2 and 3. Run 2 shows that differences
between the 1975 and 2000 flood study nearly double the damages (structure and
contents).
As expected, there is a significant increase in the losses to residential
occupancies in Run 3, because of “development” of vacant parcels.
The results are
discussed in greater detail later in this report.
Table 3.12-2. Summary of Building and Content Damage by General
Occupancy: Reedy Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$185,520
$14,271
$351,145
$14,631
$2,048,533
$33,041
Content Damage
Total
Average
Total
Average
Total
Average
$0
$0
$0
$0
$0
$7,673
$207,850
$8,660
$1,105,256
$17,827
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
Residential
Single and
Multi-Family
Residential
$99,747
3.12.2 Data Analysis
3.12.2.1 Reedy Creek Basin Characteristics and Study Data
Depth grids were created for Reedy Creek and three tributaries contributing flow to Reedy
Creek. The grids depict the depth of flooding that will be exceeded with a 1-percent
chance in any given year (100-year flood) under both existing and future development
conditions. The network of streams analyzed is shown in Figure 3.12-2.
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Figure 3.12-2. Reedy Creek Basin Stream Network
The studied reaches include approximately 3.8 miles of Reedy Creek and approximately
4.8 miles of tributary reaches.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 129 cross
sections were used: 59 on Reedy Creek and 70 on the tributaries. The flood elevation
grids were extended at the upstream end of each stream, at the confluence with each
tributary, and at relatively large areas where floodwaters are not conveyed but, rather,
pond at the elevation on the main stream. Where two stream flood elevation grids overlap
the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
3-178
The resulting existing conditions flood depth grid is shown in Figure 3.12-3.
Approximately 414 acres are within the 100-year floodplain.
Figure 3.12-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 16.57 feet. The average (mean) depth is 3.31 feet.
Approximately 12 acres are inundated at a depth greater than 10 feet.
Figure 3.12-3. Existing Conditions Flood Depth Grid
3-179
Reedy Creek Basin
100
Area Inundated (acres)
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11 12
13 14
15 16
17
Flood Depth (feet)
Figure 3.12-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.12-5. Approximately 484 acres
are within the 100-year floodplain, an increase of 80 acres or a little less than 20 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.12-6.
The maximum depth is 18.43 feet (1.86 feet greater than existing conditions). The mean
depth is 4.29 feet (about 1 foot greater than existing conditions). The area inundated by
more than 10 feet is approximately 25.7 acres (13.7 acres or more than double the
existing conditions).
3-180
Figure 3.12-5. Future Conditions Flood Depth Grid
Reedy Creek Basin
90
Area Inundated (acres)
80
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19
Flood Depth (feet)
Figure 3.12-6. Distribution of Future Condition Flood Depths
3-181
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.12-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.12-8. Note that
most (approximately 73 percent) of the differences are less than two feet. The 100-year
flood depth increases are expected to exceed two feet in approximately 132 acres within
the Reedy Creek basin. Those increases occur upstream of the Robinson Church Road
crossing of Reedy Creek Tributary #2; in most of the Reedy Creek floodplain downstream
of the confluence with Reedy Creek Tributary #2; and in the downstream half of the
portion of the Reedy Creek Tributary #1 floodplain within Mecklenburg County.
Figure 3.12-7. Increases in 100-year Flood Depths
3-182
Reedy Creek Basin
250
Area Affected (acres)
200
150
100
50
0
1
2
3
4
Difference in Flood Depths (feet)
Figure 3.12-8. Differences in 100-year Flood Depths
3.12.2.1.1 Reedy Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Reedy Creek, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 464 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding Reedy Creek. Some duplicate parcels were identified and reviewed. In all
cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 464 parcels, 135
(29%) are identified as “vacant”, an additional parcel had information in the “effective
area” field in the county assessor data indicating that a structure was on the parcel,
3-183
although no building footprint was available. The percentage of vacant parcels (29%
based on 464 parcels) shows that Mecklenburg County has significant opportunities to
manage future development and reduce future damage within the watershed.
Within these 464 parcels, there are 396 building footprints greater than 500 square feet
that are near the buffered floodplain boundary. Of these, none were surveyed as part of
the master-elevation-certificate surveys or the “gpsbfe” survey data. For the analysis, the
parcel without a building footprint that was identified as non-vacant based on the square
footage in the assessors data was carried as single structure at the centroid of the parcel,
for a total of 397 “buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
135 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 532.
3.12.3 Detailed Results for the Reedy Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Reedy Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
3-184
Table 3.12-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.12-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.12-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Reedy Creek Watershed
HAZUS®MH
Occupancy
Code
Description
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
7
Current
Inventory,
New
Floodplain
(Run 2)
15
Future
Inventory,
New
Floodplain
(Run 3)
42
5
8
8
1
1
12
13
24
62
384
373
470
The aggregate dollar damage across all building occupancy types is shown in Figure
3.12-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3.
All estimated damage are to residential structures.
Tables 3.12-4
through 3.12-7 and the associated discussion go into greater detail about the results of
the loss estimation. However, it should be noted that the escalation of estimated damage
based on future development validates the need for mitigation measures, such as
3-185
requiring future development to be 1-foot above the future one hundred year flood
elevation (as currently proposed by county staff).
$2,500,000
$2,000,000
Structure
Estimated Loss (Dollars)
Content
$1,500,000
$1,000,000
$500,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.12-9. Reedy Creek Watershed Aggregate Damage Estimation Results
Table 3.12-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3-12.4 shows that the damage (shown in percent) does not change significantly for
the existing occupancies within the floodplain between Run 1 and Run 2.
New
development, shown in Run 3, increases the estimated damage for the RES1A and
RES1E occupancies. In Run 1 and Run 2 the average structure damage levels range
between 8 and 37 percent. In Run 3, the estimated damage range increases from 9 to 41
percent. It is notable that structure damage over 50% of market value is considered
3-186
“substantial damage”8 by the Federal Insurance and Mitigation Administration. Structures
receiving over 50% estimated damage are likely to be excellent acquisition candidates.
Table 3.12-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Reedy Creek Watershed
HAZUS®MH
Occupancy
Code
Description
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
16.3
Current
Inventory,
New
Floodplain
(Run 2)
17.8
Future
Inventory,
New
Floodplain
(Run 3)
23.2
8.1
9.5
9.5
34.9
36.6
41.1
Table 3.12-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $186 thousand to $351 thousand. This corresponds to an increase of $166
thousand, or 89%. As the building inventories used for these scenarios are identical, the
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is a significant increase in estimated damage from $351 thousand to $2 million,
suffered by projected development in the floodplain. This corresponds to an increase of
$1.7 million, or 483%.
8
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-187
Table 3.12-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Reedy Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$91,779
$13,111
$220,653
$14,710
$1,253,159
$29,837
$44,089
$8,818
$78,396
$9,800
$78,396
$9,800
$49,652
$49,652
$52,096
$52,096
$716,978
$59,748
$185,520
$14,271
$351,145
$14,631
$2,048,533
$33,041
Table 3.12-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.12-4. Interestingly, the average estimated damage
(percent) does not vary greatly between the three runs. Run 1 and Run 2 are similar in
the estimated damage with the exception of the RES1E occupancy. Run 3 resembles
Run 2, in terms of percentage and even shows a reduction in the average damage for
RES1E due to additional lightly damaged structures.
Table 3.12-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Reedy Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1E
Description
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling, Split
Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
17.9
Current
Inventory,
New
Floodplain
(Run 2)
21.4
Future
Inventory, New
Floodplain
(Run 3)
27.1
8.1
10.7
10.7
38.8
44.4
38.3
Table 3.12-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.12-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $100 thousand to $208 thousand.
This corresponds to an increase of $108 thousand, or 108%. The increase in damage is
due to changes in the floodplain depth and extent between the 1975 flood study and the
recent 2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a
3-188
significant increase in estimated damage from $208 thousand to $1.1 million attributable
to projected growth. This corresponds to an increase of $897 thousand, or 432%.
Table 3.12-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Reedy Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$50,503
$7,215
$132,113
$8,808
$727,186
$17,314
$21,602
$4,320
$44,149
$5,519
$44,149
$5,519
$27,642
$27,642
$31,588
$31,588
$333,922
$27,827
$99,747
$7,673
$207,850
3-189
$8,660
$1,105,256
$17,827
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
CLARKE CREEK WATERSHED (Report Delivered February 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.13 Clarke Creek Analysis
3.13.1 Results Summary for the Clarke Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.13-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 1, 1, and 20 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.13-1. Number of Flooded Structures by General Occupancy:
Clarke Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
0
Residential
Single and MultiFamily Residential
1
1
20
1
1
20
Total
The dollar damage resulting from the analysis is shown on Figure 3.13-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $173
thousand, contents $118 thousand) and Run 2 (structure $216 thousand, contents $142
thousand). This corresponds to an increase of $43 thousand (25%) for structures and $24
thousand (21%) for contents. The increase demonstrates the effect of changes in the
floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions. It is cautioned that all damages are estimated for a
single structure and that the numbers could vary depending on the grid cell from which the
estimated depth is taken.
3-190
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
developed based on their zoning. The result is an increase in estimated structure damage
from $216 thousand to $1.1 million and a corresponding increase in estimated contents
damage from $142 thousand to approximately $551 thousand. This corresponds to an
increase of $904 thousand (418%) for structures and $409 thousand (288%) for contents.
Prior to this analysis, this escalation of potential damages based on future development
was predicted by county staff and has led to efforts requiring new development to be 1foot above the one-hundred year base flood elevation, where the BFE includes the
increased runoff from the future or ultimate build-out. These results support that effort as
the 1-foot requirement would reduce the results of Run 3 to levels more closely
resembling the results of the existing development results presented by Run 2 (see Figure
3.13-1).
$1,200,000
$1,000,000
Structure
Content
Estimated Loss (Dollars)
$800,000
$600,000
$400,000
$200,000
$Run1
Run2
Run3
Builidng Inventory and Floodplain Scenario
Figure 3.13-1. Clarke Creek Watershed Aggregate Damage Estimation Results
3-191
Table 3.13-2 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 3.13-2 shows the majority of damages are
concentrated in the residential occupancy classification in Runs 1, 2 and 3. Run 2 shows
that differences between the 1975 and 2000 flood study nearly double the damages
(structure and contents). As expected, there is an increase in the losses to residential
occupancies in Run 3, because of “development” of vacant parcels.
The results are
discussed in greater detail later in this report.
Table 3.13-2. Summary of Building and Content Damage by General
Occupancy: Clarke Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
$0
$0
$0
$0
$0
$173,044
$216,453
$216,435
$1,120,419
$56,021
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
Residential
Single and
Multi-Family
Residential
$173,044
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$117,825
$117,825
$142,056
$142,056
$551,337
$27,557
3.13.2 Data Analysis
3.13.2.1 Clarke Creek Basin Characteristics and Study Data
Clarke Creek is formed by the confluence of the North and South Prongs of Clarke Creek.
Depth grids were created for Clarke Creek and five tributaries contributing flow to Clarke
Creek. The grids depict the depth of flooding that will be exceeded with a 1-percent
3-192
chance in any given year (100-year flood) under both existing and future development
conditions. The network of streams analyzed is shown in Figure 3.13-2.
Figure 3.13-2. Clarke Creek Basin Stream Network
The studied reaches include approximately 2 miles of Clarke Creek and approximately 13
miles of tributary reaches.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 180 cross
sections were used: 24 on Clarke Creek and 156 on the tributaries. South Prong Clarke
Creek and Clarke Creek were analyzed as one continuous reach. The flood elevation
grids were extended at the upstream end of each stream, at the confluence with each
tributary, and at relatively large areas where floodwaters are not conveyed but, rather,
pond at the elevation on the main stream. Where two stream flood elevation grids overlap
the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
3-193
The resulting existing conditions flood depth grid is shown in Figure 3.13-3.
Approximately 869 acres are within the 100-year floodplain.
Figure 3.13-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 19.93 feet. The average (mean) depth is 5.34 feet.
Approximately 158.5 acres are inundated at a depth greater than 10 feet.
Figure 3.13-3. Existing Conditions Flood Depth Grid
3-194
Clarke Creek Basin
140
Area Inundated (acres)
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20
Flood Depth (feet)
Figure 3.13-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.13-5. Approximately 982 acres
are within the 100-year floodplain, an increase of 113 acres or approximately 13 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.13-6.
The maximum depth is 22.81 feet (2.88 feet greater than existing conditions). The mean
depth is 6.33 feet (about 1 foot greater than existing conditions). The area inundated by
more than 10 feet is approximately 220.6 acres (62.1 acres or almost 40 percent more
than the existing conditions).
3-195
Figure 3.13-5. Future Conditions Flood Depth Grid
Clarke Creek Basin
140
Area Inundated (acres)
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Flood Depth (feet)
Figure 3.13-6. Distribution of Future Condition Flood Depths
3-196
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.13-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.13-8. Note that
all of the differences are less than three feet and most (61 percent) are less than two feet.
The 100-year flood depth increases are expected to exceed two feet in approximately 382
acres, mostly in the downstream reaches of Clarke and Ramah Creeks. The increases
exceed two feet in small areas just upstream of the Asbury Chapel Road and Ramah
Church Road crossings of North Prong Clarke Creek.
Figure 3.13-7. Increases in 100-year Flood Depths
3-197
Clarke Creek Basin
450
Area Affected (acres)
400
350
300
250
200
150
100
50
0
1
2
3
Difference in Flood Depths (feet)
Figure 3.13-8. Differences in 100-year Flood Depths
3.13.2.1.1 Clarke Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Clarke Creek, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 249 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding Clarke Creek. Some duplicate parcels were identified and reviewed. In all
cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 249 parcels, 122
(49%) are identified as “vacant” and an additional four parcels had information in the
“effective area” field in the county assessor data indicating that a structure was on the
parcel, although no building footprint was available. The percentage of vacant parcels
3-198
(49% based on 249 parcels) shows that Mecklenburg County has significant opportunities
to manage future development and reduce future damage within the watershed.
Within these 249 parcels, there are 287 building footprints greater than 500 square feet
that are within parcels intersecting the buffered floodplain boundary. Of these, none were
surveyed as part of the master-elevation-certificate surveys or the “gpsbfe” survey data.
For the analysis, the four parcels without a building footprint that were identified as nonvacant based on the square footage in the assessors data were carried as single structure
at the centroid of each parcel, for a total of 291 “buildings” in the floodplain (including the
100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
122 vacant parcels identified earlier.
®MH
HAZUS
This structure was established to have the
default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 413.
3.13.3 Detailed Results for the Clarke Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Clarke Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
3-199
Table 3.13-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.13-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.13-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
Description
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
1
Current
Inventory,
New
Floodplain
(Run 2)
1
Future
Inventory,
New
Floodplain
(Run 3)
15
---
---
5
1
1
20
290
290
393
The aggregate dollar damage across all building occupancy types is shown in Figure
3.13-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3. All estimated damages are to residential structures within the Clarke
Creek floodplain. Tables 3.13-4 through 3.13-7 and the associated discussion go into
greater detail about the results of the loss estimation. However, it should be noted that
the escalation of estimated damage based on future development validates the need for
mitigation measures, such as requiring future development to be 1-foot above the future
one hundred year flood elevation (as currently proposed by county staff).
3-200
$1,200,000
$1,000,000
Structure
Content
Estimated Loss (Dollars)
$800,000
$600,000
$400,000
$200,000
$Run1
Run2
Run3
Builidng Inventory and Floodplain Scenario
Figure 3.13-9. Clarke Creek Watershed Aggregate Damage Estimation Results
Table 3.13-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.13-4 shows that the damage (shown in percent) for the RES1A occupancy
increases from Run 1 to Run 2 (estimated for a single structure), but decreases slightly for
Run 3. The slight reduction is caused by the additional structures (14) impacting the
overall average. In Run 1 the estimated average structure damage level is approximately
25 percent, again this is for a single building footprint. Run 2 estimates the damage to
that same structure to increase to approximately 31 percent. In Run 3, new development
increases the number of structures in the RES1A occupancy and adds several structures
in the RES1E classification. The estimated average damage, for Run 3, ranges from
approximately 28 percent to 45 percent. It is notable that structure damage over 50% of
market value is considered “substantial damage”9 by the Federal Insurance and Mitigation
9
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
3-201
Administration.
Structures receiving over 50% estimated damage are likely to be
excellent acquisition candidates.
Table 3.13-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
Description
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
25.1
Current
Inventory,
New
Floodplain
(Run 2)
31.4
Future
Inventory,
New
Floodplain
(Run 3)
28.3
---
---
45.4
Table 3.13-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $173 thousand to $216 thousand. This corresponds to an increase of $43
thousand, or 25%. As the building inventories used for these scenarios are identical, the
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is an increase in estimated damage from $216 thousand to $1.1 million, suffered
by projected development in the floodplain. This corresponds to an increase of $904
thousand, or 418%.
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-202
Table 3.13-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$173,044
$173,044
$216,453
$216,453
$789,716
$52,648
---------$330,704
$35,230
$173,044
$173,044
$216,453
$216,453
$1,120,419
$56,021
Table 3.13-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.13-4. Run 1 has an estimated damage percentage
of 34.2 percent for a single structure within the Clarke Creek floodplain.
This range
estimated damage for this structure increases to 41.2 percent for Run 2. Run 3, with an
increased number of structures in the RES1A classification and the addition of the RES1E
classification has a range of contents damage from 24 percent to 44 percent.
It is
interesting to note the reduction in estimated average damage to the RES1A occupancy
(24.1 percent) caused by the addition of a number of structures throughout the floodplain.
Table 3.13-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1E
Description
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, Split
Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
34.2
Current
Inventory,
New
Floodplain
(Run 2)
41.2
Future
Inventory, New
Floodplain
(Run 3)
24.1
---
---
48.3
Table 3.13-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.13-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $118 thousand to $142 thousand.
This corresponds to an increase of $24 thousand, or 21%. The increase in damage is due
to changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a increase
3-203
in estimated damage from $142 thousand to $551 thousand attributable to projected
growth. This corresponds to an increase of $409 thousand, or 288%.
Table 3.13-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Current Floodplain
Floodplain
Floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$117,825
$117,825
$142,056 $142,056
$375,186
$25,012
--------$176,151
$35,230
$117,825
$117,825
$142,056
3-204
$142,056
$551,337
$27,567
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
MALLARD CREEK WATERSHED (Report Delivered February 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.14 Mallard Creek Analysis
3.14.1 Results Summary for the Mallard Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.14-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 27, 31, and 125 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.14-1. Number of Flooded Structures by General Occupancy:
Mallard Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
13
14
32
Residential
Single and MultiFamily Residential
14
17
93
27
31
125
Total
The dollar damage resulting from the analysis is shown on Figure 3.14-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $1.67
million, contents $2.23 million) and Run 2 (structure $1.69 million, contents $2.25 million).
This corresponds to an increase of $29 thousand (2%) for structures and $26 thousand
(1%) for contents. The increase demonstrates the effect of changes in the floodplain
extent and depth between the 1975 flood study and newly revised (2000) floodplain map.
Based on the assumption that the revised floodplain map is more accurate than the 1975
flood study, the estimated damage results from Run 2 are representative of current
conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-205
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $1.69 million to $17.2 million and a corresponding increase in estimated
contents damage from $2.25 million to approximately $21.2 million. This corresponds to
an increase of $15.5 million (917%) for structures and $19 million (841%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.14-1).
$25,000,000
$20,000,000
Structure
Estimated Loss (Dollars)
Content
$15,000,000
$10,000,000
$5,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.14-1. Mallard Creek Watershed Aggregate Damage Estimation Results
3-206
Table 3.14-2 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 3.14-2 shows the majority of damages are
concentrated in the non-residential occupancy classifications.
Run 2 shows that
differences between the 1975 and 2000 flood study have very little impact on estimated
damages (structure and contents). As expected, there is a significant increase in the
losses to residential occupancies in Run 3, because of “development” of vacant parcels,
primarily in the non-residential classifications. The results are discussed in greater detail
later in this report.
Table 3.14-2. Summary of Building and Content Damage by General
Occupancy: Mallard Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$1,386,062
$106,620
$1,397,503
$99,822
$13,443,929
$420,123
Residential
Single and
Multi-Family
Residential
$279,098
$19,936
$296,340
$17,432
$3,784,529
$40,694
Total
Average
Content Damage
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$2,134,584
$164,199
$2,149,709
$153,551
$19,495,062
$609,221
Residential
Single and
Multi-Family
Residential
$93,744
$6,696
$104,802
$6,165
$1,719,038
$18,484
3-207
3.14.2 Data Analysis
3.14.2.1 Mallard Creek Basin Characteristics and Study Data
Depth grids were created for Mallard Creek and ten tributaries contributing flow to Mallard
Creek. The grids depict the depth of flooding that will be exceeded with a 1-percent
chance in any given year (100-year flood) under both existing and future development
conditions. The network of streams analyzed is shown in Figure 3.14-2.
Figure 3.14-2. Mallard Creek Basin Stream Network
The studied reaches include approximately 10 miles of Mallard Creek and approximately
22.5 miles of tributary reaches.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 436 cross
sections were used: 128 on Mallard Creek and 308 on the tributaries. The flood elevation
grids were extended at the upstream end of each stream, at the confluence with each
tributary, and at relatively large areas where floodwaters are not conveyed but, rather,
pond at the elevation on the main stream. Where two stream flood elevation grids overlap
the final flood elevation grid uses the greater of the two elevations.
3-208
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.14-3.
Approximately 1300 acres are within the 100-year floodplain.
Figure 3.14-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 25 feet. The average (mean) depth is 6.96 feet.
Approximately 413 acres are inundated at a depth greater than 10 feet.
Figure 3.14-3. Existing Conditions Flood Depth Grid
3-209
Mallard Creek Basin
160
Area Inundated (acres)
140
120
100
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Flood Depth (feet)
Figure 3.14-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.14-5. Approximately 1403
acres are within the 100-year floodplain, an increase of 103 acres or approximately 8
percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.14-6.
The maximum depth is 25 feet (the same as existing conditions). The mean depth is 7.00
feet (about the same as existing conditions). The area inundated by more than 10 feet is
approximately 430 acres (about 17 acres or approximately 4 percent more than the
existing conditions).
3-210
Figure 3.14-5. Future Conditions Flood Depth Grid
Mallard Creek Basin
160
Area Inundated (acres)
140
120
100
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Flood Depth (feet)
Figure 3.14-6. Distribution of Future Condition Flood Depths
3-211
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.14-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.14-8. Note that
most (97.5 percent) of the differences are less than three feet.
Approximately 77.5
percent of the increases are less than one foot. Most of the increases exceeding three
feet occur in approximately 25 acres of the Stoney Creek floodplain upstream of the
Interstate 85 crossing. The remainder of the increases exceeding three (and ranging up
to seven) feet occur upstream of the Interstate 85 crossing of Doby Creek and upstream
of the W T Harris Boulevard crossing of Doby Creek Tributary (in the vicinity of the
confluence of those two streams).
Figure 3.14-7. Increases in 100-year Flood Depths
3-212
Mallard Creek Basin
1200
Area Affected (acres)
1000
800
600
400
200
0
1
2
3
4
5
6
7
8
Difference in Flood Depths (feet)
Figure 3.14-8. Differences in 100-year Flood Depths
3.14.2.1.1 Mallard Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Mallard Creek, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 1,394 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding Mallard Creek. Some duplicate parcels were identified and reviewed.
In all cases, these turned out to be slivers in the GIS data, most likely right-of-ways that
have been either returned to the parcel, or offers of dedication that were never acted
upon. In all cases, the slivers were merged back into the main parcel. Of the 1,394
parcels, 474 (34%) are identified as “vacant”, 102 additional parcels had information in the
“effective area” field in the county assessor data indicating that a structure was on the
parcels, although no building footprint was available. The percentage of vacant parcels
3-213
(34% based on 1,394 parcels) shows that Mecklenburg County has significant
opportunities to manage future development and reduce future damage within the
watershed.
Within these 1,394 parcels, there are 1,476 building footprints greater than 500 square
feet that are within parcels intersecting the buffered floodplain boundary. Of these, 69
were surveyed as part of the master-elevation-certificate surveys or the “gpsbfe” survey
data. Unfortunately of the 69 data points from the master-elevation-certificate surveys, 9
were missing data in either the first floor elevation or the lowest adjacent grade fields
rendering the data unusable. The HAZUS default values were applied to these records.
For the analysis, the parcel without a building footprint that was identified as non-vacant
based on the square footage in the assessors data was carried as single structure at the
centroid of the parcel, for a total of 1,578 “buildings” in the floodplain (including the 100foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
474 vacant parcels identified earlier.
This structure was established to have the
HAZUS®MH default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 2,052.
3.14.3 Detailed Results for the Mallard Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Mallard Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3-214
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.14-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.14-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-215
Table 3.14-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Mallard Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1
Retail Trade
COM1/COM4 Retail Trade and
Business/Professional/
Technical Services
COM3
Personal and Repair
Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment and
Recreation
IND1
Heavy Industrial
IND2
Light Industrial
IND2/IND6
Light Industrial &
Construction
REL1
Church/Membership
Organizations
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES2
Manufactured Housing
RES3
Multi-Family Dwelling
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
2
1
Current
Inventory,
New
Floodplain
(Run 2)
3
1
Future
Inventory,
New
Floodplain
(Run 3)
3
6
1
1
1
1
1
6
7
7
7
--1
---
--1
---
5
1
1
---
---
2
5
5
60
3
5
5
---
1
1
1
1
19
4
1
27
4
1
31
4
4
125
1,551
1,547
1,927
The aggregate dollar damage across all building occupancy types is shown in Figure
3.14-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3. The majority of damages are to non-residential structures in spite of the
fact that for all runs, the residential structures either equal or greatly exceed the number of
non-residential structures. For example in Run 1 there are 14 residential and 13 non-
3-216
residential structures. In Run 3 there are 103 residential to 22 non-residential structures.
Tables 3.14-4 through 3.14-7 and the associated discussion go into greater detail about
the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development validates the need for mitigation
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
$25,000,000
$20,000,000
Structure
Estimated Loss (Dollars)
Content
$15,000,000
$10,000,000
$5,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.14-9. Mallard Creek Watershed Aggregate Damage Estimation Results
Table 3.14-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.14-4 shows that the damage (shown in percent) does not change significantly for
the existing occupancies within the floodplain between Run 1 and Run 2 indicating that
the differences between the 1975 flood study and the 2000 flood study did not have a
great impact. New development, shown in Run 3, shows the impact of flooding on new
development. In Run 1 and Run 2 the average structure damage levels range broadly
between 1 and 60 percent. Interestingly enough, Run 3 shows similar impacts as Run 1
3-217
and Run 2 to the existing inventory, but new development shows high damage estimates
indicating these parcels are very close to the floodplain. In Run 3, the estimated damages
range from 1 to 70 percent with the addition of several non-residential occupancies. It is
notable that structure damage over 50% of market value is considered “substantial
damage”10 by the Federal Insurance and Mitigation Administration. Structures receiving
over 50% estimated damage are likely to be excellent acquisition candidates.
Table 3.14-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Mallard Creek Watershed
HAZUS®MH
Occupancy
Code
COM1
COM1/COM4
COM3
COM4
COM8
IND1
IND2
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES2
RES3
Description
Retail Trade
Retail Trade and
Business/Professional/
Technical Services
Personal and Repair Services
Business/Professional/
Technical Services
Entertainment and Recreation
Heavy Industrial
Light Industrial
Light Industrial & Construction
Church/Membership
Organizations
Single Family Dwelling, 1 Story,
No Basement
Single Family Dwelling, 2 Story,
No Basement
Single Family Dwelling, Split
Level, No Basement
Single Family Dwelling, Split
Level, with Basement
Manufactured Housing
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
24.5
60.5
Current
Inventory, New
Floodplain
(Run 2)
25.2
60.5
Future
Inventory,
New
Floodplain
(Run 3)
25.2
69.5
16.8
55.1
16.8
55.1
16.8
62.5
23.5
--30.4
-----
23.5
--30.4
-----
23.5
19.2
30.4
49.4
25.8
22.3
23.0
29.2
7.8
6.4
6.4
---
0.5
0.5
28.3
35.3
37.6
45.4
14.1
45.4
14.1
45.4
21.5
Table 3.14-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
10
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
3-218
damage from $1.67 million to $1.69 million. This corresponds to an increase of $28
thousand, or 2%. As the building inventories used for these scenarios are identical, the
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is a significant increase in estimated damage from $1.69 million to $17.2 million,
suffered by projected development in the floodplain. This corresponds to an increase of
$15.5 million, or 917%.
Table 3.14-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Mallard Creek Watershed
Current Inventory,
Current Inventory, New Future Inventory, New
HAZUS®MH
Original Floodplain
Floodplain
floodplain
Occupancy
(Run 1)
(Run 2)
(Run 3)
Code
Total
Avg.
Total
Avg.
Total
Avg.
COM1
$69,566
$34,783
$81,007
$27,002
$81,007
$27,002
COM1/COM4
$90,152
$90,152
$90,152
$90,152
$1,578,160 $263,027
COM3
$196,465
$196,465
$196,465
$196,465
$196,465 $196,465
COM4
$17,644
$17,644
$17,644
$17,644
$3,889,066 $648,178
COM8
$995,357
$142,194
$995,357
$142,194
$995,357 $142,194
IND1
--------$5,831,604 $1,166,321
IND2
$16,878
$16,878
$16,878
$16,878
$16,878
$16,878
IND2/IND6
--------$650,169 $650,169
REL1
--------$205,222 $102,611
RES1A
$74,140
$14,828
$76,160
$15,232
$2,457,251
$40,954
RES1B
$21,197
$7,066
$31,714
$6,343
$31,714
$6,343
RES1D
----$1,008
$1,008
$1,008
$1,008
RES1E
$14,982
$14,982
$18,679
$18,679
$1,007,268
$53,014
RES2
$39,342
$9,835
$39,342
$9,835
$39,342
$9,835
RES3
$129,436
$129,436
$129,436
$129,436
$247,946
$61,986
Total $1,665,160
$61,673 $1,693,843
$54,640 $17,228,457 $137,829
Table 3.14-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.14-4. Run 1 and Run 2 are virtually identical in
terms of estimated damage with one notable exception.
Both Run’s have estimated
damage percentage ranging from 1 to 86 percent. As expected, the estimated damages
for Run 3 range 1 to 86 percent, but shows high content damages in the new
development.
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-219
Table 3.14-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Mallard Creek Watershed
HAZUS®MH
Occupancy
Code
COM1
COM1/COM4
COM3
COM4
COM8
IND1
IND2
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES2
RES3
Description
Retail Trade
Retail Trade and
Business/Professional/
Technical Services
Personal and Repair
Services
Business/Professional/
Technical Services
Entertainment and
Recreation
Heavy Industrial
Light Industrial
Light Industrial &
Construction
Church/Membership
Organizations
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling, Split
Level, No Basement
Single Family Dwelling, Split
Level, with Basement
Manufactured Housing
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
24.5
60.5
Current
Inventory,
New
Floodplain
(Run 2)
25.2
60.5
Future
Inventory, New
Floodplain
(Run 3)
25.2
69.5
85.9
85.9
85.9
55.1
55.1
62.5
23.5
23.5
23.5
--58.7
---
--58.7
---
48.7
58.7
74.2
---
---
25.8
27.9
29.6
28.4
10.6
7.8
7.8
---
1.2
1.2
29.7
40.8
31.2
37.4
1.8
37.4
1.8
37.4
21.2
Table 3.14-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.14-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $2.23 million to $2.25 million. This
corresponds to an increase of $26 thousand, or 1%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $2.25 million to $21.2 million attributable to projected
growth. This corresponds to an increase of $19 million, or 841%.
3-220
Table 3.14-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Mallard Creek Watershed
HAZUS®MH
Occupancy
Code
COM1
COM1/COM4
COM3
COM4
COM8
IND1
IND2
IND2/IND6
REL1
RES1A
RES1B
RES1D
RES1E
RES2
RES3
Current Inventory,
Current Inventory, New
Current Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
$91,966
$45,983
$107,091
$35,697
$181,476
$181,476
$181,476 $181,476
$502,962
$502,962
$502,962 $502,962
$28,018
$28,018
$28,018
$28,018
$1,313,871
$187,696 $1,313,871 $187,696
--------$16,290
$16,290
$16,290
$16,290
----------------$47,186
$9,437
$49,588
$9,918
$13,979
$4,660
$18,447
$3,689
----$1,244
$1,244
$7,845
$7,845
$10,789
$10,789
$16,279
$4,070
$16,279
$4,070
$8,455
$8,455
$8,455
$8,455
Total $2,228,328
$82,531
$2,254,510
3-221
$72,726
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$107,091
$35,697
$3,176,836
$529,473
$502,962
$502,962
$6,175,837 $1,029,306
$1,313,871
$187,696
$7,412,253 $1,482,451
$16,290
$16,290
$488,244
$488,244
$301,677
$150,838
$1,181,899
$19,698
$18,447
$3,689
$1,244
$1,244
$412,087
$21,689
$16,279
$4,070
$89,081
$22,270
$21,214,099
$169,713
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
IRWIN CREEK WATERSHED (Report Delivered July 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.15 Irwin Creek Analysis
3.15.1 Results Summary for the Irwin Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.15-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 222, 288, and 540 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.15-1. Number of Flooded Structures by General Occupancy:
Irwin Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
62
80
143
Residential
Single and MultiFamily Residential
160
208
397
222
288
540
Total
The dollar damage resulting from the analysis is shown on Figure 3.15-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $11.17
million, contents $25.89 million) and Run 2 (structure $18.06 million, contents $39.26
million). This corresponds to an increase of $6.9 million (62%) for structures and $13.4
million (52%) for contents.
The increase demonstrates the effect of changes in the
floodplain extent and depth between the 1975 flood study and newly revised (2000)
floodplain map.
Based on the assumption that the revised floodplain map is more
accurate than the 1975 flood study, the estimated damage results from Run 2 are
representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
3-222
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $18.06 million to $46.99 million and a corresponding increase in estimated
contents damage from $39.26 million to approximately $63.21 million. This corresponds
to an increase of $28.9 million (160%) for structures and $24 million (61%) for contents.
Prior to this analysis, this significant escalation of potential damages based on future
development was predicted by county staff and has led to efforts requiring new
development to be 1-foot above the one-hundred year base flood elevation, where the
BFE includes the increased runoff from the future or ultimate build-out. These results
support that effort, as the 1-foot requirement would reduce the results of Run 3 to levels
more closely resembling the results of the existing development results presented by Run
2 (see Figure 3.15-1).
$70,000,000
$60,000,000
Structure
Content
Estimated Loss ($)
$50,000,000
$40,000,000
$30,000,000
$20,000,000
$10,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.15-1. Irwin Creek Watershed Aggregate Damage Estimation Results
Table 3.15-2 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 3.15-2 shows the majority of damages are
concentrated in the non-residential occupancy classifications.
3-223
Run 2 shows that
differences between the 1975 and 2000 flood study has the general impact of nearly
doubling the direct damage from flooding (structure and contents). As expected, there is
a significant increase in the losses to residential occupancies in Run 3, because of
“development” of vacant parcels, primarily in the non-residential classifications.
The
results are discussed in greater detail later in this report.
Table 3.15-2. Summary of Building and Content Damage by General
Occupancy: Irwin Creek Watershed
General
Occupancy
Current Inventory,
Original Floodplain
(Run 1)
Description
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$9,009,667
$145,317
$14,597,630
$182,470
$31,246,516
$218,507
Residential
Single and
Multi-Family
Residential
$2,160,318
$13,502
$3,466,881
$16,668
$15,739,958
$39,647
Content Damage
Total
Average
Total
Total
Average
$394,584
$36,978,573
$462,232
$53,294,055
$372,686
$8,927
$2,277,782
$10,951
$9,923,260
$24,996
NonResidential
Commercial,
Industrial,
Religious,
Education
$24,464,186
Residential
Single and
Multi-Family
Residential
$1,428,241
Average
3.15.2 Data Analysis
3.15.2.1 Irwin Creek Basin Characteristics and Study Data
Depth grids were created for Irwin Creek and 6 of its tributaries, including Stewart Creek
and Kennedy Branch. The grids depict the depth of flooding that will be exceeded with a
1-percent chance in any given year (100-year flood) under both existing and future
development conditions.
The Irwin Creek stream system is shown in Figure 3.15-2.
3-224
Approximately 22.7 miles of stream were studied, including 10.9 miles of Irwin Creek and
5.3 miles of Stewart Creek.
Figure 3.15-2. Irwin Creek Stream System
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 423 cross
sections were used: 200 on Irwin Creek; 101 on Stewart Creek; and 122 on the remaining
tributaries. The flood elevation grids were extended at the upstream end of each stream,
at the confluences of tributaries and Irwin and Stewart Creeks, and at relatively large
areas where floodwaters are not conveyed but, rather, pond at the elevation on the main
stream. Where the two stream flood elevation grids overlap the final flood elevation grid
uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.15-3.
Approximately 780 acres are within the 100-year floodplain.
3-225
Figure 3.15-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is approximately 31.21 feet. The average (mean) depth
is 6.48 feet. Approximately 186 acres are inundated at a depth greater than 10 feet.
Figure 3.15-3. Existing Conditions Flood Depth Grid
3-226
Irwin Creek Basin
100
90
80
70
60
50
40
30
20
10
0
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31
Flood Depth (feet)
Figure 3.15-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.15-5. Approximately 1125
acres are within the 100-year floodplain, an increase of 345 acres or about 44 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.15-6.
The maximum depth is 31.37 feet (only 0.16 feet greater than existing conditions). The
mean depth is 7.35 feet (0.87 foot greater than existing conditions). The area inundated
by more than 10 feet is approximately 297 acres (about 111 acres or about 60 percent
more than the existing conditions).
3-227
Figure 3.15-5. Future Conditions Flood Depth Grid
Irwin Creek Basin
100
90
80
70
60
50
40
30
20
10
0
1
3
5
7
9
11
13
15
17
19
21
23
25
27
29
31
Flood Depth (feet)
Figure 3.15-6. Distribution of Future Condition Flood Depths
3-228
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.15-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.15-8. Note that
about 58 percent of the differences are less than two feet; and about 94 percent are less
than 3 feet.
The largest area (approximately 273 acres) with a more than two-foot
increase in flood depth is along both Irwin and Stewart Creeks upstream of the Southern
Railroad crossing of Irwin Creek. The areas just above the MacArthur Avenue crossing
(30 acres) and the Interstate Highway 85 and Starita Road crossings (16 acres), all on
Irwin Creek, have increases exceeding three feet.
Figure 3.15-7. Increases in 100-year Flood Depths
3-229
Irwin Creek Basin
600
Area Affected (acres)
500
400
300
200
100
0
1
2
3
4
5
Difference in Flood Depths (feet)
Figure 3.15-8. Differences in 100-year Flood Depths
3.15.2.1.1 Irwin Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Irwin Creek, the floodplain boundaries from both the 1975 flood study and
the 2000 flood study were combined and a buffer of 100-feet added.
The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 2,005 parcels that intersect the floodplain boundary (including the 100-foot
buffer) surrounding Irwin Creek. Some duplicate parcels were identified and reviewed. In
all cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 2,005 parcels, 638
3-230
(32%) are identified as “vacant” and an additional 21 parcels had information in the
“effective area” field in the county assessor data indicating that a structure was on the
parcels, although no building footprint was available. The percentage of vacant parcels
(32% based on 2,005 parcels) shows that Mecklenburg County has significant
opportunities to manage future development and reduce future damage within the
watershed.
Within these 2,005 parcels, there are 1,896 building footprints greater than 500 square
feet that are within parcels intersecting the buffered floodplain boundary. Of these, 345
were surveyed as part of the master-elevation-certificate surveys or the “gpsbfe” survey
data. For the analysis, those parcels without a building footprint identified as non-vacant
based on the square footage in the assessors data was carried as single structure at the
centroid of the parcel, for a total of 1,917 “buildings” in the floodplain (including the 100foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
638 vacant parcels identified earlier.
This structure was established to have the
HAZUS®MH default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 2,555.
3.15.3 Detailed Results for the Irwin Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Irwin Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3-231
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.15-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.15-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-232
Table 3.15-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Irwin Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Retail Trade and
Business/Professional/
Technical Services
COM2
Wholesale Trade
COM3
Personal and Repair
Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment and
Recreation
IND2
Light Industrial
IND2/IND6
Light Industrial &
Construction
REL1
Church/Membership
Organizations
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
RES3
Multi-Family Dwelling
Total No. of flooded
buildings
Total No. of buildings
w/out flooding
Current
Inventory,
Original
Floodplain
(Run 1)
3
Current
Inventory,
New
Floodplain
(Run 2)
3
Future
Inventory,
New
Floodplain
(Run 3)
20
31
4
35
5
36
5
16
23
43
1
2
2
5
1
8
1
8
26
1
3
3
116
154
297
1
1
1
12
16
43
31
222
37
288
56
540
1,695
1,629
2,015
The aggregate dollar damage across all building occupancy types is shown in Figure
3.15-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3. The majority of damages are to non-residential structures in spite of the
fact that for all runs, the residential structures either equal or exceed the number of nonresidential structures.
For example in Run 1 there are 160 residential and 62 non-
residential structures. In Run 3 there are 397 residential to 143 non-residential structures.
Tables 3.15-4 through 3.15-7 and the associated discussion go into greater detail about
the results of the loss estimation. However, it should be noted that the escalation of
estimated damage based on future development validates the need for mitigation
3-233
measures, such as requiring future development to be 1-foot above the future one
hundred year flood elevation (as currently proposed by county staff).
$70,000,000
$60,000,000
Structure
Content
Estimated Loss ($)
$50,000,000
$40,000,000
$30,000,000
$20,000,000
$10,000,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.15-9. Irwin Creek Watershed Aggregate Damage Estimation Results
Table 3.15-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Because the first two columns in Table 3.15-4 shows the variation in damage between the
1975 flood study and the 2000 flood study, some occupancies display large increases in
damage and others experience on small variations most likely controlled by their locations
relative to the floodplains. Damage to new development, estimated in Run 3, contributes
to generally increasing damage for all most occupancies.
In those cases where the
estimated average damage (in percent) drops, it gives an indication that the new
development is occurring on the fringe of the floodplain and therefore experiencing lower
overall damage.
It is notable that structure damage over 50% of market value is
3-234
considered
“substantial
Administration.
damage”11
by
the
Federal
Insurance
and
Mitigation
Structures receiving over 50% estimated damage are likely to be
excellent acquisition candidates.
Table 3.15-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Irwin Creek Watershed
HAZUS®MH
Occupancy
Description
Code
COM1/COM4 Retail Trade and
Business/Professional/
Technical Services
COM2
Wholesale Trade
COM3
Personal and Repair
Services
COM4
Business/Professional/
Technical Services
COM8
Entertainment and
Recreation
IND2
Light Industrial
IND2/IND6
Light Industrial &
Construction
REL1
Church/Membership
Organizations
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
RES3
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
22.0
Current
Inventory,
New
Floodplain
(Run 2)
37.3
Future
Inventory,
New
Floodplain
(Run 3)
43.5
43.0
12.0
54.2
11.6
53.2
11.6
43.2
46.5
38.1
3.9
5.6
5.6
10.4
3.8
14.1
21.5
14.1
38.5
4.0
15.1
15.1
21.4
24.8
28.5
9.5
12.5
12.5
32.6
32.8
40.0
25.9
30.3
35.6
Table 3.15-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $11.17 million to $18.06 million. This corresponds to an increase of $6.89
million, or 62%. As the building inventories used for these scenarios are identical, the
11
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
3-235
difference in damage is attributable to differences in the floodplain between the 1975 flood
study and the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run
3, there is a significant increase in estimated damage from $18.06 million to $46.99
million, suffered by projected development in the floodplain.
This corresponds to an
increase of $28.92 million, or 160%. Nearly $23 million of this increased damage is
created by development in three major occupancy classifications COM1/COM4 (Retail
trade/Technical Services), IND2/IND6 (Light Industrial/Construction), and RES1A (1-story
single family).
Table 3.15-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Irwin Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM2
COM3
COM4
COM8
IND2
IND2/IND6
REL1
RES1A
RES1B
RES1E
RES3
Total
Current Inventory,
Original Floodplain
(Run 1)
Total
Avg.
$38,800
$12,933
$5,378,445
$173,498
$60,234
$15,058
$3,487,059
$217,941
$15,330
$15,330
$26,566
$5,313
$1,230
$1,230
$2,003
$2,003
$1,040,384
$8,969
$11,123
$11,123
$216,451
$18,038
$892,359
$28,786
$11,169,985
$50,315
Current Inventory, New Future Inventory, New
Floodplain
floodplain
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
$65,070
$21,690
$3,233,812 $161,691
$7,576,044
$216,458
$7,706,221 $214,062
$69,643
$13,929
$69,643
$13,929
$6,567,450
$285,541
$7,024,847 $163,369
$37,148
$18,574
$37,148
$18,574
$252,556
$31,569
$252,556
$31,569
$7,035
$7,035 $12,899,606 $496,139
$22,684
$7,561
$22,684
$7,561
$1,974,658
$12,822
$8,730,551
$29,396
$14,686
$14,686
$14,686
$14,686
$309,994
$19,375
$2,052,592
$47,735
$1,167,543
$31,555
$4,942,130
$88,252
$18,064,510
$62,724 $46,986,475
$87,012
Table 3.15-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.15-4. Estimated content damage between Run 1
and Run 2 experiences increases reflecting differences between the two flood studies and
increases are generally across all occupancies. New development results in increased
estimated damage with a few occupancies increasing more than others.
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-236
Table 3.15-6. Contents Damage Percent by HAZUS®MH Occupancy Type:
Irwin Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM2
COM3
COM4
COM8
IND2
IND2/IND6
REL1
RES1A
RES1B
RES1E
RES3
Description
Retail Trade and
Business/Professional/
Technical Services
Wholesale Trade
Personal and Repair
Services
Business/Professional/
Technical Services
Entertainment and
Recreation
Light Industrial
Light Industrial &
Construction
Church/Membership
Organizations
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling, Split
Level, with Basement
Multi-Family Dwelling
Current
Inventory,
Original
Floodplain
(Run 1)
22.0
Current
Inventory,
New
Floodplain
(Run 2)
37.3
Future
Inventory, New
Floodplain
(Run 3)
43.5
43.8
53.8
56.5
56.1
55.4
56.1
43.2
46.5
35.5
3.9
5.6
5.6
19.4
3.1
25.6
28.8
25.6
55.5
4.0
15.1
15.1
25.1
31.0
32.6
9.0
16.6
16.6
25.5
19.9
25.1
42.6
52.9
64.0
Table 3.15-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.15-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $25.89 million to $39.26 million.
This corresponds to an increase of $13.36 million, or 52%. The increase in damage is
due to changes in the floodplain depth and extent between the 1975 flood study and the
recent 2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a
increase in estimated damage from $39.26 million to $6.22 million attributable to projected
growth. This corresponds to an increase of $23.96 million, or 61%.
3-237
Table 3.15-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Irwin Creek Watershed
HAZUS®MH
Occupancy
Code
COM1/COM4
COM2
COM3
COM4
COM8
IND2
IND2/IND6
REL1
RES1A
RES1B
RES1E
RES3
Current Inventory,
Current Inventory, New
Current Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
$78,104
$26,035
$130,987
$43,662
$18,663,862
$602,060 $25,920,170 $740,576
$138,586
$34,647
$176,766
$35,353
$5,537,449
$346.090 $10,429,110 $453,440
$21,707
$21,707
$52,601
$26,301
$21,030
$4,206
$230,898
$28,862
$502
$502
$4,696
$4,696
$2,944
$2,944
$33,345
$11,115
$612,310
$5,279 $1,151,532
$7,477
$5,292
$5,292
$9,755
$9,755
$85,530
$7,127
$83,783
$5,236
$725,210
$23,394 $1,032,712
$27,911
Total $25,892,527
$116,633 $39,256,355
3-238
$136,307
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$6,509,663
$325,483
$26,371,884
$732,552
$176,766
$35,353
$10,612,706
$246,807
$52,601
$26,301
$230,898
$28,862
$9,306,192
$357,930
$33,345
$11,115
$4,710,916
$15,862
$9,755
$9,755
$636,795
$14,809
$4,565,795
$81,532
$63,217,315
$117,069
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
ROCKY RIVER BASIN (Report Delivered July 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.16 Rocky River Analysis
3.16.1 Results Summary for the Rocky River Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.16-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 1, 6, and 28 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.16-1. Number of Flooded Structures by General Occupancy:
Rocky River Basin
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
1
3
Residential
Single and MultiFamily Residential
1
5
25
1
6
28
Total
The dollar damage resulting from the analysis is shown on Figure 3.16-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $2,500,
contents $1,040) and Run 2 (structure $112.4 thousand, contents $45.7 thousand). This
corresponds to an increase of $110 thousand (4,500%) for structures and $44.5 thousand
(4,300%) for contents. The increase demonstrates the effect of changes in the floodplain
extent and depth between the 1975 flood study and newly revised (2000) floodplain map.
Obviously the very large percentage increase is based on a very small number of
structures and should be used with caution. Based on the assumption that the revised
floodplain map is more accurate than the 1975 flood study, the estimated damage results
from Run 2 are representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
3-239
Run 3, damage estimates for parcels that are currently vacant were assumed to be
developed based on their zoning. The result is a sharp increase in estimated structure
damage from $112.4 thousand to $1.26 million and a corresponding increase in estimated
contents damage from $45.7 thousand to approximately $1.01 million. This corresponds
to an increase of $1.15 million (1,025%) for structures and $0.96 million (2,098%) for
contents.
As with the comparison between Run 1 and Run 2, the large change in
damages is driven by a very small number of structures.
Prior to this analysis, this
significant escalation of potential damages based on future development was predicted by
county staff and has led to efforts requiring new development to be 1-foot above the onehundred year base flood elevation, where the BFE includes the increased runoff from the
future or ultimate build-out. These results support that effort, as the 1-foot requirement
would reduce the results of Run 3 to levels more closely resembling the results of the
existing development results presented by Run 2 (see Figure 3.16-1).
$1,400,000
Structure
Content
$1,200,000
Estimated Loss ($)
$1,000,000
$800,000
$600,000
$400,000
$200,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.16-1. Rocky River Basin Aggregate Damage Estimation Results
Table 3.16-2 shows a summary of the building and content damage results by general
occupancy classification. Table 3.16-2 shows the majority of damages are concentrated
in the residential occupancy classifications. Run 2 shows that differences between the
3-240
1975 and 2000 flood study has the general impact of increasing direct damage from
flooding (structure and contents) by a factor of 50. As expected, future development
increases the residential damages (by a factor of 10), but significant increases are also
seen in the non-residential classifications. The results are discussed in greater detail later
in this report.
Table 3.16-2. Summary of Building and Content Damage by General
Occupancy: Rocky River Basin
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
>$100
>$100
$205,650
$68,550
Residential
Single and
Multi-Family
Residential
$2,446
$2,446
$112,396
$22,479
$1,060,261
$42,410
Content Damage
Total
Average
Total
Average
Total
Average
$0
$174
$174
$487,747
$162,582
$1,039
$45,553
$9,111
$517,339
$20,693
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
Residential
Single and
Multi-Family
Residential
$1,039
3.16.2 Data Analysis
3.16.2.1 Rocky River Basin Characteristics and Study Data
Depth grids were created for Rocky River, West Branch Rocky River and West Branch
Rocky River Tributary, and South Prong West Branch Rocky River and South Prong West
Branch Rocky River Tributary.
The grids depict the depth of flooding that will be
exceeded with a 1-percent chance in any given year (100-year flood) under both existing
3-241
and future development conditions. The Rocky River stream system is shown in Figure
3.16-2. Approximately 14.2 miles of stream were studied.
Figure 3.16-2. Rocky River Stream System
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches.
In all, 170 cross
sections were used: 48 on Rocky River; 75 on West Branch Rocky River and West
Branch Rocky River Tributary; and 47 on South Prong West Branch Rocky River and
South Prong West Branch Rocky River Tributary.
The flood elevation grids were
extended at the upstream end of each stream, at the confluences of tributaries and main
streams, and at relatively large areas where floodwaters are not conveyed but, rather,
pond at the elevation on the main stream. Where the two stream flood elevation grids
overlap the final flood elevation grid uses the greater of the two elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
3-242
The resulting existing conditions flood depth grid is shown in Figure 3.16-3.
Approximately 873 acres are within the 100-year floodplain.
Figure 3.16-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is approximately 17.88 feet. The average (mean) depth
is 4.12 feet. Approximately 258 acres are inundated at a depth greater than 5 feet.
Figure 3.16-3. Existing Conditions Flood Depth Grid
3-243
Rocky River Basin
160
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18
Flood Depth (feet)
Figure 3.16-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.16-5. Approximately 978 acres
are within the 100-year floodplain, an increase of 105 acres or about 12 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.16-6.
The maximum depth is 20.16 feet (2.28 feet greater than existing conditions). The mean
depth is 5.13 feet (1.01 foot greater than existing conditions). The area inundated by
more than 5 feet is approximately 460 acres (about 202 acres or 78 percent more than the
existing conditions).
3-244
Figure 3.16-5. Future Conditions Flood Depth Grid
3-245
Rocky River Basin
140
120
100
80
60
40
20
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Flood Depth (feet)
Figure 3.16-6. Distribution of Future Condition Flood Depths
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.16-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.16-8. Note that
about 45 percent of the differences are less than two feet; and about 90 percent are less
than 3 feet.
The increase in 100-year flood depth exceeds two feet for essentially the entire study
reach of Rocky River. The increase exceeds three feet on Rocky River upstream of the
East Rocky River Road crossing (29 acres) and upstream of a private road crossing, 0.75
mile downstream of East Rocky River Road (16 acres). The difference in flood depth
exceeds three feet on West Branch Rocky River upstream of the Grey Road crossing (55
acres).
3-246
Figure 3.16-7. Increases in 100-year Flood Depths
3-247
Area Affected (acres)
Rocky River Basin
500
450
400
350
300
250
200
150
100
50
0
1
2
3
4
5
6
7
Difference in Flood Depths (feet)
Figure 3.16-8. Differences in 100-year Flood Depths
3.16.2.1.1 Rocky River Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Rocky River, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 263 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding the Rocky River basin. Some duplicate parcels were identified and reviewed.
In all cases, these turned out to be slivers in the GIS data, most likely right-of-ways that
have been either returned to the parcel, or offers of dedication that were never acted
upon. In all cases, the slivers were merged back into the main parcel. Of the 263 parcels,
113 (43%) are identified as “vacant” and an additional 6 parcels had information in the
“effective area” field in the county assessor data indicating that a structure was on the
3-248
parcels, although no building footprint was available. The percentage of vacant parcels
(43% based on 263 parcels) shows that, although a small population of parcels,
Mecklenburg County has significant opportunities to manage future development and
reduce future damage within the watershed.
Within these 263 parcels, there are 253 building footprints greater than 500 square feet
that are within parcels intersecting the buffered floodplain boundary. Of these, none were
surveyed as part of the master-elevation-certificate surveys or the “gpsbfe” survey data.
The lack of MEC survey’s is supported by the very low damages estimated for the 1975
100-year floodplain boundary (Run 1: 1 structure and less than $4,000 in total damage).
For the analysis, the additional 6 parcels without a building footprint identified as nonvacant based on the square footage in the assessors data was carried as single structure
at the centroid of the parcel, for a total of 259 “buildings” in the floodplain (including the
100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
113 vacant parcels identified earlier.
This structure was established to have the
HAZUS®MH default floor area and value based on the occupancy class assigned to the
approved zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 372.
3.16.3 Detailed Results for the Rocky River Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Rocky River basin (developed from surveyed data and
parcel data) and the original 100-year floodplain boundary, based on the 1975 flood
study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3-249
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.16-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. As noted previously, the current impact is fairly limited (1 structure for Run 1
and 5 structures for Run 2). The increase between Run 2 and Run 3 is attributable to the
projected increase in potential development within the floodplain.
The values shown in Table 3.16-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-250
Table 3.16-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Rocky River Basin
HAZUS®MH
Occupancy
Description
Code
COM2
Wholesale Trade
COM8
Entertainment and
Recreation
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of buildings
w/out flooding
Current
Inventory,
Original
Floodplain
(Run 1)
-----
Current
Inventory,
New
Floodplain
(Run 2)
1
---
Future
Inventory,
New
Floodplain
(Run 3)
1
2
---
2
18
1
2
2
---
1
1
---
---
4
1
5
28
258
254
344
The aggregate dollar damage across all building occupancy types is shown in Figure
3.16-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3. The majority of damages are to residential structures and the future
development does not greatly increase the number of commercial structure impacted. For
example in Run 1 there is 1 residential and no non-residential structures. In Run 3 there
are 25 residential to 3 non-residential structures. Tables 3.16-4 through 3.16-7 and the
associated discussion go into greater detail about the results of the loss estimation.
However, it should be noted that the escalation of estimated damage based on future
development validates the need for mitigation measures, such as requiring future
development to be 1-foot above the future 100-year flood elevation (as currently proposed
by county staff).
3-251
$1,400,000
Structure
Content
$1,200,000
Estimated Loss ($)
$1,000,000
$800,000
$600,000
$400,000
$200,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.16-9. Rocky River Basin Aggregate Damage Estimation Results
Table 3.16-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Because the first two columns in Table 3.16-4 shows the variation in damage between the
1975 flood study and the 2000 flood study, some occupancies display large increases in
damage and others experience on small variations most likely controlled by their locations
relative to the floodplains. Damage to new development, estimated in Run 3, contributes
to generally increasing damage for all most occupancies.
In those cases where the
estimated average damage (in percent) drops, it gives an indication that the new
development is occurring on the fringe of the floodplain and therefore experiencing lower
overall damage.
considered
It is notable that structure damage over 50% of market value is
“substantial
damage”12
by
the
12
Federal
Insurance
and
Mitigation
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-252
Administration.
Structures receiving over 50% estimated damage are likely to be
excellent acquisition candidates.
Table 3.16-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Rocky River Basin
HAZUS®MH
Occupancy
Code
Description
COM2
Wholesale Trade
COM8
Entertainment and
Recreation
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
-----
Current
Inventory,
New
Floodplain
(Run 2)
>1.0
---
Future
Inventory,
New
Floodplain
(Run 3)
>1.0
9.4
>1.0
12.7
26.8
---
5.3
5.3
---
>1.0
>1.0
---
---
48.5
Table 3.16-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $2.4 thousand to $112.4 thousand. This corresponds to an increase of
$110.0 thousand, or 4,500%. As the building inventories used for these scenarios are
identical, the difference in damage is attributable to differences in the floodplain between
the 1975 flood study and the 2000 revision of the 100-year floodplain map. As noted
previously, the total number of structures impacted by the changes are very small and
therefore may skew the percentages.
Changes in the assumptions could lead to
variations in the results. Proceeding from Run 2 to Run 3, there is a significant increase in
estimated damage from $112.4 thousand to $1.3 million, suffered by projected
development in the floodplain.
This corresponds to an increase of $1.15 million, or
1,026%. The greatest increase in losses is seen in the residential classification with all
but approximately $200 thousand dollars of the estimated losses.
Table 3.16-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Rocky River Basin
3-253
HAZUS®MH
Occupancy
Code
COM2
COM8
RES1A
RES1B
RES1D
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
---->$100
>$100
>$100
>$100
--------$205,600 $102,800
----$77,744
$38,872
$743,195
$41,289
$2,446
$2,446
$32,677
$16,338
$32,677
$16,338
----$1,975
$1,975
$1,975
$1,975
--------$282,413
$70,603
$2,446
$2,446
$112,446
$18,741
$1,265,911
$45.211
Table 3.16-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.16-4. Estimated content damage between Run 1
and Run 2 experiences increases reflecting differences between the two flood studies and
increases are generally across all occupancies. New development results in increased
estimated damage with the addition of some non-residential development and increasing
residential losses.
Table 3.16-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Rocky River Basin
HAZUS®MH
Occupancy
Code
COM2
COM8
RES1A
RES1B
RES1D
RES1E
Description
Wholesale Trade
Entertainment and
Recreation
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling Split
Level, No Basement
Single Family Dwelling, Split
Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
-----
Current
Inventory,
New
Floodplain
(Run 2)
>1.0
---
Future
Inventory, New
Floodplain
(Run 3)
>1.0
44.6
---
9.4
31.3
>1.0
4.9
4.9
---
2.1
2.1
---
---
25.8
Table 3.16-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.16-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $1 thousand to $45.7 thousand.
This corresponds to an increase of $44.7 thousand, or 4,300%. The increased damage is
seen in a small number of structures within the floodplain. The increase in damage is due
3-254
to changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a increase
in estimated damage from $45.7 thousand to $1.01million attributable to projected growth.
This corresponds to an increase of $959.3 thousand, or 2,100%.
Table 3.16-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Rocky River Basin
HAZUS®MH
Occupancy
Code
COM2
COM8
RES1A
RES1B
RES1D
RES1E
Total
Current Inventory,
Current Inventory, New
Current Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
----$174
$174
------------$27,868
$13,934
$1,039
$1,039
$15,246
$7,623
----$2,438
$2,438
--------$1,039
$1,039
$45,727
3-255
$7,621
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$174
$174
$487,573
$243,786
$424,637
$23,591
$15,246
$7,623
$2,438
$2,438
$75,018
$18,754
$1,005,086
$35,896
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
GOOSE CREEK WATERSHED (Report Delivered October 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.17 Goose Creek Analysis
3.17.1 Results Summary for the Goose Creek Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.17-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 6, 11, and 17 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.17-1. Number of Flooded Structures by General Occupancy:
Goose Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
0
Residential
Single and MultiFamily Residential
6
11
17
6
11
17
Total
The dollar damage resulting from the analysis is shown on Figure 3.17-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, there is an increase in estimated damage between Run 1 (structure $45,640,
contents $17,300) and Run 2 (structure $135,710, contents $63,890). This corresponds
to an increase of $90 thousand (197%) for structures and $46.5 thousand (269%) for
contents. The increase demonstrates the effect of changes in the floodplain extent and
depth between the 1975 flood study and newly revised 2000 flood study map. Based on
the assumption that the revised floodplain map is more accurate than the 1975 flood
study, the estimated damage results from Run 2 are representative of current conditions.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
developed based on their zoning. The result is an increase in estimated structure damage
3-256
from $135,710 to $348,190 and a corresponding increase in estimated contents damage
from $63,890 to approximately $164,780.
This represents an increase of $212,480
(157%) for structures and $100,890 (158%) for contents.
Prior to this analysis, this
escalation of potential damages based on future development was predicted by county
staff and has led to efforts requiring new development to be 1-foot above the one-hundred
year base flood elevation, where the BFE includes the increased runoff from the future or
ultimate build-out.
These results support that effort, as the 1-foot requirement would
reduce the results of Run 3 to levels more closely resembling the results of the existing
development results presented by Run 2 (see Figure 3.17-1).
$400,000
$350,000
Structure
Content
$300,000
Estimated Loss ($)
$250,000
$200,000
$150,000
$100,000
$50,000
$Run1
Run2
Building Inventory and Floodplain Scenario
Run3
Figure 3.17-1. Goose Creek Watershed Aggregate Damage Estimation Results
Table 3.17-2 shows a summary of the building and content damage results by general
occupancy classification.
In general, Table 3.17-2 shows that the damages are
concentrated in the residential occupancy classifications. Run 2 shows that differences
3-257
between the 1975 and 2000 flood study have a relatively significant impact (structure and
contents). As expected, there is also a significant increase in the losses to residential
occupancies in Run 3, because of “development” of vacant parcels.
The results are
discussed in greater detail later in this report.
Table 3.17-2. Summary of Building and Content Damage by General
Occupancy: Goose Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$45,644
$7,607
$135,712
$12,337
$348,193
$9,693
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$17,303
$2,884
$63,890
$5,808
$164,781
$9,693
3.17.2 Data Analysis
3.17.2.1 Goose Creek Basin Characteristics and Study Data
The portion of Goose Creek basin above the confluence with Stevens Creek is located in
southeastern portion of Mecklenburg County. Depth grids were created for Goose Creek,
its major tributary, Stevens Creek, and Stevens Creek Tributary. The grids depict the
depth of flooding that will be exceeded with a 1-percent chance in any given year (100-
3-258
year flood) under both existing and future development conditions.
The network of
streams analyzed is shown in Figure 3.17-2.
Figure 3.17-2. Goose Creek Basin Stream Network
The studied reaches include approximately 1.9 miles of Goose Creek and approximately
2.8 miles of Stevens Creek and Stevens Creek Tributary.
Flood elevation grids were created interpolating between water surface elevations
determined at numerous cross sections along the study reaches. In all, 63 cross sections
were used: 27 on Goose Creek, 25 on Stevens Creek, and 11 on Stevens Creek
Tributary. The flood elevation grids were extended at the upstream end of each stream,
at the confluence with each tributary, and at relatively large areas where floodwaters are
not conveyed but, rather, pond at the elevation on the main stream. Where two stream
flood elevation grids overlap the final flood elevation grid uses the greater of the two
elevations.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
3-259
The resulting existing conditions flood depth grid is shown in Figure 3.17-3.
Approximately 152 acres are within the 100-year floodplain.
Figure 3.17-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 12.40 feet. The average (mean) depth is 2.72 feet.
Approximately 16.8 acres are inundated at a depth greater than 5 feet.
Figure 3.17-3. Existing Conditions Flood Depth Grid
3-260
Goose Creek Basin
45
Area Inundated (acres)
40
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
Flood Depth (feet)
Figure 3.17-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.17-5. Approximately 171 acres
are within the 100-year floodplain, an increase of 19 acres or approximately 12.5 percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.17-6.
The maximum depth is 14.18 feet (1.78 feet greater than existing conditions). The mean
depth is 3.37 feet (0.65 foot greater than existing conditions). The area inundated by
more than 5 feet is approximately 34 acres (17.2 acres more or double the existing
conditions).
3-261
Figure 3.17-5. Future Conditions Flood Depth Grid
3-262
Goose Creek Basin
Area Inundated (acres)
40
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Flood Depth (feet)
Figure 3.17-6. Distribution of Future Condition Flood Depths
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.17-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.17-8. Note that
about 93 percent of the differences are less than two feet and most (70 percent) are less
than one foot. The 100-year flood depth increases are expected to exceed two feet in
approximately 12.5 acres, just upstream of the Interstate Highway 485 crossing of
Stevens Creek.
3-263
Figure 3.17-7. Increases in 100-year Flood Depths
Goose Creek Basin
140
Area Affected (acres)
120
100
80
60
40
20
0
1
2
3
4
5
Difference in Flood Depths (feet)
Figure 3.17-8. Differences in 100-year Flood Depths
3-264
3.17.2.1.1 Goose Creek Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Goose Creek, the floodplain boundaries from both the 1975 flood study
and the 2000 flood study were combined and a buffer of 100-feet added. The buffered
boundary was used to select intersecting parcel and every building footprint contained
within those parcels. This process was selected because it allows Mecklenburg County to
identify all structures that are considered close to the floodplain. Structures outside the
flood depth grids do not impact the results of this analysis because they will have no net
damage, but knowing what structures are nearby was deemed advantageous to
Mecklenburg County.
There are 135 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding Goose Creek. Some duplicate parcels were identified and reviewed. In all
cases, these turned out to be slivers in the GIS data, most likely right-of-ways that have
been either returned to the parcel, or offers of dedication that were never acted upon. In
all cases, the slivers were merged back into the main parcel. Of the 135 parcels, 22
(16%) are identified as “vacant” and an additional 1 parcel had information in the “effective
area” field in the county assessor data indicating that a structure was on the parcels,
although no building footprint was available. The percentage of vacant parcels (16%
based on 135 parcels) is relatively low compared to other watersheds, but Mecklenburg
County still has opportunities to manage future development and reduce future damage
within the watershed.
Within these 135 parcels, there are 175 building footprints greater than 500 square feet
that are within parcels intersecting the buffered floodplain boundary. None of these were
surveyed as part of the master-elevation-certificate surveys or the “gpsbfe” survey data.
For the analysis, the single parcel without a building footprint that was identified as nonvacant based on the square footage in the assessors data was carried as single structure
at the centroid of the parcel, for a total of 176 “buildings” in the floodplain (including the
100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
22 vacant parcels identified earlier. This structure was established to have the HAZUS®MH
default floor area and value based on the occupancy class assigned to the approved
3-265
zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 198.
3.17.3 Detailed Results for the Goose Creek Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Goose Creek watershed (developed from surveyed data
and parcel data) and the original 100-year floodplain boundary, based on the 1975
flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
Table 3.17-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
As shown in the figure, the number of damaged
buildings increases between runs for the majority of building types. The increase in the
number of flooded buildings between Run 1 and Run 2 is due to differences in the
floodplain boundary between the 1975 flood study and the 2000 flood study map
revisions. The increase between Run 2 and Run 3 is attributable to the projected increase
in floodplain development.
The values shown in Table 3.17-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
3-266
Table 3.17-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Goose Creek Watershed
HAZUS®MH
Occupancy
Description
Code
RES1A
Single Family Dwelling,
1 Story, No Basement
RES1B
Single Family Dwelling,
2 Story, No Basement
RES1D
Single Family Dwelling
Split Level, No
Basement
RES1E
Single Family Dwelling,
Split Level, with
Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
3
Current
Inventory,
New
Floodplain
(Run 2)
5
Future
Inventory,
New
Floodplain
(Run 3)
9
3
4
4
---
1
1
---
1
3
6
11
17
170
165
181
The aggregate dollar damage across all building occupancy types is shown in Figure
3.17-9. As shown, the estimated dollar damage increases from Run 1 to Run 2 and from
Run 2 to Run 3. The majority of damages are to residential structures, further review of
the parcel data indicated that none of the parcels within the floodplain are zoned for any
other use. Tables 3.17-4 through 3.17-7 and the associated discussion go into greater
detail about the results of the loss estimation.
However, it should be noted that the
escalation of estimated damage based on future development validates the need for
mitigation measures, such as requiring future development to be 1-foot above the future
one hundred year flood elevation (as currently proposed by county staff).
3-267
$400,000
$350,000
Structure
Content
$300,000
Estimated Loss ($)
$250,000
$200,000
$150,000
$100,000
$50,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.17-9. Goose Creek Watershed Aggregate Damage Estimation Results
Table 3.17-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.17-4 shows that the damage (shown in percent) changed for the existing
occupancies within the floodplain between Run 1 and Run 2 indicating that the differences
between the 1975 flood study and the 2000 flood study impacted some structures
currently outside the 1975 100-year study and increased damage for those within the
1975 floodplain.
Ironically, new development, shown in Run 3, did not increase the
average damage (%) greatly. In Run 1 the estimated damage runs from 5 to 11 percent
while in Run 2 the average structure damage levels ranges from less than 1 percent to
nearly 31 percent. Interestingly enough, Run 3 shows similar impacts as Run 2 to the
existing inventory, but increased losses indicates that some of the new development is
within the floodplain. It is notable that structure damage over 50% of market value is
3-268
considered
“substantial
Administration.
damage”13
by
the
Federal
Insurance
and
Mitigation
Structures receiving over 50% estimated damage are likely to be
excellent acquisition candidates.
Table 3.17-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Goose Creek Watershed
HAZUS®MH
Occupancy
Description
Code
RES1A
Single Family Dwelling, 1
Story, No Basement
RES1B
Single Family Dwelling, 2
Story, No Basement
RES1D
Single Family Dwelling,
Split Level, No Basement
RES1E
Single Family Dwelling,
Split Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
11.7
Current
Inventory,
New
Floodplain
(Run 2)
12.6
Future
Inventory,
New
Floodplain
(Run 3)
13.7
5.1
9.0
9.0
---
0.7
0.7
---
30.4
38.8
Table 3.17-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, there are significant differences in the dollar damage between
the three scenarios. Between Run 1 and Run 2, there is an increase in total structure
damage from $45,644 to $135,712. This corresponds to an increase of $90 thousand, or
197%. As the building inventories used for these scenarios are identical, the difference in
damage is attributable to differences in the floodplain between the 1975 flood study and
the 2000 revision of the 100-yr floodplain map. Proceeding from Run 2 to Run 3, there is
a significant increase in estimated damage from $135,712 to $348,193 suffered by
projected development in the floodplain. This corresponds to an increase of $212,481 or
156%. As the flood depths are unchanged, the differences between Run 2 and Run 3 can
be attributed to the new development within the floodplain.
Table 3.17-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Goose Creek Watershed
13
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
3-269
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1D
RES1E
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$28,897
$9,632
$52,936
$10,587
$140,144
$15,572
$16,748
$5,583
$43,898
$10,974
$43,898
$10,974
----$840
$840
$840
$840
----$38,039
$38,039
$163,311
$54,437
$45,644
$7,607
$135,712
$12,337
$348,193
$20,482
Table 3.17-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.17-4. Run 1 has relatively limited damage while
Run 2 shows increased damages and additional residential occupancies involved in
flooding. The estimated damage in Run 1 varies from 4 to 8 percent, whereas damage in
Run 2 runs from 1 percent to 32 percent. As expected, the estimated damages for Run 3
are higher still, ranging from 1 percent to 36 percent.
Table 3.17-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Goose Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1D
RES1E
Description
Single Family Dwelling, 1
Story, No Basement
Single Family Dwelling, 2
Story, No Basement
Single Family Dwelling, Split
Level, No Basement
Single Family Dwelling, Split
Level, with Basement
Current
Inventory,
Original
Floodplain
(Run 1)
8.3
Current
Inventory,
New
Floodplain
(Run 2)
10.8
Future
Inventory, New
Floodplain
(Run 3)
12.8
4.3
8.5
8.5
---
1.6
1.6
---
31.6
36.4
Table 3.17-7 provides the estimated dollar damage to building contents by occupancy
type. Similar to the structure dollar damage shown in Table 3.17-5, there are significant
differences in the dollar damage between scenarios. Between Run 1 and Run 2, there is
an increase in total estimated contents damage from $17,303 to $63,890.
This
corresponds to an increase of $47 thousand, or 269%. The increase in damage is due to
changes in the floodplain depth and extent between the 1975 flood study and the recent
2000 revision of the 100-yr floodplain map. Between Run 2 to Run 3, there is a significant
increase in estimated damage from $63,890 to $164,781 attributable to projected growth.
This corresponds to an increase of $101 thousand, or 158%.
3-270
Table 3.17-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Goose Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
RES1B
RES1D
RES1E
Total
Current Inventory,
Current Inventory, New
Current Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
$10,188
$3,396
$22,402
$4,480
$7,115
$2,372
$20,640
$5,160
----$1,036
$1,036
----$19,812
$19,812
$17,303
$2,884
$63,890
3-271
$5,808
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$66,789
$7,421
$20,640
$5,160
$1,036
$1,036
$76,315
$25,438
$164,781
$9,693
DETERMINATION OF FINANCIAL IMPACTS FROM FLOOD STUDIES
LOWER CLARKE CREEK WATERSHED (Report Delivered
October 2002)
Prepared for:
MECKLENBURG COUNTY ENGINEERING & BUILDING STANDARDS
Charlotte, NC 28202
3.18 Lower Clarke Creek Analysis
3.18.1 Results Summary for the Lower Clarke Creek Tributary Basin
Two building inventories and two floodplain scenarios maps were combined to develop
damage estimates for the three scenarios listed above. Table 3.18-1 shows a summary of
the number of flooded buildings by general occupancy classification.
The simulation
predicted 0, 0, and 2 damaged buildings from Runs 1, 2, and 3, respectively.
Table 3.18-1. Number of Flooded Structures by General Occupancy:
Lower Clarke Creek Watershed
General
Occupancy
Description
Current Inventory,
Original
Floodplain
(Run 1)
Current
Inventory, New
Floodplain
(Run 2)
Future
Inventory, New
Floodplain
(Run 3)
NonResidential
Commercial,
Industrial, Religious,
Education
0
0
0
Residential
Single and MultiFamily Residential
0
0
2
0
0
2
Total
The dollar damage resulting from the analysis is shown on Figure 3.18-1. The damage for
each scenario shown in the figure is an aggregate across all building occupancy types.
As shown, it is estimated that all structures are outside of the 100-year floodplain or are
not damaged when the analysis is performed for Run 1 and Run 2.
The increase
demonstrates the effect of changes in the floodplain extent and depth between the 1975
flood study and newly revised (2000) floodplain map.
While the new maps can be
considered more accurate, it appears that changes between the 1975 study and the 2000
study do not impact any structures.
A comparison of the aggregate damage from Run 2 and Run 3 highlights the difference in
damage between current and future building development conditions. For the analysis in
Run 3, damage estimates for parcels that are currently vacant were assumed to be
developed based on their zoning. The result is an increase in estimated structure damage
from no damage (Run 2) to $125 thousand and a corresponding increase in estimated
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contents damage from no damage to approximately $77 thousand. Prior to this analysis,
this significant escalation of potential damages based on future development was
predicted by county staff and has led to efforts requiring new development to be 1-foot
above the one-hundred year base flood elevation, where the BFE includes the increased
runoff from the future or ultimate build-out. These results support that effort as the 1-foot
requirement would reduce the results of Run 3 to levels more closely resembling the
results of the existing development results presented by Run 2 (see Figure 3.18-1). The
results of this analysis indicate that for this watershed, Mecklenburg County could greatly
reduce, or potentially eliminate, losses due to new development through effective
floodplain management.
$140,000
$120,000
Structure
Content
Estimated Loss ($)
$100,000
$80,000
$60,000
$40,000
$20,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.18-1. Clarke Creek Tributary Aggregate Damage Estimation Results
Table 3.18-2 shows a summary of the building and content damage results by general
occupancy classification. In general, Table 3.18-2 shows the damages are concentrated
in the residential occupancy classifications. Run 2 shows that differences between the
1975 and 2000 flood study have no impact on estimated damages (structure and
contents). As expected, there is an increase in the losses to residential occupancies in
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Run 3, because of “development” of vacant parcels. It should be noted that these results
are for a small number of structures and therefore should be viewed with caution.
However, this also indicates a unique opportunity to eliminate the potential for losses.
The results are discussed in greater detail in Section 3 of this report.
Table 3.18-2. Summary of Building and Content Damage by General
Occupancy: Lower Clarke Creek Watershed
General
Occupancy
Description
Current Inventory,
Original Floodplain
(Run 1)
Current Inventory, New
Floodplain
(Run 2)
Future Inventory, New
Floodplain
(Run 3)
Structural Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$0
$0
$0
$0
$125,001
$62,500
Content Damage
Total
Average
Total
Average
Total
Average
NonResidential
Commercial,
Industrial,
Religious,
Education
$0
$0
$0
$0
$0
$0
Residential
Single and
Multi-Family
Residential
$0
$0
$0
$0
$77,407
$38,704
3.18.2 Data Analysis
3.18.2.1 Clarke Creek Tributary Basin Characteristics and Study Data
Clarke Creek Tributary enters Clarke Creek east of Mecklenburg County. The area of
Mecklenburg County draining to Clark Creek Tributary is located between the Clarke
Creek (to the north) and Mallard Creek (to the south) Basins. Depth grids were created
for Clarke Creek Tributary depicting the depth of flooding that will be exceeded with a 1percent chance in any given year (100-year flood) under both existing and future
3-274
development conditions. Clarke Creek Tributary and the adjacent basins are shown in
Figure 3.18-2.
Figure 3.18-2. Clarke Creek Tributary and Adjacent Basins
The studied reach includes approximately 1.5 miles of Clarke Creek Tributary.
Flood elevation grids were created interpolating between water surface elevations
determined at 25 cross sections along the study reach. The flood elevation grids were
extended at the upstream end of the stream and at relatively large areas where
floodwaters are not conveyed but, rather, pond at the elevation on the main stream.
Depth grids were created by subtracting the ground elevation in the digital elevation model
from the flood elevation at the corresponding cell in the elevation grid. Cells with flood
depths less than zero (those outside of the floodplain, for example) were deleted from the
flood elevation and flood depth grids.
The resulting existing conditions flood depth grid is shown in Figure 3.18-3.
Approximately 34.8 acres are within the 100-year floodplain.
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Figure 3.18-4 shows the distribution of floodplain area inundated at 1-foot depth
increments. The maximum depth is 13.12 feet. The average (mean) depth is 4.68 feet.
Approximately 12.0 acres upstream of the Street Avenue crossing are inundated at a
depth greater than 7 feet.
Figure 3.18-3. Existing Conditions Flood Depth Grid
3-276
Clarke Creek Tributary
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Flood Depth (feet)
Figure 3.18-4. Distribution of Existing Condition Flood Depths
The future conditions flood depth grid is shown in Figure 3.18-5. Approximately 36.8
acres are within the 100-year floodplain, an increase of only 2 acres or less than 6
percent.
The distribution of floodplain area inundated at various depths is shown in Figure 3.18-6.
The maximum depth is 13.33 feet (0.21 feet greater than existing conditions). The mean
depth is 4.84 feet (0.16 foot more than existing conditions). The area inundated by more
than 7 feet is approximately 14.6 acres (2.6 acres or approximately 21.7 percent more
than the existing conditions).
3-277
Figure 3.18-5. Future Conditions Flood Depth Grid
Clarke Creek Tributary
14
12
10
8
6
4
2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Flood Depth (feet)
Figure 3.18-6. Distribution of Future Condition Flood Depths
3-278
Subtracting the existing conditions flood depth grid from the future conditions flood depth
grid yields a grid showing the increase in 100-year flood depths expected as a result of
development within the basin. That grid is shown in Figure 3.18-7.
The distribution of changes in 100-year flood depths is shown in Figure 3.18-8. Note that
about 97 percent of the differences are less than one.
The 100-year flood depth
increases are expected to exceed one foot in two small areas totaling 1.2 acres.
Figure 3.18-7. Increases in 100-year Flood Depths
3-279
Clarke Creek Tributary
40
Area Affected (acres)
35
30
25
20
15
10
5
0
1
2
Difference in Flood Depths (feet)
Figure 3.18-8. Differences in 100-year Flood Depths
3.18.2.1.1 Lower Clarke Creek Tributary Parcel and Building Data Review
To determine the parcels and building footprints that may be potentially impacted by
inundation from Clarke Creek Tributary, the floodplain boundaries from both the 1975
flood study and the 2000 flood study were combined and a buffer of 100-feet added. The
buffered boundary was used to select intersecting parcel and every building footprint
contained within those parcels. This process was selected because it allows Mecklenburg
County to identify all structures that are considered close to the floodplain. Structures
outside the flood depth grids do not impact the results of this analysis because they will
have no net damage, but knowing what structures are nearby was deemed advantageous
to Mecklenburg County.
There are 89 parcels that intersect the floodplain boundary (including the 100-foot buffer)
surrounding Clarke Creek Tributary.
Some duplicate parcels were identified and
reviewed. In all cases, these turned out to be slivers in the GIS data, most likely right-ofways that have been either returned to the parcel, or offers of dedication that were never
acted upon. In all cases, the slivers were merged back into the main parcel. Of the 89
parcels, 42 (47%) are identified as “vacant” and there were no parcels with information in
3-280
the “effective area” field in the county assessor data indicating that a structure was on the
parcels, although no building footprint was available. The percentage of vacant parcels
(47% based on 89 parcels) shows that Mecklenburg County has significant opportunities
to manage future development and reduce future damage within the watershed.
Within these 89 parcels intersecting the buffered floodplain boundary, there are 48
building footprints greater than 500 square feet. Of these, none were surveyed as part of
the master-elevation-certificate surveys or the “gpsbfe” survey data. The HAZUS default
values were applied to these records. For the analysis, the parcel without a building
footprint that was identified as non-vacant based on the square footage in the assessors
data was carried as single structure at the centroid of the parcel, for a total of 48
“buildings” in the floodplain (including the 100-foot buffer).
For the future conditions or “build-out”, a single structure was placed at the centroid of the
42 vacant parcels identified earlier. This structure was established to have the HAZUS®MH
default floor area and value based on the occupancy class assigned to the approved
zoning of the parcel (see Tables A-1 and A-2 in Appendix A). Therefore the total
“buildings” within the floodplain for the built-out inventory is assumed to be 90.
3.18.3 Detailed Results for the Lower Clarke Creek Tributary Basin
To determine the potential impacts of floodplain map modifications, three damage
estimates have been developed, as follows:
1. Run 1 – current inventory, original floodplain. Estimated losses based on the current
building inventory within the Lower Clarke Creek watershed (developed from surveyed
data and parcel data) and the original 100-year floodplain boundary, based on the
1975 flood study.
2. Run 2 – current inventory, new floodplain. Estimated losses based on the current
building inventory, expanded to include structures within the new floodplain that were
previously outside the floodplain, and the newly revised (2000) floodplain boundary.
3. Run 3 – future inventory, new floodplain. Estimated losses to a projected future
inventory that incorporates projected growth (“build out”), relative to the new floodplain
boundary.
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Table 3.18-3 presents the number of buildings damaged by flooding (i.e., non-zero dollar
damage) by occupancy under each of the three scenarios. As noted in Section 2, a
HAZUS®MH occupancy class was associated with each occupancy type in the
Mecklenburg assessor database.
Should the number of flooded buildings increased
between Run 1 and Run 2 this would have been due to differences in the floodplain
boundary between the 1975 flood study and the 2000 flood study map revisions. The
increase seen between Run 2 and Run 3 is attributable to the projected increase in
floodplain development.
The values shown in Table 3.18-3 are presented to give the reader perspective on the
relative number of flooded buildings, which is useful when viewing the subsequent tables.
Table 3.18-3. Number of Flooded Buildings by HAZUS®MH Occupancy Type:
Lower Clarke Creek Watershed
HAZUS®MH
Occupancy
Description
Code
RES1A
Single Family Dwelling,
1 Story, No Basement
Total No. of flooded
buildings
Total No. of unflooded
buildings
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory,
New
Floodplain
(Run 2)
0
Future
Inventory,
New
Floodplain
(Run 3)
2
0
0
2
48
48
88
The aggregate dollar damage across all building occupancy types is shown in Figure
3.18-9. As shown, there are no estimated damages in Run 1 or Run 2. As expected,
there is estimated dollar damage increases from Run 2 to Run 3. All of the damages are
to residential structures and further study indicated that only a few parcels that intersect
the floodplain are zoned non-residential. Tables 3.18-4 through 3.18-7 and the associated
discussion go into greater detail about the results of the loss estimation. However, it
should be noted that the escalation of estimated damage based on future development
validates the need for mitigation measures, such as requiring future development to be 1foot above the future one hundred year flood elevation (as currently proposed by county
staff).
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$140,000
$120,000
Structure
Content
Estimated Loss ($)
$100,000
$80,000
$60,000
$40,000
$20,000
$Run1
Run2
Run3
Building Inventory and Floodplain Scenario
Figure 3.18-9. Clarke Creek Tributary Aggregate Damage Estimation Results
Table 3.18-4 presents the overall average damage percentage calculated for building
structures by occupancy, through the application of the various depth-damage curves.
Table 3.18-4 shows that there is no damage (shown in percent) estimated for Run 1 and
Run 2 indicating that the differences between the 1975 flood study and the 2000 flood
study do not impact any structures. New development, shown in Run 3, shows the impact
of flooding on the new structures.
In Run 3, the damages are estimated to be
approximately 43% although this is for a very limited number of structures. Due to the low
number of structures impacted, Mecklenburg County may be in a position to control the
conditions seen in Run 3 through floodplain regulation. It is notable that structure damage
over 50% of market value is considered “substantial damage”14 by the Federal Insurance
and Mitigation Administration. Structures receiving over 50% estimated damage are likely
to be excellent acquisition candidates.
Table 3.18-4. Average Structure Damage Percent by HAZUS®MH Occupancy
Type: Lower Clarke Creek Watershed
14
According to NFIP guidelines, structures suffering “substantial damage” must be brought up to
current floodplain management standards for new (post-FIRM) construction (i.e., removed from the
flood hazard area or elevated to or above the level of the base flood elevation, or floodproofed if it
is nonresidential).
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HAZUS®MH
Occupancy
Code
RES1A
Description
Single Family Dwelling, 1 Story,
No Basement
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory, New
Floodplain
(Run 2)
0
Future
Inventory,
New
Floodplain
(Run 3)
42.9
Table 3.18-5 shows the dollar damage to building structures that is computed when the
individual building damage percentages are multiplied by the structure/parcel valuation.
As with the previous table, the differences in the dollar damage between the three
scenarios help identify the source of the damage. Between Run 1 and Run 2, no losses
are estimated and as the building inventories used for these scenarios are identical, the
difference in damage, or lack thereof, is attributable to differences in the floodplain
between the 1975 flood study and the 2000 revision of the 100-yr floodplain map.
Proceeding from Run 2 to Run 3, there is a increase in estimated damage from no
damage to $125,000, attributable to projected development in the floodplain. As indicated
in previous tables, the estimated damages are to a limited number of structures placed at
the centroid of the parcel. This clearly indicates that Mecklenburg County can seek ways
to avoid these losses through effective floodplain regulation.
Table 3.18-5. Structure Dollar Damage by HAZUS®MH Occupancy Type
Lower Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
Total
Current Inventory,
Current Inventory, New Future Inventory, New
Original Floodplain
Floodplain
floodplain
(Run 1)
(Run 2)
(Run 3)
Total
Avg.
Total
Avg.
Total
Avg.
$0
$0
$0
$0
$125,000
$62,500
$0
$0
$0
$0
$125,000
$62,500
Table 3.18-6 presents the overall average damage percentage calculated for building
contents by occupancy type. The estimated damage for contents will typically follow the
same general trend as seen in Table 3.18-4. Run 1 and Run 2, as expected, show no
estimated damage. The estimated damages for Run 3 are approximately 53%, but again
this value is for a small number of new structures.
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Table 3.18-6. Contents Damage Percent by HAZUS®MH Occupancy Type
Lower Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
Description
Single Family Dwelling, 1
Story, No Basement
Current
Inventory,
Original
Floodplain
(Run 1)
0
Current
Inventory,
New
Floodplain
(Run 2)
0
Future
Inventory, New
Floodplain
(Run 3)
53.2
Table 3.18-7 provides the estimated dollar damage to building contents by occupancy
type.
Similar to the structure dollar damage shown in Table 3.18-5, no losses were
estimated for Run 1 and Run 2 indicating that the changes in the floodplain depth and
extent between the 1975 flood study and the recent 2000 revision of the 100-yr floodplain
map do not impact any structures. Between Run 2 to Run 3, there is an increase in
estimated damage from no damage to $77 thousand attributable to projected growth.
Table 3.18-7. Contents Dollar Damage by HAZUS®MH Occupancy Type
Lower Clarke Creek Watershed
HAZUS®MH
Occupancy
Code
RES1A
Total
Current Inventory,
Current Inventory, New
Current Floodplain
Floodplain
(Run 1)
(Run 2)
Total
Avg.
Total
Avg.
$0
$0
$0
$0
$0
$0
$0
3-285
$0
Future Inventory, New
Floodplain
(Run 3)
Total
Avg.
$77,407
$38,704
$77,407
$38,704
3.19 Beaverdam Creek Analysis
3.19.1 Results Summary for the Beaverdam Basin
The analysis for this watershed has not been performed. This page is intentionally left
blank as a placeholder for the analysis results at the County’s discretion.
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3.20 Clear Creek Analysis
3.20.1 Results Summary for the Clear Creek Basin
The analysis for this watershed has not been performed. This page is intentionally left
blank as a placeholder for the analysis results at the County’s discretion.
3-287
3.21 Gar Creek Analysis
3.21.1 Results Summary for the Gar Creek Basin
The analysis for this watershed has not been performed. This page is intentionally left
blank as a placeholder for the analysis results at the County’s discretion.
3-288
3.22 Paw Creek Analysis
3.22.1 Results Summary for the Paw Creek Basin
The analysis for this watershed has not been performed. This page is intentionally left
blank as a placeholder for the analysis results at the County’s discretion.
3-289
3.23 Steele Creek Analysis
3.23.1 Results Summary for the Steele Creek Basin
The analysis for this watershed has not been performed. This page is intentionally left
blank as a placeholder for the analysis results at the County’s discretion.
3-290
4. Recommendations for Data Enhancements
During the course of this project, a variety of data was utilized and as such, several data
related issues arose. In most cases, assumptions were made and the project moved
forward. ABS feels that this project greatly enhanced our knowledge of the issues that
municipalities will likely encounter when attempting to use local data in the HAZUS®MH
Flood Model.
Some of the issues warrant further review and discussion by
Charlotte/Mecklenburg staff. To support the discussion within the County, this section of
the report will identify a variety of potential enhancements that can be made to the
County’s data that will enhance the utilization of the data within the County. As there were
so many data layers and databases used, the sections are simplified as much as possible
to prevent confusion. As a result, there may be some repetitive recommendations.
4.1
County Tax Assessor Data Enhancement
During the processing of the data for the analysis, one potential enhancement was quickly
identified. There are several parcels throughout the County that has had more than one
building or structure developed upon it. The Assessor data itself, however, does not
contain structure specific attributes. For example, there are some parcels with several
apartment buildings. The Assessor data, identifies the number of structures, but has a
single valuation and a single total floor area for each building. The ideal enhancement
would be to provide a unique identifier defined by the parcel number (column one) and the
building number (column two).
The two columns together uniquely identify a single
building and the record contains all attribute data for that distinct structure. This would
include the total floor area, the valuation, number of stories and other attributes currently
provided by the Assessor. The County Assessor could use the functionality of MS Access
or other relational database systems to sum the individual records for each parcel and
thereby produce the tax bill for the owner.
Another data enhancement would be to improve the definition of the building footing or
foundation type. This would require the County Assessor to work with building owners to
4-1
identify the foundation type when paying their taxes, or capture this information as the
building changes hands. This information will have great value when the County uses
HAZUS ®MH for future analyses.
4.2
Integration of Building Footprints and Parcel Data
Two components of the data provided by the County include the countywide parcel data
layer that could be joined to the Tax Assessor rolls and the building footprint coverage.
As noted earlier in this report, ABS performed an analysis that assigned the parcel
identification value to the building footprint data layer. It is recommended that the County
perform this effort again, but with a little more manual intervention and the addition of the
Building Id as discussed in Section 4.1 above. As noted above, this would allow GIS
operators to perform building specific analyses, using parameters unique to the structure
from the Assessor rolls.
This task is not entirely simple. There will be issues that the County will need to address,
such as those buildings that cross multiple parcels. How the building footprint and the
parcel numbers are assigned in this case will require some thought. As noted in Section
4.3, this may require revisions to the Parcel coverage. In other cases, the County may
need to determine if there is a limit on the size of the structure to be included in this
process. ABS eliminated all footprints less than 500 square foot in size. The County may
wish to do something similar, or it may want to carry every structure within the data
coverage.
4.3
Merging Survey Data and Assignment Data to Building Footprints
Similar to the assignment of building footprints and the parcel data, there are opportunities
to enhance future analysis efforts by reviewing and assigning the various survey data
tables to the building footprint database and data layer. Prior to performing this effort, it is
recommended that the County build a master survey database that merges the four
datasets used in this analysis together. Duplicate data can be eliminated and the most
current and accurate data can be preserved.
4-2
With a master database, the County could then assign the unique identifier for the survey
data point to the building footprint it represents. This would greatly enhance the Assessor
data already joined by parcel and building Id by providing the flood specific elements such
as first floor elevation and lowest adjacent grade.
4.4 Revisit Parcel Boundary Inconsistencies
As seen in Figure 4-1 below, there are some parcels within the County’s parcel boundary
data layer where the building footprints seem to have been removed from the a parcel
layer, and the Assessor data had no associated data.
The digital orthophotography
supplied by the County was used to aid in the review of these areas. In essence, the
parcel level data were no longer attached to the parcel boundary, rather the attributes
were attached to smaller footprint-sized polygons. In this analysis it created the potential
of an occupied parcel being identified as vacant. Additionally, the survey data did not
always include every structure on a given parcel – thereby reducing the potential for
accurately capturing the total exposure. For illustration, Figure 4-1 shows a highlighted
parcel that has several condominium buildings within its boundary. Some buildings have
apparently been cropped out of the parcel boundaries and the associated assessor data
assigned to each footprint. Unfortunately, the highlighted area is indicated as a vacant
parcel in the Tax Assessor data. For the McAlpine Creek study, the parcel was left as
vacant and it was assumed that additional residential construction occurred in the future.
This issues raises the potential to either underestimate the existing exposure or the
potential to overestimate future development.
4-3
Figure 4-1. ArcView Image of Parcel Data and Survey Points
4.5
Building Valuation
For the 305 surveyed structures that intersect a parcel polygon, a comparison of the
building valuation with the assessor data showed that the two data sources were at times
significantly different. As such, the assessor data were applied for all building valuations.
Although this potential valuation problem was avoided for the damage estimation, the data
inconsistency may be of interest to Mecklenburg staff.
4-4
A. Reference Tables
Table A-1. Mapping of Mecklenburg Use Codes into HAZUS®MH Occupancy
Codes
Meck.
Use
Mecklenburg Code
Code
Description
01
SINGLE FAMILY
RESIDENTIAL
02
MANUFACTURED
HOME
03
MOBILE HOME
04
CONDO
05
PATIO HOME
06
CONDO HIGH RISE
07
SINGLE FAMILY
RESORT
08
--09
TOWNHOUSE
10
COMMERCIAL
11
12
13
14
15
16
17
18
19
20
21
22
Source of
HAZUS®MH
Applied
Occupancy HAZUS®MH Occupancy
Structure
Code
Code Description
Damage Curve
RES1
Single Family Dwelling
FIA
RES2
Mobile Home
FIA
RES2
RES3
RES1
RES3
RES1
Mobile Home
Multi-Family Dwelling
Single Family Dwelling
Multi-Family Dwelling
Single Family Dwelling
FIA
Galveston
FIA
Galveston
FIA
----RES3
Multi-Family Dwelling
COM1/COM4 Retail Trade and
Business/Professional/
Technical Services
CONVENIENCE STORE COM1
Retail Trade
CAR WASH
COM3
Personal and Repair
Services
DEPARTMENT STORE COM1
Retail Trade
SUPERMARKET
COM1
Retail Trade
SHOPPING CENTERCOM1
Retail Trade
MALL
SHOPPING CENTERCOM1
Retail Trade
STRIP
OFFICE
COM4
Business/Professional/
Technical Services
OFFICE HIGH RISE
COM4
Business/Professional/
Technical Services
MEDICAL BUILDING
COM7
Medical Office/Clinic
MEDICAL CONDO
COM4
Business/Professional/
Technical Services
RESTAURANTS
COM8
Entertainment &
Recreation
FAST FOODS
COM8
Entertainment &
A-1
--Galveston
Wilmington
Wilmington
Galveston
Wilmington
Wilmington
Wilmington
Wilmington
Wilmington
Wilmington
Wilmington
Wilmington
Wilmington
Galveston
23
24
BANKS
OFFICE CONDO
25
COMMERCIAL/SERVICE COM4
26
27
28
29
30
31
SERVICE STATION
AUTO SALES/SERVICE
PARKING GARAGE
MINIWAREHOUSE
LAB/RESEARCH
DAY CARE
COM1
COM1
COM10
COM2
IND1
COM3
32
33
THEATER
LOUNGE/NIGHTCLUB
COM9
COM8
34
BOWLING ALLEY
COM8
35
COMMERCIAL CONDO
COM4
37
HOTEL/MOTEL HIGH
RISE
FURNITURE
SHOWROOM
HOTEL/MOTEL
INDUSTRIAL
RES4
Source of
Applied
HAZUS®MH Occupancy
Structure
Code Description
Damage Curve
Recreation
Depository Institutions
Wilmington
Business/Professional/
Wilmington
Technical Services
Business/Professional/
Wilmington
Technical Services
Retail Trade
Wilmington
Retail Trade
Wilmington
Parking
Wilmington
Wholesale Trade
Wilmington
Heavy Industrial
Galveston
Personal and Repair
Galveston
Services
Theaters
Wilmington
Entertainment &
Wilmington
Recreation
Entertainment &
Wilmington
Recreation
Business/Professional/
Wilmington
Technical Services
Temporary Lodging
Wilmington
COM1
Retail Trade
Wilmington
RES4
IND2/IND6
Wilmington
Galveston
LIGHT
MANUFACTURING
HEAVY
MANUFACTURING
LUMBERYARD
PACKING PLANT/FOOD
PREP
BOTTLER/BREWERY
WAREHOUSE CONDO
WAREHOUSE
PREFAB WAREHOUSE
RURAL HOMESITE
COLD
STORAGE/FREEZER
TRUCK TERMINAL
SERVICE GARAGE
IND2
Temporary Lodging
Light Industrial &
Construction
Light Industrial
Galveston
IND1
Heavy Industrial
Galveston
COM2
IND3
Wholesale Trade
Foods/Chemicals/Drugs
Wilmington
Galveston
IND3
COM2
COM2
COM2
RES1
COM2
Foods/Chemicals/Drugs
Wholesale Trade
Wholesale Trade
Wholesale Trade
Single Family Dwelling
Wholesale Trade
Galveston
Wilmington
Wilmington
Wilmington
FIA
Galveston
COM2
COM3
Wholesale Trade
Personal and Repair
Services
Galveston
Galveston
Meck.
Use
Code
38
39
40
41
42
43
44
46
47
48
49
50
51
52
53
Mecklenburg Code
Description
HAZUS®MH
Occupancy
Code
COM5
COM4
A-2
Meck.
Use
Code
54
55
56
60
61
HAZUS®MH
Occupancy
Code
COM2
----RES3
RES3
Source of
Applied
HAZUS®MH Occupancy
Structure
Code Description
Damage Curve
Wholesale Trade
Wilmington
--------Multi-Family Dwelling
Wilmington
Multi-Family Dwelling
Wilmington
RES3
RES3
Multi-Family Dwelling
Multi-Family Dwelling
Galveston
Wilmington
----REL1
----Wilmington
Wilmington
64
69
70
Mecklenburg Code
Description
OFFICE/WAREHOUSE
----GARDEN APARTMENT
TOWNHOUSE
APARTMENT
DUPLEX/TRIPLEX
HIGH-RISE
APARTMENT
----INSTITUTIONAL
71
CHURCH
REL1
72
EDU2
COM6
RES5
RES1
Hospital
Institutional Dormitory
Single Family Dwelling
Galveston
Wilmington
Wilmington
COM3
78
COUNTRY CLUB
COM8
79
AIRPORT
COM4
Personal and Repair
Services
Entertainment &
Recreation
Entertainment &
Recreation
Business/Professional/
Technical Services
Wilmington
77
SCHOOL:COLLEGEPRIVATE
HOSPITAL-PRIVATE
ORPHANAGE
SINGLE FAMILY
RESIDENTIAL
MORTUARYCEMETERY
CLUB-LODGE
----Church/Membership
Organizations
Church/Membership
Organizations
Colleges/Universities
82
83
84
86
87
88
89
91
FOREST-PARK
PUBLIC SCHOOL
PUBLIC COLLEGE
OTHER COUNTY
OTHER STATE
OTHER FEDERAL
OTHER MUNICIPAL
UTILITY
EDU1
EDU2
GOV1
GOV1
GOV1
GOV1
COM4
Wilmington
Wilmington
Galveston
Wilmington
Wilmington
Galveston
Galveston
93
94
96
98
99
PETRO-GAS
BLANK
BLANK
VALUE-IMPR
NEW PARCEL
COM1
---------
Schools/Libraries
Colleges/Universities
General Government
General Government
General Government
General Government
Business/Professional/
Technical Services
Retail Trade
---------
62
63
73
74
75
76
COM8
A-3
Wilmington
Wilmington
Galveston
Galveston
Wilmington
---------
Table A-2. Estimated Typical Floor Area and Cost per Square Foot by
Mecklenburg Occupancy Code
County Assessor
Occupancy Code
01
02
04
05
06
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
37
38
39
40
41
42
43
47
48
49
Estimated Typical
Floor Area from
Assessor’s Data (ft2)
2,290
1,450
1,110
1,300
3,860
1,320
9,970
2,070
1,800
91.980
42,240
348,370
27,930
19,750
256,180
15,790
3,270
6,620
2,960
6,480
1,120
87,750
2,560
25,830
164,720
6,230
78,220
8,130
51,080
8,660
59,040
1,050
129,800
24,970
24,220
47,570
59,300
17,470
5,600
3,650
31,800
14,470
A-4
HAZUS®MH Cost
Models for Valuation
($/ft2)
63.59
26.38
57.24
54.73
90.19
54.18
41.90
82.68
37.26
37.67
43.57
93.10
39.15
61.26
82.61
76.63
76.26
57.68
80.95
114.01
67.73
34.47
16.01
25.30
18.32
15.19
77.82
57.82
70.11
33.21
24.03
41.77
61.28
38.30
42.96
27.67
32.44
48.76
6.84
28.70
25.70
18.93
County Assessor
Occupancy Code
51
52
53
54
55
56
60
61
62
63
64
69
70
71
72
73
74
76
77
78
83
84
86
88
89
91
99
Estimated Typical
Floor Area from
Assessor’s Data (ft2)
37,520
45,510
8,790
36,680
1,927,640
290,830
11,970
6,890
2,340
44,990
45,410
2,480
7,260
9,690
15,080
43,780
31,210
8,560
4,520
11,420
29,130
30,970
30,080
24,430
17,010
970
4,760
HAZUS®MH Cost
Models for Valuation
($/ft2)
42.15
23.32
32.32
29.57
68.63
80.11
36.22
33.54
52.14
44.37
76.74
52.74
54.74
72.81
55.37
86.56
60.13
63.85
49.86
95.73
52.42
69.00
69.94
87.64
81.35
83.00
0.44
Note: See Table A-1 for occupancy code definitions and assignment to HAZUS
occupancy classifications
A-5
Table A-3. HAZUS®MH Content Value as a Percentage of Structure Value
HAZUS®MH Occupancy
RES1
RES2
RES3A
RES3B
RES3C
RES3D
RES3E
RES3F
RES4
RES5
RES6
COM1
COM2
COM3
COM4
COM5
COM6
COM7
COM8
COM9
COM10
IND1
IND2
IND3
IND4
IND5
IND6
AGR1
REL1
GOV1
GOV2
EDU1
EDU2
Contents Value (Percent of Structure
Value)
50
50
50
50
50
50
50
50
50
50
50
100
100
100
100
100
150
150
100
100
50
150
150
150
150
150
100
100
100
100
150
100
150
A-6
Table A-4. HAZUS®MH Foundation Distribution (default) and Adjusted
Foundation Distribution Used in Mecklenburg County
Foundation Type
HAZUS Default for
North Carolina
Mecklenburg
Adjusted
Foundation
Distribution Based
on Assessor Data
Solid Pier (or
Pile Wall
Post)
0
0
0
1
1
Basement (or
Garden Level) Crawlspace
23
35
5
20
A-7
30
Fill
0
Slab on
Grade
42
1
42