Floodplain Analysis and
Mapping Standards Guidance
Document
Floodplain Mapping Project
July 2008
Prepared for:
Charlotte-Mecklenburg Storm Water Services, Flood Mitigation Program
700 North Tryon Street
Charlotte, NC 28202
Prepared by:
Dewberry & Davis, Inc.
6135 Lakeview Road
Suite 400
Charlotte, NC 28269
Floodplain Analysis and Mapping Standards
Guidance Document
Floodplain Mapping Project
July 2008
TABLE OF CONTENTS
1.
Introduction
Overview of this Documents ...............................................................................................1
Mecklenburg County Floodplain Mapping ........................................................................1
2.
Field Data Collection Standards
Channel Cross Section Information .....................................................................................3
Estimating Field Data Collection of Cross Section Information ....................................3
Survey Cross Section Locations .....................................................................................4
Cross Section Survey Accuracy......................................................................................4
Stream Crossing Information ..............................................................................................5
Identification of Stream Crossings..................................................................................5
Verification of Existing Stream Crossing Information...................................................5
Field Survey Stream Crossings.......................................................................................6
LIDAR Specifications.....................................................................................................6
Aerial Orthophotography ................................................................................................6
3.
Hydrology Analysis Standards
Hydrologic Model ...............................................................................................................8
Storm Pattern ......................................................................................................................8
Precipitation Depths ............................................................................................................9
Land Use .............................................................................................................................9
Existing Land Use ........................................................................................................10
Right-of-Way ...............................................................................................................11
Future Land Use............................................................................................................11
Target Subbasin Size .........................................................................................................13
Time of Concentration .......................................................................................................13
Hydrologic Routing ..........................................................................................................14
Channel Routing ..........................................................................................................14
Storage Routing ............................................................................................................15
Balancing Models ..............................................................................................................16
Flow Change Points ...........................................................................................................16
4.
Hydraulic Analysis Standards
Hydraulic Model ................................................................................................................17
Starting Boundary Condition .............................................................................................17
Channel Bank Station Assignment ....................................................................................18
Cross Section Spacing........................................................................................................18
Cross Sections at Bridges/Culverts....................................................................................19
Bridge/Culvert Annotation.................................................................................................19
Contraction/Expansion Coefficient....................................................................................19
Roughness Coefficient .......................................................................................................20
Ineffective Flow Areas.......................................................................................................20
Modeling Buildings ...........................................................................................................21
Minor Obstructions ............................................................................................................21
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July 2008
Table of Contents i
Documentation of Input Data.............................................................................................22
Community Encroachment Area........................................................................................22
FEMA Floodway ...............................................................................................................22
5.
Model Calibration Standards
Hydrologic Calibration .....................................................................................................24
Hydraulic Calibration.........................................................................................................25
6.
Mapping Standards
Floodway Delineation around Buildings ..........................................................................27
Community Floodplain Boundary Delineations ................................................................27
Seamless Panels ................................................................................................................28
Letters of Map Change.......................................................................................................28
Flood Insurance Study .......................................................................................................28
FIRM Panel Format ...........................................................................................................29
GIS Concerns ..................................................................................................................31
Appendices
Appendix “A” - Project Memorandum Dated 08/28/07 (Revised 8/31/07) Survey
Comparison Findings Mecklenburg County Floodplain Mapping Project
Appendix “B” - Sample GPS Cross Section Points Calculation
Appendix “C”- Additional Information on Existing and Future Land Use
Appendix “D” - Project Memorandum Dated 10/10/07 (Revised 10/29/07) Comparison
and Sensitivity Analysis of Charlotte CIPs and FEMA FIS Mecklenburg County
Floodplain Mapping Project
Appendix “E” - Sample Data Collection for Stream Crossings
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July 2008
Table of Contents ii
1.
Introduction
Overview
The purpose of this document is to assure consistency among mapping consultants and
quality control when creating Flood Insurance Rate Maps (“FIRMs”) in Mecklenburg
County. This document is the primary technical analysis and floodplain mapping
guidance document to be used for floodplain mapping efforts associated with the
Charlotte-Mecklenburg Storm Water Services Floodplain Mapping Project. The
standards presented in this document were developed during the initial phase of the
Project in 2007. It is the intent that the standards described in this document are
consistent with guidelines specified in the Federal Emergency Management Agency
(FEMA) Guidelines and Specifications for Flood Hazard Mapping Partners (G&S) and
the Mapping Activity Statement (MAS) agreement between FEMA and Mecklenburg
County. However, it is anticipated that this document will need to be updated
periodically as updates may be made to the G&S and/or MAS, or as standards are
modified at the discretion of Mecklenburg County.
Floodplain Mapping in Mecklenburg County
Accurate and updated floodplain maps are important to public health and safety in our
community. Newer floodplain maps will work in tandem with regulations to:
•
•
•
•
Ensure safer construction for new or redeveloped buildings in/near floodplains
Communicate the flood risk to current and potential floodplain property owners
Determine where flood insurance is needed and properly rate the policies
Assist with planning and prioritizing flood mitigation projects, like acquisitions
In an effort to protect public health and safety, the current floodplain maps and
regulations focus on the current risk of flooding as well as the future risk. The floodplain
maps contain floodplain boundaries and information that estimates the likelihood of
flooding under existing conditions and estimates the likelihood of flooding in the future.
The purpose of providing information regarding the likelihood of increased flooding in
the future is to assist in making appropriate floodplain decisions today. The future
conditions floodplain information provided on the floodplain maps estimates the potential
long term flooding limits resulting from increased fill in the floodplain and increased
impervious area.
The County’s Flood Mitigation Program within Charlotte-Mecklenburg Storm Water
Services (hereinafter referred to as CMSWS), through inter-local agreements, administers
the National Flood Insurance Program (NFIP) throughout the County, which includes the
City of Charlotte, and the Towns of Cornelius, Davidson, Huntersville, Matthews, Mint
Hill, Pineville, and the unincorporated areas of Mecklenburg County. The floodplain
maps and floodplain regulations in Mecklenburg County have unique characteristics such
as the Community Encroachment Area and Future Conditions Floodplain. The floodplain
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mapping consultant must review the floodplain ordinances and storm water quality
ordinances for the County, City and Towns to assure that the maps meet the needs of
these jurisdictions. CMSWS maintains a progressive floodplain management program
and is a pioneer in flood map modernization efforts. CMSWS initiated an unprecedented
comprehensive effort to study and remap all of the FEMA-regulated floodplains in the
County using custom GIS-based tools.
CMSWS’ effective DFIRMs represented state-of-the-art technical and mapping
information when they were developed and still provide a greater level of detail than
found on FEMA mapping in most communities. However, the community is ever
changing and the information and analysis the DFIRMs are based on is becoming quickly
outdated. Mecklenburg County has experienced explosive growth in the last ten years
resulting in numerous developments in floodplain areas, as well as, upland areas. In
addition, there have been many projects (both private and CMSWS initiated) that impact
floodplain mapping through modifications to streams or floodplain corridors (e.g. stream
restoration, flood mitigation projects, stream crossings, etc.). In striving towards the goal
of providing accurate mapping in a cost-effective manner to the community, CMSWS
commenced a floodplain map maintenance initiative in 2007. The initiative generally
referred to as the Floodplain Mapping Project, will re-map a portion (anticipated to be 10
– 20%) of the FEMA floodplains on a recurring annual basis. This map maintenance
effort will provide a means to incorporate changes in the watersheds and streams, correct
errors and deficiencies in the existing maps, and incorporate improved mapping data and
methods.
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2. Field Data Collection Standards
This section provides guidance for assessing the amount of pre-modeling field data
collection that is recommended for a given floodplain mapping phase, and for
specifications for conducting field data collection. Discussion of field data collection
tasks are categorized by channel cross section information and stream crossing structure
information below.
Channel Cross Section Information
Channel cross section information refers to geometry and feature information that is
collected for natural stream channels at selected locations (i.e. cross sections). This
information is one of the key elements of hydraulic floodplain modeling as it is directly
used to assess the flood carrying capacity of a channel. For the purposes of the
Floodplain Mapping Project, field data collection of cross section information is
primarily used to supplement and/or verify information obtained from other sources such
as countywide Light Detection and Ranging (LiDAR), previous Effective hydraulic
models– it is not intended to be solely sufficient to support hydraulic modeling described
in later chapters (i.e., field data collection will only be collected at a portion (generally 30
– 50%) of the cross section locations necessary for hydraulic modeling). Guidelines for
field data collection associated with channel cross section information are summarized
below.
Estimating Field Data Collection of Cross Section Information
Evaluation of available data sources, data accuracy considerations, and data collection
cost considerations have been performed to develop a procedure for estimating field data
collection of cross section efforts for a given floodplain mapping phase. The evaluation
included a comparison of channel cross section information generated from field survey
versus LiDAR, which is included in Appendix A. Based on this evaluation, the field data
collection of cross section information can be estimated as:
Field Cross Section Survey:
# Field Data Cross Section Locations = Stream Length to be Modeled (feet) / 1,100
Example: If modeling three watersheds with 28 miles of streams, then the total target number of locations
to collect cross section information in the field would be approximately 134 as calculated below:
# locations = (28 miles * (5,280 feet/mile)) / 1100 = 134.4
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Survey Cross Section Locations
Though the overall average cross section field data collection spacing is one cross section
every 1,100 feet, the actual locations for field data collection should be determined by the
mapping consultant and CMSWS. The evaluation of cross section information sources
indicated that LiDAR is reasonably accurate within the channel overbanks for wider,
open streams, with relatively shallow base flow. Whereas, it is generally less accurate for
narrower streams obscured by vegetation or with deeper base flow. From this, it is
recommended that field data collection efforts be more concentrated on the narrower
streams and less concentrated on the wider streams. All other field data collection
specifications (e.g. survey codes, photograph requirements, naming conventions, etc.)
should be collected in accordance with the FEMA G&S (primarily Appendix N).
Survey Cross Section Locations:
CMSWS must approve cross section locations prior to survey
Cross Section Survey Accuracy
In an effort to optimize the balance of the amount of data collected, accuracy, and cost
effectiveness of field data collection efforts, two variations of channel cross section
survey techniques are to be utilized – full survey-grade channel survey, and a “relative
GPS” channel survey. The full survey-grade channel survey is the more traditional
approach in which field measurements are tied to a survey-grade reference, resulting in
precise horizontal and vertical coordinates for all channel measurements (i.e. surveygrade X, Y, and Z). The relative GPS channel survey involves the same (or very similar)
channel measurements; however, the measurements are tied to a non survey-grade
reference. The horizontal reference is generally to be taken from a mapping-grade GPS
position. The vertical reference is generally to be taken from Countywide LiDAR
information. Other horizontal and vertical reference sources that result in equal to or
greater accuracy levels may be used if the aforementioned sources are not feasible or
available. For both methods, the reference point location should be off the top of bank
(approximately 50’ where feasible). The coordinates and elevation of the remaining
cross section points are determined from offset distances and level readings relative to the
reference point. Appendix B provides an example of how raw relative GPS cross section
measurements are translated to X, Y, and Z coordinates.
It is anticipated that the total number of channel cross section surveys (i.e. stream length /
1,100) should be interspersed and roughly equally split (i.e. 50% / 50%) between full
survey-grade channel survey and relative GPS channel survey techniques. This will
result in cross sections for modeling placed 1/500’ on average. However, the mapping
consultant shall coordinate with CMSWS to determine the appropriate split and final
location for each type of cross section survey.
Cross Section Survey Accuracy:
1) 50% full detailed survey
2) 50% relative GPS survey
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Stream Crossing Information
Stream crossing information refers to geometry and feature information that is collected
for culvert, bridge, or dam/weir structures that cross a modeled stream. Similar to natural
cross sections, this information is a key element of hydraulic floodplain modeling as
structures can have a significant impact on flooding conditions in a stream. For the
purposes of the Mecklenburg County Floodplain Mapping Project, field data collection of
stream crossing information is primarily used to supplement and/or verify information
previously collected and used in the previous Effective studies. However, new field
survey will be necessary for newer structure crossings that were not in the previous
Effective studies and/or that have been significantly modified from that as depicted in the
previous Effective studies. A 3-step process should be used to identify and locate stream
crossings, verify existing information, and collect new stream crossing information. The
intended end result of the stream crossing information collection process is to have
accurate spatial, schematic, and tabular information for all modeled stream crossings
organized in a consistent format.
Identification of Stream Crossings
Stream crossings should be identified and located by reviewing the most recent aerial
orthophotography in conjunction with Effective HEC-RAS models. The aerial
photography must be less than two years old. The mapping consultant shall
add/incorporate the spatial location (approximate centroid of where the structure crosses
the stream) and related information about each crossing into the CMSWS’ master stream
crossing GIS file and summary table (to be provided by CMSWS), such that all known
stream crossings should have a spatial point location and basic descriptive information.
Crossings that are identified on the aerial photography and are in the Effective HEC-RAS
models should be verified as described directly below. Crossings that are identified, but
not in Effective HEC-RAS models shall be flagged for new field survey (described
following verification procedures).
Georeference Stream Crossings:
Provide a spatial point location for all bridges, culverts and dams/weirs in GIS file
Verification of Existing Stream Crossing Information
Stream crossings identified in the Effective HEC-RAS models should be verified as
indicated below:
•
In-Office Verification: Review stream crossing sketches and photographs (from
Effective survey books) with Effective HEC-RAS models (and other pertinent
information as available) to verify/confirm consistency between survey data and
model data for bridges and structures. Flag crossings that appear to be
inconsistent for field verification and/or full survey. Crossings that appear to be
consistent should be recorded accordingly in CMSWS’ master stream crossing
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•
information. Information on these crossings in the current floodplain mapping
phase will be obtained primarily from the Effective information.
Field Verification: Field verify basic geometry/dimension (e.g. size and shape)
and material for all remaining culvert/pipe or weir crossings, and any bridges
flagged in previous step. Collect new photographs at each structure (4 total looking upstream, looking downstream, upstream face, downstream face) and
modify Effective sketch information (or create new if changes are significant).
Flag structures that have been significantly modified from that depicted in the
Effective information for full field survey. Crossings that appear to be consistent
should be recorded accordingly in CMSWS’ master stream crossing information.
Information on these crossings in the current floodplain mapping phase will be
obtained primarily from the Effective information.
Confirm Consistency for Stream Crossings:
Update stream crossing database to confirm consistency
Field Survey of Stream Crossings
Information for any stream crossings flagged in the previous steps (i.e., crossings where
Effective information is not available or it is not accurate) should be collected in the field
in accordance with FEMA G&S (primarily Appendix N in the G&S). New stream
crossing information shall be incorporated in CMSWS’ master stream crossing database.
Information on these crossings in the current floodplain mapping phase will be obtained
directly from the new survey information.
Stream Crossing Database:
Update Stream Crossing Database to include all information
LiDAR Specifications
Countywide LiDAR provided by CMSWS should be used to assist in field cross section
surveys and field verification, and to supplement field survey for floodplain analysis and
mapping. LiDAR data should meet or exceed specifications set forth by the North
Carolina Floodplain Mapping Program (NCFMP) and be no more than two years old at
the beginning of a mapping project.
LiDAR:
Meet or exceed NCFMP standards and be no more than two years old
Aerial Orthophotography
Countywide aerial photography provided by CMSWS should be used to support field
verification and floodplain analysis and mapping. The aerial photography shall be no
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more than 2 years old at the beginning of a mapping project and meet the standard
County specifications (true color, ½ foot uncompressed pixel resolution).
Aerial Photography:
Meet or exceed County standards and be no more than two years old.
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3. Hydrology Analysis Standards
This section provides guidance and standards for selected hydrologic parameters and
analyses associated with floodplain modeling. Hydrologic parameters and analyses not
discussed below are left to the discretion of CMSWS and the mapping consultant.
Hydrologic Model
The use of rainfall-runoff models is the preferred method to estimate the peak discharge
for the various storm events. The current Effective FIRMs and FIS were created using
HEC-1 to perform the computations. The Floodplain Mapping Project will employee the
latest version of the HEC-HMS program accepted by FEMA to compute the necessary
peak discharges.
Hydrologic Model:
The latest version of the Army Corp of Engineers’ HEC-HMS program is the
preferred method for performing the rainfall-runoff calculations
Storm Pattern
Two types of storm patterns commonly used in the Charlotte-Mecklenburg area are the 6hour “balanced” storm (often used for City of Charlotte Capital Improvement Projects
(CIPs) and new development design) and the 24-hour Soil Conversation Service (SCS)
Type II distribution (used with the FEMA Effective Flood Insurance Study (FIS)). The
two storm patterns were compared and evaluated in context with City, County, and
FEMA objectives and guidelines.
The evaluation indicates the 24-hour SCS Type II storm distribution is more appropriate
for the Floodplain Mapping Project for the following reasons:
•
•
Per section C.1.1.3 of FEMA G&S (Appendix-C: Guidance for Riverine Flooding
Analyses and Mapping), rainfall duration must exceed the time of concentration
of the watershed and must be large enough to capture all the excess rainfall as
well as provide reasonable runoff and sediment volumes when performing storage
analyses. Sensitivity analysis indicates that the time of concentration for some of
the larger County watersheds (e.g. Little Sugar-Briar, McAlpine, etc.) is greater
than 6 hours.
Similarly, section C.1.1.3 of FEMA G&S document indicates that the design
storm producing the highest flood discharge/water-surface elevation for the
flooding source should be used. Sensitivity analyses indicate the 24-hour SCS
Type II storm typically generates higher peak flows than the 6-hour storm.
Storm Pattern:
24-Hour Duration with SCS Type II Distribution, a minimum 72-hour storm
simulation with a 1 minute time step
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Precipitation Depths
Intensity-Duration-Frequency (IDF) information presented in the Charlotte-Mecklenburg
Storm Water Design Manual (CMSWDM) (dated 1993) specify precipitation depths to be
used for the various design storm events (e.g. 2- through 100-year storms) and patterns.
The rainfall depths presented in CMSWDM were compared with results of a recent
United States Geological Survey (USGS) precipitation study (SIR 2006-5017) prepared
in 2006. The USGS study developed several independent families of IDF curves based
on different precipitation gage networks and data samples.
Based on a comparison and evaluation of precipitation depth sources and
recommendations in the USGS publication, it was deemed that the 24-hour precipitation
depths from the combined “NOAA dataset plus aggregated USGS site representing the
CRN initial dataset” family with no area reduction factors (presented in Table 1),
hereafter referred to as the “combined” dataset, should be used for the Floodplain
Mapping Project.
Table 1. Precipitation Depths for the
Floodplain Mapping Project
Storm Event
2-year
10-year
25-year
50-year
100-year
500-year
Precipitation Depth
(inches)
3.06
4.80
5.76
6.51
7.29
9.23
NOTES:
1. Precipitation values taken from combined "NOAA dataset plus
aggregated USGS site" IDF presented in SIR 2006-5017
The USGS combined precipitation depths are slightly higher in the 100-year storm, but
equal to or slightly lower in the smaller (higher frequency) storms, than those presented
in the CMSWDM for a 24-hour storm duration.
Precipitation Depths:
24-Hour Precipitation Depths from the Combined “NOAA Dataset Plus
Aggregated USGS Site Representing the CRN Initial Dataset” Family with No
Aerial Reduction factors
Land Use
Land use is often used in floodplain analysis as an indirect indicator of the percent
imperviousness of a watershed, which has a significant effect on subsequent surface
runoff and associated hydrologic peak flow calculations. The Effective Flood Insurance
Rate Maps (FIRMs) include floodplain mapping based on both existing and future land
use conditions. Information for obtaining/developing existing and future land use for the
Floodplain Mapping Project is described below. The existing and future land use layers
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will be used with land use-soil type lookup tables (approved and/or provided by
CMSWS) to develop curve number calculations for hydrologic modeling.
Existing Land Use
The existing land use layer developed by the CMSWS, City Administrative/GIS Services
shall be used as the “basis” for the existing land use on the Project. This layer is
regularly maintained from aerial photography and development plans, and believed to be
the best available data at the countywide scale. See Appendix C for additional
information and history of the existing land use.
Land Use Categories
The existing land use coverage shall be reduced to 12 categories for use in the hydrologic
analysis. The twelve categories are as follows:
Table 2: Land Use Categories
•
•
•
•
•
•
•
•
•
•
•
•
1 - woods/brush
2 - open space (golf courses, mining, fields), > 2 acres residential
3 - > 1/2 to 2 acres residential
4 - 1/4 to 1/2 acre residential
5 - < 1/4 residential, townhouses
6 - institutional; schools, hospitals, government offices
7- light industrial
8 - heavy industrial
9 - light commercial - office parks, hotels, dense residential (> 7 dwelling units
per acre)
10 - heavy commercial - car parks, malls, retail
11 - water bodies, usually ponds greater than 2 ac in size
12 - transportation, multilane roads, interstates
Land Use Categories for Hydrologic Analysis:
The twelve categories listed in Table 2 shall be used to estimate curve numbers.
Verification of Existing Conditions Land Use
Although the existing conditions land use layer is believed to be the best available data,
additional verification and modification to the base information is necessary. The
mapping contractor will overlay the land use coverage containing the twelve categories
over the most recent aerial photographs to verify that the land use category contained in
the GIS data is appropriate. The mapping contractor will perform a comparison of the
land use coverage against the most recent aerial photographs to identify discrepancies.
The mapping contractor will bring these discrepancies to the attention of CMSWS.
CMSWS will resolve issues with the land use coverage. A detailed description of the
land use coverage verification process is contained in Appendix C.
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Verification of Existing Land Use Coverage:
Verify accuracy of land use coverage using most recent aerial photographs.
Report discrepancies to CMSWS.
Treatment of Street Right-of-Way
Estimating runoff from the street right-of-way has the potential for variation between
mapping consultants. If a right-of-way is included in the existing land use coverage but
not the future land use coverage, it is difficult to determine the effect of the right-of-way
on the results. For consistency, the floodplain mapping consultant shall create a GIS
coverage showing the street right-of-way under existing conditions given the Land Use
Code 12 – transportation. This GIS coverage shall be duplicated in the future conditions
land use coverage and used to estimate future conditions curve numbers.
Treatment of Street Right-of-Way:
Create GIS coverage for street right-of-way under existing conditions and
duplicate for future conditions
Creation of Final Existing Conditions Land Use Coverage for Modeling
After consultation with CMSWS to resolve any discrepancies between the GIS coverage
and the most recent aerial photography, the mapping contractor shall create the final GIS
coverage for estimating curve numbers. See Appendix C for step by step processing and
conditioning of the existing land use. The final coverage as well as the intermediate
versions of the coverage shall be delivered to CMSWS.
Creation of Final Existing Conditions Land Use Coverage for Modeling:
Mapping consultant shall deliver final and intermediate versions of the existing
conditions land use coverage to CMSWS
Future Land Use
The future land use layer developed by the Charlotte-Mecklenburg Planning Department
is to be used as the “basis” for the future land use. This layer is regularly maintained
from zoning cases and district/area plans, and is believed to be the best available data at
large scale. Future land use categories vary and can be up to ninety (90) groups (not
listed for clarity). See Appendix C for additional background, history and categories of
the future land use. Unlike the existing land use data which is countywide, the future
land use data only covers the City ETJ. As with the existing land use, additional
verification and modification to the base information is necessary to ensure
appropriateness for floodplain mapping and comparability with the existing land use.
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Translation of Future Land Use Categories
The land use categories of the base future land use layer should be condensed into the
same twelve categories as used in the existing land use coverage, using the translation
table provided in Appendix C.
Land Use Categories for Hydrologic Analysis:
Translate Planning Commission future conditions land use categories into the 12
land use categories from existing conditions using translation table contained in
Appendix C.
Verification of Future Conditions Land Use
As with the existing conditions land use layer, additional verification and modification to
the base information is necessary. As indicated previously, the future land use layer only
covers the City of Charlotte ETJ. Thus, future land use for any watershed areas that
extend beyond the ETJ will have to be developed from other sources. Potential data
sources include existing land use coverage, the 2015 land use file that was used for the
current Effective mapping, aerial photographs, City/County zoning layers, land use
information from other municipalities in the Mecklenburg County. The mapping
contractor shall coordinate with CMSWS to assess the most appropriate data source for
developing future land use beyond the City of Charlotte ETJ. The mapping contractor
shall also verify and modify the overall future land use layer (for both areas inside and
outside of the City ETJ) to ensure its appropriateness for floodplain mapping. The future
conditions land use within the floodplain must be revised. The current Planning
Commission future land use layer depicts all these areas as greenways/open space. These
areas should be evaluated in context with other data sources (e.g. existing land use,
zoning, etc.) and modified as necessary, to ensure that the classifications are reasonable
for floodplain analysis. In general, the existing land use conditions within the Community
Floodplain Boundary shall be duplicated in the future conditions land use coverage.
Verification of Future Land Use Coverage:
Develop future land use coverage for areas outside City ETJ. Verify accuracy of
future land use coverage using existing conditions land use coverage, aerials, and
zoning to determine appropriate land use category. Report discrepancies to
CMSWS
Creation of Final Future Conditions Land Use Coverage for Modeling
After consultation with CMSWS to resolve any discrepancies between the GIS coverage
and the most recent aerial photography, the mapping contractor shall create existing and
future land use coverages for presentation to a watershed specific Task Force. The
Floodplain Mapping Project includes a watershed specific Task Force for each remapping
zone. The Task Force will be composed of volunteers that owned property in the
remapping zone and one member from the Storm Water Advisory Committee who will
act as the chairperson. The purpose of the Task Force is to review the existing and future
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land use coverages created by the mapping consultant and recommend appropriate
changes to the land use information. The Task Force will reach consensus on the existing
and future land use information in the form of final land use maps. The mapping
consultant will use these maps to estimate curve numbers for the hydrologic analysis.
Refer to Appendix C for a detailed description of the construction of the final GIS
coverage. The final coverage as well as the intermediate versions of the coverage shall be
delivered to CMSWS.
Creation of Final Future Conditions Land Use Coverage for Modeling:
Mapping Consultant shall deliver final and intermediate digital and hardcopy
versions of the future conditions land use coverage to CMSWS
Target Subbasin Size
Subbasins are areas that are used to sub-divide a watershed area into smaller hydrologic
components to reflect more localized hydrologic patterns and ultimately to improve the
accuracy of peak flows calculated along the main streams to be modeled. Section C.1.1.3
of the FEMA G&S states that “when the unit hydrograph method is used in developing
hydrographs, subwatershed drainage areas shall be appropriately defined within the limit
that the unit hydrograph is able to reflect watershed response to changing conditions”.
The current Effective FEMA hydrologic models are based on “larger scale” subbasins
with a typical size between 150 - 200 acres. The use of large sub-watershed areas
generally meet the criteria under Section C.1.1.3 however these “larger scale” basins do
not capture pertinent information/features useful for “smaller scale” comparison and City
CIPs.
Sensitivity analysis and evaluation of subbasin size was conducted to assess the
appropriateness of using smaller subbasin sizes for the Floodplain Mapping Project.
Based on the results of the analyses and evaluation, it was deemed that a target 60 acre
subbasin size should be used for the Project. The sensitivity analysis is included in
Appendix D. It is believed that this additional target size captures additional hydrologic
detail that will be useful for smaller scale comparisons and initiatives and provide
reasonable/consistent results, while not excessively increasing the burden of additional
hydrologic processing and computations.
Target Subbasin Size:
60 acres
Time of Concentration
One of the most variable hydrologic parameters is the computation of time of
concentration. In an effort to standardize the time of computation for floodplain mapping
purposes, the floodplain mapping consultant shall use the method described in Chapter 3,
Urban Hydrology for Small Watersheds (Technical Release 55), Natural Resource
Conservation Service (1986). In this method the time of concentration is computed using
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sheet flow, shallow concentrated flow and channel flow. The flow length for sheet flow
must be a maximum of 100 feet in urban areas and 300 feet in rural areas.
Time of Concentration:
Method described in Chapter 3- Urban Hydrology for Small Watersheds
Hydrologic Routing
Hydrologic routing is often used in floodplain analysis to account for peak flow
attenuation that occurs as a flood wave travels through a natural drainage system. In the
context of the Floodplain Mapping Project, routing specifically refers to routing of a
flood wave through an open channel system (including culverts and bridges) (i.e.,
channel routing) or through storage areas such as off-line ponds or areas behind large
embankments (i.e. storage routing).
Channel Routing
Channel routing occurs in floodplain areas where peak flows are attenuated as a result of
water expanding and slowing down upon entering the floodplain. Channel routing is
often accounted for through the use of hydrologic routing reaches, which route a
hydrograph through a channel or pipe from one subbasin outlet to the next downstream
subbasin outlet.
Channel Routing – Streams with HEC-RAS Modeling
The Modified Puls method shall be used for channel routing along reaches with a HECRAS model. The HEC-RAS model shall be used to develop the required Modified Puls
storage-outflow parameters used in the hydrologic model. As stated in the HEC-HMS
documentation, “It affects attenuation where one sub-reach gives maximum attenuation and
increasing the number of subreaches approaches zero attenuation. This parameter is necessary
because the travel time through a subreach should be approximately equal to the simulation time
step. A good estimate is to divide the actual reach length by the product of the wave celerity and
the simulation time step. It can also be a calibration parameter in some cases.” The consultant
shall follow the HEC-HMS guidance to set the initial value of sub-reaches.
Steady State HEC-RAS Modeling Channel Routing:
Modified Puls Method.
Channel Routing - No HEC-RAS Modeling
The lag method or Muskingum-Cunge method shall be used for channel routing along
routing reaches where no HEC-RAS modeling is being conducted (e.g. routing reaches
upstream of the streams to be modeled/mapped). The mapping contractor should attempt
to use a single method if possible; however, the method selected should be modified and
finalized as appropriate during model calibration (described in later sections).
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Streams with No HEC-RAS Modeling:
Lag Method or Muskingum-Cunge Method
Storage Routing
Storage routing occurs when flow is impeded by a dam, embankment, or other feature
and the structure (e.g. culvert pipe, weir, etc.) that allows water to pass through the
feature does not have the capacity to pass the full natural flow. This causes the water to
be “backed up” and stored behind the feature, thus reducing the peak flow. Storage
routing is often accounted for through the use of storage elements, which route an input
hydrograph through an outlet structure (e.g. spillway).
Storage Routing - Streams with HEC-RAS Modeling
Storage routing is incorporated in the Modified Puls method described for channel
routing above, therefore it should not be performed along reaches where steady-state
HEC-RAS model is being conducted, except for in-line ponds and locations with
excessive backwater conditions (e.g. where backwater impacts a significant portion of the
routing reach, upstream tributaries and/or other routing reaches).
Level pool storage routings are appropriate where the hydrograph is attenuated
significantly. Significant attenuation is a routing where the attenuation exceeds 10
percent (the outflow peak discharge is more than 10 percent less than the inflow
hydrograph peak discharge). Typically, 10 percent attenuation can occur when the
floodplain restriction (culvert, bridge, etc.) fill height exceeds the opening height by a
factor larger than two. Fill height to opening height ratios of three almost always result
in more than 10 percent attenuation. Therefore, the consultant shall include storage
routing requirement for restrictions with fill height to opening height ratios larger then
two. This requirement will be relaxed if modeling indicates that the attenuation at these
locations is less than 10 percent. No other storage routing should be performed unless
deemed appropriate during calibration.
Storage Routing Along Streams with HEC-RAS Modeling:
None, except for in-line ponds and areas with excessive backwater conditions.
Level-Pool routing for areas with excessive backwater conditions.
Storage Routing - Streams with no HEC-RAS Modeling
The level-pool method should be used for all in-line ponds that have a drainage area
greater than 100 acres where the pond is covered by a permanent maintenance easement
granted to the City of Charlotte, Mecklenburg County or one of the Towns along reaches
with no HEC-RAS modeling.
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Storage Routing Along Streams with no HEC-RAS Modeling:
All in-line ponds and off-line ponds with a drainage area greater than 100 acres
All culverts/bridges with fill height to open ratio greater than 2
As indicated above, certain hydrologic routing parameters/considerations may be subject
to modification during model calibration. Any modifications resulting from calibration
should be physically or process-logic (e.g. change in method) based. In addition, care
should be taken such that routing is not “double counted” (i.e. overlapping channel and
storage routing used for the same area).
Balancing Channel/Storage Routing with HEC-RAS model Results
It is necessary to balance the HEC-HMS peak discharge results with the HEC-RAS peak
water surface elevations results. In a typical hydrologic and hydraulic project, the storage
volumes are obtained from the HEC-RAS model and transferred to the HEC-HMS
model. The mapping consultant begins the process by creating a HEC-RAS model and
using a family of peak discharge values to estimate the water surface elevations and
generate storage volumes. The storage volumes are transferred to the HEC-HMS model
and peak discharges are calculated. These peak discharges are transferred to the HECRAS model and new peak water surface elevations and storage volumes are calculated.
The mapping consultant is required to continue this iterative process until the difference
between the peak discharges in successive runs is less than 10 percent.
Balancing Channel/Storage Routing with HEC-RAS Modeling:
Peak Discharges between runs within 10 %
Flow Change Points Along Reach
The accurate estimation of peak discharge along long reach such as those contained in a
FEMA Flood Insurance Study is of paramount importance. In an effort to accurately
estimate the change in peak discharge, it is necessary for the hydrologic model to, at a
minimum, contain a flow combination point at each tributary to the main stream reach.
This increase in peak discharge shall be included in the hydraulic model at the confluence
of the tributary and main reach. If there is a long section of the main reach with a
tributary, the mapping consultant shall supply a peak discharge change point in the
hydraulic model when the peak discharge has changed by 10% in the hydrologic model.
Flow Change Points:
There must be a peak discharge change at the confluence of all tributaries to the main
reach or where the peak discharge is increasing by 10%
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4.
Hydraulic Analysis Standards
This section provides guidance and standards for selected hydraulic parameters and
analyses associated with floodplain modeling. Hydraulic parameters and analyses not
discussed below are left to the discretion of CMSWS and the mapping consultant.
Hydraulic Model
The preferred method for computing the necessary hydraulic information for the FIRM
and FIS is the latest version of the Army Corp of Engineers HEC-RAS computer program
accepted by FEMA for floodplain mapping. The floodplain mapping consultant will use
the steady state option for the hydraulic analysis. This hydraulic analysis will incorporate
peak discharges for the 24-hour duration storm for the 2-, 10-, 25-, 50-, 100- (existing
conditions), 100- (future conditions), and 500-year storm events. The HEC-RAS models
will contain 3 plans for each watershed as follows:
1. There will be a plan for the natural or floodplain analysis containing peak
discharges for all of the storm events. This will be labeled using the title
“Floodplain”.
2. There will be a plan for the FEMA Floodway (0.5 ft surcharge) using the 100year existing conditions peak discharge and the title “FEMA Floodway.”
3. There will be a plan for the Community Floodway (0.1 surcharge) using the 100year modified existing conditions peak discharge and the title “Community
Floodway.”
The use of unsteady state HEC-RAS modeling by the floodplain mapping consultant to
verify or calibrate the results of the steady state model is permissible provided that the
CMSWS is provided with a scope of services describing the necessity for the unsteady
state analysis and agrees to the additional analysis.
HEC-RAS Model Structure:
Steady State HEC-RAS model will have 3 plans with the appropriate titles
Starting Boundary Condition
The floodplain mapping consultant will select the subcritical flow regime for the
floodplain models. In the case of a known starting water surface elevation (i.e., at the
County boundary line with an adjacent County), the floodplain mapping consultant will
use the known elevation. If the downstream water surface elevation is unknown, the
floodplain mapping consultant will use the normal depth routine to determine the starting
water surface elevation. The mapping consultant will use normal depth to determine the
starting water surface elevation for tributaries at the confluence with the main stream.
Starting Boundary Condition:
1) Use known water surface elevation at County Boundary if available otherwise
use normal depth
2) Use normal depth for tributaries at confluence with main stream
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Channel Bank Station Assignment
The assignment of channel bank stations in hydraulic modeling is important as it has an
impact on hydraulic conveyance calculations, as well as, it can impact the assignment of
encroachment stations in floodway analysis and mapping.
Channel banks for hydraulic cross sections (e.g. cross sections in the hydraulic model)
should be assigned using the following guidelines:
Condition 1: Hydraulic Cross Section in the Vicinity of Field Channel Survey
At locations where field channel survey is available in the vicinity of the
hydraulic cross section, channel bank stations should be assigned based on
general engineering judgment as well as consideration of bank locations noted in
the survey.
Condition 2: Hydraulic Cross Section not in the Vicinity of Field Channel Survey
At locations where no field channel survey is available in the vicinity, bank
stations should be assigned based on natural slope break points depicted in
existing County mapping (e.g. LiDAR). The bank stationing should be
verified/checked with field channel survey information (e.g. compare channel
widths) along the same channel or along other channels with similar contributing
drainage areas, and/or with other available data (e.g. aerial photography) to ensure
bank stationing estimates are reasonable.
At a minimum, channel bank stations must be placed at a point below the existing
conditions 100-year storm event elevation, but not within the channel top of bank.
Channel Bank Station Assignment:
Based on field survey and/or other available physical data
Cross Section Spacing
Cross section spacing is a function of stream size, slope and the uniformity of cross
section shape. Cross sections shall be placed at representative locations along a stream
reach and at locations where changes occur in discharge, slope, shape, roughness etc. An
approximate average target cross section spacing of 500 feet is to be used for the Project,
with tighter spacing for areas with more abrupt changes and looser spacing for flatter,
uniform areas.
Target Cross Section Spacing:
500 feet
Cross Sections at Bridges/Culverts
The mapping contractor shall adopt standard practice for modeling flow through
bridge/culvert structures by including four cross sections appropriately spaced (see
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Chapter 5 of the HEC-RAS Hydraulic Reference Manual) upstream and downstream of
bridge/culvert structures.
Cross Sections at Bridges/Culverts:
All bridges and culverts must contain four appropriately spaced cross sections
Bridge/Culvert Geometric Data Annotation
The HEC-RAS models will follow a naming convention. The bridge/culvert geometric
data will be annotated with the crossing street name and any assumed information
associated with the bridge/culvert routing coding. The proposed is to facilitate duplication
of results and the use of these models by professionals working on Letters of Map
Change, Capital Improvement Projects, Transportation Improvement Projects and private
projects.
Bridge/Culvert Geometric Data Annotation:
All bridges and culverts must be properly annotated
Contraction/Expansion Coefficients
The floodplain mapping consultant will select the contraction and expansion coefficients
according to the procedure described in Chapter 5, HEC-RAS Hydraulic Reference
Manual. Typically, the values will be 0.1 for contraction and 0.3 for expansion for
channel cross sections and 0.3 for contraction and 0.5 for expansion for abrupt transitions
near structures (cross sections 2 and 3 per the Hydraulic Reference Manual).
Contraction and Expansion Coefficients:
Contraction and expansion coefficients shall be selected using the procedure
described in the HEC-RAS Hydraulic Reference Manual
Channel and Overbank Roughness Coefficients
The roughness coefficients for the channel and overbanks often have significant
variability. It is important that CMSWS have consistency regarding roughness coefficient
selection between floodplain mapping studies. The floodplain mapping consultant should
use the process for computing roughness coefficients and tables found in Chapter 5,
“Open Channel Hydraulics”, Chow, (1959) or other source if approved by CMSWS.
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Channel and Overbank Roughness Coefficients:
Roughness Coefficients will be computed using the principals described in “Open
Channel Hydraulics”.
Ineffective Flow Areas
The use of ineffective flow areas to simulate conditions in areas with low velocity flow is
expected. The floodplain mapping consultant shall select areas for ineffective flow
pursuant to the recommendation contained in Chapter 3 (channel cross sections) and
Chapter 5 (structure cross sections), HEC-RAS Hydraulic Reference Manual.
Ineffective Flow Areas:
Ineffective Flow Areas must be included in areas with low velocity flow.
Modeling Buildings
Being an urbanized area, there are many buildings in the floodplain area within
Mecklenburg County. Buildings can obstruct and/or impede the flow of flood waters
(similar to other obstructions such as dense vegetation) thus impacting the hydraulic
capacity of a floodplain area.
The impact of buildings should be accounted for in the Floodplain Mapping Project in
areas where it is deemed that building density and/or size would present a significant
impact on floodplain hydraulics, using the following general guidelines:
Condition 1: Numerous Buildings in the Floodplain
For areas where there are numerous buildings in the floodplain (e.g. many houses
in the floodplain along a stream), the impacts of these buildings should be
accounted by adjusting the Manning’s n values used in the cross section. The
following table gives ranges of Manning’s n values to account for varying levels
of building obstructions. It is recommended that horizontal variation in
Manning’s n values be used (rather than generalized values assigned to the
channel and the left and right overbanks).
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Table 2. Building Obstruction Manning's n Categories
Building
Obstruction
Level
Range Manning's n
Low
0.05 - 0.07
Medium
0.08 - 0.12
Examples
Scattered buildings in
floodplain with low to medium
natural ground roughness
(e.g. 0.035 – 0.05)
Typical residential subdivision
in floodplain with low to
medium natural ground
roughness
High
0.15 - 0.20
High density buildings in
floodplain
Condition 2: Few Very Large Buildings in the Floodplain
In areas where there are few but very large buildings that by themselves comprise
a significant obstruction of the floodplain area (e.g. a large industrial/warehouse
building in a narrow floodplain area), the mapping consultant should consider the
use of blocked obstructions to model the building.
It is anticipated that adjusting the Manning’s n value (as specified in Condition 1) will be
used for the vast majority of situations. The use of blocked obstructions (as specified in
Conditions 2) should be used very sparingly and only for exaggerated cases due to the
manual and localized nature of this method.
Modeling Buildings:
Accounted for through the use of Manning’s n adjustments (general case) or
blocked obstructions (extreme case)
Minor Channel Obstructions
In general, utility crossings (e.g., sewer aerials) are not to be included as block
obstruction because they only impact a short length of channel and a relatively minor
percentage of cross section area. The mapping consultant shall use the Cowan equation to
estimate the increase in channel roughness to account for minor obstructions such as
sanitary sewer aerial crossings. (See Estimating Hydraulic Roughness Coefficients,
Agricultural Engineering, v.37, no. 7, p. 473-475).
Minor Channel Obstructions:
Account for minor channel obstructions using the Cowan’s equation.
Documentation of Input Data
The HEC-RAS program allows comments to be added to input files. The consultant shall use the
comment field for data entries that are not clearly defined in the Flood Insurance Study (FIS) that
accompanies the model and data entries that fall outside of commonly accepted parameter ranges
or when a value is assigned to a parameter to achieve a means (such as calibration) that might be
handled differently by another modeler, such as might occur later when better data are available.
The consultant shall use the comment field in situation where the modeler assigned an atypical
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Manning’s n value as a method for representing a specific site condition (such as the presence of
buildings).
Documentation of Input Data:
Note reason for values that are outside a commonly accepted parameters ranges
Floodway Modeling
The floodway represents the portion of a channel or other watercourse and the adjacent
land area that should be reserved/maintained to carry the base flood without increasing
flood elevations by more than a specified maximum tolerance. In most communities,
there is one floodway that is based on a maximum 1-foot surcharge tolerance. However,
there are two floodways on the FIRMs for Mecklenburg County – the FEMA Floodway
and the Community Encroachment Boundary.
Community Encroachment Area
The Community Encroachment Area shall be determined using a 0.1 foot maximum
surcharge using modified 100-year existing conditions base flood discharges. The 100-year
existing conditions discharge will be modified to account for the future loss of storage
due to filling of the floodplain fringe.
The mapping consultant will begin the process by determining the Community
Encroachment Area Boundary Line using the traditional encroachment analysis with a
0.1 foot surcharge. The loss of storage due to future fill will be computed by assuming
that there will be fill up to the revised Community Encroachment Area Boundary Line on
all parcels which are not owned by Local Government for park or greenway use. The
assumed filled shall not be placed within 100 feet from top of bank in order to account
for SWIM buffer restrictions. The mapping consultant will compute revised storage
values and use these values to compute the modified 100-year peak discharges using
HEC-HMS. The mapping consultant will use the modified 100-year peak discharges to
re-compute the Community Encroachment Area Boundary Line and the process will be
repeated for one more iteration.
Modeling Community Encroachment Boundary:
Use 0.1 ft surcharge with the modified 100-year existing conditions discharges
accounting for future fill in the floodplain
FEMA Floodway
The FEMA Floodway Boundary Lines will be determined in a manner similar to the
Community Encroachment Boundary Lines with a 0.5 foot surcharge using the modified
existing conditions base flood discharges. The consultant will use the peak discharge
values developed to compute the Community Encroachment Area and the traditional
floodway encroachment analysis to determine the FEMA Floodway Boundary Lines.
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Modeling FEMA Floodway:
Use 0.5 ft surcharge with modified 100-year (existing conditions) peak discharges
accounting for future fill in the floodplain
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5. Model Calibration Standards
This section provides guidance and standards for selected hydrologic and hydraulic
calibration and/or verification methods associated with floodplain modeling. Calibration
typically entails comparison of computed results (e.g. peak flows and flood stages) with
previously recorded/observed results under similar conditions, previous studies, and/or
published estimation methods (e.g. regression equations). Once calibration is completed
and it is determined that the models can be expected to yield dependable results, the
models can be directed toward analyzing many different flood events and different land
use scenarios.
For steady-state hydraulic modeling, initial hydrologic calibration is often performed
prior to hydraulic calibration; however, calibration is often an iterative process. The
mapping contractor should perform the adequate number of iterations to ensure the model
results are within limits as recommended in the section below.
Hydrologic Calibration
Prior to finalizing the hydrologic analysis, comparisons between historic conditions and
modeled conditions should be made, and parameters adjusted, until satisfactory fits are
obtained. Hydrologic calibration is typically performed by adjusting subbasin lag times,
initial abstractions, curve numbers, and/or peaking coefficients, as justifiable, to better
match computed peak flows and hydrograph time to peaks with observed values or
previous studies. It is important to note that adjustment/refinement of hydrologic
parameters for calibration should generally be based on physical information when
possible. Global changes to hydrologic parameters should be used with caution for
variables that are more subjective (e.g. adjusting Manning’s n values for sheet flow in
subbasin lag calculation for different grass cover types). However, in all cases adjusted
parameters should stay within the range of generally accepted values for a given physical
condition. There is no set of rules that specify when adequate matching is achieved.
However, it is recommended that the peak flow and total volume comparison be matched
within 10%, and the time to peak at the gaging station should be within 30 minutes,
where possible. Hydrologic calibration for the Project should be performed using the
conditional approach indicated below.
Condition 1: Calibration in Watersheds with Historical Precipitation and Stream
Flow Gage Data
For watersheds with detailed historic precipitation and flood hydrograph data
hydrologic calibration should be performed by performing initial hydrologic
analysis with actual recorded rainfall data and then comparing the key results (i.e.
peak flow, time to peak, and total hydrograph volume) to the stream gage data.
When calibrating the hydrologic model, the consultant shall progress as follows:
•
Start at the top of the watershed and progress to the bottom of the
watershed during calibration;
•
Adjust curve number ( +/- 4) so that total runoff volume matches as close
as possible at measured locations;
•
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•
•
•
•
Adjust time parameters. Subbasin lag times are generally one of the most
subjective hydrologic parameters, thus there is more flexibility in
adjusting this parameter to help match peak flow and time to peak.
Cross-check with USGS regression equations (this serves as a
reasonableness check since gage data provide better site-specific
information than regional regression equations).
If multiple rain gages are available within the watershed, spatial
distribution in rainfall distribution should be considered for the calibration
run.
Other hydrologic parameters (e.g. base flow) can be considered if
necessary and justifiable.
It may be necessary to perform several iterations of these parameters to calibrate the
models. The consultant shall fully document each stage of the calibration process.
Condition 2: Calibration in Watersheds with Historical Stream Flow Gage Data
Only
For watersheds that have stream gage data, but no precipitation data, adjacent
watershed rain gage data can be used. The rest of the calibration procedures
would be the same as described above.
Condition 3: Calibration in Watersheds without Historical Stream Flow Data
For watersheds that do not have historic stream flow data, computed key results
should be compared with regression estimates, Effective FIS information, and
other available studies. The hydrologic models should be calibrated using the
techniques outlined above, if deemed appropriate. Any remaining significant
discrepancies should be noted and documented.
For all conditions, it is recommended for the mapping consultant to compare flows with
current Effective information, regression equation, and/or other available data to assess
the reasonableness of flows and to provide a preliminary indication of how the analysis
and subsequent floodplain maps may differ from the current Effective maps.
Calibration of Hydrology:
Peak flow and total volume comparison should match within 10%, and the time to
peak at the gauging station should be within 30 minutes for streams with historical
stream and flow gage data. For streams without gage data, flows should be compared
to other available data and adjusted as appropriate.
Hydraulic Calibration
Hydraulic calibration is used to ensure flood stages computed in the hydraulic model are
realistic with actual conditions. Hydraulic calibration is typically performed by adjusting
Manning’s n, inlet/outlet coefficients, and/or contraction/expansion coefficients, as
justifiable, to better match computed flood stages with observed values or previous
studies. As with hydrologic calibration it is important to note that adjustment/refinement
of hydraulic parameters for calibration should generally be based on physical information
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when possible. Global changes to hydraulic parameters should be used with caution for
variables that are more subjective (e.g. using 0.06 instead of 0.05 for overbank
Manning’s n values to represent a low density of buildings in the floodplain). However,
in all cases adjusted parameters should stay within the range of generally accepted values
for a given physical condition. Hydraulic calibration for the Project should be performed
using the conditional approach indicated below.
Condition 1: Calibration along Streams with Historical Stage Gage Data
For streams with detailed historic flood stage data, such as those with flood stage
gages, hydraulic calibration shall be performed to match the known flood stages
within 0.5 feet, where possible. When calibrating the hydraulic model, the
consultant shall adjust parameters in the following order of priority:
• Roughness Coefficients;
• Minor adjustments to peak discharge;
• Inlet/outlet coefficients & contraction/expansion coefficients;
• Conveyance- effective flow areas for large cross sections and areas near
culverts.
It may be necessary to perform several iterations adjusting these parameters to
calibrate the models. The consultant shall fully document each stage of the
calibration process.
Condition 2: Calibration along Streams with Historic High Water Mark Data
For streams with more scattered and/or limited historic flood stage data, such as
high water marks or citizen witness, the mapping consultant should first review
the available information to assess the consistency, adequacy, and flood frequency
of the data, and then perform hydraulic calibration as deemed appropriate. Due to
the more variable nature of this type of historic data, calibration for this condition
may not be as precise or close to that done for Condition 1.
Condition 3: Calibration along Streams with No Historic Flood Stage Data
For streams with no or unreliable historic flood stage data, computed flood stages
should be compared with the Effective FIS and/or other previous studies. The
hydraulic models should be calibrated using the techniques outlined above, if
deemed appropriate. Any remaining significant discrepancies should be noted
and documented.
Calibration of Hydraulics:
Peak stage should be within 0.5 feet for streams with historical stage gage data,
where possible. For streams without gage data, flows should be compared to other
available data and adjusted as appropriate.
For all conditions, it is recommended for the mapping consultant to compare computed
water surface elevations with current Effective information and/or other available data to
assess the reasonableness of peak stages and to provide a preliminary indication of how
the analysis and subsequent floodplain maps may differ from the current Effective maps.
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6.
Mapping Standards
This section provides guidance and standards for selected floodplain and floodway
mapping considerations. Mapping standards not discussed below should be consistent
with FEMA Mapping Guidelines & Specifications (G&S).
Floodway Delineation Around Buildings
Floodways can have a very significant impact on the potential uses and value of property,
but the delineation of floodway boundaries can be subjective. Initial floodway
delineations should be based more strictly on direct results from the hydrologic and
hydraulic analysis. However, the initial delineations will likely need to be refined to
incorporate features that are not directly accounted for in hydrologic and hydraulic
models. In the case where initial floodway lines are drawn through buildings, the lines
should be generally evaluated and adjusted as follows. If less than 50% of the building is
within the initial floodway boundary, the line should be modified to generally exclude the
building. If more than 50% of the building is within the initial floodway boundary, the
line should remain as is. This is a general rule of thumb. The mapping contractor should
use appropriate engineering judgment and coordinate with CMSWS to determine the
final floodway boundaries for unique or complex situations to ensure that the delineations
are reasonable (e.g. do not weave in and out of buildings).
Floodway Delineation
Include building in floodway if more than 50% of building would be in floodway.
Coordinate with CMSWS on final delineation for unique or complex areas.
Community Floodplain Boundary Delineation
The FIRMs in Mecklenburg County contain a floodplain delineation of the 100-year
future conditions storm event known as the “Community Floodplain”. The Community
Floodplain boundary extends upstream of the last cross section in the detailed study. The
mapping contractor shall delineate the Community Floodplain boundary upstream of the
last detailed cross section by projecting the 100-year future conditions base flood
elevation at the last cross section along its contour.
Community Floodplain Delineation:
Extend Community Floodplain boundary upstream of the last cross section of the
detailed study along the contour of the Community Base Flood Elevation at the last
cross section.
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Merging Effective and New Flood Hazard Data
Seamless Panels
The final mapping of the complete countywide dataset must be completed by stitching
the various floodplain spatial files together to merge restudied and non-restudied
floodplains, integrate backwater mapping, identify and resolve mismatches, and deal with
issues of topology. The final result should be a single, comprehensive floodplain file for
the whole county suitable for publication. The most up-to-date topographic data should
be used for mapping all restudies (no more than 2 years old when hydrology and
hydraulic analysis begins). If there is a significant change in topography at an area of tiein between a restudied stream and a non-restudied stream, the new topographic data
should be used to blend the tie-in.
Merging Data:
1) Create one seamless dataset for the entire County
2) Use the most up-to-date topographic data for tie-in with unstudied stream
LOMCs
An additional aspect of merging effective and new flood hazard information is the
process of identifying all effective LOMRs and determining if they are still valid or
superseded by the revision. LOMRs on non-restudied streams will still be valid and
should be incorporated into DFIRMs and the FIS report. Any LOMRs on restudied
streams will be superseded by the new analysis and should not be incorporated into the
DFIRMs and FIS Report.
Letters of Map Change:
1) Include Effective LOMRs on non-restudied streams shown on re-mapped panel.
2) Supersede LOMRs on restudied stream.
Flood Insurance Study
The FIS Report must be updated to merge effective and new flood hazard information.
The joint Mecklenburg County/ North Carolina Floodplain Mapping Program (NCFMP)
format FIS Report must be used as the format for future revisions to ensure consistency.
The FIS Report should comply with the NCFMP Graphical Specifications for Floodplain
Mapping Contractors, Section NC.12 (Dated November 2007) and FEMA’s G&S,
Appendix J, where applicable. The Mecklenburg County effective FIS has 5 volumes
and each volume should only be revised and should only be reissued if flood hazard data
in that volume is affected by the revision. The standard FIS Report tables (Summary of
Discharges, Floodway Data, Manning’s “n”, etc) must be updated to merge effective and
new flood hazard information and to integrate backwater mapping and resolve issues of
mismatch at tie-in locations.
Flood Insurance Study:
Update FIS using most current Mecklenburg County/NCFMP format
Floodplain Analysis and Mapping Standards
Guidance Document
Floodplain Mapping Project
July 2008
Page 28 of 31
FIRM Panel Format
Once all of the effective and new data is merged, final mapping specifications should be
applied to the newly studied streams. Base Flood Elevations (BFEs) must be created and
mapped for all newly studied streams. These BFEs must be mapped on the digital Flood
Insurance Rate Map (DFIRM) panel along the stream in accordance with FEMA’s G&S
at correct spacing and locations, including profile inflection points, structures, and panel
edges. Upon review and acceptance by CMSWS, the complete floodplain lines and BFEs
for a newly detailed study stream are ready for creating a seamless, countywide flood
theme.
After the effective and new flood hazard information are merged in the production
geodatabase, DFIRM panel production should begin. The DFIRM panel production
process should be largely automated through the creation of GIS map document files,
referred to as MXDs. MXD files can be created for each panel in a batch-process. The
MXDs should also specify the symbology to be used for each map feature in compliance
with FEMA’s G&S Appendix K and the North Carolina Floodplain Mapping Program
(NCFMP) DFIRM Graphic Specifications. The MXDs should also apply the map collar
(i.e., legend, border notes and title block) based on a template set up for the project. For
the Mecklenburg County revision, the customized template developed by the NCFMP to
merge the unique features of the Mecklenburg County 2004 effective maps with the panel
scheme and grid of the NCFMP maps should be used.
Update FIRM Panel:
Update FIRM Panel using most current Mecklenburg County/NCFMP format
DFIRM features should be attributed in the production geodatabase as they are created
and map annotation for map features (e.g., roads, flood sources, flood zones, corporate
limits, and corner coordinates) should be created by simply pulling the names from the
database and applying the right font, size, and style according to FEMA’s G&S, NCFMP
DFIRM Graphic Specifications, and the FEMA- and NCFMP-approved Mecklenburg
County DFIRM panel prototype. The unique characteristics of the Mecklenburg County
DFIRM panels include:
•
Panel border:
- Flood Hazard Data Table (FHDT) has Future Land Use Conditions and
Community Encroachment Lines
- Mecklenburg County Benchmark Reference
- Charlotte-Mecklenburg Storm Water Services references and websites
•
Panel Image:
- Mecklenburg County supplied vector planimetric base map building footprints
and street centerlines
- Mecklenburg County Benchmark symbology
Floodplain Analysis and Mapping Standards
Guidance Document
Floodplain Mapping Project
July 2008
Page 29 of 31
- Community Encroachments Areas
- Future Conditions floodplain
The following documents outline standard FEMA and NCFMP specifications for DFIRM
mapping, the Flood Insurance Study (FIS Report), and DFIRM Database products:
•
•
•
•
•
•
•
•
•
•
DFIRM Mapping
-NCFMP Graphical Specifications for Floodplain Mapping Contractors, Sections
NC.1 through NC.11. Dated November 2007.
-FEMA G&S, Appendix C: Guidance for Riverine Flooding Analysis and
Mapping, Sections C.6, C.7.2, and C.8.
-FEMA G&S, Appendix E: Guidance for Shallow Flooding Analysis and
Mapping, Sections E.1 through E.5.
-FEMA G&S, Appendix K: Format and Specifications for Flood Insurance Rate
Maps, Sections K.1 through K.2.
FIS Report
-NCFMP Graphical Specifications for Floodplain Mapping Contractors, Section
NC.12. Dated November 2007.
-For other specifications, FEMA G&S, Appendix J: Format and Specifications for
Flood Insurance Study Reports. Sections J.1 through J.5.
DFIRM Database
FEMA G&S, Appendix L: Guidance for Preparing Draft Digital Data and DFIRM
Database.
Disparities with Adjacent County Panels
Gaps currently exist between the limit of study within Mecklenburg County and the limit
of study within contiguous counties for various streams that flow in and out of
Mecklenburg County. Discrepancies with tie-ins (gaps) across contiguous County data
should be identified before engineering modeling begins and resolved for all restudied
streams. If a discrepancy is created as part of the new analysis, Mecklenburg County
should notify the NCFMP of the discrepancy and provide a courtesy copy of final
modeling and mapping for the area of concern for the NCFMP to archive for the next
map maintenance revision for the contiguous county that requires an update to match the
newly studied Mecklenburg County data. Since all cross sections were converted from a
letter system to a number system with the recent NCFMP revision to the DFIRMs, tie-ins
of cross section numbering with contiguous counties will be manageable.
For non-restudied streams where a gap or mismatch occurs between Mecklenburg County
and a contiguous county, Mecklenburg County should archive the mismatch to be
resolved during future revisions when the streams of concern are restudied.
Floodplain Analysis and Mapping Standards
Guidance Document
Floodplain Mapping Project
July 2008
Page 30 of 31
GIS Concerns
Symbology, Format, Tables, Content
All flood hazard and associated data on the DFIRM panels, in the DFIRM database, and
in the FIS report should comply with the current Mecklenburg County/NCFMP DFIRM
Graphic Specifications and NCFMP DFIRM Style File to ensure consistency with the
“look and feel” of the mapping products. Data must also comply with FEMA G&S,
Appendices J, K, and L.
Retain Current Map Format
The Mecklenburg DFIRMs should have a mixed base map format due to the nature of
statewide mapping. A combination of orthophotos (for contiguous counties) and vector
base map data (for Mecklenburg County) should be used on all shared county DFIRMs,
to comply with North Carolina statewide mapping standards. All DFIRMs showing only
Mecklenburg County should only have vector base data. The most current vector base
data should be used for Mecklenburg County for corporate limits, roads, and building
footprints. The mapping contractor must use the joint Mecklenburg County/NCFMP
DFIRM prototype for Mecklenburg County used to create the final FIRM panels issued
in 2007.
A FEMA standard format DFIRM database will be required for submission to FEMA’s at
preliminary for this revision. The database must be a complete, countywide database that
includes both unrevised and newly studied data and must comply with FEMA G&S,
Appendix L for database creation. The DFIRM database must match exactly the
information printed on the preliminary DFIRM panels that are issued to the community.
Mapping Format:
Mapping format must match format for panels issued in 2007.
Floodplain Analysis and Mapping Standards
Guidance Document
Floodplain Mapping Project
July 2008
Page 31 of 31
APPENDIX A
PROJECT MEMORANDUM DATED 08/28/07 (REVISED 8/31/07)
SURVEY COMPARISON FINDINGS
MECKLENBURG COUNTY FLOOD PLAN
MAPPING PROJECT
Project Memorandum
To:
David Goode, PE, CFM
From: Neal Banerjee, PE, CFM
Date: 8/28/07 (REVISED 8/31/07)
Re:
Survey Comparison Findings
Mecklenburg County Floodplain Mapping Project
David,
Dewberry has performed a comparison of stream channel geometries taken from actual field
survey (collected for previous projects) versus that derived from the 2007 LiDAR information at
selected locations within the Little Sugar Creek and Briar Creek watersheds. The effort was
authorized per an email from you dated 8/23/07 and performed under Task 2.2 (Sensitivity
Analysis) of our existing contract. This memorandum summarizes the approaches, findings,
and recommendations of the comparison evaluation, and represents the “deliverable” of the
agreed effort.
Comparison Methodology/Approach
Channel geometries were compared by comparing both channel cross section plots and
channel profile plots at selected sample locations. Channel cross sections from actual field
channel survey (collected for previous projects) were plotted versus those generated from an
ESRI 3-D terrain surface developed from 2007 LiDAR data. Effective HEC-RAS model cross
section information was also plotted, where available, for general reference. Cross sections
were compared at the following eight (8) locations:
• Little Sugar Creek – upstream of Princeton Avenue (stream station 97205)
• Little Sugar Creek – downstream of Princeton Avenue (stream station 61985)
• Little Sugar Creek – downstream of Princeton Avenue (stream station 61440)
• Dairy Branch – upstream Cumberland Avenue (stream station 970)
• Briar Creek – off Windsor Drive (stream station 3610)
• Briar Creek – off Hungerford Place (stream station 19365)
• Edwards Branch – downstream of Sheffield Avenue (stream station 11455)
• Briar Creek Tributary #1 – off Henshaw Road (stream station 270)
Channel profiles from the Effective models were plotted versus those developed from the
aforementioned ESRI terrain surface (using a digitized stream centerline from 2007 aerials as
the profile alignment). Actual field data information was not used for comparison profiles due to
the lack sufficient survey information to generate an adequate profile. Profiles were compared
at the following three (3) locations:
• Little Sugar Creek – downstream of Princeton Avenue (between stream stations 61875
and 60940)
• Little Sugar Creek – along Villa Hermosa Drive (between stream stations 52555 and
51545)
• Briar Creek – along Museum Drive (between stream stations 19835 and 18275)
Attachment A contains the cross section plots/comparisons developed at each sample location.
Attachment B contains the profiles plots/comparisons developed at each sample location.
Ae
A-1
December, 2006
Project Memorandum
Comparison Results
Comparison of the cross section and profile plots indicate the level of agreement between
channel geometries as defined by field survey versus those derived from the new LiDAR data is
generally good for the larger, more open streams. At sample locations on Little Sugar Creek
and Briar Creek channel cross sections compared reasonably well, with inverts generally within
2 feet and the general shape and area appearing to be within 15%. However, at the sample
location on Dairy Branch, which is a smaller, more covered tributary, agreement between LiDAR
and field survey was significantly lower, with the invert being close to 5 feet off. Although there
was only one sample location on a smaller, more vegetated stream, it is likely that similar
discrepancies may occur. In general the Effective FEMA cross sections compared reasonably
well with the field survey, with the exception of a location on Edwards Branch, where both the
invert and shape were off noticeably.
The profile comparisons at all locations showed significant discrepancies between the Effective
information and that derived from LiDAR, with differences in invert ranging between less than
0.5 feet to more than 6 feet. The profile also fluctuates vertically (i.e. goes up and down)
considerable, which is often undesirable in profile mapping. One likely reason for the
discrepancies and the fluctuation is the profile alignment digitized from the aerials may not
necessarily correspond to the lowest channel elevation in the LiDAR data (i.e. the alignment
from the aerials may run up on the channel banks which are steep and rise sharply). This may
explain why the cross sections generally match better than the profiles.
Based on sample spot checks (not shown in the attachment), comparison of LiDAR data with
field data in upland areas (areas 20 or more feet off the top of channel bank) generally showed
very good agreement (generally with 0.1 – 0.4 feet).
Conclusions and Recommendations
The following conclusions and recommendations are offered based on the result of this effort:
• 2007 LiDAR appears to generally be accurate in overbank/upland areas and adequate
for floodplain mapping
• Based on the results from the samples locations, one can deduce that the LiDAR
reasonably depicts the general shape and area of the channel for larger, more open
streams with shallow baseflow. The level of accuracy/agreement decreases
considerably for smaller, narrower streams that are more obscured by vegetation or
have deeper base flows. In both cases, profiles directly generated from LiDAR may be
questionable. Due to the variability in the streams within Mecklenburg County, it is
recommended that the LiDAR be supplemented with actual stream survey. Survey
supplement every 1,000-ft on average appears to be a reasonable suggested
supplement.
• The Effective cross section information appears to generally be reasonably consistent
with field survey. It appears that the Effective information could potentially be used to
reduce the amount of field survey collected in the current project, however, there are
issues associated with trying to just capture and blend the channel portion of the
Effective survey (horizontal stationing, areas between effective cross sections, etc.)
which may reduce the potential cost savings.
Attachment A – Channel Cross Section Comparison Plots
Attachment B – Channel Profile Comparison Plots
P:\50006632\Adm\Correspondence\Memos\082807srvysen2Cnty\srvysentvtymemoREV.doc
AL
A-2
December, 2006
Project Memorandum
ATTACHMENT A –
Cross Section Comparison Plots
l
A-3
December, 2006
Field Survey Vs. '07 LiDAR
Little Sugar Creek (DS Princeton Ave - Sta 61985)
From Dancy No-Impact 2006-2007
616.0
614.7
614.0
614.1
613.1
612.9
612.0
610.4
Elevation
610.0
608.2
608.0
606.0
606.4
605.9
604.0
604.1
604.0
602.0
601.4
600.6
600.0
598.7
598.0
INV
LiDAR
Field Survey
FEMA Xsec
603.4
600.0
601.4
600.6
598.2
INV
596.0
0
20
40
60
80
100
120
140
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-4
December, 2006
Field Survey Vs. '07 LiDAR
Little Sugar Creek (DS Princeton Ave - Sta 61440)
From Schonberg No-Impact 2007
614.0
612.0
611.6
610.7
610.0
609.6
608.5
Elevation
608.0
608.3
607.7
607.6
606.4
606.1
605.6
605.1
606.0
604.0
605.5
605.5
605.2
604.6
604.0
603.6
603.0
602.2
601.3
602.0
600.1
600.0
598.7
598.0
598.0
LiDAR
Field Survey
FEMA Xsec
602.0
601.8
600.7
600.6
600.0
598.6
597.4 597.2
596.0
INV
0
20
40
60
80
100
120
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-5
December, 2006
Field Survey Vs. '07 LiDAR
Little Sugar Creek (US Tryon Street - Sta 97205)
From Hidden Valley Stream Restoration 2007
710.0
708.7
708.0
706.4
Elevation
706.0
704.0
702.0
705.0
704.7
704.1
703.6
703.1
702.3
704.4
703.6
LiDAR
Field Survey
FEMA Xsec
702.7
701.3
700.7
700.0
699.8
698.4
697.8
698.0
698.3
697.0
696.0
0.0
20.0
40.0
60.0
80.0
100.0
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-6
December, 2006
Field Survey Vs. '07 LiDAR
Dairy Branch (US Cumberland Ave - Sta 970)
From Schwieman No-Impact 2005
632.0
630.0
628.0
Elevation
626.0
624.0
623.5
623.5623.4
623.4
623.5
623.3
622.3
622.0
622.9
LiDAR
Field Survey
FEMA Xsec
622.6
622.3
622.3
620.4
620.0
618.4
618.0
INV
616.7
615.9
616.0
614.0
0.0
20.0
40.0
60.0
80.0
100.0
120.0
140.0
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-7
December, 2006
Field Survey Vs. '07 LiDAR
Edwards Branch (DS Sheffield Ave - Sta 11455)
From Sheffield No-Impact 2005
696.0
694.0
693.45
693.5
692.0
692.5
692.33
692.0
691.1
690.93
690.4
Elevation
690.99
690.0
688.0
693.7
693.02
LiDAR
Field Survey
FEMA Xsec
687.8
687.45
686.3
686.0
686.2
684.9
684.5
684.40
683.71 683.8
684.0
682.78
682.0
0.0
20.0
40.0
60.0
80.0
100.0
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-8
December, 2006
Field Survey Vs. '07 LiDAR
Briar Creek (Off Hungerford Pl - Sta 19365)
Briar Creek Sewer Inteceptor 2002
626.0
624.0
Elevation
622.0
623.9
623.5
622.3
621.5
620.0
620.0
618.6
618.0
616.0
614.0
LiDAR
Field Survey
FEMA Xsec
617.3
617.3
616.6
616.2
615.8
615.3
614.0
613.9
613.8
613.2
613.2
614.3
612.7
612.0
610.0
-10.0
10.0
30.0
50.0
70.0
90.0
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-9
December, 2006
Field Survey Vs. '07 LiDAR
Briar Creek (Off Windsor Dr - Sta 3610)
Briar Creek Sewer Inteceptor 2002
590.0
588.0
588.9
587.0
586.0
585.3
584.2
583.7
Elevation
584.0
583.2
582.0
581.9
LiDAR
Field Survey
FEMA Xsec
580.8
580.0
580.1
578.6 578.8
578.0
577.5
577.2
576.1 576.0
576.0
574.8
574.0
572.0
-10.0
576.3
576.1
576.0
575.9
575.6
575.1
573.9
10.0
30.0
50.0
70.0
90.0
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-10
December, 2006
Field Survey Vs. '07 LiDAR
Briar Creek Trib #1 (Off Henshaw Rd - Sta 270)
Briar Creek Sewer Inteceptor 2002
598.0
596.0
595.7
596.0
594.0
593.0
Elevation
592.0
591.2
590.8590.9
590.6
590.6
590.0
588.8
588.1
588.0
LiDAR
Field Survey
586.7
586.1
586.0
584.9
584.0
584.1
584.0
582.3
582.3
581.9
580.7
580.7
580.5 580.2
579.9
582.0
580.0
578.0
0.0
10.0
20.0
30.0
40.0
50.0
60.0
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-11
December, 2006
Project Memorandum
ATTACHMENT B –
Profile Comparison Plots
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-12
December, 2006
Effective FEMA Vs. '07 LiDAR
Little Sugar Creek (Sta 61875 - 60940) - DS Princeton Ave
603.0
602.5
602.0
601.5
Elevation
601.0
600.5
LiDAR
FEMA Profile
600.0
599.5
599.0
598.5
598.0
597.5
60800
61000
61875
61409
60938
61200
61400
61600
61800
62000
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-13
December, 2006
Effective FEMA Vs. '07 LiDAR
Little Sugar Creek (Sta 52555 - 51545) - Along Villa Hermosa Dr
584.0
583.0
582.0
Elevation
581.0
580.0
579.0
52557
578.0
577.0
52196
576.0
575.0
574.0
51400
LiDAR
FEMA Profile
51911
51545
51600
51800
52000
52200
52400
52600
52800
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-14
December, 2006
Effective FEMA Vs. '07 LiDAR
Briar Creek (Sta 19835 - 18275) - Along Museum Dr
618.0
617.0
616.0
LiDAR
FEMA Profile
Elevation
615.0
614.0
613.0
19837
612.0
19365
611.0
610.0
609.0
18000
18854
18275
18500
19000
19500
20000
Distance
Appexdix A - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
A-15
December, 2006
APPENDIX B
SAMPLE GPS CROSS SECTION POINTS
SAMPLE GPS CROSS SECTION POINTS CALCULATION
XS_ID
ROD_HGT
LHT-007
0.92
Reference point 1
(Required)
Reference point 2
(Optional)
1445518.64
521683.37
616.27
1445457.54
521683.75
LOB
ROB
REL_X
STA
REL_Z
DESC
CALC_X
CALC_Y
CALC_Z
XS_ID
0
7
16.5
17
21
25
25.5
31
-21
-14
-4.5
-4
0
4
4.5
10
0.92
4.28
9.76
10.12
10.75
10.5
10.02
6.78
GR
GR
TE
H2O
H2O
H2O
TE
GR
1445518.64
1445511.64
1445502.14
1445501.64
1445497.64
1445493.64
1445493.14
1445487.64
521683.37
521683.37
521683.37
521683.37
521683.37
521683.37
521683.37
521683.37
616.27
612.91
607.43
607.07
606.44
606.69
607.17
610.41
LHT-007
LHT-007
LHT-007
LHT-007
LHT-007
LHT-007
LHT-007
LHT-007
38
59
17
38
4.8
3.62
GR
GR
1445480.64
1445459.64
521683.37
521683.37
612.39
613.57
LHT-007
LHT-007
XCOORD
YCOORD
ZCOORD
Co-Ordinate
Location
XS Bearing Determination
DX
DY
Calc. Angle
XS Bearing
Calculated Angle
Measured Angle
Angle Used
-61.1
0.380
180
180
180
Notes:
`
Cross-Section Profile
618.00
616.00
Elevation (ft.)
614.00
612.00
610.00
608.00
606.00
604.00
0
10
20
30
40
50
60
70
Station (ft.)
Appendix B - CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User's Guide - FINAL
B-1
December, 2006
APPENDIX C
ADDITIONAL INFORMATION ON EXISTING AND
FUTURE LAND USE
Additional Information on Existing and Future
Land Use
Existing Land Use
History
The City Storm Water Services (CSWS) existing land use coverage was originally created by
Ogden and Woolpert in 1992 using city/county topographic maps and 1990 aerial photographs.
CSWS has since its creation maintained and updated the existing land use coverage using actual
ground information based on commercial site plans and aerial photography, which is usually
flown every other year. The existing land use consists of 12 land use condensed/groups as
follows:
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
1 - woods/brush
2 - open space (golf courses, mining, fields), > 2 acres residential
3 - > 1/2 to 2 acres residential
4 - 1/4 to 1/2 acre residential
5 - < 1/4 residential, apartments, multifamily
6 - institutional; schools, hospitals, government offices
7- light industrial
8 - heavy industrial
9 - light commercial - office parks, hotels
10 - heavy commercial - car parks, malls
11 - water bodies, usually ponds greater than 2 ac in size
12 - transportation, multilane roads, interstates
Procedure for Processing Existing Land Use Coverage
The overall intent of the procedures below is to help ensure that land use information used is to
the appropriate level of detail for floodplain analysis.
1. Obtain the latest existing land use layer from CMSWS, City Administration/GIS.
2. Create study area polygon encompassing the watersheds to be studied and clip existing
land use to study area boundary.
3. Add the field names in the GIS file as described below or as dictated by MCSWS:
o Field [LU_CODE] is a short numeric field and represents numeric land use code
for each feature land use type. This field should be matched and populated with
data from the original shape file field name [LUSECODE]
o Field [LU_DESC] is a 60 character text field and represents the description of the
land use associated with the land use code.
o Field [ACRES] is a double numeric field and represents each land use feature
area calculated in acres.
Appendix C – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
C-1
December 2006
4.
5.
6.
7.
o Field [LU_SOURCE] is a 10 character text field and represents the source of the
land use information. ([LU_SOURCE] is “City SWS” if a features land use
attribute is unchanged from original obtained from the city and “[<MAPPING
PARTNER NAME>]”).
o Field [DATE_CRRNT] is a 10 character text field and represents the date of the
land use feature is current since it was last obtained from the city.
([DATE_CRRNT] is “[<DATE MAPPING PARTNER OBTAINED THE
LAND USE SHAPEFILE FROM THE CITY>]” if a features land use attribute is
unchanged from original obtained from the city and “[<DATE MAPPING
PARTNER LAST UPDATED/MODIFIED THE FEATURE>]” if the land use
feature attribute is changed/updated).
o Field [NOTES] is a 100 character text field containing the description of
supporting information used for the modification/updates of land use feature
attributes where applicable.
Check metadata files for the prevailing date of the land use and aerial photographs. If the
aerial photographs are relatively more recent than the existing land use, check and update
the existing land use using the more current aerial photographs. It is quicker and more
efficient to screen and update areas where there is recent development by overlaying
current preliminary development plans layer (available from Mecklenburg County GIS).
Flag areas where there are obvious discrepancies between the existing land use layer’s
land use and corresponding land use type deciphered from the aerial photography.
Report discrepancies to CMSWS.
Land use features/polygons flagged as a result of discrepancies are updated by editing the
attributes in fields [LU_CODE] and [LU_DESC] consistent with the preliminary
development layer and aerial photographs.
Mapping consultant shall provide a copy of all coverages (rec’d from the City, rec’d from
County GIS, and the developed final coverage) to Flood Mitigation for the Project File.
Future Land Use
History
The future land use layer developed by the Charlotte-Mecklenburg County Planning Department
is to be used as the “basis” for the future land use on the Project. This layer is believed to be the
best available data at large scale. The future land use is developed in a tiered system in the order
from lowest to highest priority as:
1. District plans
2. Area plans
3. Approved rezoning cases
4. FEMA floodplain areas
In this order, the district plan information is used as the base and if there is an area plan
available it is used to overwrite district plan data and so on. FEMA floodplains are designated
as greenway/open space and overwrite district, area, or rezoning cases. Area plans and rezoning
cases have relatively high level of public involvement/approval while district plans (originally
developed in the early to mid 1990's) had limited public involvement/approval. The future land
use is generally updated every 3-4 months. The future land use layer is categorized into
Appendix C – CLT-BC Technical Reference
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December 2006
multiple (can be as many as ninety) groups. The need for compatibility and comparability
between the existing the future land use categories in the current mapping effort has been
emphasized by CMSWS. In pursuance of this objective, a translation table for condensing the
multiple categories of the base future land use layer in to twelve (12) land use groups based on
the existing land use categories has been developed as shown in Table 01 below. The
translation table maps future land use categories on to existing land use categories depending on
the compatibility of categories. Certain future land use categories such as the mixed uses are not
directly compatible with the existing land use. In such cases, the comparability of curve
numbers, percent imperviousness and/or engineering judgment should be considered in
assigning suitable existing land use translation code.
Procedure for Processing Future Land Use Coverage
Unlike the existing land use data which is countywide, the future land use data only covers the
City ETJ. As with the existing land use, additional verification and modification to the base
information is necessary to ensure appropriateness for floodplain mapping and comparability
with the existing land use. The general steps to process the future land use layer are outlined
below.
1. Obtain the latest future land use layer from the Charlotte-Mecklenburg Planning
Department (Note: Mecklenburg County Planning department is deemed to have the most
detailed and accurate future land use coverage).
2. Clip future land use to study area boundary developed for existing land use.
3. Add the field names in the GIS file as described above for the existing land use layer, or
as dictated by MCSWS.
4. Update/condense the future land use categories in the data base using the translation
table. This is easily done using Microsoft Access.
5. Create land use polygons for right-of-way areas that are not included in the base future
land use layer (i.e. the future land use layer does include polygons for many right-ofways – there are gaps in the GIS layer) and populate them as road/transportation.
6. Create future land use polygons for any areas outside the ETJ within the study
watersheds. Potential data sources may include zoning, the existing land use layer, the
future land use file developed for the Effective studies, aerial photographs, greenway
plans, and others.
7. QC future land use in conjunction with the existing land use and other available sources,
and update/modify areas with the any better data (potential sources listed in the previous
step) as appropriate. FEMA floodplain areas in the base future land use layer are
assigned to the open space land use category. Specifically revisit these areas to develop
more suitable future land use from data such as a combination of CSWS existing land use
data and Mecklenburg County Parks and Recreation Greenway Master Plans.
8. Mapping consultant shall provide a copy of all coverages (rec’d from the City, rec’d from
County GIS, and the developed final coverage) to Flood Mitigation for the Project File.
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December 2006
TABLE 01 - TRANSLATION TABLE FOR FUTURE LAND USE PROCESSING
Future Land Use Category (Code)
Existing Land Use Translation (Code)
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.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
[<GREENWAY (GREENWAY)
INDUSTRIAL (IND1)
INDUSTRIAL - LIGHT (IND2)
INDUSTRIAL-HEAVY (IND3)
INSTITUTIONAL (INST)
INSTITUTIONAL_CHURCH (INST_CH)
INSTITUTIONAL_MEDICAL (INST_MED)
MULTI-FAMILY (MF)
MULTI-FAMILY <= 12 DUA (MF12)
MULTI-FAMILY <= 17 DUA (MF17)
MULTI-FAMILY <= 22 DUA (MF22)
MULTI-FAMILY <= 25 DUA (MF25)
MULTI-FAMILY <= 8 DUA (MF8)
MULTI-FAMILY > 25 DUA (MFGT25)
MOBILE HOME (MH)
SINGLE FAMILY/MULTI-FAMILY (MIX1)
SINGLE FAMILY/MULTI-FAMILY/INSTITUTIONAL/OFFICE/RETAIL (MIX10)
MULTI-FAMILY/OFFICE (MIX11)
MULTI-FAMILY/LIMITED RETAIL (MIX12)
MULTI-FAMILY/RETAIL (MIX13)
MULTI-FAMILY/UTILITY (MIX14)
MULTI-FAMILY/OFFICE/RETAIL (MIX16)
RESEARCH/OFFICE/RETAIL (MIX17)
INSTITUTIONAL/PARK (MIX18)
OFFICE/RETAIL (MIX19)
SINGLE FAMILY/OFFICE (MIX2)
OFFICE/INDUSTRIAL (MIX20)
MULTI-FAMILY/GREENWAY (MIX21)
MULTI-FAMILY/RESEARCH (MIX22)
MULTI-FAMILY/OPEN SPACE (MIX23)
MULTI-FAMILY > 12 /OFFICE/RETAIL (MIX24)
MULTI-FAMILY >12 /OFFICE/RETAIL/INDUSTRIAL (MIX25)
SINGLE FAMILY/MULTI-FAMILY/INSTITUTIONAL/OFFICE (MIX27)
MULTI-FAMILY/INSTITUTIONAL/OFFICE/RETAIL (MIX28)
SINGLE FAMILY/MULTI-FAMILY <= 8 DUA (MIX3)
INSTITUTIONAL/OFFICE/RETAIL (MIX30)
OFFICE/WAREHOUSE (MIX34)
Appendix C – CLT-BC Technical Reference
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User’s Guide - FINAL
C-4
>2 ACRES - RESIDENTIAL & OPEN SPACE (2)
INDUSTRIAL - HEAVY (8)
INDUSTRIAL - LIGHT (7)
INDUSTRIAL - HEAVY (8)
INSTITUTIONAL AREAS (6)
INSTITUTIONAL AREAS (6)
INSTITUTIONAL AREAS (6)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
0.5 TO 2 ACRES RESIDENTIAL (3)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
INDUSTRIAL - LIGHT (7)
INSTITUTIONAL AREAS (6)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
INDUSTRIAL - LIGHT (7)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
COMMERCIAL - LIGHT (9)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
COMMERCIAL - LIGHT (9)
INDUSTRIAL - LIGHT (7)
December 2006
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
76.
77.
OFFICE/RETAIL/LIGHT INDUSTRIAL (MIX35)
INSTITUTIONAL/RETAIL (MIX36)
SINGLE FAMILY/MULTI-FAMILY/OFFICE (MIX4)
SINGLE FAMILY/MULTI-FAMILY/INSTITUTIONAL (MIX5)
SINGLE FAMILY/MULTI-FAMILY/RETAIL (MIX6)
SINGLE FAMILY/OFFICE/RETAIL (MIX7)
SINGLE FAMILY/MULTI-FAMILY/RESEARCH/RETAIL (MIX8)
SINGLE FAMILY/MULTI-FAMILY/OFFICE/RETAIL (MIX9)
N/A (N/A)
NON-RESIDENTIAL O/C (NONRES1)
OFFICE (OFFICE1)
OFFICE/BUSINESS PARK (OFFICE2)
OFFICE/BUSINESS PARK/RESEARCH (OFFICE3)
OFFICE/BUSINESS PARK/LIGHT INDUSTRIAL (OFFICE4)
OFFICE/BUSINESS PARK/INDUSTRIAL (OFFICE5)
PARK/OPEN SPACE (OPSPACE)
>
PARKING (PARKING)
PRIVATE RECREATION (PRIVREC)
RESIDENTIAL/OFFICE (RES_OFF)
RESIDENTIAL/OFFICE/RETAIL (RES_OFF_RETAIL)
RESIDENTIAL/RETAIL (RES_RETAIL)
RESIDENTIAL/OFFICE (RES22_OFF)
RESEARCH (RESEARCH)
RESIDENTIAL (RESID)
RESIDENTAIL <= 12 DUA (RESID12)
RESIDENTIAL <= 17 DUA (RESID17)
RESIDENTIAL <= 22 DUA (RESID22)
RESIDENTIAL <= 4 DUA (RESID4)
RESIDENTIAL <= 5 DUA (RESID5)
RESIDENTIAL <= 6 DUA (RESID6)
RESIDENTIAL <= 8 DUA (RESID8)
RESIDENTIAL > 22 DUA (RESIDGT22)
RETAIL (RETAIL)
SINGLE FAMILY <= 1 DUA (SF1)
SINGLE FAMILY <= 3 DUA (SF3)
SINGLE FAMILY <= 4 DUA (SF4)
SINGLE FAMILY <= 5 DUA (SF5)
SINGLE FAMILY <= 6 DUA (SF6)
SINGLE FAMILY <= 8 DUA (SF8)
TRANSIT SUPPORTIVE DEVELOPMENT - MIXED (TSD-M)
Appendix C – CLT-BC Technical Reference
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User’s Guide - FINAL
C-5
INDUSTRIAL - LIGHT (7)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
0.5 TO 2 ACRES RESIDENTIAL (3)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
2 ACRES - RESIDENTIAL & OPEN SPACE (2)
TRANSPORTATION INCLUDING ROW (12)
> 2 ACRES - RESIDENTIAL & OPEN SPACE (2)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (9)
COMMERCIAL - LIGHT (7)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
COMMERCIAL - HEAVY (10)
0.5 TO 2 ACRES RESIDENTIAL (3)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
0.25 TO 0.5 ACRES RESIDENTIAL (4)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
COMMMERCIAL - LIGHT (9)
December 2006
78.
79.
80.
81.
TRANSIT SUPPORTIVE DEVELOPMENT - RESIDENTIAL (TSD-R)
UTILITY (UTIL)
WAREHOUSE/DISTRIBUTION (WARE)
WATER (WATER)
< 0.25 ACRES RESIDENTIAL/APT/ROW HOUSES (5)
INDUSTRIAL - LIGHT (7)
INDUSTRIAL - HEAVY (8)
STANDING WATER (11)
NOTES
1.
2.
Older base future land use coverage data contained codes "single fam", "residgt12", "resid22_no" and "comm" which have been deleted from the latest
base future land use coverage
The transportation category coded "road" will be added to the base future land use coverage during processin
Appendix C – CLT-BC Technical Reference
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December 2006
Appendix C – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
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December 2006
APPENDIX D
PROJECT MEMORANDUM DATED 10/10/07 (REVISED 10/29/07)
COMPARISON AND SENSITIVITY ANALYSIS OF
CHARLOTTE CIPS AND FEMA FIS
MECKLENBURG COUNTY FLOODPLAIN
MAPPING PROJECT
Project Memorandum
To:
David Goode, PE, CFM
From: Neal Banerjee, PE, CFM
Date: 10/10/07 (REVISED 10/29/07)
Re:
Comparison and Sensitivity Analysis of Charlotte CIPs and FEMA FIS
Mecklenburg County Floodplain Mapping Project
David,
Dewberry has performed a comparison and sensitivity analyses on selected Charlotte Storm
Water Services (CSWS) Capital Improvement Projects (CIPs) and FEMA Flood Insurance Study
(FIS) hydrologic and hydraulic analyses per our Task 2, Task Order #1 scope, dated 8/18/07.
The overall goal of the effort was to investigate potential measures that may be implemented by
Mecklenburg County Storm Water Services (MCSWS), CSWS, or both, to facilitate/improve the
interoperability of the hydrologic/hydraulic methods and results between the CIPs and the
County/FEMA analyses. The effort generally included a review and comparison of hydrologic
and hydraulic approaches/standards used by CSWS and MCSWS, sensitivity evaluation on
several hydrologic parameters, and development of recommendations. This memorandum
summarizes the approaches, findings, and recommendations from the comparison and
sensitivity analyses, and represents the “deliverable” of the agreed effort.
Executive Summary
Overall, there does not appear to be a consistent pattern between overall peak flows generated
from City CIP versus County/FEMA modeling – sometimes County/FEMA peak flows are higher,
sometimes CIP peak flows are higher. It is believed that this variability is due to the number of
parameters that influence peak flow. Based on the comparison and evaluation conducted as
part of this effort, the following recommendations to the County and City are offered below:
• The 24-hr SCS Type II storm distribution using rainfall depths derived from the USGS
combined IDF data without application of area-depth reduction factors is recommended
for the upcoming County floodplain mapping efforts. It is further recommended that this
storm pattern being used (or at least checked) for City CIP analyses.
• A 50-acre average target subbasin size should be considered for the upcoming County
floodplain mapping efforts. The City should continue using the smaller subbasin sizes
(typically 15 – 30 acres) for City CIP projects.
• The Modified Puls channel routing method is recommended for streams in which HECRAS (or similar hydraulic) models exist for both County floodplain mapping and City CIP
efforts. Level pool routing is recommended for ponds above the threshold subbasin
size and behind large embankments where excessive backwater conditions exist (e.g.
where backwater impacts upstream tributaries and/or other routing reaches).
• A minimum 72-hour storm simulation with a 1-minute time step is recommended for the
upcoming County floodplain mapping efforts. Similarly, a minimum 12-hour storm
simulation with a 1-minute time step is recommended for City CIP projects.
Background
MCSWS manages the major (drainage area > 1 sq. mi.) drainage system, including the
County/FEMA regulated streams and floodplain areas, whereas CSWS manages the minor (< 1
D1
December, 2006
Project Memorandum
sq. mi. drainage area) drainage system which is comprised of the structures, pipes, and smaller
channels providing localized drainage for streets and property. Since the focus and scale are
different (large-scale FEMA analysis versus small-scale local analysis), MCSWS and CSWS
have traditionally use different independent analytical approaches for assessment and
management of their respective systems. This difference in approaches has complicated the
direct comparison and assessment of inter-relationships between County and City projects. In
developing map/analytical standards for the floodplain mapping updates, MCSWS (in
coordination with CSWS) wished to investigate potential measures that may be implemented by
MCSWS, CSWS, or both, to facilitate/improve the interoperability of the methods and results
between the approaches. Dewberry was tasked to perform this investigation.
Review/Comparison of Hydrologic/Hydraulic Approaches and Standards
As indicated above the scale, focus, and approaches of the City CIPs and the County/FEMA
floodplain management efforts are different. Table 1 summarizes some of the primary City CIPs
and County/FEMA approaches and standards.
D2
December, 2006
Project Memorandum
Table 1. Comparison of City CIP Versus County/FEMA Approaches and Standards
Parameter
Project Scale
Project Focus
Primary Guidance
Document
Base Map/Survey
Hydrologic Model
Storm Pattern / Rainfall
Depth
Subbasins
Loss Methodology
Hydrograph
Transform/Lag Time
Routing
Calibration
Hydraulic Model
Cross Sections
Other Hydraulic
Parameters
Calibration
City
County/FEMA
Overall Project Information
Neighborhood level (typ. 200 - 500
Major stream watershed level (typ 1 ac drainage area)
40 sq mi drainage area)
Support planning, design, and
Develop floodplain/floodway mapping
construction of drainage system
along major streams for regulatory
improvements
purposed and mitigation projects
Charlotte-Mecklenburg Storm Water
FEMA Guidelines and Specifications
Design Manual
for Flood Hazard Mapping Partners
Detailed project-specific survey
Countywide GIS supplemented with
including channel survey every 50'
survey at structure crossings and
min, drainage system features, walls,
2-ft min contour mapping, etc.
sporadic channel areas
Hydrologic Parameters/Standards
Typically HEC-1/HMS, occasionally
HEC-1/HMS
SWMM or others
Charlotte 6-hour storm using
SCS 24-hr Type II storm using
Charlotte IDF depths
Charlotte IDF depths
Typically 15 - 30 acres in size,
considering influence of closed
Average 150 acres in size, based on
ground topography
drainage system
Typically SCS Curve Number
SCS Curve Number
Typically SCS Unit Hydrograph Using
SCS Unit Hydrograph Using TR-55
TR-55 Lag Time Method
Lag Time Method
Often normal depth channel routing
and level pool behind significant
embankment in existing conditions.
Level pool behind significant
embankments in future conditions
where storage can be maintained
Effective FIS uses automated
through acquisition of permanent
Modified Puls
easements.
Typically none, although peak flows
Effective FIS uses limited calibration
usually compared with FEMA,
to selected gages using global
regression, and/or other studies.
multipliers
Hydraulic Parameters/Standards
Typically HEC-RAS for open systems
and HGL based calcs for closed
HEC-RAS
system
Typically spaced every 200 feet or
less on average
Spaced every 500 feet on average
Standard use of ineffective areas at
Standard use of Manning's n values,
structure crossings and loss
coefficients. More conservative
ineffective areas at structure
crossings, and loss coefficients
Manning's n values.
Calibration to 1995 and 1997 storms
along selected streams
Typically none
D3
December, 2006
Project Memorandum
Comparison of Hydrologic Parameters and Peak Flows for Selected Projects
Dewberry compiled hydrologic parameter information and resulting peak flows for projects
where the information was readily available (e.g. provided by the City or was available inhouse). A summary of the compiled information is provided in Table 2. Attachment A provides
additional comparison information.
Table 2. Hydrologic Input Parameter and Result Comparison
City CIP
Project /
Neighborhood
Drainage
Area (ac)
Avg
Sub
(ac)
Q100EX
(cfs)
239
162
14
81
469
County/FEMA
CN
Tp
(min)
Avg
Sub
(ac)
Q100EX
(cfs)
747
553
78.3
77.5
21
23.4
112
154
235
1002
79.4
43.2
276
14
734
75.9
422
14
1322
CN
Tp
(min)
%Diff
Q100EX
698
683
76.4
77.9
30
15
-7%
24%
112
1868
79.5
25.2
86%
45
150
686
65.4
25.2
-7%
87.5
NA
106
1270
85.7
30
-4%
CherokeeScotland
Ivey's Pond
US Park Road
Pond/Eastbern
East
Providence
MyrtleMorehead
Shillington
Place
Louise Avenue
Nightingale
Lane
Jefferson
480
403
19
18
1227
1468
74.5
85.6
NA
22.2
154
141
1294
1580
78.9
82.8
30
22.2
5%
8%
173
864
16
26
670
1898
83.1
75.8
10.8
15
198
108
602
1535
84.5
73.7
34.8
55.2
-10%
-19%
Wilkinson
Tunnel
1875
268
3511
75.8
29.4
94
1669
60.2
75
-52%
NOTES:
1. Information obtained from CIP reports and/or hydrologic models and Effective FEMA HEC-1 models
2. Avg Sub = average subbasin size; Q100EX = 100-yr Existing Peak Flow, Tp = Calculated time from peak rainfall
to peak runoff; %Diff Q100EX taken as difference from City CIP values
As the table indicates, there does not appear to be a clear general pattern in peak flows
developed in City CIPs versus County/FEMA analysis. In some cases the CIP flows are higher,
in others, the County/FEMA flows are higher. Furthermore, the difference in peak flows varies
considerably (between 4% - 86%). In many cases, the differences in peak flows can be at least
partially explained by comparing the CN and Tp values. In general terms, peak flow increases
with increasing CN values and decreasing Tp values. For example, the difference in peak flows
on the Jefferson project is 19% (County/FEMA flow is 19% lower than CIP flow). In this case
the CNs are close, however, the Tp for the CIP is dramatically lower (15 minutes versus 55.2
minutes). There are other factors which impact the peak flow (e.g. precipitation, hydrologic
routing, model specifications, etc.). The following paragraphs discuss some of these other
potential influential parameters in more detail.
D4
December, 2006
Project Memorandum
Hydrologic/Hydraulic Parameter Investigation and Sensitivity Analysis
Dewberry investigated several of the hydrologic/hydraulic parameters listed in the table above in
further detail to provide a preliminary assessment of the response of the models to parameter
modifications, and the potential benefits (e.g. potential to improve model interoperability) and
costs (e.g. extra effort/cost required) to implement the modification in the models for either the
City or County approaches. The evaluations generally entailed modifying the selected
parameters (one at a time) in the existing Effective County/FEMA models and/or CSWS CIP
models at “representative” sample locations, re-running the models to gage the sensitivity of the
parameters, and then trying to extract trends from the analyses. The more detailed evaluations
focused on hydrologic approaches, however, the hydraulics models were used in some cases to
support the evaluations. The general approaches and findings are summarized below for each
parameter evaluated.
Storm Pattern / Rainfall Depth
Storm pattern was evaluated by comparing design hyetographs and peak hydrographs for the
Charlotte 6-hr 100-year storm and the SCS 24-hr Type II 100-year storm. Although the storm
pattern differs, both the City CIP and County/FEMA analyses use the Charlotte IntensityDuration-Frequency (IDF) information presented in the Charlotte-Mecklenburg Storm Water
Design Manual (CMSWDM) to specify rainfall depths. Comparison of the two different storm
patterns reveals the following findings and trends:
• The total rainfall depth of the 24-hr storm is approximately 30% greater than the 6-hr
storm (6.96 in vs. 5.34 in). The maximum intensity (over any 5-min period) is
approximately the same at 0.8 inches, however, it occurs sooner in the 6-hr storm (at 3
hours versus at 12 hours in the 24-hr storm). Attachment B is a graphical comparison
of the two storm patterns.
• At the individual subbasin level (direct runoff from an individual subbasin, keeping all
other parameters equal, based on hypothetical subbasins ranging between 5 acres and
1 square mile), the 24-hr SCS Type II storm flows tend to be approximately 28% higher
than those produced by the 6-hr Charlotte storm.
• At the City project/FEMA watershed level (runoff at the outlet of a project watershed
with multiple subbasins, routings, etc., based on sample runs with Briar-Little Sugar, Six
Mile, Cherokee/Scotland, Ivey’s Pond, and Park Road models), the 24-hr peak flow is
higher than the 6-hr, but the difference is more variable (generally 5% - 25%), averaging
around 15%.
The rainfall depths presented in CMSWDM were compared with results of a recent USGS
precipitation study (SIR 2006-5017) prepared in 2006. The USGS study developed several
independent families of IDF curves based on different precipitation gage networks and data
samples. Based on the statistical analysis, the study indicated that most appropriate family of
IDF curves to use for deriving general precipitation frequency characteristics in Mecklenburg
County is the combined “NOAA dataset plus aggregated USGS site representing the CRN initial
dataset” family, hereafter referred to as the “combined” dataset. Comparison of CMSWDM
versus the USGS combined precipitation reveals the USGS combined rainfall depths are slightly
higher in 100-yr storm, but equal to or slightly lower in the smaller (higher frequency) storms. All
storm depths for the same duration storms (e.g. 6-hr or 24-hr) are within 5 percent. Differences
in rainfall depth between the 24-hr versus the 6-hr duration storms are slightly higher in the
combined dataset versus the CMSWDM information (e.g. 34% difference in the USGS
combined versus 30% in the CMSWDM for the 100-year storm). Table 2 shows a comparison
of rainfall depths for the different information sources and storm durations.
D5
December, 2006
Project Memorandum
Table 3. Rainfall Depth Comparison
6-hour
Storm
Event
2
10
25
50
100
Design
Manual
IDF
2.28
3.72
4.38
4.92
5.34
2006
USGS
Combined
2.22
3.54
4.27
4.85
5.46
24-hour
%
Difference
-3
-5
-3
-1
2
Design
Manual
IDF
3.12
4.80
5.76
6.48
6.96
2006
USGS
Combined
3.06
4.80
5.76
6.51
7.29
% Diff 24-hr vs. 6-hr
%
Difference
-2
0
0
0
5
Design
Manual
IDF
37
29
32
32
30
2006
USGS
Combined
38
36
35
34
34
NOTES:
1. Design Manual IDF values from Charlotte-Mecklenburg Storm Water Design Manual (1993)
2. 2006 USGS Combined values from using combined "NOAA dataset plus aggregated USGS site" IDF
presented in SIR 2006-5017
The existing County/FEMA HEC-1 models include an area-depth reduction factor, which
reduces the precipitation amount as the contributing drainage increases. However, using the
24-hr storm and having watershed sizes typically being around 20 sq miles in Mecklenburg
County, precipitation reductions are mild (around 3% or less).
Review of this information in context with FEMA guidelines and specifications indicates the 24hour storm duration is more appropriate for County/FEMA floodplain analysis. The main factors
for concluding this are:
• The time of concentration for some of the larger watersheds (e.g. Briar-Little Sugar,
Irwin-Sugar, etc.) appears to be greater than 6 hours. FEMA guidelines dictate that the
storm durations should be longer than the time of concentration
• The 24-hr storm produces a more conservative peak flows. FEMA guidelines indicate to
generally use the higher peak-producing storm event when more than one is being
considered
• the 24-hr storm is a standard duration that is often used for watershed hydrology on this
scale and it is recommended in previous studies
The rainfall depth source to use is less sensitive, as the two IDF sources produce very similar
depths (within 5%). The benefits of using the USGS combined depths are: they are based on
more recent analysis which includes 15+/- more years of data than the CMSWDM depths and
they produce more conservative depths in the 100-year storm. However, using the USGS
depths would represent a diversion from the CMSWDM which is/has been the primary resource
for hydrologic assessment in Mecklenburg County, so that may be a factor to consider. Since
aerial reduction does not appear to have a significant impact, and using it potentially
complicates interpretation of the hydrologic results, it is probably simplest not to use it.
Storm Pattern / Rainfall Depth Recommendation: Given the above factors, it is
recommended that the 24-hr SCS Type II storm distribution using rainfall depths derived from
the USGS combined IDF data without application of area-depth reduction factors be initially
used for the upcoming floodplain mapping efforts. However, it may be appropriate to re-visit
these recommendations (especially related to the rainfall depths and area-depth reductions)
during the hydrologic model calibration process. The 24-hr SCS II storm could also be used for
City CIP projects. However, if the City did not desire to switch to the 24-hour storm, it is
recommended that the 100-year storm be run with the 24-hour SCS Type II storm (in addition to
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December, 2006
Project Memorandum
the 6-hour storm models currently being developed) to provide a more direct comparison with
County/FEMA models.
Average Subbasin Size
As indicated in Table 1, the average subbasin size differs considerably between typical City CIP
and County/FEMA hydrologic analysis. The City CIP subbasins are smaller and more detailed
to support the much more detailed/small-scale level of evaluation. The subbasin delineations
for selected City CIP projects were compared with the equivalent County/FEMA basins to first
glean additional insight (besides just size) on the impact of the different subbasin delineations
on the hydrologic analysis. The general findings/patterns of this comparison include:
• In general the existing County/FEMA subbasins along a given stream appear to be
delineated at arbitrary outlet points, whereas the City CIP basins are generally
delineated at more strategic locations (e.g. ponds/roads/culverts etc.)
• The minimum size basin that would be necessary to pick up the primary drainage pattern
for a City CIP project appears to be generally in the 60+/- range (i.e. the subbasin size to
have at least one subbasin for each primary stream/conveyance system) (based on
samples from Ivey’s Pond, Cherokee/Scotland, Taragate Farms, and Derita-Allen Hills).
• The influence of the closed system infrastructure (e.g. pipe systems) on subbasin
delineation is generally more significant as average subbasin size decreases. However,
once the subbasins size reaches 60+/- acres and above, the closed system
infrastructure does not appear to generally have a significant affect on delineation. In
other words, projects using an average subbasin size of less than 60 acres should
consider the effects of the closed system inventory, whereas, the impact of the closed
system infrastructure is generally not significant (and thus may not need to be
considered) for projects using an average subbasin size greater than 60 acres.
• In addition to specific CIPs, CSWS is currently working on a watershed ranking project
which will be used to help prioritize CIPs. It is understood hydrologic calculations based
on 50-acre basins will be used in the watershed ranking process. The City has
expressed a preference to use 50-acre subbasins for the floodplain mapping efforts.
A sensitivity analysis was conducted to assess the impact of subbasin size on the overall
watershed peak flows. The sensitivity analysis generally entailed modifying several City CIP
hydrologic models by grouping the CIP subbasins into the approximate 60-acre basins as
discussed above (i.e. minimum number of subbasins that would still differentiate the primary
drainage patterns). Results of the sensitivity analysis indicate that having more subbasins tends
to result in slightly higher (generally 1% - 4%) peak flows (based on sample runs from
Cherokee/Scotland, Ivey’s Pond, Jefferson, Park Road, Briar-Little Sugar, Four Mile, and Six
Mile). This would loosely suggest that peak flows for a watershed tend to increase as average
subbasin size decreases.
Average Subbasin Size Recommendation: Given the above factors, it is recommended that
an average target subbasin size of between 50 and 60 acres be considered for the upcoming
floodplain mapping efforts. Although this increased detail in basin delineation would likely not
have a significant affect on peak flows on the FEMA streams, it is believed that this size may be
a good middle ground for providing more frequent and strategic peak flow comparisons that will
allow for increased interoperability between City and County/FEMA models and be assist with
the City’s watershed ranking. Using 50 – 60 acre target subbasins would entail developing 2.5
– 3 times the subbasins and associated hydrologic parameters versus using 150 acre basins, so
there would definitely be an impact on the level of effort. It is estimated that decreasing the
target subbasin size from 150 acres to 50 – 60 acres will increase the level of effort between
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December, 2006
Project Memorandum
35% – 75%, depending on the level of automation for some of the hydrologic parameter
calculations. If the same detailed level of hydrologic parameter development evaluation that is
assumed for the 150-acre basins is used for 50 – 60 acre basins (most noticeably with Tcs and
routings), then the % increase would be closer to 75%. A detailed level of effort/fee for the
hydrology has not been developed at this time, however, it is estimated that the 35% - 75%
increased effort would potentially relate to roughly $20,000 - $45,000 in extra fees. These
estimates are preliminary, so this recommendation and associated level of effort may need to be
revisited when hydrologic/hydraulic analysis begins. No change to City CIP subbasin size is
recommended since the CIP basins are developed at the scale necessary to conduct drainage
analysis and design calculations.
Channel/Storage Routing
City CIP analyses often use the Muskingum-Cunge 8-Point routing for channels and level pool
routing for significant storage areas (based on CIPs reviewed for this effort), whereas, the
County/FEMA models use a Modified Puls Method approach. The Muskingum-Cunge 8-Point
approach is based on normal depth calculations where the Modified Puls method has the ability
to consider storage computed from water surface profiles including variable geometry and
structures. A sensitivity analysis was conducted to assess the impact the two methods have on
peak flow attenuation. The sensitivity analysis generally entailed replacing the MuskingumCunge routing with a Modified Puls based routing in the CIP hydrologic models. Parameters for
Modified Puls routing (i.e. storage-outflow relationships) were developed from the CIP HECRAS hydraulic models. The following observations are offered from the results of the sensitivity
analysis:
• The Modified Puls method generally results in higher attenuation (i.e. peak flow
reduction), especially in urban areas with multiple stream crossings and differing
floodplain geometries.
In the samples used in the sensitivity analysis
(Cherokee/Scotland, Shillington, Nightingale, and Taragate Farms) the overall peak
flows at the watershed outlet were generally 10 – 15% lower using Modified Puls. In
rural areas, the difference would likely be less.
• The Muskingum-Cunge 8-Point method generally shows minimal (typically < 2% based
on sample runs from projects mentioned above) attenuation in individual channel routing
reaches along CIP streams. The Modified Puls method generally shows more
attenuation, however, it varies considerably (generally 2% - 15%).
• The Modified Puls method is believed to be more accurate (especially in urban areas)
since the channel storage can be obtained from the HEC-RAS models (if available),
which “sees” all the cross sections in the reach (including culverts), rather than being
derived from normal depth calculations based on a single “representative” cross section.
A separate sensitivity analysis was not conducted for storage routing (e.g. ponds, areas behind
large embankments, etc.), however, the following observations are offered based on a review of
CIP and County/FEMA models, and previous experience:
• It is believed that Modified Puls (based on HEC-RAS results) and Level Pool storage
routing techniques will produce similar attenuation in most circumstances.
• The Level Pool technique may be more appropriate for actual in-line ponds or extreme
backwater conditions (e.g. behind CSX railroad on Briar Creek where backwater extend
onto multiple streams and/or across multiple routing reaches), however, for typical urban
roadway embankments the Modified Puls method is believed to be adequate and is
recommended if Modified Puls is used for channel routing.
• The Modified Puls storage-outflow values in the County/FEMA Effective models are
believed to have been largely derived from an automated normal depth calculation type
D8
December, 2006
Project Memorandum
•
•
approach, which in many cases is not consistent with the storage-outflow values that can
be derived from actual HEC-RAS results.
FEMA Guidelines and Specifications (Appendix C, page C-23, 1st full paragraph) indicate
that if storage is accounted for behind structures (culverts, bridges, etc.), the storage
area should be included in the floodway (i.e. to restrict encroachments), or the flows
downstream of the structure should be computed based on routing the hydrograph
through the reduced storage area.
In-line ponds that are upstream of County/FEMA-mapped streams are not accounted for
in the existing County/FEMA Effective models. Project experience (e.g. Park Road Park
Lake, Ivey’s Pond, etc.) indicate these ponds may have a significant impact peak flow
reduction.
Channel/Storage Routing Recommendation: Given the above factors, it is recommended
for both City and County/FEMA efforts that Modified Puls routing based on HEC-RAS outflowstorage values be used for channel routing (including typical roadway embankments), where
HEC-RAS modeling exists, for existing conditions models. Muskingum-Cunge 8-Point or some
other more approximate routing technique may be appropriate for channel routing reaches in
which HEC-RAS modeling does not/will not exist, or potential for more conservative future
conditions modeling. The recommended routing technique for non-FEMA streams may need to
be revisited during hydrologic and hydraulic modeling and/or calibration. Level pool routing is
recommended for all in-line ponds with drainage areas greater than the target subbasin size
(e.g. 50 - 60 acres) (or some other specified minimum size to be determined later) and
potentially at large embankments with excessive backwater conditions.
Using the Modified Puls method as recommended would require additional effort. The modeler
would need to develop a rating curve based on a range of flows in HEC-RAS, which in turn
would be used in the hydrologic HEC-1/HMS model. This process may complicate the analysis
(especially for improvement analysis where multiple iterations between HEC-RAS and HEC1/HMS could be required when the hydraulic system is altered such that it significantly impacts
the rating curves). However, it is believed that using Modified Puls would increase the effort for
performing routing calculations minimally in existing conditions analysis (probably 5%+/-) and
possibly 10 – 20% for improvement analysis. In addition, available City pond information
indicates that there are approximately 17 ponds within the Briar-Little Sugar watershed with a
contributing drainage area greater than 50 acres, and 10 ponds with drainage area greater than
120 acres. Detailed outlet structure (e.g. weirs, riser-barrels, etc) information would be
necessary to conduct level pool routing for these ponds. The availability of this data may
influence the decision to use as a minimum drainage area threshold before considering for
level-pool routing. It is estimated that it would cost between $500 and $1,000 per pond to
collect the outlet structure information if it was not already available. Another potential
consideration (not necessarily a recommendation) for the upcoming County/FEMA floodplain
mapping efforts is to use the recommended routing for just the existing conditions modeling, and
either remove channel routing (i.e. use simple lag) or use a simplified or reduced routing method
for the future land use runs. If this were implemented, it would also help improve the
interoperability between the City and County/FEMA analyses.
Model Specifications
City CIP analyses often use model specifications (i.e. control specifications) with a 1 or 5 minute
time and a simulation duration ranging from 6 hours to 24 hours. The County/FEMA models
use a 5 minute time step with an approximate 3.5 day simulation duration. A sensitivity analysis
was conducted to assess the sensitivity of the time step variable. The analysis consisted of
D9
December, 2006
Project Memorandum
changing sample City CIP models that used a 1 minute time step to a 5 minute time step, and
converting the selected County/FEMA models from a 5 minute time step to a 1 minute time step.
The following observations are offered based on the sensitivity analysis:
• Peak flow appears to generally increase as the time step decreases. In the samples
used in the sensitivity analysis the overall peak flows at the watershed outlet were
generally 3 - 5% higher using a 1 minute time step versus a 5 minute time step for both
modified City CIP models and County/FEMA models.
The following additional observations are offered based on a review of CIP and County/FEMA
models, and previous experience:
• Although it did was not evident in the samples used for the sensitivity analysis, it is
believed that the difference in peak flows could increase for smaller-scale projects with
flashy peaks, since a longer time step may “miss the peak”.
• If using HEC-1 with simple lag routing using an RT card (e.g. this is sometimes used to
simply lag or shift a hydrograph through a closed system without accounting for any
attenuation), the lag time is specified as an integer of the time step. Thus, changing the
time step in models with this use of the RT card, requires changing the lag value in the
RT card.
• Model simulation duration does not affect the peak flow as long as the model is
simulated long enough to capture the peak. For the County/FEMA watershed the peak
flow occurs within 28 hours or less from the commencement of rainfall. The time to
recede back to base flow appears to be longer than 83 hours (i.e. longer than the
simulation time) for the larger watersheds. For City CIPs the peak flow generally occurs
within 6 hours or less, and the time to recede back to bass flow appears to be 12 hours
or less (generally 8 hours or less). Having the full hydrograph generally would not be
necessary for CIP and/or County/FEMA analyses, but may be beneficial for some uses
(e.g. calibration, figuring recession times, etc.)
Model Specifications Recommendation: Given the above factors, it is recommended that
a 1 minute time step be used for the upcoming floodplain mapping efforts. Although a
significant change in peak flows is not expected, it would improve the interoperability with
most City CIP models. It is believed that the extra computation processing and or data
storage requirements associated with the smaller time step would be negligible on today’s
computers. A simulation duration equal or longer than 72 hours is recommended for
County/FEMA analysis. For City CIP analysis the same 1 minute time step, and a minimum
12 hour simulation duration is recommended.
Hydraulic Parameters
In general, many of the hydraulic parameters/approaches used by the City and County/FEMA
are similar. This is largely due to the fact that both agencies use HEC-RAS for modeling
streams. The primary difference is the City CIPs extend into drainage systems that are
predominately closed systems (e.g. pipe systems), whereas, the County/FEMA models are
almost all open system with a few closed system components (e.g. under Independence Blvd).
The City typically uses HGL programs (e.g. spreadsheets, StormCAD, etc.) for the closed
systems. The current Effective County/FEMA models simplify the limited close systems so they
can still be modeled in HEC-RAS. Since the scale and purpose of the hydraulic analysis for the
City CIP and County/FEMA efforts are different and essentially independent (unlike the
hydrologic analysis, where the two approaches “overlap”), and the fact that both agencies use
similar approaches, no changes to the current approaches are offered at this time.
D10
December, 2006
Project Memorandum
Conclusions
Dewberry has reviewed and compared hydrologic and hydraulic approaches/standards used by
the City for CIPs and by the County/FEMA for floodplain mapping efforts, and has conducted
sensitivity analysis on several of the hydrologic parameters. Overall, there does not appear to
be a consistent pattern between overall peak flows generated from City CIP versus
County/FEMA modeling – sometimes County/FEMA peak flows are higher, sometimes CIP
peak flows are higher. It is believed that this variability is due to the number of parameters that
influence peak flow. Sensitivity evaluation of several the hydrologic parameters revealed
general trends and tendencies in the data. Storm pattern and rainfall depth significantly
influence the County/FEMA flows to be higher, whereas, channel/storage routing, basin size,
and model specifications influence the CIP flows to be higher.
Based on the comparison and evaluation conducted as part of this effort, the following
recommendations to the County and City are offered below:
• The 24-hr SCS Type II storm distribution using rainfall depths derived from the USGS
combined IDF data without application of area-depth reduction factors is recommended
for the upcoming County floodplain mapping efforts. It is further recommended that this
storm pattern being used (or at least checked) for City CIP analyses.
• A 50-acre average target subbasin size should be considered for the upcoming County
floodplain mapping efforts. The City should continue using the smaller subbasin sizes
(typically 15 – 30 acres) for City CIP projects.
• The Modified Puls channel routing method is recommended for streams in which HECRAS (or similar hydraulic) models exist for both County floodplain mapping and City CIP
efforts. Level pool routing is recommended for ponds above the threshold subbasin
size and behind large embankments where excessive backwater conditions existing
(e.g. where backwater impacts upstream tributaries and/or other routing reaches).
• A minimum 72-hour storm simulation with a 1-minute time step is recommended for the
upcoming County floodplain mapping efforts. Similarly, a minimum 12-hour storm
simulation with a 1-minute time step is recommended for City CIP projects.
References
Charlotte-Mecklenburg Storm Water Services (CSWS), (date varies). Capital Improvement
Project Information (reports and/or models) for the following projects: Cherokee-Scotland, East
Providence, Ivey’s Pond, Jefferson, Louise Avenue, Myrtle-Morehead, Nightingale Lane, Park
Road, Shillington, and Wilkinson Tunnel
CSWS, 1993. Charlotte Mecklenburg Storm Water Drainage Design Manual.
City of Charlotte, 2006 (frequently updated). Charlotte Land Development Standards Manual.
Federal Emergency Management Agency (FEMA), 2004. Flood Insurance Study –
Mecklenburg County, North Carolina and Incorporated Areas.
FEMA, 2003. Guidelines and Specifications for Flood Hazard Mapping Partners.
Unities State Geological Survey (USGS), 2006, Frequency of Annual Maximum Precipitation in
the City of Charlotte and Mecklenburg County, North Carolina, through 2004. SIR 2006-5017.
D11
December, 2006
Project Memorandum
ATTACHMENT A –
Charlotte CIP versus FEMA Hydrologic Comparison
D12
December, 2006
ATTACHMENT A: HYDROLOGIC INPUT AND PEAK FLOW COMPARION
CITY CIP VERSUS COUNTY/FEMA ANALYSIS
Mecklenburg Floodplain Mapping Project
October 2007
Project
Cherokee-Scotland
Ivey's Pond
CIP Information
Ave
Basin Hydrololgic 100-yrEX
Model
Qp (cfs)
Area
0.02 HEC-1
747
Drainage
Area (sq mi) # Basins
0.37
17
0.25
2
100-yrFU 10-yrEX FEMA HEC- Drainage
CN-EX Tqp (min) Qp (cfs) Qp (cfs)
# Basins
1 ID
Area
78.3
21
971
485 BC68C
0.35
2
0.13 HEC-1
553
77.5
23.4
43.2 NA
Park Road
0.73
2
0.37 HEC-1
1002
79.4
East Providence
Myrtle-Morehead
0.43
0.66
20
30
0.02 HEC-1
0.02 XP-SWMM
734
1322
75.9
87.5 NA
Shillington Place
0.75
25
0.03 HEC-HMS
1227
74.5 NA
45
602
306 FMRB8
0.7
1181
329 SM2C
956 LSC44C
0.47
0.66
1709
772 MM47C
0.72
2155
861 LSC32C
793
360 MM60
1197 MASR7R
1837 SGTC13CC
568 LLS8C
NA
Louise Avenue
0.63
22
0.03 HEC-1
1468
85.6
22.2
Nightingale Lane
Jefferson
0.27
1.35
11
33
0.02 HEC-HMS
0.04 HEC-1
670
1898
83.1
75.8
10.8
15 NA
Wilkinson Tunnel
2.93
7
0.42 HEC-1
3511
75.8
29.4
4633
0.24
1
FEMA Information
Ave
100-yrEX
Basin
CN-EX
Qp
Area
0.18
698
76.4
Tqp
100-yrFU 10-yrEX
Qp (cfs) Qp (cfs)
30
684
399
Notes
Comparion of East Trib above Ivey's pond
(no pond attenuation) (HEC-1 ID 2C). At
outlet below Ivey's Pond, comparison is
773 FEMA vs. 262 CIP (has 3 ponds total
in area) in 100yrEX.
CIP flows does not include pond
attenuation. After pond attenuation, CIP
100yrEX flow drops to 650. Eastburn CIP
(US of pond) 100yrEX is 983 for apprx .67
sq mi area.
50% attenuation at Winthrope Ridge Rd in
upper portion of basin
Primarily closed system
The attenuation affect at street crossings
in the existing conditions is minimal.
Attenuation was provided for the future
model. Basin file corrupt so time to peak
not available.
Consultant also ran future w/ attenuation
(100yr=1498,10yr=884)
CIP model is calibrated using earlier
models.
0.24
683
77.9
15
776
403
4
0.18
1868
79.5
25.2
1879
1307
2
4
0.24
0.17
686
1270
65.4
85.7
25.2
30
948
1381
330
772
3
0.24
1294
78.9
30
1390
757
0.66
3
0.22
1580
82.8
22.2
1669
870
0.31
1.18
1
7
0.31
0.17
602
1535
84.5
73.7
34.8
55.2
622
1733
379
901
2.63
18
0.15
1669
60.2
75
1669
CN in FEMA model calibrated using a
760 multiplication factor of 0.8
Notes:
1. all CIPs run with Charlotte 6-hr storm, FEMA with SCS 24-hr Type II storm
SUMMARY
Project
Cherokee-Scotland
Ivey's Pond
Park Road
East Providence
Myrtle-Morehead
Shillington Place
Louise Avenue
Nightingale Lane
Jefferson
Wilkinson Tunnel
CIP 100yrEX FEMA 100 %Diff
CIP Tp
747
698
-7%
21
553
683
19%
23.4
1002
1868
46%
43.2
734
686
-7%
45
1322
1270
-4%
NA
1227
1294
5%
NA
1468
1580
7%
22.2
670
602
-11%
10.8
1898
1535
-24%
15
3511
1669
-110%
29.4
FEMA Tp
30
15
25.2
25.2
30
30
22.2
34.8
55.2
75
%Diff
30%
-56%
-71%
-79%
NA
NA
0%
69%
73%
61%
D13
December, 2006
Project Memorandum
ATTACHMENT B –
Charlotte 6-hour Versus SCS 24-hour Precipitation Input Plot
D14
December, 2006
Attachment B: Rainfall Intensity and Cumulative Precipitation Comparison:
6-hr 100-yr vs. 24-hr 100-yr (5-min input intervals)
0.9
0.8 in/5-min Maximum Intensity
8
0.8 in/5-min Maximum Intensity
0.8
6.96 in Cumulative Depth
6
5.34 in Cumulative Depth
0.6
5
0.5
4
0.4
3
0.3
Cumulative Precipitation
Incremental Precipitation
0.7
7
2
0.2
0.1
1
0.0
0
0
2
4
6
8
10
12
14
16
18
20
22
24
TIME (HR)
D15
December, 2006
APPENDIX E
SAMPLE DATA COLLECTION
FOR STREAM CROSSINGS
Sample Data Collection for Stream Crossings
Mapping Contractor shall develop a spreadsheet to monitor and verify data collection for stream crossings. An example spreadsheet
could have the following field names with description and sample data for Edwards Branch Station 11843 (EDB – 012) shown below.
(See Figure XX at the end of this appendix for a complete spreadsheet showing stream crossings data collection on Edwards Branch)
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-1
December 2006
Stream Crossing Sketch (EDB-012_SKT.JPG)
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-2
December 2006
Field Photo of Downstream Channel (EDB-012_DSC.JPG)
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-3
December 2006
Field Photo of Downstream Structure Face (EDB012_DSF.JPG)
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-4
December 2006
Field Photo of Upstream Channel (EDB-012_USC.JPG)
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-5
December 2006
Field Photo of Upstream Structure Face (EDB-012_USC.JPG)
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-6
December 2006
Figure A – Stream Crossing Verification for Edwards Branch (Note: Figure broken in
two parts to fit in page)
Break in Figure A
at this column
Break in Figure A
continues from this
column
Appendix E – CLT-BC Technical Reference
Charlotte Benefit/Cost Model
User’s Guide - FINAL
E-7
December 2006