Floodplain Mapping
at a Local Level
Tim Trautman, PE, CFM
Flood Mitigation Program Manager
July 24, 2008
Floodplain Mapping at a
Local Level
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Overview
Introduction to Charlotte-Mecklenburg Floodplains
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Floodplain Mapping History
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Community Floodplain
Map Maintenance
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Background, Mapping Purpose & Goals
Map Changes (LOMC’s)
Strategy
Map Maintenance Implementation
Future of Map Maintenance
Charlotte-Mecklenburg
 “Top of the hill”
 5% of City floodplain
 Urban, national average is 7.5%
floodplain
Uptown Charlotte
Mecklenburg County
Watersheds
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~350 miles of FEMA
streams
32 watersheds
2000+ miles of non-FEMA
streams & open/piped
drainage systems
All water in Mecklenburg
County (except Catawba
River) originates in
Mecklenburg County
History of Floodplain
Mapping
Floodplain mapping identifies areas of flood hazards
and is required for communities participating in the
National Flood Insurance Program.
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National Flood Insurance Act of 1968
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Required mapping of flood-prone areas
Made flood insurance available in communities that
meet floodplain management requirements
Flood Disaster Protection Act of 1973
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Mandated purchase of flood insurance for insurable
structures within flood-prone areas as condition of
receipt of Federal or federally related financing
Benefits of
Floodplain Maps
To provide accurate flood risk information to reduce
loss of life and property
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Flood Insurance
Guide future development away from flood hazard areas
Require that new and substantially improved buildings
be constructed to resist flood damage
Provide homeowners with financial assistance after
floods, especially after smaller floods that do not
warrant federal disaster aid
Used to guide local Flood Mitigation Strategies
Many, Many, More……
Benefits of
Floodplain Maps
WHY should Floodplain maps updated?
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More accurate Flood hazard information to reduce loss
of life and property
Keep up with changing flood hazards
Accurately communicate Risk
Benefits of Local Ownership
& Maintenance of Maps
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Floodplain Mapping is an effective mitigation technique
Desire to Have Updated, Accurate Maps
Floodplain Modeling is Not Easy In Highly Urban Areas
Ability to customize for local needs (Future Conditions And
Two Floodways)
Local Knowledge of Conditions
Missing the floodway by a few feet or elevations by a couple
of feet can mean differences of $Millions on property values
Community input and engagement
Local Ownership –
Mapping as a CTP
LOCAL COMMUNITY RESPONSIBILITIES
 Assist ($$ and time) with restudy
 Developing maps/studies that meet local and FEMA needs
 Maintaining maps to meet local and FEMA needs
KEYS TO SUCCESS
 Experienced In-house Staff to Review Models, Manage
Consultants
 Ability to Hire 3rd Party Consultant to Review Studies
 Good Relationship w/ FEMA
Floodplain Map History
Charlotte-Mecklenburg Overview
•Land Use and Topographic Data
1997
•Future Build-out District Plans
1999
•Adopted (Local – Regulatory)
5/22/00
•Effective (FEMA - Flood
Insurance)
2/4/04
•Map Maintenance
Strategy/Planning
2005-2006
•Map Maintenance Implementation
Floodplain Map History
Overview - Floodplain Mapping in 1999
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1st CTP east of Mississippi
Pilot Study
Decision to map to Future Conditions
Interim Policy
Develop new maps
Adopt locally
FEMA approval
Pilot Study & Map
Development
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Mallard Creek Watershed
 Mix of urban and rural
GIS data used:
 Topography
 Soil Types
 Landuse
 Past, present and
future
Pilot Study
Existing Landuse:
Pilot Study
Future Landuse:
Pilot Study Results:
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FEMA Floodplain (1999 landuse)
 Average change: +1.9’
FLUM Floodplain (Future landuse)
 Average change: +4.3
Set Interim Policy
New Construction must be 5.7’
above FEMA BFE until new
maps are adopted locally
Developing the
Floodplain Maps
1999 - 2000
Floodplain on Plans
February 2004 FIRM
Floodplains
Community (Future Conditions) Floodplain
FEMA (Existing
Conditions) Floodplain
FEMA Floodway
Community Encroachment Area
Floodplain
Regulations
and the
Flood
Insurance
Rate Map
(FIRM)
FEMA Floodplain
Area
Community Floodplain
Area
Mapping Results
Flooded Structures
32% Increase
3500
3000
2500
2000
1500
1000
500
0
FEMA '75
Current '04
Future '04
Mapping Results
Flooded Structures
32% Increase
59% Increase
3500
3000
2500
2000
1500
1000
500
0
FEMA '75
Current '04
Future '04
Mapping Results
Flooded Structures Structure Flooding
Avoided
3500
1300
3000
2500
2000
1500
1000
500
0
FEMA '75
Current '04
Future '04
Remapping Cost in 1999
Total Cost = $ 2 Million
Local Map Adoption
Prior to FEMA
Pros
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Ensure construction above new, accurate BFE’s
Property owners get heads up on flood insurance
needs, flood risk etc
Provides a chance to get public input on maps prior to
FEMA Submittal
Provides data to assist with Mitigation Planning
Interim Policy
Local Map Adoption
Prior to FEMA
Cons
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Have two sets of maps and regulations
Confusing to designers, developers, reviewers
Model Management
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Construction in Floodway, which model to use?
Approval from whom?
Delay of FEMA Process
Enforcement of “New”
Maps
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New Floodplain terms
2 Floodplains, 2 Floodways
Higher standards on Maps  Higher standards in ordinance
Significant updates to Floodplain Ordinance
Map Maintenance Program
WHY should Floodplain maps be Maintained?
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Floodplain Maps are Quickly Outdated
Current FIRM Panels
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Topographic Information 1997
Land use/Land cover 1997
Building footprints 1995
Streets 2002
Floodplain Modeling 2002
Explosive Growth “Development” in watershed
Communicating Risk that has potentially changed
Protecting citizens through Flood Insurance
Map Maintenance Strategy
Charlotte-Mecklenburg Approach to Map Maintenance
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Formulated Strategy in 2004 – 2005
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Developed Criteria & Ranking Scheme 2006
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Watershed based approach
Continuous program (1-2 Watershed/Zones per year)
Evaluate each watershed every 3-5 years
Evaluate priorities (Re-rank) 1-2 years
Local Implementation in 2007 (no Federal Funding)
Expanded Implementation in 2008 (Federal cost
share)
Map Maintenance
Charlotte-Mecklenburg Approach to Map Maintenance
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Two Components
Letter’s of Map Change
(LOMR, LOMR-F, LOMA)
Other Risk-based Changes
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Physical watershed changes
Climatological Changes
Technical Changes/Advancements
Letter of Map Change
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LOMC Pilot Program
Effective July 2006
State Pilot to North
Carolina
Local – Mecklenburg
County
Regional - Denver Urban
Drainage District
LOMCs Processed
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Conditional and Final
Letters of Map Revision
FEMA MT-2 Forms only
No MT-1 Forms: LOMRFs, LOMA, CLOMAs and
CLOMR-Fs
Benefits of Local LOMC
Review & Ownership
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Promotes local ownership of Flood Maps
Increases ability to maintain map data at local level
Increased effectiveness in maintain Floodplains and
Models (Floodzone Lookup)
Compress Floodplain Permitting & CLOMR Review
Meetings with applicant
Local knowledge of the floodplains
Site visits
Map Maintenance
Criteria
FEMA CATEGORIES
1.
2.
3.
Physical watershed changes
Climatological Changes
Technical Changes/Advancements
Charlotte-Mecklenburg Criteria
 Age of Effective Analysis
 Floodplain Development
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Floodplain Development Permits
LOMAs/LOMR-Fs
Effective LOMR’s
Map Maintenance
Criteria
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Historical Flooding Information
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Flooding Exceeding Estimated Base Flood Elevation
NFIP Claims Factors Affecting Hydrology
Population Growth
Building Permits
Minor System Capital Improvement Projects
Factors Affecting Hydraulics
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Construction of New Bridges or Culverts
Major Stream Capital Improvement Projects
Map Maintenance
Criteria
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OTHER FACTORS (Current and future)
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Changes in Topography (LIDAR comparison)
Land Use Comparison (Existing & Future)
Climate Change (precipitation data)
Technology (models, LIDAR, etc)
Map Maintenance Zones /
Watersheds
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~350 miles of FEMA streams
7 watershed based zones
Zone
Steam Length
Description
(Miles)
Reedy/Goose
37.9
LSC/Briar
37.5
McDowell/Gar
27.8
McAlpine/Four Mile
81.9
Sugar/Irwin
60.8
Long/Paw
51.2
Mallard/Rocky River
65.8
Map Maintenance Zones
Ranking Procedure
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Collected data within each Zone/Watershed
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Computed quantities per criteria
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GIS datasets
GIS computations
Assigned points to each Zone/Watershed based on
each criteria
Computed total points per Zone/Watershed
Normalized using stream miles
Ranking
Spreadsheet
Results
Re-Mapping Zone Ranking Using
Normalized Data
Zone
Ranking
Description
Steam Length
(Miles)
Total
Normalized
Points
Points
1
Reedy/Goose
37.9
475
12.5
2
LSC/Briar
37.5
265
7.1
3
McDowell/Gar
27.8
175
6.3
4
McAlpine/Four Mile
81.9
460
5.6
5
Sugar/Irwin
60.8
300
4.9
6
Long/Paw
51.2
240
4.7
7
Mallard/Rocky River
65.8
215
3.3
Coordination with
Statewide Map Maintenance
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Currently working toward increased coordination of
Map Maintenance needs ($ per year)
Estimate Cost “need” ($) per Watershed/Zone
Convey short term need ($) to State – 1-2 years
Fit local data within state format/scheme
Implementing Map
Maintenance Strategy
2007 - present
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Year 1
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Create mapping-level framework for future years
(Mapping Standards Guidance Document)
1 Watershed/ Zone Briar & Little Sugar Creeks
Federal funding not yet available for map
maintenance
$600,000 local (including seed $)
Year 2
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2 Watersheds/Zones
$900,000 FEMA FY08 funding
$300,000 local
Floodplain Mapping
Standards
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Meet FEMA Guidelines and
Specifications
Ensure quality and consistency
in watersheds
Include lessons learned from
previous maps & experience
Technical standards developed
as part Task Order 1
Standards will be used
throughout map maintenance
effort
Floodplain Mapping
Standards
Technical Standards Categories - Highlights
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Field Cross Section
Survey:
Field Data Collection
Standards
# Field Data Cross Section Locations = Stream Length to be Modeled (feet) / 1,100
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Channel Cross Section Information
Stream Crossing
Information
Geo-reference Stream Crossings:
Provide a spatial point location for all bridges, culverts and dams/weirs in GIS file
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Hydrology Analysis LiDAR:
Standards
Storm Pattern
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Target Subbasin Size
Storm Pattern:
Duration withModels
SCS Type II Distribution, a minimum 72-hour storm
 24-Hour
Balancing
simulation with a 1 minute time step
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Flow Change Points
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Meet or exceed NCFMP standards and be no more than two years old
Target Subbasin Size:
60 acres
Floodplain Mapping
Standards
Technical Standards Categories - Highlights
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Target Cross
Section Spacing:
Hydraulic Analysis
Standards
500 feet
Cross Section Spacing
Modeling Buildings:
 Accounted
Modeling
Buildings
for through
the use of Manning’s n adjustments (general case) or
blocked obstructions (extreme case)
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Floodways (FEMA & Community)
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Modeling FEMA Floodway:
Model
Calibration
Use 0.5 ft surcharge with modifiedStandards
100-year (existing conditions) peak discharges
accounting for future fill in the floodplain
Mapping Standards
Peak flow
Floodway
Delineation
and total volume
comparison shouldaround
match withinBuildings
10%, and the time to
Calibration of Hydrology:
Community Floodplain Boundary Delineations
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
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to other available data and adjusted as appropriate.
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.
Floodplain Mapping
Standards
Summary
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Developed by Charlotte-Mecklenburg and Contractor
Performed 2-3 pilot/sensitivity studies (ex – Considered
using steady state models)
Reviewed by group of City & County Engineers
Reviewed by Independent QA/QC Contractor
Comment period for Local Engineering Community
Endorsement by appointed Storm Water Advisory
Committee
Adjustments to document as needed during mapping
Public Involvement
Charlotte-Mecklenburg “Sandwich
Approach”
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Base Data Collection
Public Input (Land Use
Task Force)
H & H, Floodplain Analysis
Public Input (watershed
map meetings)
Panel & FIS Production
Public Input (FEMA
process)
Implementing Map
Maintenance Strategy
YEAR 1 – Briar & Little Sugar Creeks
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Drainage Area of 50 square
miles
Stream Length of 40 miles
Contracts ~ $600,000
 Mapping Contractor
 QA/QC Contractor
Implementing Map
Maintenance Strategy
YEAR 1 – Briar & Little Sugar
Creeks
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Topographic information
 LiDAR
 Detailed Survey Channel
Aerial Photography
Updated Streets
Updated building locations
Updated stream crossing
information
Existing Conditions
Land Cover
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Estimates “existing”
impervious area
Land use maps and
aerials
No straight adoption of
the land use
Aerial photography to
verify current
development
Future Conditions
Land Cover
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Estimating full build out
conditions
Planning Commission’s Land
Use plans as starting point
80 future land use categories
Translate to 12 categories
impervious area categories for
floodplain mapping
Comparison – Existing to Future
Public Involvement
Floodplain Land Use Task
Force
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Live or have ownership interests
within watershed
10 meetings
Contentious on land use map
translation for floodplain
mapping
H & H, Floodplain
Analysis
YEAR 1 – Briar & Little Sugar
Creeks
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Meat of the Sandwich
Currently Underway
Implementing Mapping
Standards Guidance Document
Implementing Land Use maps
into Hydrology
Next Steps
YEAR 1 – Briar & Little Sugar
Creeks
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FINISH THE SANDWICH
 FEMA & State Coordination
 Public Input (watershed map
meetings)
 Panel & FIS Production
 Public Input (FEMA process)
 New Effective Maps & FIS
Next Steps
YEAR 2
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2 Watersheds/Zones
Drainage Area of 125 square miles
Stream Length of 120 miles
FEMA Mapping Process
YEARS 3 - 5
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4 Watersheds/Zones Remain
55% Stream Length Remains ~200 miles
Leverage FEMA/State Funding
Future of Map
Maintenance
Map Maintenance Discussions
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All Digital Maps with print on demand
Data Federation
 Technology/Data storage & ownership
Data Stewardship
 Custodian with varying levels of
responsibility
Integrated, mapping, assessment and
planning life cycle
Future of Map
Maintenance
Future of Map
Maintenance
Charlotte-Mecklenburg comprehensive approach
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Identify Risk
 Hazard Mapping
 Map Maintenance (keep risk
current)
Access Risk
 Flood Mitigation Planning
 All-Hazards Planning
 Updated land data
 Elevation Certificates
Future of Map
Maintenance
Charlotte-Mecklenburg comprehensive approach
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Communicate Risk
 Floodzone web application
 Merge map data with other data to better
communicate risk
Mitigate Risk
 Implement elements in Mitigation Plans
 Acquisitions
 Elevations
Closing Thoughts…
Take ownership of managing Flood Risks
 Various levels of stewardship in mitigation
 Don’t need to do it all yourself (partnerships)
Think holistically
 Maps, Regulations, Mitigation Plans, Mitigation Projects,
Communication
Live in the present AND future
 Communicate Existing Risk
 Communicate and Regulate Future Risk
 Determine what type of “Future” mapping is appropriate
QUESTIONS &
BREAK