1. A Holistic and Integrated Approach to Floodway Modeling/Delineation
NC CFS Training Seminar October 2002
A Holistic and Integrated Approach to Floodway Modeling/Delineation
Presentation by:
Michael A. Hanson, P.E., LEED AP
Shweta Chervu, P.E., CFM
May 17, 2011
Course 1

2. NC CFS Training Seminar October 2002
INTRODUCTION
NC CFS Training Seminar October 2002
Floodway is a tool to assist communities in balancing development within the floodplain against the resulting increase in flood hazard.
Appendix C: Guidelines and Specifications for Flood Hazard Mapping Partners.
Need to include a map with color coded community floodways
Course 1

3. NC CFS Training Seminar October 2002
BACKGROUND
NC CFS Training Seminar October 2002
State Level Flood Studies
County Level Flood Studies
Community Level Flood and Master Planning Studies
Limited Detailed Studies (LDS) with Non-Encroachment Areas
Letter of Map Revisions (LOMRs)-Better Methodology Cases
Bridge Replacements-Memorandum of Agreement ( MOA)
Story about the height of floodway- One case
Fencing
Overhang
NC map
Course 1

4. NC CFS Training Seminar October 2002
OVERVIEW
NC CFS Training Seminar October 2002
Traditional Floodway Modeling Issues
Non Traditional Floodway Modeling Issues
A Holistic and Integrated Floodway Modeling Approach
Community Floodway Expert System
Implementation and Advantages
Future Work
Story about the height of floodway- One case
Fencing
Overhang
NC map
Course 1

5. HEC RAS STEADY STATE FLOODWAY ROUTINE
NC CFS Training Seminar October 2002
Floodway/Encroachments
Surcharge -1 foot
Backwater Computations
Equal Conveyance Reduction
Optimization
BACKWATER CONTROL
UNSTABLE AT BRIDGES
DEPENDENT ON CONSTRICTIONS
Include a HEC-RAS blocked obstruction and truncated XS
NFIP
Change the heading
Source: IS-9 Managing Floodplain Development Through The National Flood Insurance Program (NFIP)
Course 1

6. Traditional Floodway Modeling Issues
NC CFS Training Seminar October 2002
Bridges &Culverts
Expanded Channel Sections
Oxbows & meanders
Incised channels
Lakes
Parking Lots
Confluences
Levees
Inline structures
Interchanges
Floodway has been computed using a 1.0 foot of surcharge using a traditional steady state one dimensional model.
Examples in the next slides are taken from cases from a variety of hydraulic and hydrologic (H&H) studies, including countywide flood mapping studies; watershed based flood studies, and local stormwater management studies.
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7. Bridges AND CULVERTS
Encroachments defined inside the bridge opening area
Negative and Excessive surcharges causing incorrect floodway delineation.

8. Incised CHANNEL VS. EXPANDED CHANNEL
Liberal use of ineffective
flow area can lead to a narrow
floodway.
100-year contained inside
the bank stations.
Encroachment stations are
set at the bank stations/
outside the 100-year.

9. WATER BODIES, OXBOWS AND MEANDERS
NC CFS Training Seminar October 2002
Floodway defined inside the
water body in the RAS model.
Revised during the mapping process.
Floodway defined inside
the oxbows.
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10. NON-TRADITIONAL FLOODWAY MODELING ISSUES
Permissible Water Depth, Velocity & Discharge
Debris, Bank Erosion, Stream Stability, Bridge/Culvert Scour
Existing Buildings and Fill in the Floodway
Dams & Levees
Environmental Protection Measures: Water Quality Regulations
Special Designations: Stream Corridors as Wild or Scenic River
Flood Hazards of Special Concern-Costal Erosion & Dam Breaks
Source: IS-9 Managing Floodplain Development Through The National Flood Insurance Program (NFIP) Pages 5-21 to 5-28
Page Floodway Analysis

11. NC CFS Training Seminar October 2002
FLOOD DEPTH & Velocity
NC CFS Training Seminar October 2002
Flood Depth 15 to 20 feet
Flood Velocity feet/sec
Washington Encroachments defined in ft/sec 100-year overbank flood velocities
Establish velocity difference between the 100-year and floodway run
Depth of proposed fill in the floodplain will be between feet.
Establish a permissible depth of fill in the floodplain between 0-5 feet.
EX- Minnesota
SHOULD THERE BE A MINIMUM FLOOD DEPTH VELOCITY CRITERIA FOR THE FLOODWAY
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12. NC CFS Training Seminar October 2002
FLOOD DISCHARGE
NC CFS Training Seminar October 2002
Loss of flood storage area due to encroachments
LSC- DA- 51 sq. mile, urban, had channel storage in it - The reduced discharge due to removal of storage by the floodway caused 30% increase in flows. Delineated Community Floodway.
BC-DA-22 sq. mile, urban, had channel storage in it - The reduced discharge due to removal of storage by the floodway caused 20% increase in flows. Delineated Community Floodway.
Related to other recurrenance interval flows and found out its was comparable with 500yr discharges (2%-6%)
SIGNIFICANT REACH STORAGE, START CONSIDERING YEAR FLOWS TO COMPUTE FLOODWAY LIMITS
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13. Sediment Transport Conservation Easement Bankfull Discharge
Altering Design Parameters
SEDIMENT TRANSPORT ANALYSIS AT STREAM RESTORATION SITE

14. Debris / OBSTRUCTION
Historically, one of contributing factor of bridge and culvert failure has been debris and blockage during a flood event.
Perched Culverts
Include a floodway run with 1-foot, 2-foot etc. debris run to evaluate the worst case scenario.
CHECKING THE SURCHARGE VALUES WITH DEBRIS RUN IN THE FLOODWAY ANALYSIS

15. Bridge Scour EVALUATION
NC CFS Training Seminar October 2002 =
SHOULD THE BRIDGE SCOUR ANALYSIS BE PERFORMED FOR THE STRUCTURES IN STUDIED STREAM
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16. RESEARCH: Bridge Scour EVALUATION
NC CFS Training Seminar October 2002
Analysis indicates a positive correlation
between increase in surcharge and
scour depth
Theoretical analysis with prepared resources; should anybody express enough research on similar variables
Note: This plot is based on the preliminary research.
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17. NC CFS Training Seminar October 2002
BANK EROSION
NC CFS Training Seminar October 2002
Effective Discharge
Urban Stream
1-year to 5-year events are
not included in FEMA study.
Urban Streams : Effective Discharge: Urban Stream from 1 year to 2-year, currently not included in FEMA.
Look at the velocities of smaller events from the floodway geometry and flag velocities which are higher than 5ft/sec.
Revise areas with greater than 10ft/sec becomes a pressurized condition.
Development will be triggering the erosion found in Walnut Creek Basin- City of Raleigh
RELATIONSHIP WITH BANK EROSION/ EFFECTIVE DISCHARGE WITH INCREASING VELOCITIES
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18. NC CFS Training Seminar October 2002
INLINE STRUCTURES
NC CFS Training Seminar October 2002
Narrow Floodway downstream
of an inline structure. Not accounting
for a dam break scenario
Include a picture from Carolina Pines or Kit Creek
SHOULD WE START LOOKING AT THE DAM BREAK SCENARIO TO COMPUTE THE FLOODWAY
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19. HOLISTIC & INTEGRATED FLOODWAY MODELING
NC CFS Training Seminar October 2002
Resolve both Traditional and Non-Traditional Floodway Modeling Issues
Restrictive Requirement / Higher CRS Rating/Discounts to Policy Holder/ Smart Development
Use Holistic and Integrated Floodway Modeling Approach
Better Coordination with Community/Floodplain Manager/Residents
COMMUNITY FLOODWAY EXPERT SYSTEM (CFES)
The overall floodway modeling/delineation approach need to be reevaluated by accounting variables such as type of community, type of streams, land use, water quality, erosion-stream stability, type of modeling technique etc.
Using a Community Floodway Expert System: A matrix tool that will input vast parameters ranging from type of stream, land use, type of structures, historical data and will provide the integrated floodway analysis approach which can be delineated in different color code
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20. CFES: INPUTS AND DATA COLLECTION
NC CFS Training Seminar October 2002
Readily Available Data
Type of streams (rural; urban and mixed)
FIS and Risk Map Database
Land use/Zoning
Historical Flooding Data
Flood control structures: Dams, Levees
Coordination with Regulatory Agencies
Integrating data from other agencies like National Oceanic and Atmospheric Administration (NOAA), Natural resources Conservation Service (NRCS), United States Army Corps of Engineering (USACE) , Environmental Protection Agencies (EPA ):Water quality-Clean Water Act-404 Streams and Wetlands; Department of Transportation
Order is missing
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21. Debris Moderate
Scour Depth
15 feet
Design Velocity
5 feet/sec

22. COMMUNITY FLOODWAY EXPERT SYSTEM (CFES)
Type of Stream ,Critical Land Use, Number of Bridges, Number of Culverts Number of existing structures in floodway, Debris , Blockage, Number of structures in floodplain, Depth of flooding Floodway Storage, Bank Erosion, Water Quality, Dams, Levees, Historic Flooding Data, Gage Data etc
CFES Inputs
Surcharge- 0.0, 0.1, 0.2, 0.5 and 1.0
Model Type- Steady or Dynamic
Discharge- 100-year & 500-year
Sediment Transport / Bridge Scour
Permissible Floodway Depth/ Velocity
Integrated
Floodway
Analysis
Delineation
GREEN
BLUE
RED
ORANGE

23. CFES: TOOL SNAPSHOT CFES Inputs Delineation Integrated Floodway
Analysis
Delineation

24. NC CFS Training Seminar October 2002
“BLUE” FLOODWAY
NC CFS Training Seminar October 2002
“Blue” code includes consideration for culvert / bridge scour, debris, bank erosion and stream stability.
Delineate the floodway
Bridge Scour Analysis
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25. NC CFS Training Seminar October 2002
“RED” FLOODWAY
NC CFS Training Seminar October 2002
“Red” code for urban stream where detailed modeling, and calibration has been performed due to occurrence of severe flood, critical structures, dam break analysis, levees etc.
Delineate the floodway
Calibrated model
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26. NC CFS Training Seminar October 2002
“GREEN” FLOODWAY
NC CFS Training Seminar October 2002
“Green” designation, where stringent water quality regulations dictate any development within the floodplain.
Delineate the floodway
Revised floodway set at the stream buffer limits
Sedimentation
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27. “ORANGE” FLOODWAY
“Orange” code shows areas such as hazardous waste management and landfill sites near the floodplain.
Slurry wall to stop transfer of containments

28. IMPLEMENTATION & ADVANTAGES
NC CFS Training Seminar October 2002
Risk MAP: Computing Avoided Risk
MT-2 Forms
Restrictive Requirement: Community Rating System , Premium Discounts to Policyholders
CFES can be adopted by the communities in their H&H master planning studies
Focus on variables which are of utmost importance to them and eliminate variables which are not applicable to their community
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29. NC CFS Training Seminar October 2002
Future Work
NC CFS Training Seminar October 2002
Research/ Analysis of Variables
Input from the Communities
Risk MAP Integration
True Digital Delivery Integration
Cost / Benefit Analysis
Include a risk map from the document; do a little research on the tie-in between your idea and it integration with RISK MAP
Theoretical analysis with available resources; should anybody express enough interest to research similar variables
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30. NC CFS Training Seminar October 2002
CONCLUSION
NC CFS Training Seminar October 2002
REDEFINING FLOODWAY: Floodway is a tool to assist communities in balancing development within the floodplain and identify risk against the resulting increase in flood hazard and minimize environmental damage ( to stream stability, riparian and aquatic habits, etc. )
Include a risk map from the document; do a little research on the tie-in between your idea and it integration with RISK MAP
Course 1

31. NC CFS Training Seminar October 2002
QUESTIONS?
NC CFS Training Seminar October 2002
Contact Information:
Michael A. Hanson, P.E., LEED AP
ID:
Shweta Chervu, P.E., CFM
ID:
Course 1

32. References
Bridge Scour left impage
Bridge Scour Right Impate:
Debris Picture: