1-Day of 2-DHow Are The Results Of Hydraulic Models Used To Manage Floodplain Development Under The NFIP?

Eric Simmons, FEMA Region IX

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Idealized Schematic of Floodplain Mapping

Final Flood Maps

Digital Base Mapping

Flood Data /

Database

Hydraulic Modeling

Streamflow & Meterological

Data

Flood FlowsHydrologic Analysis

+ =

Field Data & Surveys

Topographic Data & Ground Elevations

-

-

Initial Data Initial Outputs

Input Analyses Final Products

Floodplain Boundarie

s

Floodway Stations

Water Surface

Elevations

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Considerations For The NFIP Model Results Are Used For Floodplain

Management:• Flood hazard areas• Flood elevations• Regulatory floodways

Differences between 1-D and 2-D Computer Programs?

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Considerations For The NFIP Model Results Are Used For Floodplain

Management:• Flood hazard areas• Flood elevations• Regulatory floodways

Differences between 1-D and 2-D Computer Programs?• Momentum, velocity averaging, vertical variations, etc.• Riverine, coastal and alluvial floodplains• Cross sections (with prescribed flow direction) vs. nodes• Steady vs. unsteady (peak discharge vs. hydrograph)• Ability to perform automated encroachment analysis• Model setup time, costs, equipment, run times, industry

expertise, stakeholder acceptance, etc.• Cross section spacing and mesh resolution

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Considerations For The NFIP Model Results Are Used For Floodplain

Management:• Flood hazard areas• Flood elevations• Regulatory floodways

Differences between 1-D and 2-D Computer Programs?• Momentum, velocity averaging, vertical variations, etc.• Riverine, coastal and alluvial floodplains• Cross sections (with prescribed flow direction) vs. nodes• Steady vs. unsteady (peak discharge vs. hydrograph)• Ability to perform automated encroachment analysis• Model setup time, costs, equipment, run times, industry

expertise, stakeholder acceptance, etc.• Cross section spacing and mesh resolution

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Advantages of 2-D Computer Programs Flow Splits Very Wide Floodplain Braided Channels Scour Analysis Sediment and Pollutant Transport Estuary Dynamics Other?

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1-D Model Links: Profile, Baseline & SFHA 1% Annual

Chance Water-Surface Elevations (for development/new building and flood insurance purposes) determined from Flood Profile along profile baseline (or table in the FIS)

Flood Insurance Study (FIS) (digital & hard-copy)

Mapped Floodplain (SFHA) on

FIRM Panels(digital &

hard-copy)

● Baseline ► Model ► Flood Profile ► Floodplain.Hydraulic Model

Hydraulic Baseline in Floodplain

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Floodplain Management Floodplain Management is a Decision Making

Process That Aims to Achieve Wise Use or Nonuse of the Floodplain

With the National Flood Insurance Program over 21,000 communities nationwide have adopted a floodplain management process based on, at least, a Flood Insurance Rate Map

Flood Insurance Rate Maps:• High Hazard Areas (flood zone boundary)• Base Flood Elevations (flood profile & profile baseline) or

Depths• Regulatory Floodways (in riverine areas)

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My Questions For Today• What issues does FEMA face in using 2-D and

unsteady models?• What underlying assumptions of each model are

valid/invalid?• When do the benefits outweigh costs for another

analysis type?• Should we let a 2-D hydraulic model delineate

the mapped floodplain boundaries (or a separate GIS application as typically done with 1-D analyses)?

• How can/should 1% annual chance flood elevations be recorded for individual structures (to the nearest tenth of a foot) using 2-D models?

• Can 2-D programs efficiently model floodways? If not, should NFIP floodplain management and mapping criteria be changed?

• For which analyses should 1-D and 2-D modeling be linked?