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Managing Floodplain Development: Insights from 1-D and 2-D Hydraulic Modeling Under the NFIP

This presentation details the applications of hydraulic models in floodplain management under the National Flood Insurance Program (NFIP). It contrasts 1-D and 2-D modeling approaches, examining their impact on flood hazard mapping, flood elevations, and regulatory floodways. Key considerations include model setup, costs, and the effectiveness of each model type in various floodplain scenarios. We explore the issues faced by FEMA in implementing 2-D models, the validity of modeling assumptions, and how these models inform decision-making for over 21,000 communities nationwide.

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Managing Floodplain Development: Insights from 1-D and 2-D Hydraulic Modeling Under the NFIP

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  1. 1-Day of 2-D How Are The Results Of Hydraulic Models Used To Manage Floodplain Development Under The NFIP? Eric Simmons, FEMA Region IX

  2. Idealized Schematic of Floodplain Mapping Topographic Data & Ground Elevations Floodplain Boundaries   =     +  Water Surface Elevations    Floodway Stations Streamflow & Meterological Data Hydrologic Analysis Flood Flows Hydraulic Modeling Flood Data / Database Digital Base Mapping Final Flood Maps - - Initial Data Initial Outputs Input Analyses Final Products Field Data & Surveys

  3. 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?

  4. 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

  5. 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

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

  7. 1-D Model Links: Profile, Baseline & SFHA • ●Baseline► Model ► Flood Profile ► Floodplain. • 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) Hydraulic Model Mapped Floodplain (SFHA) on FIRM Panels (digital & hard-copy) Hydraulic Baseline in Floodplain

  8. 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)

  9. My QuestionsFor 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?

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