Approximate Floodplain Mapping - Procedures and Approaches to Data Challenges Troy Thielen, CFM Brett Addams, CFM May 18, 2010

Procedures and Approaches to Data Challenges Overview 1. Approximate Floodplain Mapping Defined 2. Study Area 3. Procedure/Methodology 4. Challenges Encountered

Approximate Study – Zone A DFIRM

Challenges Encountered in Flat Terrain Case Study: Northwest Ohio’s Black Swamp region Source: Ohio History Central

Challenges Encountered in Flat Terrain Methodology 1. Hydrology 2. Hydraulics 3. Mapping

Hydrology u Flows calculated using USGS Regression Equations u Input parameters calculated by using ArcHydro, via StreamStats

)1(*)(*)(*)(    WSLDARC 100 = Peak flow for a 100-yr recurrence interval RC DA 8510  SL W = Regression constant (dependent on project location) = Drainage area (sq. miles) = Slope = Open water or wetlands (percentage of drainage area) USGS Regression Equation Hydrology

StreamStats Web-based application that computes streamflow statistics for both gaged and ungaged sites. Includes a map-based user interface that uses ArcIMS, ArcSDE, ArcGIS and ArcHydro. Hydrology

Hydrology StreamStats

Hydrology

Hydrology StreamStats Shapefiles

Vector Stream Network Digital Elevation ModelHydrology ArcHydro Inputs

Hydraulics u Hydraulic Model: HEC-RAS u HEC-GeoRAS extension of ArcGIS used for pre- and post-processing GIS inputs u Process: 1. Pre-processing of GIS Data 2. Import to HEC-RAS 3. HEC-RAS modeling 4. Export HEC-RAS data to GIS

Hydraulics Cutting Cross-sections

Hydraulics Channel Manually Added to XS Geometry

Model Input - Stream Centerline - Flowpaths - XS Cut Lines Model Output - Bounding Polygons - Floodplain Boundaries (bp001) - XS Cut Lines Terrain Grid Depth Grid Water Surface Grid Hydraulics

Mapping u Post-Processing of raw GeoRAS floodplain polygon output u Review for accuracy u Clean up data anomalies u Adjust mapping to meet FEMA standards

Mapping Post-Processing

Challenges Encountered in Flat Terrain u Hydrology – difficulty in defining drainage divides and flow paths u Hydraulics – Floodplain outputs may indicate need for model corrections u Island and Ponds – Segmented floodplain outputs require further analysis u Lateral Flow Areas – Flat terrain can artificially amplify flooding effects

Challenges Encountered in Flat Terrain Challenge: Hydrology u Automated delineation of hydrologic features can result in erroneous results u Analysis and manual correction is required

Challenge: Hydrology Basin Delineation

Challenge: Hydrology Ditch Network

Challenges Encountered in Flat Terrain Challenge: Hydraulics u Analysis of floodplain outputs may require review of modeling data

Challenge: Hydraulics Raw HEC-GeoRAS output

Challenge: Hydraulics Model Run 1

Challenge: Hydraulics Model Run 2

Challenge: Hydraulics Run 1 vs. Run 2

Challenges Encountered in Flat Terrain Challenge: ISLANDS and PONDS u Very minor terrain variation can result in numerous islands and disconnected ponds u Result: –Highly segmented floodplain output generated

Challenge: Islands and Ponds Raw HEC-GeoRAS Output

Depth Grids Subtract Ground DEM values from Water Surface DEM values

Depth Grids Depth Grid – Stretched

Depth Grids Depth Grid – Classified Values

Challenge: Islands and Ponds Islands only slightly above the calculated WSEL

Challenge: Islands and Ponds Disconnected ponds at very shallow depth

Challenges Encountered in Flat Terrain Challenge: LATERAL FLOW AREAS u Unreasonable split flow situations u Unreasonably large flooding effects along portions of the floodplain

Challenge: Lateral Flow Areas Raw HEC-GeoRAS Output

Challenge: Lateral Flow Areas

Conclusion Troy Thielen, CFM Brett Addams, CFM (312) Questions