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Approximate Floodplain Mapping - Procedures and Approaches to Data Challenges Troy Thielen, CFM Brett Addams, CFM May 1

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 Approximate Floodplain Mapping Defined Study Area Procedure/Methodology Challenges Encountered.

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Approximate Floodplain Mapping - Procedures and Approaches to Data Challenges Troy Thielen, CFM Brett Addams, CFM May 1

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

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

  3. Approximate Study – Zone A DFIRM

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

  5. Challenges Encountered in Flat Terrain Methodology • Hydrology • Hydraulics • Mapping

  6. Hydrology • Flows calculated using USGS Regression Equations • Input parameters calculated by using ArcHydro, via StreamStats

  7. - = + 0 . 757 0 . 232 0 . 295 Q ( RC ) * ( DA ) * ( SL ) * ( W 1 ) - 100 10 85 Hydrology USGS Regression Equation Q = Peak flow for a 100-yr recurrence interval 100 RC = Regression constant (dependent on project location) DA = Drainage area (sq. miles) SL = Slope - 10 85 W = Open water or wetlands (percentage of drainage area)

  8. 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. http://water.usgs.gov/osw/streamstats/ssonline.html

  9. Hydrology StreamStats

  10. Hydrology

  11. Hydrology StreamStatsShapefiles

  12. Hydrology Digital Elevation Model Vector Stream Network ArcHydro Inputs

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

  14. Hydraulics Cutting Cross-sections

  15. Hydraulics Channel Manually Added to XS Geometry

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

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

  18. Mapping Post-Processing

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

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

  21. Challenge: Hydrology Basin Delineation

  22. Challenge: Hydrology Ditch Network

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

  24. Challenge: Hydraulics Raw HEC-GeoRAS output

  25. Challenge: Hydraulics Model Run 1

  26. Challenge: Hydraulics Model Run 2

  27. Challenge: Hydraulics Run 1 vs. Run 2

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

  29. Challenge: Islands and Ponds • Raw HEC-GeoRAS Output

  30. Depth Grids • Subtract Ground DEM values from Water Surface DEM values Water Surface - Ground Surface = Depth

  31. Depth Grids • Depth Grid – Stretched

  32. Depth Grids • Depth Grid – Classified Values

  33. Challenge: Islands and Ponds • Islands only slightly above the calculated WSEL

  34. Challenge: Islands and Ponds • Disconnected ponds at very shallow depth

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

  36. Challenge: Lateral Flow Areas • Raw HEC-GeoRAS Output

  37. Challenge: Lateral Flow Areas

  38. Conclusion Troy Thielen, CFM thielentr@cdm.com Brett Addams, CFM addamsbh@cdm.com (312) 346-5000 Questions

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