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 1. Approximate Floodplain Mapping Defined 2. Study Area 3. Procedure/Methodology 4. 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 1. Hydrology 2. Hydraulics 3. Mapping

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

7. 295.0232.0 8510 757.0 100 )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

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

9. Hydrology StreamStats

10. Hydrology

11. Hydrology StreamStats Shapefiles

12. Vector Stream Network Digital Elevation ModelHydrology ArcHydro Inputs

13. 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

14. Hydraulics Cutting Cross-sections

15. Hydraulics Channel Manually Added to XS Geometry

16. 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

17. 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

18. Mapping Post-Processing

19. 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

20. 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

21. Challenge: Hydrology Basin Delineation

22. Challenge: Hydrology Ditch Network

23. Challenges Encountered in Flat Terrain Challenge: Hydraulics u 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 u Very minor terrain variation can result in numerous islands and disconnected ponds u 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

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 u Unreasonable split flow situations u 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