approximate floodplain mapping - procedures and approaches to data challenges troy thielen, cfm...
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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
Challenges Encountered in Flat Terrain
Case Study:
Northwest Ohio’s Black Swamp region
Source: Ohio History Central
Hydrology
Flows calculated using USGS Regression Equations Input parameters calculated by using
ArcHydro, via StreamStats
295.0232.08510
757.0100 )1(*)(*)(*)(
-- += WSLDARCQ
Q100
= 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
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
Hydraulics
Hydraulic Model: HEC-RAS HEC-GeoRAS extension of ArcGIS used for
pre- and post-processing GIS inputs Process:
1. Pre-processing of GIS Data
2. Import to HEC-RAS
3. HEC-RAS modeling
4. Export HEC-RAS data to GIS
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
Post-Processing of raw GeoRAS floodplain polygon output Review for accuracy Clean up data anomalies Adjust mapping to meet FEMA standards
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
Challenges Encountered in Flat Terrain
Challenge: Hydrology
Automated delineation of hydrologic features can result in erroneous results
Analysis and manual correction is required
Challenges Encountered in Flat Terrain
Challenge: Hydraulics
Analysis of floodplain outputs may require review of modeling data
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
Depth Grids
Subtract Ground DEM values from Water Surface DEM values
Water Surface
Ground Surface
Depth
=
-
Challenges Encountered in Flat Terrain
Challenge: LATERAL FLOW AREAS
Unreasonable split flow situations Unreasonably large flooding effects along
portions of the floodplain