Slide 1

```Approximate Floodplain Mapping Procedures and Approaches to
Data Challenges
Troy Thielen, 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
 Flows calculated using USGS Regression
Equations

Input parameters calculated by using
ArcHydro, via StreamStats
Hydrology
USGS Regression Equation
100
= ( RC) * ( DA) 0.757 * ( SL10 -85 ) 0.232 * (W + 1) -0.295
100
RC
= Peak flow for a 100-yr recurrence interval
= Regression constant (dependent on project location)
DA = Drainage area (sq. miles)
SL10 -85 = Slope
W=
Open water or wetlands (percentage of drainage area)
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
Hydrology
StreamStats
Hydrology
Hydrology
StreamStats Shapefiles
Hydrology
Vector Stream
Network
Digital Elevation
Model
ArcHydro Inputs
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
Hydraulics
Cutting Cross-sections
Hydraulics
Channel Manually Added to XS Geometry
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
Mapping
 Post-Processing of raw GeoRAS floodplain
polygon output

Review for accuracy

Clean up data anomalies

Adjust mapping to meet FEMA standards
Mapping
Post-Processing
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
Challenge: Hydrology
Basin Delineation
Challenge: Hydrology
Ditch Network
Challenges Encountered in Flat Terrain
Challenge: Hydraulics
 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
 Very minor terrain variation can result in
numerous islands and disconnected ponds

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
 Unreasonable split flow situations
 Unreasonably large flooding effects along
portions of the floodplain
Challenge: Lateral Flow Areas
Raw HEC-GeoRAS Output
Challenge: Lateral Flow Areas
Conclusion
Questions
Troy Thielen, CFM
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