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

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

Hydrology

StreamStats

Hydrology

Hydrology

StreamStats Shapefiles

Vector StreamNetwork

Digital ElevationModel

Hydrology

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

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

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

Water Surface

Ground Surface

Depth

=

-

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

Troy Thielen, CFMthielentr@cdm.com

Brett Addams, CFMaddamsbh@cdm.com

(312) 346-5000

Questions