1. Backwater Induced Floodplain Assessment
November 30, 2006
GIS in Water Resources
Michael Konieczki

2. Introduction Federal Highway Administration FEMA regulations: Problems
Bridges in national highway system need crash tested rails (1968)
FEMA regulations:
100-yr Floodplain cannot rise by more that 1 foot for bridge construction or modifications
Problems
Older bridges
Effects of railings little understood

3. TxDOT Regulations Must pass 50-yr event
Checked for FEMA compliance of 100 yr event
Passage of 100-yr event not always needed
Overtopping of a local street bridge in Austin. Source: City of Austin

4. Project Goals Develop models for analysis
Pre-processing
Post-processing
Analyze backwater induced floodplains
How significant are bridge structures?
Railings?
World Peace (At least with FEMA)

5. Study Area
Colorado River
Lake Austin to SH 130
Lake Austin
SH 130

6. Data needs Pre-constuction Post-construction Flowlines Waterbodies
Basins
DEM’s (Digital Elevation Model)
Roads
Counties
Post-construction
use HEC-GeoRAS

7. HEC-GeoRAS GIS tools for HEC-RAS River models can be
Basic or comprehensive
Bridges modeled simply
Assumes rails clog with debris
Height is to top of rail
Possibly a conservative estimate
HEC-GeoRAS calculated floodplain. Source: HEC-GeoRAS use’s’ manual

8. Methods I (The Good) NHD data Texas data
Used network tools to select information for Colorado River
Built a geodatabase with that information
Texas data
Obtained through online resources
Roads
Counties
100yr floodplain

9. Methods II (The Bad) Processing a DEM into a TIN
1st: tried to process entire basin – failure
2nd: processed Travis County – success

10. Methods III (The Ugly) GeoRAS Load NHD data Draw cross-sections
Good luck!
Draw cross-sections
Data entry of whatever you couldn’t load

11. Goals Redux Develop models for analysis
Pre-processing
Post-processing
Analyze backwater induced floodplains
How significant are bridge structures?
Railings?
World Peace (At least with FEMA)

12. Methods IV – A New Hope Calibrate GeoRAS model to 100yr floodplain
For the SH 130 bridge only super extreme events will matter anyway
Simulate extreme event (200-yr?)
Change bridge geometry - account for railings
Compare
This may show us the limitations of RAS

13. Future Work Possibly change HEC-RAS model
Better simulate bridge deck and railing systems
Develop a tool via model builder
Allow for easy comparison of GeoRas results

14. Acknowledgements Tyler Janzen for his NHD help
Nishesh Mehta for his help with CO basin DEM

15. Questions???