Floodplain Delineation for Unsteady Flow: Colorado River Study

1. Floodplain Delineation of Unsteady Flow Using HEC-RAS Final Presentation Presented By: Kevin Donnelly

2. Study Area • The Colorado River between Wirtz Dam and Inks Dam (Lake LBJ) and its two major tributaries, Sandy Creek and the Llano River • Lake LBJ is 21.15 miles long and 10,800 feet wide at its widest point. The lake covers 6,375 acres, and its capacity is 138,500 acre-feet. Inks Dam

3. Objectives • Develop a terrain model combining 30-meter DEM data, 2-foot contours, and UNET cross sections • Develop new cross sections along the Colorado River (Lake LBJ), Llano River, and Sandy Creek using GeoRAS • Simulate flooding that occurred in November 2000 • Import resulting water surface profiles back into GIS were the spatial extent of the flooding can be better seen and understood

4. UNET Cross Section HEC-RAS Aligning Cross Sections

5. TIN Input Data

6. Adjusting Cross Sections • Interpolate cross sections (500 ft) in HEC-RAS • Use Generate Report function to export cross sections to GIS • Create 3D XS using ArcView scripts developed by Eric Tate • Erase cross section points outside of the channel from interpolated cross sections • Form polygon outlining cross sections • Where 2-foot contours are available (Colorado River) • Convert 30-meter DEM to points • Select grid-points within 30-meters of 2-foot contours • Select grid-points that intersect XS bounding polygon • Erase selected grid-points

7. Adjusting Cross Sections • Where 2-ft contours are not available • Buffer cross section bounding polygon • Clip DEM to buffered polygon • Resample cross sections using Eric Tate’s scripts • Cross section points within the channel remain unchanged • The elevation of the DEM at the end of each cross section and the cross section elevation at the channel bank is determined • The cross section elevation of each point outside the channel is adjusted to create a smooth transition between the original cross section and the DEM

9. Combining TIN Inputs

10. TIN Creation

11. Developing GIS Data

12. Unsteady Flow Data from DSS

13. HEC-RAS Results

14. GeoRAS Results

15. Problems/Limitations • TIN is very large (~ 5 million triangles), GeoRAS requires < 100,000 cells to delineate floodplain • Floodplain was delineated using 30-foot cells • Channels are filled-in in areas away from cross sections, forming humps in the TIN • Incorrect water depths and gaps in floodplain delineation • Did not affect RAS calculations, but limited where cross sections could be cut • Too many points in cross section created using GeoRAS (> 400) • Not enough cross sections to correctly delineate floodplain • Floodplains are delineated for each time step separately

16. TIN Anomalies

17. Not Enough Cross Sections

18. Solutions • Interpolate cross sections to a much smaller maximum separation ~ 50’ • Utilize export channel only option in HEC-RAS • Create a cross section TIN, convert to grid, then points and use as point elevations for larger TIN • Use Cross Section Points Filter to reduce number of point below 400 • Break up reaches, delineating individual portions of the floodplains, and then merge them together

19. Questions kevin@mail.utexas.edu