1. Terrain for the Lower Colorado River Flood Damage Evaluation Project Erin Atkinson, Halff Associates, Inc. Rick Diaz, Lower Colorado River Authority Symposium on Terrain Analysis for Water Resources Applications December 18, 2002

2. Project Description Flood Damage Evaluation Project (FDEP) cooperative effort between –Fort Worth District, U.S. Army Corps of Engineers –Lower Colorado River Authority Detailed, basin-wide approach for studying hydrology and hydraulics (H&H)

3. Project Description Modeled, simulated, and computed frequency based – –Rainfall –Runoff –Reservoir elevations –Stream flood elevations Used detailed topographic mapping along the main stem of the Colorado River

4. Study Area - Hydrology Colorado River drainage area –40,000 square miles Lower Colorado River contributing basin –Downstream of O.H. Ivie Reservoir –18,300 square miles

5. Lower Colorado River Drainage Area

6. Study Area - Hydraulics Upstream limit of study –US Hwy 190 near San Saba, TX Downstream limit of study –Intracoastal waterway at Matagorda Bay 480 miles of detailed hydraulics

7. US HWY 190 Near San Saba, TX Intracoastal Waterway At Matagorda Bay

8. Hydraulic Modeling HEC-RAS – Unsteady flow Automation – Geometry extraction HEC-GeoRAS –River centerline –Cross sections –Bank lines –Flow lines Surface model – Triangulated Irregular Network (TIN)

9. Available Terrain Data 1.Aerial Mapping 2.USGS NED 3.Lake Bathymetry 4.Field Surveys

10. Aerial Mapping Traditional aerial mapping by ADR Mapped the Colorado River corridor to the approximate FEMA 500-yr boundary 2’ contours along corridor 1’ contours for some urban areas Spot elevations Planimetrics – edge of water

11. Aerial Mapping Contours and Spot Elevations

12. USGS NED 30 meter Digital Elevation Model (DEM) Used for basin-wide hydrology Assembled by UT-CRWR

13. USGS NED Data Used For Basin-Wide Hydrology

14. Lake Bathymetry Highland Lakes system Spot elevations acquired from hydro surveys TWDB –Lake Austin, Town Lake LCRA –Lake Buchanan, Inks Lake, Lake LBJ, Lake Marble Falls, Lake Travis

15. Lake Buchanan Lake LBJ Lake Marble Falls Lake Travis Lake Austin Town Lake Highland Lakes Inks Lake

16. Lake Bathymetry Inks Lake Spot Elevations

17. Field Surveys Channel cross sections of Colorado River Survey locations –Bridges –55 XS upstream of Lake Buchanan –110 XS downstream of Town Lake Averaged 2 cross sections per mile

18. Field Surveys

19. River Channel Problem – Not enough field surveys to adequately model the river Solution – Channel interpolation Choices – –HEC-RAS interpolator –GIS interpolator

20. HEC-RAS Interpolation Interpolation after geometry extraction Linear interpolation –Elevation values –From cross section to cross section Overbank interpolated along with channel

21. HEC-RAS Cross Section Interpolation

22. GIS Interpolation Interpolation before geometry extraction Linear interpolation –Elevation values “Curvilinear” interpolation –From cross section to cross section Overbank is not interpolated

23. GIS Interpolation Method Required elements –Centerline –Edge of water polygon –Survey cross sections Generate intermediate cross sections Connect survey and intermediate XS at equal intervals along the XS Interpolate elevations along connections

24. Centerline Edge of Water Survey Cross Section

25. GIS Interpolation Advantages Cross section interpolation lines can be used during the creation of the terrain surface Channel geometry generated for entire river Channel interpolation occurs without interpolating the overbank areas

26. Data Prioritization 1.Aerial Mapping 2.Field Surveys 3.Lake Bathymetry 4.NED

27. Data Pre-Processing Clip lake bathymetry with edge of water –Edge of water from aerial planimetrics –Low lake levels –Islands Clip NED with aerial mapping boundary –Boundary + 500 feet (transition zone)

28. Boundary Between Aerial Mapping and NED Without Buffer

29. Surface Model Triangulated Irregular Network (TIN) –Surface represented by network of triangles –Allows for variability in density –Can use multiple sources of base data TIN created with ArcInfo workstation –More stable than ArcView 3.x –Process larger datasets –More control over data inputs

30. Wire Frame of TIN Data Structure

31. TIN Data Types Aerial Mapping –Contours – hard lines –Spot elevations – mass points Interpolated Channel – break lines Lake Bathymetry – mass points NED – mass points

32. NED ADR Spot Elevations ADR Contours and Spot Elevations Bathymetry

33. TIN Limitations for FDEP Project Size of TIN datasets –Too much data, scratch files > 2 GB –20 subareas HEC-GeoRAS –Centerline has to overlap with TIN –Added “tails” to the TINs

34. TIN with a “Tail” for HEC-GeoRAS

35. Terrain Integration Results 1.One dataset rather than four 2.Cross section extraction at any location 3.Efficient update of hydraulic models

36. Questions?