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

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)

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

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

Lower Colorado River Drainage Area

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

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

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)

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

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

Aerial Mapping Contours and Spot Elevations

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

USGS NED Data Used For Basin-Wide Hydrology

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

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

Lake Bathymetry Inks Lake Spot Elevations

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

Field Surveys

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

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

HEC-RAS Cross Section Interpolation

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

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

Centerline Edge of Water Survey Cross Section

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

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

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 feet (transition zone)

Boundary Between Aerial Mapping and NED Without Buffer

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

Wire Frame of TIN Data Structure

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

NED ADR Spot Elevations ADR Contours and Spot Elevations Bathymetry

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

TIN with a “Tail” for HEC-GeoRAS

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

Questions?