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 190Near San Saba, TX

Intracoastal WaterwayAt 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 MappingContours 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 LakesInks 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-RASCross SectionInterpolation

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 + 500 feet (transition zone)

Boundary BetweenBoundary BetweenAerial MappingAerial Mapping

and NEDand NEDWithout BufferWithout 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 FrameWire Frameof TIN Dataof TIN Data

StructureStructure

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 Contoursand 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?