terrain for the lower colorado river flood damage evaluation project erin atkinson, halff...
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![Page 1: Terrain for the Lower Colorado River Flood Damage Evaluation Project Erin Atkinson, Halff Associates, Inc. Rick Diaz, Lower Colorado River Authority Symposium](https://reader036.vdocuments.us/reader036/viewer/2022062518/56649cc55503460f9498e6c6/html5/thumbnails/1.jpg)
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
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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)
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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
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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
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Lower Colorado River Drainage Area
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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
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US HWY 190Near San Saba, TX
Intracoastal WaterwayAt Matagorda Bay
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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)
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Available Terrain Data
1. Aerial Mapping
2. USGS NED
3. Lake Bathymetry
4. Field Surveys
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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
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Aerial MappingContours and
Spot Elevations
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USGS NED
• 30 meter Digital Elevation Model (DEM)
• Used for basin-wide hydrology
• Assembled by UT-CRWR
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USGS NED Data Used For Basin-Wide
Hydrology
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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
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Lake Buchanan
Lake LBJ
Lake Marble Falls
Lake Travis
Lake Austin
Town Lake
Highland LakesInks Lake
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Lake Bathymetry
Inks Lake
Spot Elevations
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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
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Field Surveys
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River Channel
• Problem – Not enough field surveys to adequately model the river
• Solution – Channel interpolation
• Choices – – HEC-RAS interpolator– GIS interpolator
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HEC-RAS Interpolation
• Interpolation after geometry extraction
• Linear interpolation– Elevation values– From cross section to cross section
• Overbank interpolated along with channel
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HEC-RASCross SectionInterpolation
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GIS Interpolation
• Interpolation before geometry extraction
• Linear interpolation– Elevation values
• “Curvilinear” interpolation– From cross section to cross section
• Overbank is not interpolated
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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
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Centerline
Edge of Water
Survey Cross Section
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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
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Data Prioritization
1. Aerial Mapping
2. Field Surveys
3. Lake Bathymetry
4. NED
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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)
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Boundary BetweenBoundary BetweenAerial MappingAerial Mapping
and NEDand NEDWithout BufferWithout Buffer
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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
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Wire FrameWire Frameof TIN Dataof TIN Data
StructureStructure
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TIN Data Types
• Aerial Mapping– Contours – hard lines– Spot elevations – mass points
• Interpolated Channel – break lines
• Lake Bathymetry – mass points
• NED – mass points
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NED
ADR Spot Elevations
ADR Contoursand Spot Elevations
Bathymetry
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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
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TIN with a “Tail”for HEC-GeoRAS
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Terrain Integration Results
1. One dataset rather than four
2. Cross section extraction at any location
3. Efficient update of hydraulic models
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Questions?