terrain for the lower colorado river flood damage evaluation project erin atkinson, halff...
TRANSCRIPT
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?