Hydrogeomorphic methods for the regional evaluation of flood hazards |
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| Authors: | Victor R Baker |
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| Institution: | (1) Department of Geological Sciences, University of Texas at Austin, 78712 Austin, Texas, U.S.A. |
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| Abstract: | The “upstream” approach to flood hazard evaluation involves the estimation of hydrologic response in small drainage basins.
This study demonstrates the application of geomorphology to such studies in a region of unusually intense flooding in central
Texas. One approach to flood hazard evaluation in this area is a parametric model relating flood hydrograph characteristics
to quantitative geomorphic properties of the drainage basins. A preliminary model uses multiple regression techniques to predict
potential peak flood discharge from basin magnitude, drainage density, and ruggedness number. After mapping small catchment
networks (4 to 20 km2) from remote sensing imagery, input data for the model are generated by network digitization and analysis by a computer-assisted
routine of watershed analysis.
The study evaluated the network resolution capabilities of the following data formats: (1) large-scale (1:24,000) topographic
maps, employing Strahler's “method of v's”, (2) low altitude black-and-white aerial photography (1:13,000 and 1:20,000 scales),
(3) NASA-generated aerial infrared photography at scales ranging from 1:48,000 to 1:123,000, and (4) Skylab Earth Resources
Experiment Package S-190A and S-190B sensors (1:750,000 and 1:500,000 respectively). Measured as the number of first order
streams or as the total channel length identified in small drainage areas, resolution is strongly dependent on basin relief.
High-density basins on the Edwards Plateau were poorly depicted on orbital imagery. However, the orbital network definition
of low-relief basins on the inner Texas Coastal Plain is nearly as accurate as results from large-scale topographic maps.
Geomorphic methods are also useful for flood hazard zonation in “downstream” flood plain areas. Studies of the Colorado River
valley near Austin, Texas, easily distinguished infrequent (100- to 500-year recurrence interval), intermediate (10- to 30-year),
and frequent (1- to 4-year) hazard zones. These mapping techniques are especially applicable to the rapid regional evaluation
of flood hazards in areas for which there is a lack of time and money to generate more accurate engineering-hydraulic flood
hazard maps. |
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