Stream-channel response to floods, with examples from central Texas

Abstract

The principle that most geomorphic work is accomplished by relatively frequent events of moderate intensity requires modification for application to stream channels in certain climatic and physiographic settings. Small drainage basins in regions of highly variable flood magnitudes appear to have a high potential for catastrophic response. Flash-flood potential for small basins can be regionally mapped by computing the standard deviation of the logarithms of the annual flood peaks. Highly right-skewed flood-frequency distributions indicate that a high potential exists in certain arid regions of the southwestern United States and in the seasonal subtropical-to-steppe climate region of central Texas. High-magnitude flood response is also promoted by physiographic factors, such as hillslope morphology, soils, rock type, and drainage density. The relative importance of overland flow, which produces intense flood peaks, versus interflow and ground-water flow, which produce more uniform streamflow, appears to integrate both the climatic and the physiographic influences on the potential for catastrophic floods. Another factor in realizing the climatic-hydrologic potential for catastrophic stream-channel response is the resistance of the channel itself to scour. Small limestone streams in central Texas show significant channel modification only during the rare high-magnitude floods characteristic of that region. This is mainly because of the high response threshold required to scour bouldery alluvium and dense valley-bottom vegetation. Effects of especially intense floods on such streams include the following: entrainment of jointed bed rock and boulders as much as 3 m in diameter, uprooting of trees that usually bind coarse-grained point bars, macroturbulent transport of boulders even over divides into adjacent drainages, local scour of chutes on meander bends, and passive boulder deposition on other preflood valley-bottom surfaces.