Thresholds and the spatial variability of flood power during extreme floods

Abstract

Analysis of the effects of extreme events on overall landscape form has shifted geomorphic attention away from a focus on the magnitude and frequency relationship of discharge to a focus on flood power. This paper analyzes the variation in flood power both at-a-station and downstream in a basin of differing lithologies, and discusses the geologic and geomorphic controls on flood power. Design floods ranging from the 2-year discharge tothe 500-year discharge are routed trough a drainage basin, and channel boundary shear stress and unit stream power are calculated using HEC-2 along 21 cross valley transects. Because of the strong lithologic control on valley width and channel slope, an irregular downstream pattern of flood power emerges which reflects primarily the pattern in valley width. Narrow valleys correspond to a greater percent of channel conveyance during large magnitude floods and increasing rates of depth increases - thus maximizing flood power in these zones. The opposite occurs in anomalously wide valleys where flood power is minimized. A downstream spatial pattern emerges reflecting both local and general controls. For a given probability event, flood power varies at least threefold within the basin with a maximum occuring in a specific downstream section. Holding flood power constant produces an extremely wide range in flow probabilities. In order to establish a link between critical flood power magnitude and flow frequency, minimum flood powers are developed from an analysis of previous work on “catastrophic” flooding for humid, alluvial channels. These minimum thresholds tentatively correspond to 100 N/m 2 or 300 W/m 2 for shear stress and unit stream power, respectively. Discharges required to attain these critical flood powers vary from events roughly twice the 100-year flood to a maximum of roughly 18 times the discharge of the 100 year flood.