Influence of Bed-Load Transport on Flow Resistance of Step-Pool Channels

Abstract

This study investigates the influence of bed-load transport on flow resistance and bed stability in steep step-pool channels. A total of 86 flume experiments was performed. Stable step-pool sequences were formed with increasing discharge under clear-water conditions. The addition of fine bed-load material to the flow led to a decrease in bed roughness and an increased mobility of step-forming clasts. Velocity measurements showed that bed-load transport significantly decreased flow resistance and increased near-bed velocity for the conditions investigated. A set of existing resistance equations was tested against our data, of which the hydraulic geometry relation performed best. However, none of the existing equations was able to reproduce flow resistance during active bed-load transport. Two new resistance equations based on a hydraulic geometry approach are proposed for conditions with and without bed-load transport. The increase in near-bed velocity associated with bed-load transport was identified as the main cause for the observed increase in step mobility associated with bed-load transport. For flows with bed-load transport, step-forming clasts were mobilized at discharges some 10% to 30% lower than under clear-water conditions.