A LABORATORY STUDY OF ALLUVIAL CHANNELS WITH UNSTEADY FLOW

摘要

A laboratory experimental programme has investigated the response of straight alluvial regime channels to the passage of unsteady flows, when allowed to develop freely. A triangular hydrograph was created by a stepwise increase and subsequent decrease in flow rate. The peak flow rates were selected such that the water velocities over the flood plain were kept below the threshold of motion. The base flow rates were selected such that the main channel was just below the threshold of motion. The width of the main channel and the water surface elevation were monitored during the experiment. It was found that, following the passage of the unsteady flow, the width of the main channel had increased, the depth decreased and the channel side slope increased. The channel planform exhibited changes in the width of channel which were associated with the changes in the bed level. During increasing discharge two-dimensional dunes developed and during recession they were suppressed or their migration ceased as sediment transport rate reduced. Bankfull discharge and sediment concentration were reduced in the adjusted channel. The results of this work may help to improve our understanding of the response of a river to flood flows.