A series of laboratory experiments have been carried out to investigate the channel forming process under unsteady flow conditions, when channel boundaries are allowed to develop freely. To simulate unsteady flows, a stepped triangular unit hydrograph was used. The peak flow rates were set such that the flood plains were not mobile. The base flow rates were also set such that the main channel dimensions were stable. The change in width of the main channel, the water surface elevation, and sediment transport rate, as channel adjustment criteria, were recorded during the experiment. The results show that the width of the main channel and the channel side slope increased, and the depth decreased in response to the passage of the flood. The sediment concentration was usually larger on the rising than on the falling limb for the equivalent discharge. When channel conditions changed from bankfull to overbank, the sediment concentration decreased. A significant observation was that the stage was higher on the falling limb than on the rising limb for the equivalent discharge. As the discharge increased, two-dimensional dunes developed and during the recession they were suppressed or their migration ceased as sediment transport rates reduced. The results of this work may also improve our understanding of river channel response under flood flows.
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