ABSTRACT
In this study, a hydraulic experiment on a mobile bed was conducted with focus on the process of dune fading and bed resistance fluctuation during a transition state under unsteady flow. The process was observed through two discharge hydrographs of 300 minutes and 60 minutes. It was revealed that the bed resistance began to decrease in the discharge-increase period. The dune height decreased over time in the unsteady flow hydrograph, and the bed resistance decreased in accordance with the dune height. The dunes completely disappeared in the 300-minute hydrograph, however, not in the 60-minute one, and reformed in the discharge recession period of the latter. A bivalency of water depth to the discharge was observed in the 300-minute case, in which the flow velocity coefficient increased much more than in the 60-minute case. Comparison of the two hydrographs suggested that there was a riverbed and resistance response delay that followed the discharge curve during the transition process.
