| Sand waves are known to occur due to interaction of flow、 sediment transport and bed topography. However、 although some experiments have been conducted on responses of bed topography to temporal flow changes in actual rivers、 there has been few quantitative explanation on transition phenomena、 such as formation and dissipation of sand waves. It has been reported that bed configuration differs between the flow increase and decrease processes、 even if the flow rate is the same in the transition processes. The water level becomes high due to formation of sand waves in the flow increase process、 whereas the resistance and water level are low in the flow decrease process due to the absence of sand wave formation. In this study、 a hydraulic experiment on the process of river bed configuration during consecutive unsteady flow waveforms was conducted. The flow rate was increased gradually by 15 liters/sec. from 100 liters/sec.、 which was a stable dune formation and was set as the initial condition of the observation. Steady state was maintained for 90 minutes at each flow rate、 and flow was increased in four stages until the dune dissipated at 160 liters/sec. Bed topography was observed during the 90-minute steady state of each flow rate. After the dune dissipated at 160 liters/sec.、 flow was decreased in the same way in four stages (15 liters/sec. each)、 until it returned to the initial condition of 100 liters/sec. Steady state was maintained for 90 minutes at each flow rate to observe bed topography likewise. Dunes did not dissipate between the initial and peak flow rates、 and then flat bed was observed when the dunes dissipated at the peak of flow. However、 in the reversal process、 the river bed remained flat from the peak to initial flow rates and dunes were not generated. The bivalency - dune and flat bed - were observed at a certain stage between the initial and peak flow rates. Evaluation using the coefficient of velocity suggested that some areas tended to have dune beds while others had flat beds、 i.e.、 there was significant dependence on the initial bed topography. |
