Abstract
In this study, we aimed to elucidate the detailed hydraulic mechanism of long term persistance of turbidity in reservoirs, since the phenomeman cause water quality problem in the downstream river of the dams. For that purpose, we made field measurements on velocity disribusion characteristic under a thermally stratified condition and grain size of suspended sediment after a flood event in Isawa Dam Reservoir in Japan. A current meter with high temporal resolution was used in order to measure the velocity distribution that could influence on long term suspension of fine grained sediment in the reservoir. The device was moored during the measurement in the reservoir at intended depths by using the apparatus that was assembled with rope, angle bars, submerged floats, and self-jamming pulleies. From the obtained data, velocity distribution was analyzed as well as the swaying condition of the current meter during the measurement. The grain size of the suspended sediment sampled in this study showed almost homogenious distribution from the upper layer to the middle layer and concentrated around 10μm. The velocity distribution was rather uniform regardless of the depth. Therefore, we presumed that such velocity distribution and fluctuation could effect on keeping suspended sediment from falling, and cause active diffusion in epilimnion in the reservoir.
