Kazuo Ashida

Environmental impacts of coordinated sediment flushing

The environmental impacts of sediment flushing are predicted by a one-dimensional model for prediction and evaluation of sediment flushing (PESF1D). The model computes one-dimensional unsteady flow and non-equilibrium transport of non-uniform sediment mixtures. In the sediment transport module, bed material load is taken as sediment mixtures for calculating the bed evolution and wash load concentration is computed in consideration of the mixing action between the fine sediments in the flowing water and bed porewater. The influences of tributaries on the concentrations are taken into account in diffusion equations of wash load and suspended load. The non-uniform property of the bed material load is modeled by introduction of a mixing layer, transition layer and deposition strata. PESF1D was calibrated on the basis of the observations during the concerted flushing at Dashidaira and Unazuki reservoirs in 2001. The suspended solid concentrations predicted for potential flushing in 2002 are in good agreement with the measurements during the concerted flushing in July 2002. The dissolved oxygen concentration of the lower Kurobe River would not have strong relationship to the suspended solid concentration. The sediment budget shows the riverbed has been eroded after completion of Unazuki dam in 2001.