Effects of sediment replenishment on riverbed material size distribution and attached algal biomass in the downstream reaches of a dam

Sediment replenishment is an effective method for resupplying depleted sediment and detaching overgrown algae in the downstream reaches of a dam. In this study, we used empirical data to examine the effects of sediment replenishment on bed material size and algal biomass in the downstream reaches of the Futase Dam, Chichibu City, Saitama Prefecture, Japan. Assuming that algae detach from bed materials when they are moved by water flow, we calculated the tractive force on the riverbed (tau) and allotted a threshold bed material size in motion (D-cri) for each given tau. The resulting bed material in the downstream reaches of the dam in any year was typically finer than that in the previous year when flooding in the rainy season transported a large volume of sediment. Algal biomass was lower when monthly D-cri exceeded 2 mm, versus when it was less than 2 mm. These results suggest that replenishment of fine bed materials accelerates algal detachment and restricts the accumulation of algal biomass by reducing bed stability.