| Sediment to deficient areas in downstream may promote detaching overgrown algae, which is frequently old and is low nutritional quality. The system of permanently supplying sediment, like sediment bypass tunnels, is important for detaching overgrown algae, because algae can recover their biomass within several months. We assessed annual algal biomass in downstream reaches of a dam equipped with sediment bypass tunnels, for estimating the capacity of yearly limiting the amount of algal biomass. For the assessment, we developed models for calculating algal biomass. The models contain many variables, because algae is easily affected by several factors (ex. flow regimes, sediment volume, etc.). After, confirming the accuracy of our model for calculating algal biomass, downstream conditions were simulated with and without sediment from the bypass tunnel. A comparison of these two scenarios indicated that the algal biomass was detached not only in the flood season (summer and autumn) when supplying sediment from bypass tunnels, but also in the normal season (winter and spring). This result may be attributable to the increased quantities of fine river-bed materials in the downstream reaches by the sediment from the bypass and the increased tractive force on the riverbed, which accelerated algal detachment, resulting in a reduction of algal biomass. |
