PHYSICAL ENVIRONMENT EVALUATION OF THE SPAWNING FOR AYU BASED ON ESTIMATION OF FLOW FIELD AND SEDIMENT BEHAVIOR USING ALB DATA IN THE LOWER ASAHI RIVER

There has been an ecological concern that the rigid riverbed causes degradation of fishery habitats in vegetated rivers. However, investigations for the actual habitats have been conducted locally using limited data related to the river bathymetry, flow fields, and sediment dynamics. This study presents a method to evaluate the potential of suitable spawning for Ayu using a PHABSIM model with flow field data. This study also examines the key mechanism for forming a loose riverbed piled with gravels that provides a favorable environment for the spawning. For that purpose, we obtained ALB data of high-resolution bathymetry, conducted numerical simulations of the flow field, and estimated the tractive force for each grain size of riverbed sediment at Gion area in the lower Asahi River. Results showed that numerical simulations validated by our field observations are able to estimate the time and spatial variation of the suitable spawning area induced by small floods. They also presented that the loose riverbed condition is not generated and maintained mainly by the bed load transport of major gravels due to large flooding, but by wash-out of sands from the riverbed because of small floods.