METHODS TO EVALUATE SEDIMENT RUNOFF DURING HEAVY RAINFALL IN THE MOUNTAINOUS RIVERS

In the mountainous rivers, a huge amount of sediment is supplied due to landslides and debris flows during heavy rainfall, which causes flood disasters with huge amount of sediment in downstream reaches. This study proposes a method to predict sediment runoff by combining a rainfall-runoff model and a unit river model to compute sediment transport. The proposed model is compared with a method to predict sediment runoff by considering the sediment erosion from the supplied sediment. As a result of computation for the Gofukuya-river flood disaster in 2019, the computational result of the proposed method underestimates the actual sediment runoff, because the sediment coarsening occurs in mountainous rivers during the flood event, which implies the importance of sediment supply settings in the mountainous rivers.