Flood simulation model coupled with woody vegetation wash-out in the lower Asahi River, Japan

Flooding with a discharge of approximately 4500 m³/s occurred in the lower Asahi River because of the heavy rain event in western Japan in July 2018. This flooding engendered wash-out situation of vegetation such as woody and herbaceous species and lodging of bamboo grove. Earlier research works attempted to clarify the critical wash-out condition of woody vegetation during flooding. However, two challenges still need to be solved: (1) the tractive force and moment because of drag force acting on woody vegetation, were estimated assuming without vegetation was washed out, and (2) the mechanism of woody vegetation wash-out was evaluated considering only a unique parameter of either the tractive force or moment, but not the both. Accordingly, the critical wash-out condition in the lower Asahi River in 2018 was not reasonably elucidated by the earlier frameworks. Considering those limitations, this study proposed a novel numerical simulation model for the woody vegetation wash-out in rivers during flooding. The mechanism of the flow resistance reduction because of the woody vegetation wash-out was also taken into consideration. In this study, vegetation conditions and topo-bathymetric data were accurately extracted using an airborne laser bathymetry (ALB) system for the targeted domain in the lower Asahi River. The simulated results demonstrated that the proposed model can improve the reproducibility of the wash-out situation after the flooding, compared with models that did not incorporate both the woody vegetation height distribution and transition process of woody vegetation wash-out during flooding.