Author(s): Daisuke Harada; Shinji Egashira

Linked Author(s): Shinji Egashira

Keywords: Sediment transport; Driftwood; Flood disaster; 2-D computation

Abstract: The Akatani river flood disaster in Japan, in 2017 is characterized as the obvious longitudinal sediment sorting and a huge amount of driftwood supply from upstream mountainous areas, that were likely to increase the damage of flood disasters in downstream areas. This study aims to develop methods to simulate such flood disasters. Particullary, we employed a bedload formula in which the sediment transport rate is proportional to the 2.5th power of non-dimensional bed shear stress in order to evaluate the obvious longitudinal sediment sorting. In addition, we propose methods to simulate the behavior of large amounts of driftwood in the flow field with active sediment transportation by employing the convection-diffusion equation. The flood flow and sediment tranport is simulated using 2-D depth-integrated governing equations. To analyze a large amount of driftwood in the flood flow, driftwood pieces are assumed to behave as neutral buoyant particles, for this assumption enables the introduction of the convection-diffusion equation. Further, assuming that the erosion and deposition of driftwood take place in proportion to sediment erosion and deposition and also assuming that driftwood accumulation occurs at artificial structures such as bridges, the convection-diffusion equation is coupled with the storage equation of driftwood in the channel bed. 2-D numerical simulations are conducted for the Akatani River. The simulation results indicate that the flood flow can be significantly affected by driftwood deposition at the bridge in terms of water level, velocity distribution, and sediment deposition. The sediment deposition downstream of the bridges is efficiently simulated when driftwood deposition is incorporated into simulations, which indicates that driftwood deposition can affect the morphological changes of the river channel significantly. The comparison between the field and simulation results addressing driftwood deposition in the field demonstrates that the simulation method proposed in this study can reproduce driftwood behavior. Overall, since the proposed methods makes it possible to simulate the behavior of a large number of driftwood pieces, it can be applied to the management of hazards and rivers, such as the Akatani River.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221350

Year: 2022