Author(s): Menglu Qin; Daisuke Harada; Shinji Egashira

Linked Author(s):

Keywords: Water-sediment inundation prediction; Landslides and debris flows; Multi-hazards; Rainfall and sediment runoff; Sediment transport process

Abstract: Water-sediment inundation, often observed in plains during torrential rainfall, is caused by the rapid deposition of large amounts of sediment carried by floodwaters from mountain streams containing substantial sediment delivered from landslides and debris flows. This study aimed to improve the prediction accuracy of water-sediment inundation hazards by integrating a catchment-scale rainfall and sediment runoff model, which incorporates slope stability analysis for landslide occurrence and a point mass system-based debris-flow runout model, with a depth-integrated two-dimensional flow and sediment transport model. This approach allows the proposed method to predict the temporal variation of the conditions of water and sediment inflows in plain areas for 2-D water-sediment inundation analysis. The proposed method was applied to simulate such multi-hazards in the Uchi River basin in Japan during Typhoon Hagibis in October 2019. The simulation results, validated against observed data, included predictions of landslide and debris-flow occurrence and distribution, sediment budgets within the catchment, and the size and depth of deposited sediment on the plain. Results showed that sediment produced by landslides and debris flows is partly stored in the river bed and remains on the mountain slopes, indicating that such event-based hazards can have long-term effects. Additionally, the study found that water-sediment inundation risk is particularly high in small tributaries, where sediment inflows during torrential rainfall often significantly exceed the channel’s transport capacity, resulting in rapid bed aggradation that substantially reduces the channel’s capacity.

DOI: https://doi.org/10.64697/IAHR-APD2024_P196

Year: 2024