196 / 2024-02-27 13:25:50

Modeling of Water-Sediment Inundation Process Incorporating with a Rainfall-Sediment Runoff model

Water-sediment inundation prediction,Sediment transport processes,Landslide and debris flow,Rainfall and sediment runoff,Sediment sorting,Bed variation

In recent years, the heightened occurrence of rainfall-induced landslides and debris flows has altered the dynamics of flood inundation in fluvial plain areas, emphasizing floods characterized by substantial rapid sediment deposition alongside active channel changes and bed variations. The accuracy of riverbed variation prediction is crucial for predicting such hazard because it is the dominant driving factor in determining where and when inundation is likely to occur. However, accurate prediction remains a challenge, primarily because of the difficulty in obtaining appropriate upstream boundary conditions for the computation of two-dimensional riverbed variation in plain areas, particularly with respect to sediment discharge hydrographs at each size class.

Therefore, this study aims to enhance the accuracy of predicting flow and sediment inundation by integrating a watershed-scale rainfall and sediment runoff model into a two-dimensional riverbed variation model. This integration enables the model to automatically predict the appropriate upstream boundary conditions for the two-dimensional riverbed variation computation.

The proposed model was applied to simulate the large flood and sediment inundation disaster that occurred in the Uchi-kawa River Basin in Miyagi Prefecture, Japan, during Typhoon Hagibis in October 2019. Rainfall and sediment runoff simulation was conducted in mountainous terrains. A distributed rainfall–runoff model using a diffusion wave scheme was employed for rainfall–runoff analysis, along with slope stability analysis for predicting landslide occurrences and a mass point system-based debris flow runout analysis. Surface flow, subsurface flow, and debris flow from hillslope areas eventually converged into unit channel networks, where the unit channel was defined as the channel reach between its upstream and downstream confluence points, with sediment transport transitioning to bedload and suspended-load modes within the channel networks. Based on the computations of these sediment transport processes, flood and sediment flow conditions at the mountain valley outlet can be predicted over time, serving as upstream boundary conditions for the two-dimensional riverbed variation and inundation computation in the plain area. The simulation results, including predictions of landslide and debris flow occurrences in terms of volume and spatial distribution, riverbed sediment grain size distribution, and the extent, depth, and grain size of inundated sediment in the plain area, were validated against observed data. Consequently, the proposed model is applicable for predicting multi-composited flood and sediment hazards across a watershed in high resolution of the hazard conditions.