This study aimed to determine the depositional effect and improve the identification of debris flow risk zones. To accomplish this goal, we developed a two-dimensional debris flow model (Deb2D) based on a hyperbolic conservation form of the mass and the momentum balance equation with consideration of the erosion–entrainment effect as well as the depositional effect. In this model, we implemented a widely-used rheological equation—the Voellmy equation—and a quadtree adaptive grid-based shallow-water equation. This model was applied to two study sites to assess the depositional effect. The impact area, volume of soil loss, maximum velocity, inundated depth, and erosion depth resulting from the debris-flow modeling were compared with the field data. The simulation results with/without deposition were evaluated using the receiver operating characteristic method. The implementation results of the erosion–entrainment model with deposition showed superior accuracy when estimating the damage range and flow time.

Landslides and debris flows are significant disasters that frequently occur on hillslopes, often resulting in casualties and property damage when they take place near residential areas. Specifically, in regions with dams or reservoirs, landslides and debris flows can raise the top of dead storage, reducing the effective storage capacity of these facilities. Additionally, debris flows entering reservoirs can generate tsunami-type waves, potentially leading to overflow-induced flooding and the collapse of hydraulic structures. Numerical modeling has been widely employed to mitigate such disasters. However, most studies utilized three-dimensional hydrodynamics or smoothed particle hydrodynamics, focusing primarily on laboratory-scale events without considering critical processes such as erosion, entrainment, and deposition. These processes are essential for accurately simulating debris flow dynamics. To address these limitations, this study developed a multi-layer dynamics simulation model based on shallow water equations that consider erosion, entrainment, and deposition mechanics, enabling the analysis of field-scale events. The model's performance was validated through theoretical and laboratory experiments. The 2020 Sanyang Reservoir collapse event in South Korea was selected as a case study to evaluate the model's applicability. Scenario-based analyses were conducted, considering debris flow characteristics and reservoir water level conditions, to explore various potential outcomes. The results highlighted the correlation between debris flow momentum and wave scale, with the maximum momentum of the debris flow identified as a strong predictor of the wave's magnitude.

Landslides and debris flows are significant disasters that frequently occur on hillslopes, often resulting in casualties and property damage when they take place near residential areas. Specifically, in regions with dams or reservoirs, landslides and debris flows can raise the top of dead storage, reducing the effective storage capacity of these facilities. Additionally, debris flows entering reservoirs can generate tsunami-type waves, potentially leading to overflow-induced flooding and the collapse of hydraulic structures. Numerical modeling has been widely employed to mitigate such disasters. However, most studies utilized three-dimensional hydrodynamics or smoothed particle hydrodynamics, focusing primarily on laboratory-scale events without considering critical processes such as erosion, entrainment, and deposition. These processes are essential for accurately simulating debris flow dynamics. To address these limitations, this study developed a multi-layer dynamics simulation model based on shallow water equations that consider erosion, entrainment, and deposition mechanics, enabling the analysis of field-scale events. The model's performance was validated through theoretical and laboratory experiments. The 2020 Sanyang Reservoir collapse event in South Korea was selected as a case study to evaluate the model's applicability. Scenario-based analyses were conducted, considering debris flow characteristics and reservoir water level conditions, to explore various potential outcomes. The results highlighted the correlation between debris flow momentum and wave scale, with the maximum momentum of the debris flow identified as a strong predictor of the wave's magnitude.