Debris flow in South Korea predominantly occur during the concentrated heavy rainfall of the summer monsoon season. With over 70% of the country's territory consisting of mountainous terrain, the risk of debris flows and landslides is exceptionally high compared to other disasters. South Korea is thus considered one of the countries with a very high vulnerability to these hazards. The model proposed in this study utilizes high-resolution aerial LiDAR DEM-based terrain data (with a resolution of 1m x 1m) to analyze the topographical stability of slopes and simulate rainfall flow.
We assume that areas with extreme changes in rainfall vector velocity on slopes with low stability generate abrupt impact forces, leading to a higher likelihood of debris flow occurrence. We identify points with these characteristics using GIS overlay analysis. The reliability of this technique will be validated by comparing it with representative real-world cases of debris flow occurrence.
Takahashi(2007) classified the initiation of debris flows in steep mountainous areas during concentrated rainfall into three cases: (1) natural slope failure, (2) gully erosion and incision on valley floors and slopes, and (3) collapse of natural earthen dams. Similarly to Japan, the initiation of natural slope failure in domestic areas is primarily due to an increase in pore pressure resulting from rainfall infiltration. Considering this, the following assumptions are made in this study to estimate the starting point of debris flows:
(1) During rainfall events, water generally collects on slopes through various temporary drainage networks and converges in catchment areas, forming temporary channels and moving down the slopes. (2) Debris flows are more likely to occur in areas with low stability index grades. (3) The flow generated by rainfall temporarily alters velocity and impact force along the channels, influenced by the topography. The probability of debris flow occurrence is higher at points where there is a rapid change in velocity vectors and impact force. (4) After a sufficient cumulative rainfall(200ml), we assume the pore water pressure of soil on the slope is close to the saturated state.
The soil composed of fine particles maintains the slope in a state of cohesion. When fluid flow generated by rainfall induces shear stress in the slope, exceeding the yield stress limit due to cohesion, the soil particles mix with water and begin to move. The movement of debris flow can be considered as the movement of a viscous fluid, and rheological factors such as yield stress and viscosity play a crucial role in determining the mobility of the failure surface (Jeong, 2010).
To reflect the characteristics of debris flow behavior in South Korea, this study simulated the flow and extent of debris flow based on specific similarity concentrations using the FLO-2D model.
Figure 1 shows the conceptual process of estimating the occurrence points of debris flows proposed in this study.
The locations where the flow velocity undergoes a rapid change, and the impact force abruptly increases along the path are determined through overlapping analysis. To consider the flow motion of the discharge during concentrated rainfall events, a numerical analysis program based on rheological models is used to simulate the hydraulic flow and calculate the similar velocity vectors, thus identifying the points of abrupt change in the watercourse.
The reliability of the analysis results is evaluated by comparing them with the actual locations of debris flow occurrence obtained through field surveys. Figure 2. shows an overview of the flowchart for this study.