# Simulation of Debris-Flow Runout Near a Construction Site in Korea
## 5 Discussion  
This study used slope angles to simulate debris-flow runout distances. In each of four watersheds, slopes were plotted along the debris-flow direction (Figure 17). Because each slope section shows just one point of inflection (PI), the simulations used average slopes on either side of the PI. In Watershed A, the PI is located 183 m from the bottom of the slope. The average slope of the lower part is 16.2 degrees and that of the upper part is 31.6 degrees. In Watershed B, which has its PI at 119 m, the average slope of the lower part is 7.2 degrees and that of the upper part is 32.5 degrees. Watershed C has a PI at 322 m with average slopes of 18.2 degrees and 31.7 degrees in the lower and upper parts, respectively. In Watershed D, the PI is at 74 m and the average slopes are 6.2 degrees below the PI and 18.2 degrees above. The simulations show a debris flow in Watershed C approaching within 9.6 m of the target site. This result reflects the relationship between debris runout distances and slope angles, with Watershed C having steeper average slopes, across both parts, than the other watersheds. The steeper slopes mean that the speed of debris flow does not reduce much until arriving at the bottom of the slope. The debris flow in Watershed C, therefore, came nearer to the target site than in the four watersheds. In the case of Watershed B, the slope below the PI is a very gentle 7.2 degrees while the upper part has the steepest average slope angle in all the watersheds. This would induce fast debris flow on the upper slope, but below the PI the gentle slope area significantly reduces the debris flow speed and the flow energy. Therefore, in Watershed B, the simulated debris flow does not reach the target site even though the debris flow starting point in Watershed B is, among the watersheds, the nearest to the target site. Based on the simulation results, the slope angle is a major factor influencing debris-flow runout distances. Debris runout distances are determined by several factors, in addition to slope angle, including the volume of debris, quantity of water, physical and rheological properties of the soil. Debris-2D considers the volume of debris and yield stress are significant in calculating runout distance. [PERSON] and [PERSON] [36] discussed model sensitivity to yield stress and the volume of debris. They explained that the effect of varying volume of debris was three to four times larger than that of varying yield stress in case  
Figure 16: Simulation of maximum flow speeds over the flow period in each watershed. (**a**) Watershed A, (**b**) Watershed B, (**c**) Watershed C, (**d**) Watershed D.  
of Chi-Chi debris flow. As mentioned in Section 3.2, [PERSON] and [PERSON] [43] found that, if yield stress is 100 times larger, the front location of the affected area only reduces by 50 m. However, if the volume of debris flow is just 10 times larger, the front location increases by almost 300 m. The greater influence of debris volume is reflected here in the similar pattern of runout distances among the watersheds. Watersheds C and D which have larger debris volumes, in combination with rainfall water volume, have longer runout distances than the other watersheds (Table 2). In particular, Watershed D has the largest area and the largest debris-flow volume in the study area. This may explain why the simulation shows the longest runout distance in Watershed D even though the average slope is gentler than in the other watersheds. The major advantage of the methodology used in this study is that the dynamics of debris flows can be simulated accurately. In addition, initial mass volume and occurrence location were evaluated by integrating the proposed landslide susceptibility model with a mechanically based debris flow model. Therefore, this study has successfully delineated the dangerous zones and affected area of debris flows from multiple watersheds, and it will be considered as an important reference of countermeasure engineering for the construction site.