# Simulation of Debris-Flow Runout Near a Construction Site in Korea
## 4 Simulation of Debris Flow
### Results of the Numerical Simulation of Debris Flow  
There are four watersheds near the construction site, to the west of Mt Goobong, as shown in Figure 8. Information on each watershed is listed in Table 2. In the modeling, it is assumed that the mass of debris-flow at the initiation area comes from a landslide triggered on the slopes. The areas of the landslide and sources of mass used in the simulations were those with a landslide probability over 70%. Based on the regolith mapping, the thickness of loose soil on the slope of Mt Goobong was set to 1.5 m uniformly. On this assumption, the total volume of dry debris across all four watersheds is 24,726.78 m\({}^{3}\) (Figure 9).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Watenshed Area [m\({}^{2}\)]** & **Total Dry Debris Volume [m\({}^{3}\)]** & **Designed Debris-Flow Volume \(\dagger\) [m\({}^{3}\)]** & **Area for Collecting Rainfall [m\({}^{3}\)]** & **Designed Water Volume by Rainfall [m\({}^{3}\)]** \\ \hline A & 67,283.35 & 6612.56 & 10,966.11 & 33,778.37 & 555.54 & 13,996.33 \\ B & 55,942.89 & 3174.32 & 5264.21 & 14,144.71 & 2326.80 & 5860.97 \\ C & 78,296.82 & 3594.74 & 5961.43 & 45,122.06 & 742.58 & 18,696.67 \\ D & 13,946.30 & 11,345.36 & 18,814.54 & 75,789.91 & 12,467.44 & 31,404.13 \\ \hline \hline \end{tabular}  
* \(\dagger\) calculated as \(V_{d}\)/\(C_{v}\) (\(C_{c}=0.603\)), \(\ddagger\) calculated as \(V_{water}\)/\(\bar{I}-C_{v}\)) \(V_{d^{\ast}}\) dry debris volumes, \(C_{v}\): equilibrium solid volume by Rainfall [\(\dagger\)m\({}^{3}\)]  
\end{table}
Table 2: Information of watershed area and total volume of debris in each watershed.  
Figure 8: Distribution of watersheds in the simulation area overlain on the landslide prediction map.  
In preparing for damage by the debris flow, it is necessary to consider the worst-case scenario, i.e., when all potential mass becomes debris flow. The worst triggering hazard condition was considered in the simulation. A severe landslide-triggered debris flow occurred in Chooncheon on 27 July 2011 and 15 July 2013. According to Korean Meteorological Agency data, the accumulated rainfall amounts from 26 to 27 July 2011 were 427 mm, and 265 mm from 14 July to 15 July 2013. The simulation used the accumulated rainfall as water input to determine the debris-flow volume. Taking the worst-case scenario, the debris flow was assumed conceptually to involve the entire volume from bottom erosion. However, [PERSON] and [PERSON] [36] set assumption that the entrained volume of debris flow from bottom erosion is not much compared with total volume of debris flow which is majorly composed of source volume of debris flow. Therefore, Debris-2D considers the initial volume of debris flow as major volume of debris flow to calculate the total volume. The increase of volume by bottom erosion can be included by assigning sufficient initial volume of debris flow. For the simulation, based on the inclined plane test of soil samples collected from the field, the average yield stress was set to 2800 Pa in the saturated condition. This calibrated value of yield stress is typical for a flow mass with a high proportion of coarse sized debris or rock, and therefore high friction, as is shown in Figures 4 and 5.  
Figure 10 illustrates, in three dimensions, the eventual deposition locations in the four watersheds. The debris in Watersheds A and D flows across Jamboree Road, which is located between Mt Goobong and the site, before deposition. In Watershed B the debris is deposited on the mild slopes before reaching Jamboree Road. However, in Watershed C the final deposition of debris is very close to the southern part of the construction site. Simulated flows and debris thicknesses for each watershed are depicted in Figures 11-15.  
Figure 9: Map of effective rainfall collection areas (yellow) in each watershed. The purple areas denote initial mass distributions and the cyan triangle is the peak of Mt Goobong.