## 4 Results and Discussion
### Dam Failure Scenarios  
The danger of non-warning release from the dams in the northern part of the Imjin river basin has been recorded in history, but simulation still presents a number of limitations due to the lack of information of the northern part, and the simulated discharge may not reflect accurately the actual  
Figure 4: The simulated and observed discharges at Gunnam station in 4 flood events (2009, 2010, 2011, and 2012) after considering dam impact. Figure 3: The simulated and observed discharges at Gunnam station in 4 flood events (2009, 2010, 2011, and 2012) after considering dam impact. amount of discharge in the basin. In the release period, this may cause danger for those living downstream. Therefore, four dam failure scenarios were designed based on dam water level and amount of water released from the dam including: 100 percent, 80 percent, 50 percent, and 20 percent of maximum dam storage. The simulated results based on these four dam failure scenarios could help to recognize the potential hazard caused by dam impact of the northern part of the Imjin river basin. Firstly, the water storage capacity of the dam was defined using the GIS Functional Surface toolset, then the remaining water storage was also defined based on the percentage of the water storage capacity corresponding to four flood scenarios. The threshold water level set up in the model was computed using the relationship between the water level and storage of Hwanggang dam calculated in Section 4.1. The amount released, remaining storage, and threshold water level corresponding to four flood scenarios are shown in Table 4.  
To run all flood scenarios caused by dam failure, in this study, the event in 2010 was simulated with the starting time of the dam failure at 10:00 am on 2 September 2010. The simulation results of peak discharges in Gunnam station and Hwanggang dam are shown in Table 5, and simulated discharge at Gunnam station and outflow at Hwanggang dam based on four dam failure scenarios are illustrated in Figure 5.  
The hazardous danger of dam failure is clearly shown in the case of 100% or 80% of the water amount being released from the dam, with the sudden increase of downstream discharge and the extremely high peak flow at Hwanggang dam and Gunnam station. In detail, for the case of 100% of the water amount being released from the dam, the discharge in Gunnam station increases rapidly and reaches the peak flow of 22,536.5 m\({}^{3}\)/s within only 2 hours after dam failure, more than eight times compared to the observed peak flow in the same period. For other flood scenarios, the peak flow in Gunnam is lower, and the time of peak becomes slower. Especially in the case of 20% of the water amount being released from the dam, the simulated peak flow is less than the observations, showing that in reality, during this event, the amount of water released from the dam is probably greater than 20% of its storage. In Hwanggang dam, the threshold water level for dam safety can be changed in the range of 90 m to 105.8 m. When the dam level gets to the flood level at 103.7 m, it warns of an emergency case, and the dam may release a large amount of water stored in dam. For four designed flood events, the scenario of 100 percent of water being released is the most dangerous case with the peak flow of \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scenarios** & **Dam Water Level (m)** & **Amount Released (10\({}^{6}\) m\({}^{3}\))** & **Remaining Storage (10\({}^{6}\) m\({}^{3}\))** \\ \hline
100\% & 73 & 451.4 & 0 \\
80\% & 92.1 & 361.1 & 90.3 \\
50\% & 98.5 & 225.7 & 225.7 \\
20\% & 102.6 & 90.3 & 361.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Threshold water level corresponding to 4 dam failure scenarios.