## 3 Results of Dam Operation  
### Results of Historical Data  
The concept of resilience was applied to the Hwacheon Dam using historical data. About 54% of the annual precipitation in Korea is concentrated during the flood season (June to September). Therefore, the hydropower reservoir in Korea was built for power generation, but since 1973, it has been contributing to flood control by setting a water level limit during the flood season. The limiting water level operation method causes losses in  
Figure 6: Scenario of dam operation at Hwacheon Dam. (**a**) Target of dam water level; (**b**) Target of outflow discharge. terms of power generation. To confirm this, resilience was applied using historical data before and after 1973. Resilience was compared in 1981 and 1986, when the average annual inflow was like that of 1971, before the operation of the limited water level during the flood season. In Table 2, hydropower data, operation data, and resilience of hydropower reservoirs by year were calculated and presented. Figure 7 compares dam water levels by year. In Figure 7, in 1971, when there was no limiting water level during the flood season, the water level in the dam recovered the fastest to normal high-water level (NWL) (Point A). In 1981 and 1986, the water level of the dam was operated below EL175 m during the flood season due to the limited water level (Point B & C), and the hydropower reservoir was operated by restoring the water level to NWL at the end of the flood season (Point D). As a result of applying the resilience defined in this study using historical data for the period, it was found that the resilience of 1971, when it was operated without a limiting level, was greater. There is a clear difference in resilience during the flood period. Comparing generation, discharge, and resilience by year, we can confirm the importance of water level recovery in terms of power generation. Theoretically, to increase the amount of power generation, it is necessary to restore the dam level to NWL to secure an effective head. Therefore, in 1971, when the water level was restored the fastest, the resilience was the highest at 0.61 (all period). At this time, the annual total power generation amounted to 422,421 MWh in 1971, producing the largest amount of electricity. On the other hand, in 1981 and 1986, it was operated below the limit water level during the flood season, and the resilience was 0.59 and 0.51, respectively (Figure 8). Power generation also decreased to 385,426 MWh (91.2%) and 376,201 MWh (89.1%), respectively. The average annual outflow discharge in 1981 is less than in 1986, but the average annual generation is higher. The reason can be confirmed by the average annual dam water level. In 1981, it was confirmed that the water level recovered more rapidly during the flood period. In fact, when comparing the power generation in June, the power production in June 1981 was 38,547 MWh, which was almost twice as high as 20,826 MWh in 1986. Through this, it can be said that the resilience defined by the dam water level is related to the power generation \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Contents** & **1971** & **1981** & **1986** \\ \hline Annual Mean Water Level (EL.m) & 171.6 & 171.1 & 169.3 \\ \hline Annual Mean Inflow (m\({}^{3}\)/s) & 100.91 & 116.64 & 98.13 \\ \hline Annual Mean Outflow (m\({}^{3}\)/s) & 88.84 & 81.82 & 85.35 \\ \hline Spillway Discharge (m\({}^{3}\)/s) & 6329 & 14,556 & 7666 \\ \hline Power Generation (MWh) & 422,421 & 385,426 & 376,201 \\ \hline Resilience (All Season) & 0.6100 & 0.5901 & 0.5153 \\ \hline Resilience (Flood Season) & 0.7741 & 0.7464 & 0.5354 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of hydrological data and resilience (All seasons are full year and the Flood season is from 21 June to 20 September). Figure 7: Comparison of dam water level by year. in hydropower plants. As shown in Figure 8, operation results in 1971, which had high resilience regardless of period, produced more electricity than results in 1981 and 1986.