## 3 Results
### The Water Level  
The flood process responses to the application of the IFCS in Dongting Lake under one of the largest floods in the 20 th century, i.e., 1998 type flood (24 June to 22 September), was calculated based on the numerical model. Figure 7 shows the fluctuation of the water level in West Dongting Lake (WDL, Nanzui station), South Dongting Lake (SDL, Yingtian station), and East Dongting Lake (EDL, Chenglingji station). The results show a significant difference in the flood stage with the IFCS and the peak water level significantly decreases in the second and third flood wave period. Figure 7a shows that the water level is higher than that under the natural condition due to the narrowing of the flow cross section by the dike before the gate opens when the water level is less than the control level. The water level dramatically decreases, e.g., the second and the third flood wave, when the water level is higher than the control level. The average reductions of the water levels are about 0.18 m and 0.15 m in the second and third flood wave period, respectively. The variation of water level at Yingtian station (see Figure 7b) and Chenglingji station (see Figure 7c) is similar to the Nanzui station. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Natural Condition**} & \multicolumn{2}{c}{**First Flood Wave**} & \multicolumn{2}{c}{**Second Flood Wave**} & \multicolumn{2}{c}{**Third Flood Wave**} \\ \cline{2-7}  & \(\Delta h\) **(m)** & \(V_{e}\) **(10\({}^{9}\) m\({}^{3}\))** & \(\Delta h\) (m)** & \(V_{e}\) **(10\({}^{9}\) m\({}^{3}\))** & \(\Delta h\) (m)** & \(V_{e}\) **(10\({}^{9}\) m\({}^{3}\))** \\ \hline WDL & 1.25 & 0.24 & 2.76 & 0.52 & 1.13 & 0.22 \\ \hline SDL & 0.87 & 0.49 & 1.77 & 0.99 & 0.84 & 0.47 \\ \hline EDL & 0.73 & 0.71 & 1.58 & 1.54 & 0.78 & 0.76 \\ \hline Total & - & 1.44 & - & 3.05 & - & 1.45 \\ \hline
**IFCS** & \(\Delta h\) **(m)** & \(V_{e}\) **(10\({}^{9}\) m\({}^{3}\))** & \(\Delta h\) (m)** & \(V_{e}\) **(10\({}^{9}\) m\({}^{3}\))** & \(\Delta h\) (m)** & \(V_{e}\) **(10\({}^{9}\) m\({}^{3}\))** \\ \hline WDL & 0.78 & 0.15 & 2.71 & 0.52 & 1.13 & 0. For Yingtian station, there is a short period when the water level is still higher than that without IFCS, even when the gate is opened (i.e., the water level is higher than control level) during the first flood wave period (see Figure 7b). This can be explained by the fact that the gate opening period is too short (i.e., about 1 day) and the water level is only slightly higher than the control level, resulting in a limited excess floodwater being be mitigated into the inner lake. Therefore, the reduction of the water level is slight. However, the water level reduction is significant in the second and third flood wave, i.e., the average reduction of the water levels are about 0.10 m and 0.20 m, respectively. For Chenglingji station, the average reduction of the water levels are about 0.25 m and 0.18 m in the second and third flood wave period, respectively. The highest water level is the most important factor in estimating the potential flood loss. Hence, the second and third flood wave were chosen to analyse the change of the highest water level after applying the IFCS due to the highest water level in the WDL occurring in the second flood wave and the highest water level in the SDL and EDL occurred in the third flood wave (see Figure 6). Table 4 shows the difference in the peak water level in the second and third flood wave before and after the use of IFCS at all the selected hydrological stations. During the second flood wave, the peak water level considerably decreases at Nanzui station (in the south of WDL), Lujiao station and Chenglingji station (in the EDL). However, it only decreases a little at Xiaobezui station (in the north of WDL) and Yingtian station (in the SDL). As for the third flood period, the peak water level decreases at least 0.32 m at all  
Figure 7: The water level process during the flood season (24 Jun. to 22 Sep. 1998) in Dongting Lake with and without IFCS.