## 4 Discussion  
### Flood Control Effectiveness of IFCS and its Operation  
The hydrodynamic model was applied in the Dongting Lake area in order to evaluate the flood mitigation ability of the IFCS. We compared the effective flood control capacity and the difference of  
Figure 10: Simulation of fluctuation of water level at Chenglingji station. Figure 9: The layout of the IFCS in East Dongting Lake. maximum water levels of the same hydrograph with/without IFCS, and the hydrographs in 1998 were chosen as an example. In the natural conditions, the effective flood control capacity (EFC) is limited, because the safe floodwater occupies most of the capacity of the lake prior to the arrival of the flood wave. Take the East Dongting lake as an example, the EFC is only about 0.71 billion m\({}^{3}\) during the first flood wave under the natural condition (see Table 3). However, the EFC could be greatly increased after the application of the IFCS, as the safe floodwater is not allowed to enter the inner lake. Therefore, the EFC increases to 6.37 billion m\({}^{3}\), which is about nine times larger than that in the natural condition, as the EDL is nearly empty before the first flood wave arrives (see Figure 11a). During the second flood wave, the natural EFC of EDL increases to 1.54 billion m\({}^{3}\), while it becomes 7.28 billion m\({}^{3}\) with the IFCS. It is about 4.7 times larger than that in the natural condition, because the water level in EDL is still very low before the second flood wave arrives (see Figure 11b). As for the third flood wave, the flood control capacity of Dongting Lake could still be increased to 2.16 billion m\({}^{3}\), which is nearly 2.8 times larger than that under the natural condition due to the fact that the water level in the inner lake is still lower than that in the main channel, which indicated that there was still extra floodwater storage capacity in the EDL (see Figure 11c). The Chinese Central Government proposed a policy known as \"return land to lake\" after the 1998 flood to control the water level in Dongting Lake [5]. The lake embankment would be removed and the land surrounding the Dongting Lake would be converted into a lake area to increase the capacity of the lake. The \"return land to lake\" policy has a positive effect on the flood control in Dongting Lake area, which was investigated by [PERSON] (2017) [18] while using a numerical simulation method. The water surface area has been gradually increased in recent years [23], but it is proceeding very slowly, because it has dramatic effect on the people who live in that area, since they have to move out of that place. However, the IFCS increases the flood control capacity by exploring and developing the potential flood control capacity of the lake itself, and thus it has little effect on the people who live in the lake's surrounding area. The operation of the hydraulic gates in the IFCS has a significant effect on flood mitigation. If the operating water level; (i.e., the gate will be opened when the water level is over this level near the gate) is too high, as a consequence, the gate would open relatively late, which indicated that there is not enough time to let the excess floodwater enter into the inner lake. Therefore, there is still extra water storage capacity in the inner lake and the peak water level could be further reduced when comparing Case 1 with Case 2. If the operating water level is too low, the floodwater would enter the inner lake too early. This part of the floodwater would occupy some part of the lake storage capacity, resulting in  
Figure 11: Water level distribution in Dongting Lake before the flood wave arrives in 1998. (**a**) 3 July; (**b**) 22 July; and, (**c**) 16 August. there not being enough effective capacity to store the excess floodwater when the peak flood discharge arrives. Hence, the reduction of the peak water level would decrease when comparing Case 3 with Case 2.