## 1 Introduction  
Flood disaster causes the greatest losses among natural hazards and it has become more severe and most frequent in recent years due to climate change, urbanization, and water infrastructures. [1; 2; 3]. Economic losses from flood hazards have considerably increased over the recent decades [4]. Great floods result in great losses to the world as well as in China. During 1990-1998, most of the global significant flood disasters occurred in China [1], especially in China's Yangtze River Basin, where about 40% of the national GDP in China is produced due to its abundant water. On the other hand, its economic development is constrained by these flood disasters, according to the Flood chronology of China. Upstream dams on rivers (e.g., Three Gorges Dam (TGD, China) [5], Hoover Dam (America) [6], and Wivenhoe Dam (Australia) [7], etc.) can greatly regulate the inflow from their catchments, which can effectively reduce flooding. However, floodwater from the vast downstream area cannot be regulated, as there is no dam to control the flow. Relatively, the downstream river valleys, as well as the delta, have a higher population density and properties and any floods can cause huge losses, thus excessive floodwater management is most important. For example, there are two dams with a flood control capacity of 1.97 billion m\({}^{3}\) in the Brisbane River catchment, but Brisbane city on the river delta was flooded twice in the 20 th century, and the city was flooded again, leading to 35 deaths and $2.55 billion economic loss in 2011 [7]. The TGD is one of the largest dams in the world, with a flood control capacity of 22.1 billion m\({}^{3}\), but it only controls 56% of the Yangtze River catchment area (see the shadowed area in Figure 1); floodwater from the downstream area cannot be controlled, even where the mega-cities, like Wuhan, Nanjing, and Shanghai, are situated. When the floods come from the middle or lower part of the Yangtze River basin, the TGD cannot effectively regulate the floodwater for the purpose of disaster mitigation, as happened in Brisbane in 2011. Therefore, it is worthwhile to investigate the flood control strategy in the downstream area of the dam. In the current study, we take the Dongting Lake, which is located downstream of the TGD, as an example to investigate how a lake can achieve a more effective role in mitigating a flood disaster in the middle or low part of a large river.  
Generally, a large lake (i.e., the mean surface area is greater than 500 km\({}^{2}\)[8]) plays an important buffer role for temporal floodwater detention during flood seasons [9; 10]. However, a flood disaster still occurs when the incoming excessive floodwater is higher than a lake's effective storage capacity, which usually becomes very small relative to its total storage, because safe floodwater occupies most of its total storage (small/medium scale flood) as the dead storage before the peak flood wave arrives [11]. For example, the severe flood-period of 1998 in China resulted in 3600 deaths and wreaked more than $30 billion in damages [1], even though its upstream dams and downstream lakes played some role in mitigating the loss. An innovative flood control scheme (IFCS) proposed by [PERSON] [11; 12] could be used to mitigate the flood disasters in a large lake to improve the effective flood control capacity of the lake to reduce the flooding loss. The IFCS consists of a by-pass canal with connecting shuice gates around the lake, which can be used to mitigate flood disasters by properly operating the shuice gates. In this paper, the feasibility study of applying the new flood control scheme (IFCS) to the Dongting Lake was conducted while using the hydrodynamic module of MIKE 21 FM (flow model) with a long-time simulation (June to September 1998). The main objectives were to: (1) investigate the flooding process in Dongting Lake; (2) apply and design the IFCS into Dongting Lake; and, (3) examine its feasibility for the flood mitigation in Dongting Lake and its downstream region.