applied sciences

Article Mitigation Using an Innovative Flood Control Scheme in a Large Lake: Dongting Lake,

Yizhuang Liu 1,2 , Shu-Qing Yang 2, Changbo Jiang 1,3,4,*, Muttucumaru Sivakumar 2 , Keith Enever 5, Yuannan Long 1,3,4 , Bin Deng 1,3,4, Usman Khalil 2 and Lingshi Yin 1

1 School of Hydraulic Engineering, Changsha University of Science & Technology, Changsha 4100114, China; [email protected] (Y.L.); [email protected] (Y.L.); [email protected] (B.D.); [email protected] (L.Y.) 2 School of Civil, Mining and Environmental Engineering, University of Wollongong, NSW 2522, Australia; [email protected] (S.-Q.Y.); [email protected] (M.S.); [email protected] (U.K.) 3 Key Laboratory of Dongting Lake Aquatic Eco-Environmental Control and Restoration of Hunan Province, Changsha 410114, China 4 Key Laboratory of Water- Sciences and Water Disaster Prevention of Hunan Province, Changsha 410114, China 5 School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, NSW 2522, Australia; [email protected] * Correspondence: [email protected]; Tel.: +86-0731-85258437

 Received: 6 May 2019; Accepted: 13 June 2019; Published: 17 June 2019 

Featured Application: The Innovative flood control scheme could be applied to the flood-prone lake areas to mitigate the flood disaster.

Abstract: A large lake plays an important role in mitigating flood disasters in its nearby regions during the flooding period; however, the effect is limited, because most of its storage capacity becomes dead storage prior to the arrival of the flood wave. In the study, an innovative flood control scheme (IFCS) is applied to Dongting Lake (the second largest freshwater lake in China) to alleviate flood disasters. MIKE 21 FM was used to examine its feasibility to mitigate flood disasters. One of the largest floods in the 20th century, the 1998-type flood, was modelled and the maximum water levels with/without IFCS were compared. The result shows that the effective flood control storage could be at least doubled when compared with the natural condition once IFCS was applied. The peak flood level in the Dongting Lake could be lowered by at least 0.32 m at the Chenglingji station in the same flood passage of Dongting Lake. The case study reveals that, after applying IFCS, the hydraulic gates play a very important role in floodwater regulation and further study should be conducted to find the optimized operation for each gate in the flood control scheme system.

Keywords: flood mitigation; Dongting Lake; flood control capacity; peak water level

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 , urbanization, and water infrastructures. [1–3]. 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 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.

Appl. Sci. 2019, 9, 2465; doi:10.3390/app9122465 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x 2 of 16

Appl.its economic Sci. 2019, 9 development, 2465 is constrained by these flood disasters, according to the Flood chronology2 of 16 of China. Upstream on (e.g., Three Gorges (TGD, China) [5], Hoover Dam (America) [6], and WivenhoeUpstream Dam dams (Australia) on rivers (e.g., [7], etc.) Three can Gorges greatly Dam regulate (TGD, the China) inflow [5 from], Hoover their Damcatchments, (America) which [6], canand Wivenhoeeffectively Damreduce (Australia) flooding. [7 However,], etc.) can greatlyfloodwa regulateter from the the inflow vast fromdownstream their catchments, area cannot which be regulated,can effectively as there reduce is no flooding. dam to control However, the flow. floodwater Relatively, from the the downstream vast downstream river valleys, area cannotas well beas theregulated, delta, have as there a higher is no population dam to control density the and flow. properties Relatively, and theany downstream can cause river huge valleys, losses, as thus well excessiveas the delta, floodwater have a higher management population is most density important. and properties For example, and any there floods are two can dams cause with huge a losses, flood thus excessive floodwater management is most important. For example, there are two dams with a control capacity of 1.97 billion m3 in the Brisbane River catchment, but Brisbane city on the flood control capacity of 1.97 billion m3 in the Brisbane River catchment, but Brisbane city on the river was flooded twice in the 20th century, and the city was flooded again, leading to 35 deaths and $2.55 billiondelta was economic flooded loss twice in 2011 in the [7]. 20th The century, TGD is andone of the the city largest wasflooded dams in again, the world, leading with to a 35 flood deaths control and $2.55 billion economic loss in 2011 [7]. The TGD is one of the largest dams in the world, with a flood capacity of 22.1 billion m3, but it only controls 56% of the Yangtze River catchment area (see the 3 shadowedcontrol capacity area in of Figure 22.1 billion 1); floodwater m , but it onlyfrom controls the downstream 56% of the area Yangtze cannot River be controlled, catchment areaeven (see where the theshadowed mega-cities, area inlike Figure Wuhan,1); floodwater Nanjing, and from Shangh the downstreamai, are situated. area cannot When bethe controlled, floods come even from where the middlethe mega-cities, or lower likepart Wuhan,of the Yangtze Nanjing, River and basin, Shanghai, the TGD are situated.cannot effectively When the regulate floods comethe floodwater from the formiddle the purpose or lower of part disaster of the mitigati Yangtzeon, River as happened basin, the TGDin Brisbane cannot in eff 2011.ectively Therefore, regulate it the is worthwhile floodwater for to investigatethe purpose the of flood disaster control mitigation, strategy as in happened the downstre in Brisbaneam area in of 2011. the dam. Therefore, In the itcurrent is worthwhile study, we to takeinvestigate the Dongting the flood Lake, control which strategy is located in the downstream downstream of area the ofTGD, the dam.as an Inexample the current to investigate study, we how take athe lake Dongting can achieve Lake, a which more iseffective located role downstream in mitigating of the a TGD,flood asdisaster an example in the tomiddle investigate or low how part a lakeof a largecan achieve river. a more effective role in mitigating a flood disaster in the middle or low part of a large river.

Figure 1. YangtzeYangtze River basin.

Generally, a large lake (i.e., the mean surface area is greater than 500 km2 [8]) plays an important Generally, a large lake (i.e., the mean surface area is greater than 500 km2 [8]) plays an important buffer role for temporal floodwater detention during flood seasons [9,10]. However, a flood disaster 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, 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 which usually becomes very small relative to its total storage, because safe floodwater occupies most its total storage (small/medium scale flood) as the dead storage before the peak flood wave arrives [11]. of its total storage (small/medium scale flood) as the dead storage before the peak flood wave arrives For example, the severe flood-period of 1998 in China resulted in 3600 deaths and wreaked more than [11]. For example, the severe flood-period of 1998 in China resulted in 3600 deaths and wreaked more $30 billion in damages [1], even though its upstream dams and downstream lakes played some role than $30 billion in damages [1], even though its upstream dams and downstream lakes played some rolein mitigating in mitigating the loss.the loss. An An innovative innovative flood flood control control scheme scheme (IFCS) (IFCS) proposed proposed by by Yang Yang [11 [11,12],12] could could be beused used to mitigateto mitigate the the flood flood disasters disasters in ain large a large lake lake to improveto improve the the eff ectiveeffective flood flood control control capacity capacity of ofthe the lake lake to reduceto reduce the the flooding flooding loss. loss. The The IFCS IFCS consists consists of a by-passof a by-pass canal with connectingwith connecting sluice sluice gates gatesaround around the lake, the whichlake, which can be can used be toused mitigate to mitigate flood disastersflood disasters by properly by properly operating operating the sluice the sluice gates. In this paper, the feasibility study of applying the new flood control scheme (IFCS) to the Dongting gates. LakeIn was this conducted paper, the while feasibility using thestudy hydrodynamic of applying modulethe new offlood MIKE control 21 FM scheme (flow model)(IFCS) withto the a Dongtinglong-time simulationLake was conducted (June to September while using 1998). the The hydrodynamic main objectives module were to:of MIKE (1) investigate 21 FM (flow the flooding model) processwith a long-time in Dongting simulation Lake; (2)(June apply to September and design 1998). theIFCS The main into Dongtingobjectives Lake;were to: and, (1) (3)investigate examine the its floodingfeasibility process for the floodin Dongting mitigation Lake; in Dongting (2) apply Lake and and design its downstream the IFCS region.into Dongting This is theLake; first and, attempt (3) examineto investigate its feasibility the application for the offlood the IFCSmitigation in Dongting in Dongting Lake whileLake usingand its MIKE downstream 21 FM. This region. study This may is providethe first attempt useful information to investigate regarding the application flood mitigation of the IFCS in in Dongting Dongting Lake Lake and while other using similar MIKE lakes. 21 FM. Appl. Sci. 2019, 9, 2465 3 of 16

2. Materials and Methods In this study, a hydrodynamic module of MIKE 21 FM was employed to investigate the IFCS’s effects on the flooding process in Dongting Lake. The hydrodynamic module was used to model the flow in the waterways of Dongting Lake. The required new dike with several hydraulic gates in the lake was designed to regulate the river flows into Dongting Lake. Mike 21 FM is a two-dimensional (2D) depth-averaged hydrodynamic model while using the fine volume method to solve the shallow water equations. The governing equations are:

∂h ∂hu ∂hv + + = 0 (1) ∂t ∂x ∂y

p ! ∂u ∂u ∂u ∂η u u2 + v2 ∂2u ∂2u + u + v + g + g = vt + (2) ∂t ∂x ∂y ∂x C2h ∂x2 ∂y2 p ! ∂v ∂v ∂v ∂η v u2 + v2 ∂2v ∂2v + u + v + g + g = vt + (3) ∂t ∂x ∂y ∂y C2h ∂x2 ∂y2 in which h = η + d is the total water depth, d is static water depth, η is water level; t is the time; x and y are in Cartesian coordinates; u and v are the corresponding velocity components in x and y directions, R R = 1 η = 1 η 2 respectively, u h d udz, v h d vdz; g is the gravitational acceleration (m/s ); C is the Chezy − − coefficient; and, vt is turbulence viscosity coefficient. The hydrodynamic module of MIKE 21 FM has the capability of modeling the process of wetting and drying as water enters and leaves the area, which means that it is suitable for simulating water flow onto the dry land when the water level is relatively low in the lake. Many researchers have used the MIKE 21 FM model to investigate a large lake’s hydrodynamic process, like the Poyang Lake (China) [13], Dongting Lake (China) [14,15], and Vembanad Lake (India) [16], hence, only a brief description of the model is presented here.

2.1. Study Area Dongting Lake is located at the northern end of Hunan Province in China, which lies between 28◦200–30◦200 N, 111◦300–113◦150 E (see Figure2). It is the second-largest freshwater lake in China. The whole lake is divided into three main water bodies (West Dongting Lake, WDL; South Dongting Lake, SDL; and, East Dongting Lake, EDL). The total area of Dongting Lake covers approximately 2670 km2 in the flood season and it shrinks to 710 km2 in the annual dry season [17]. The lake receives water from the Yangtze River through three channels (Songzi, Hudu, and Ouchi) and four local rivers (Xiang, Zi, Yuan, and Li) and discharges to the Yangtze River via Chenglingji , where a flood control station is situated (Figure2). During the period from 1951 to 2010, the three channels contribute approximately 31.3% of Dongting Lake’s inflow, while the four rivers account for 58.2% of its inflow [17], and 100% of lake water flows to downstream via the Chenglingji channel. Appl. Sci. 2019, 9, 2465 4 of 16 Appl. Sci. 2019, 9, x 4 of 16

Figure 2. Location of Dongting Lake. The lake collects water from the Yangtze River through ‘three channels’ (Songzikou, Taipingkou, and Ouchikou) an andd ‘four rivers’ (Xiang, Zi, Yuan, and Li) and it discharges to the Yangtze RiverRiver atat Chenglingji.Chenglingji. 2.2. Calibration and Validation of the Model 2.2. Calibration and Validation of the Model A 2D depth-averaged hydrodynamic model based on MIKE 21 FM was applied to investigate the A 2D depth-averaged hydrodynamic model based on MIKE 21 FM was applied to investigate flooding process in the Dongting Lake area, which includes a part of the Yangtze River (i.e., Jingjiang the flooding process in the Dongting Lake area, which includes a part of the Yangtze River (i.e., section), Dongting Lake, and its main . The topography data of the Dongting area and the Jingjiang section), Dongting Lake, and its main tributaries. The topography data of the Dongting area Yangtze River was based on the surveyed data, with a resolution from 30–200 m updated in 2003 and the Yangtze River was based on the surveyed data, with a resolution from 30–200 m updated in and 2016, respectively. Unstructured triangular grids that can be adapted to the complex shape of the 2003 and 2016, respectively. Unstructured triangular grids that can be adapted to the complex shape Dongting Lake area and vary in different grid sizes in any region were used to construct the model. of the Dongting Lake area and vary in different grid sizes in any region were used to construct the In order to improve the computational efficiency and accuracy, a higher mesh resolution (30 m) was model. In order to improve the computational efficiency and accuracy, a higher mesh resolution (30 used in the channel and a coarse mesh (700 m) covered the Dongting Lake. Therefore, a total of m) was used in the channel and a coarse mesh (700 m) covered the Dongting Lake. Therefore, a total 118,542 nodes and 210,341 triangular elements were included in the grid system with a minimum and of 118,542 nodes and 210,341 triangular elements were included in the grid system with a minimum maximum element size of 30m and 700m, respectively. Daily catchment inflows from Yangtze River and maximum element size of 30m and 700m, respectively. Daily catchment inflows from Yangtze and four rivers were specified as the upstream boundary conditions in the model. Luoshan gauging River and four rivers were specified as the upstream boundary conditions in the model. Luoshan station on the Yangtze River was set as the downstream boundary of the model while using the rating gauging station on the Yangtze River was set as the downstream boundary of the model while using curve that was observed at Luoshan gauging station. the rating curve that was observed at Luoshan gauging station. The model has been calibrated to obtain the proper Manning’s roughness coefficient (M), which is The model has been calibrated to obtain the proper Manning’s roughness coefficient (M), which the most important parameter in hydrodynamic modeling of rivers and lakes, while using the daily is the most important parameter in hydrodynamic modeling of rivers and lakes, while using the daily measured data during the flood period in 1996 (i.e., from 11 July to 2 August). In the current model, measured data during the flood period in 1996 (i.e., from 11 July to 2 August). In the current model, the value of M was initially set at 32 m1/3/s and then modified during the calibration. The calibrated the value of M was initially set at 32 m1/3/s and then modified during the calibration. The calibrated M ranges from 32 m1/3/s to 60 m1/3/s in Yangtze River; 45 m1/3/s to 50 m1/3/s in the three channels, M ranges from 32 m1/3/s to 60 m1/3/s in Yangtze River; 45 m1/3/s to 50 m1/3/s in the three channels, and and 32 m1/3/s to 60 m1/3/s in Dongting Lake, as well as in the four rivers, which agree well with the 32 m1/3/s to 60 m1/3/s in Dongting Lake, as well as in the four rivers, which agree well with the previous previous studies [18]. studies [18]. A simulation was conducted to validate the current model while using the observation data from A simulation was conducted to validate the current model while using the observation data from the flood period in 1998 (i.e., from 6 June to 17 September). The minimum time step was set to 2 s and the flood period in 1998 (i.e., from 6 June to 17 September). The minimum time step was set to 2 s and the run time is about seven days using a computer with i7-4790 CPU (3.6 GHz, Intel Core, LENOVO, the run time is about seven days using a computer with i7-4790 CPU (3.6 GHz, Intel Core, LENOVO, Beijing, China). The Nash–Sutcliffe efficiency coefficients (NSE) [19] of water level and the difference Beijing, China). The Nash–Sutcliffe efficiency coefficients (NSE) [19] of water level and the difference of the maximum water level between the measured and simulated data are given in Table1 to estimate of the maximum water level between the measured and simulated data are given in Table 1 to the accuracy of the current numerical model. estimate the accuracy of the current numerical model. Table1 shows that all of the NSE values are over 0.87, the R 2 values are greater than 0.928, Table 1 shows that all of the NSE values are over 0.87, the R2 values are greater than 0.928, and and the RMSE values are less than 0.484, which indicate that the calculated results agree well with the RMSE values are less than 0.484, which indicate that the calculated results agree well with the observations. Generally, the NSE values are close to previous studies [20]. Table 1 also shows that the Appl. Sci. 2019, 9, 2465 5 of 16

the observations.Appl. Sci. 2019, 9, xGenerally, the NSE values are close to previous studies [20]. Table1 also5 of shows16 that the difference of the maximum water level between the measured and simulated data is less than 0.16difference m at of all the selected maximum stations water inlevel Dongting between Lake,the measured which indicatedand simulated that data the is current less than model 0.16 m could representat all theselectedflood stations well. Figurein Dongting3 shows Lake, the which time indicated series of that water the level current at themodel main could hydrologic represent stationsthe in Dongtingflood well. Lake Figure. The 3 shows results the show time thatseries the of water model level appears at the main to simulate hydrologic the stations noticeable in Dongting water level Lake. The results show that the model appears to simulate the noticeable water level fluctuation of fluctuation of the lake successfully, since the calculated results include the flood rising and falling the lake successfully, since the calculated results include the flood rising and falling process and the process and the flood peak value agrees well with the measured data. Although there are some flood peak value agrees well with the measured data. Although there are some differences between differencesthe simulated between data the and simulated measured data data, and the measured numerical data, simulations the numerical reasonably simulations agree with reasonably the agreeobservations with the observations overall, which overall, indicated which that indicated the current that model the currentcan dynamically model can simulate dynamically the flooding simulate the floodingprocess in process Dongting in DongtingLake. Lake.

(a) (b)

(c) (d)

(e) (f)

FigureFigure 3. Comparison 3. Comparison of of water water levellevel process during during the the flood flood season season (June (June to September) to September) in 1998. in(a) 1998. Nanzui; (b) Xiaohezui; (c) Xiangyin; (d) Yingtian; (e) Lujiao; and, (f) Chenglingji. (a) Nanzui; (b) Xiaohezui; (c) Xiangyin; (d) Yingtian; (e) Lujiao; and, (f) Chenglingji. Appl. Sci. 2019, 9, 2465 6 of 16 Appl. Sci. 2019, 9, x 6 of 16

TableTable 1.1. TheThe Nash–SutcliNash–Sutcliffeffe eefficiencyfficiency coecoefficientsfficients (NSE), R2 andand the the root root mean mean square square error error (RMSE) (RMSE) calculated at the maincalculated hydrological at the stationsmain hydrologic in Dongtingal stations Lake. in Dongting Lake.

MaximumMaximum Water Water Level Level (m) (m) Stations NSE Difference (m) R2 2 RMSE Stations NSE Measured Simulated Difference (m) R RMSE Measured Simulated Nanzui 0.890 35.36 35.52 0.16 0.928 0.478 Nanzui 0.890 35.36 35.52 0.16 0.928 0.478 XiaohezuiXiaohezui 0.873 0.873 35.14 35.14 34.59 34.59 −0.150.15 0.929 0.929 0.484 0.484 − XiangyinXiangyin 0.936 0.936 34.28 34.28 34.37 34.37 0.09 0.09 0.961 0.961 0.376 0.376 YingtianYingtian 0.949 0.949 34.26 34.26 34.36 34.36 0.10 0.10 0.967 0.967 0.363 0.363 LujiaoLujiao 0.986 0.986 34.14 34.14 34.26 34.26 0.12 0.12 0.992 0.992 0.239 0.239 Chenglingji 0.981 34.00 33.98 0.02 0.995 0.308 Chenglingji 0.981 34.00 33.98 −−0.02 0.995 0.308

2.3. The The Conception Conception of of IFCS IFCS and its Preliminary Design in Dongting Lake Besides the existing dike surrounding a a lake, lake, th thee IFCS IFCS needs needs one one more more dike dike (the (the inner inner dike) dike) parallel to the existing dike to form a by-pass canal that mainly consists consists of of the the inner inner dike dike in in the the lake; lake; sluice gates are installed in the inner dike (see Figure 4 4).). TheThe innerinner dikedike couldcould separateseparate thethe innerinner lakelake and main channel. Thus, Thus, the the excess excess floodwater floodwater could be diverted into the inner lake for storage, storage, while the safe floodwater floodwater will be discharged to downstre downstreamam without storage by closing the gates when the safe floodflood appears.appears. Therefore,Therefore, e ffeffectiveective flood flood control control capacity capacity could could be greatlybe grea increased,tly increased, since since the safethe safeflood flood would would not occupynot occupy the inner the inner lake’s lake’s capacity. capacity.

Figure 4. TheThe schematic schematic of of the the innovative innovative flood flood control scheme (IFCS).

The applicationapplication ofof the the IFCS IFCS in in Dongting Dongting Lake Lake needs needs to require to require designing designing the inner the inner dikes dikes and sluice and sluicegates ingates the lake,in the and lake, Figure and5 aFigure shows 5a the shows preliminary the preliminary design, where design, the innerwhere dike the is inner represented dike is representedby thick and by solid thick blue and lines solid and blue the lines hydraulic and the gates hydraulic are marked gates are by marked numbers by with numbers void redwith circles void redwith circles a cross with inside. a cross The inside. inner dikesThe inner are built dikes along are built the main along channel, the main which channel, can easilywhich be can recognized easily be recognizedaccording toaccording the water to bodythe water in the body dry in season the dry of se Dongtingason of Dongting Lake (see Lake Figure (see5 b).Figure The 5b). area The of area the ofinner the Lakeinner ofLake WDL, of WDL, SDL, andSDL, EDL and is EDL 190 kmis 1902, 560 km km2, 5602, and km2, 969 and km 9692, respectively.km2, respectively. The water The water level levelin the in inner the inner lake lake should should drop drop to itsto its ecological ecological water water level, level, which which is is corresponding corresponding toto basicbasic water requirement [21] [21] (i.e., 29.0 m in WDL, 27.5 m in SDL, and 26 m in EDL, which are calculated while using the lake morphology [22]) [22]) before the arrival of the the flood flood wave wave in in the the wet wet season season [11]. [11]. Therefore, Therefore, the Dongting LakeLake couldcould bebe nearly nearly empty empty and and thus thus the the lake lake has has su sufficientfficient eff ectiveeffective capacity capacity to storeto store the theexcess excess floodwater. floodwater. The The sluice sluice gates gates will will be opened be open toed let to the let excessthe excess floodwater floodwater enter enter into into the innerthe inner lake lakewhen when the peak the floodpeak waveflood arrives wave [arrives11], and [11], the safeand floodwaterthe safe floodwater could be discharged could be todischarged the downstream to the downstreamthrough the mainthrough channel the main by closing channel the by sluice closing gates. the sluice gates. Appl. Sci. 2019, 9, x 7 of 16

Appl. Sci. 20192019,, 9,, x 2465 77 of of 16

(a) (b)

Figure 5. The application of the IFCS in Dongting Lake. (a) The layout of the IFCS; (b) The water body

in the dry season of Dongting Lake (December 2006). (a) (b) 3. Results Figure 5. TheThe application application of of the the IFCS IFCS in in Dongting Dongting Lake. ( (aa)) The The layout layout of of the the IFCS; IFCS; ( (bb)) The The water water body body inFor the a dry major season flood, of Dongting such as Lakein 1998, (December the operatin 2006). g water level of the gate could be determined based on the safe water level, which is a designed water level for flood protection [23] at each 3. Results 3.hydrological Results station (see Table 2). We take gate #8, for example, to explain how the IFCS works to mitigateFor aflooda majormajor disasters. flood,flood, such such In the as as incurrent in 1998, 1998, thestudy, the operating operatin the gate waterg will water levelbe openedlevel of the of gatewhenthe couldgate the couldwater be determined belevel determined is greater based basedthanon the the on safe safe the water watersafe level, water level which (i.e.,level, is32.50 which a designed m atis Chenglingjia waterdesigned level station),water for flood level until protection for the flood water [23protection level] at each reaches hydrological[23] itsat peakeach hydrologicalvalue.station For (see the Table station 19982). type We(see takeflood, Table gate the2). #8,floodingWe for take example, processgate #8 to, canfor explain beexample, divided how to theinto explain IFCS three workshow main the toflood mitigateIFCS waves, works flood i.e., to mitigatefirstdisasters. flood flood wave In the disasters. (3–11 current July), In study, the second current the gateflood study, will wave be the (2 opened 2–30gate July),will when be and opened the the water third when level flood the is wavewater greater (16–25level than is August) thegreater safe waterthan(see Figure. the level safe (i.e.,6). water 32.50 level m at(i.e., Chenglingji 32.50 m at station), Chenglingji until thestation), water until level the reaches water its level peak reaches value. its For peak the 1998value. type For flood,the 1998 the type flooding flood, process the flooding can be dividedprocess intocan be three divided main into flood three waves, main i.e., flood first floodwaves, wave i.e., Table 2. The safe water level at each hydrological station (unit: m). first(3–11 flood July), wave second (3–11 flood July), wave second (22–30 flood July), wave and (2 the2–30 third July), flood and wave the third (16–25 flood August) wave (see(16–25 Figure August)6). (see Figure. 6). Safe Water Safe Water Safe Water Station Table 2. The safe waterStation level at each hydrological stationStation (unit: m). Level Level Level Station Safe WaterTable Level2. The safe Stationwater level at Safe each Water hydrological Level station Station (unit: m). Safe Water Level Nanzui 34.20 Xiangyin 33.59 Lujiao 33.03 NanzuiSafe 34.20 Water XiangyinSafe 33.59 Water LujiaoSafe 33.03 Water Station Station Station Xiaohezui 33.85Level33.85 Yintian Yintian 33.10Level 33.10 ChenglingjiChenglingji 32.50Level32.50

Nanzui 34.20 Xiangyin 33.59 Lujiao 33.03

Xiaohezui 33.85 Yintian 33.10 Chenglingji 32.50

Figure 6. TheThe flooding flooding process in Dongting La Lakeke at selected stations in 1998.

Figure 6. The flooding process in Dongting Lake at selected stations in 1998. Appl. Sci. 2019, 9, 2465 8 of 16

3.1. The Effective Flood Control Capacity The effective flood control capacity (EFC) can be calculated, as follows:

Ve = S ∆h = S (Hmax h ) (4) × × − i 3 2 where Ve (m ) is the effective flood control capacity, S (m ) is the surface area of each lake, Hmax (m) is the maximum water level in each flood wave, and hi (m) is the incipient rising water level. Table3 shows the EFC of each part of Dongting Lake (i.e., WDL, SDL, and EDL) for the 1998 type flood before and after the application of the IFCS. For the natural condition, the total EFC of Dongting Lake is about 1.44 billion m3, 3.35 billion m3, and 1.45 billion m3 in the first, second, and third flood wave, respectively. The largest EFC occurred in the second flood wave due to the biggest variation range of the water level (∆h). The IFCS has a dramatic effect on the ∆h in EDL. The ∆h significantly increases in EDL, which is about nine times, 4.7 times, and 2.8 times higher than that in the natural condition for the first, second, and third flood wave, respectively. Therefore, the EFC has significantly increased in EDL, i.e., 6.37 billion m3, 7.28 billion m3, and 2.16 billion m3 in the first, second, and third flood wave, respectively. For the WDL and SDL, the ∆h decreases a little in the first and the second flood wave and it is almost the same in the third flood wave. Hence, the EFC decreases a little or it is nearly the same as that in the natural condition. However, due to the EDL contributing the most EFC in Dongting Lake (see Table3), the total EFC of Dongting Lake significantly increases, which is about 4.7 times, 2.8 times, and two times higher than that under the natural condition during the first, second, and third flood wave, respectively, after the application of the IFCS.

Table 3. Comparison of effective flood control capacity before and after using the IFCS. WDL: West Dongting Lake; SDL: South Dongting Lake; EDL: East Dongting Lake.

First Flood Wave Second Flood Wave Third Flood Wave Natural Condition 9 3 9 3 9 3 ∆h (m) Ve (10 m ) ∆h (m) Ve (10 m ) ∆h (m) Ve (10 m ) WDL 1.25 0.24 2.76 0.52 1.13 0.22 SDL 0.87 0.49 1.77 0.99 0.84 0.47 EDL 0.73 0.71 1.58 1.54 0.78 0.76 Total - 1.44 - 3.05 - 1.45 9 3 9 3 9 3 IFCS ∆h (m) Ve (10 m ) ∆h (m) Ve (10 m ) ∆h (m) Ve (10 m ) WDL 0.78 0.15 2.71 0.52 1.13 0.22 SDL 0.54 0.30 1.48 0.83 0.84 0.47 EDL 6.57 6.37 7.51 7.28 2.4 2.16 Total - 6.82 - 8.65 - 2.85

3.2. The Water Level The flood process responses to the application of the IFCS in Dongting Lake under one of the largest floods in the 20th century, i.e., 1998 type flood (24 June to 22 September), was calculated based on the numerical model. Figure7 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. Figure7a 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 Figure7b) and Chenglingji station (see Figure7c) is similar to the Nanzui station. Appl. Sci. 2019, 9, x 9 of 16 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 inAppl. the Sci. second2019, 9, 2465and third flood wave period, respectively. The variation of water level at Yingtian9 of 16 station (see Figure 7b) and Chenglingji station (see Figure 7c) is similar to the Nanzui station. For YingtianFor Yingtian station, station, there there is a isshort a short period period when when the the water water level level is isstill still higher higher than than that that without without IFCS, IFCS, eveneven when when thethe gate gate is is opened opened (i.e., (i.e., the the water water level level is is higher higher than than control control level) level) during during the the first first flood flood wavewave period (see(see FigureFigure7 b).7b). This This can can be be explained explaine byd by the the fact fact that that the gatethe gate opening opening period period is too is short too short(i.e., about(i.e., about 1 day) 1 and day) the and water the levelwater is level only is slightly only slightly higher thanhigher the than control the level,control resulting level, resulting in a limited in aexcess limited floodwater excess floodwater being be mitigated being be intomitigated the inner into lake. the Therefore,inner lake. the Therefore, reduction the of reduction the water levelof the is waterslight. level However, is slight. the However, water level the reduction water level is significant reduction inis thesignificant second in and the third second flood and wave, third i.e., flood the wave,average i.e., reduction the average of the reduction water levels of the are water about 0.10levels m are and about 0.20 m, 0.10 respectively. m and 0.20 For m, Chenglingji respectively. station, For Chenglingjithe average station,reduction the of average the water reduction levels are of aboutthe water 0.25 mlevels and are 0.18 about m in the0.25 second m and and 0.18 third m in flood the secondwave period, and third respectively. flood wave period, respectively.

(a) (b)

(C)

FigureFigure 7. 7. TheThe water water level level process process during during the the flood flood season season (24 (24 Jun. to to 22 22 Sep. Sep. 1998) 1998) in in Dongting Dongting Lake Lake withwith and and without without IFCS. IFCS. ( aa)) Nanzui Nanzui station; station; ( (b)) Yingtian Yingtian station; and, (c) Chenglingji station.

TheThe highest highest water water level level is is the the most most important important factor factor in in estimating estimating the the potential potential flood flood loss. loss. Hence, Hence, thethe second second and and third third flood flood wave wave were were chosen chosen to to an analysealyse the the change change of of the the highest highest water water level level after after applyingapplying the the IFCS IFCS due due to to the the highest highest water water level level in in the the WDL WDL occurring occurring in in the the second second flood flood wave wave and and thethe highest highest water water level level in in the the SDL SDL and and EDL EDL occurred occurred in in the the third third flood flood wave wave (see (see Figure Figure 66).). Table 4 4 showsshows the the difference difference inin thethe peakpeak water water level level in in the the second second and and third third flood flood wave wave before before and and after after the the use useof IFCS of IFCS at all at the all selectedthe selected hydrological hydrological stations. stations. During During the secondthe second flood fl wave,ood wave, the peak the peak water 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 Xiaohezui 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 Appl. Sci. 2019, 9, x 10 of 16 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 Xiaohezui 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 of the selected hydrological stations. It indicates that the IFCS could mitigate about 0.83 Appl. Sci. 2019, 9, 2465 10 of 16 billion m3 excess floodwater at the flood peak point. of theTable selected 4. The hydrological difference between stations. the It indicatespeak water that level the before IFCS couldand after mitigate using aboutIFCS at0.83 selected billion m3 excesshydrology floodwater stations at the (unit: flood m). peak point.

Table 4. The difference betweenSecond the peakPeak water level before and after using IFCS Third at selected Peak hydrology Station stations (unit: m). Natural Condition IFCS Difference Natural Condition IFCS Difference Second Peak Third Peak NanzuiStation 35.65 35.20 −0.45 35.33 34.99 −0.34 Natural Condition IFCS Difference Natural Condition IFCS Difference XiaohezuiNanzui 34.75 35.65 34.67 35.20 −0.080.45 35.3334.85 34.9934.54 −0.340.35 − − Xiaohezui 34.75 34.67 0.08 34.85 34.54 0.35 Yingtian 34.14 34.13 −−0.01 34.51 34.19 −−0.32 Yingtian 34.14 34.13 0.01 34.51 34.19 0.32 Lujiao 33.93 33.54 −−0.39 34.29 33.97 −−0.32 Lujiao 33.93 33.54 0.39 34.29 33.97 0.32 − − ChenglingjiChenglingji 33.54 33.54 33.12 33.12 −0.420.42 34.034.0 33.6733.67 −0.330.33 − −

3.3. The Operation of the IFCS The results show that the IFCS could effectively effectively lower the water level during the flood flood season, especially in the EDL (East Dongting Lake)Lake) areaarea (see(see Tabletable 44).). However,However, thethe operationoperation ofof thethe IFCSIFCS hashas great eeffectffect on the waterwater levellevel reductionreduction and the waterwater level could be even higher than the natural condition if the the wrong wrong operation operation is is chosen. chosen. Therefore, Therefore, we we built built another another model model to toexplore explore the the effect effect of ofthe the gate gate operation operation on onthe thewater water level level reduction. reduction. The The1996 1996 type typeflood flood was waschosen, chosen, as there as therewas only was onlyone big one flood big flood peak peakin this in year this year(Figure (Figure 8) and8) andIFCS IFCS was wasonly only adopted adopted in the in East the East Dongting Dongting Lake Lake (see (Figuresee Figure 9) to9 avoid) to avoid the theoperation operation in each in each lake lake affecting affecting the the other. other. Table Table 55 showsshows the operating waterwater level of the gate, i.e., when the water level reaches this value, then the gate will be opened to mitigate the floodwaterfloodwater intointo thethe innerinner lake.lake.

Figure 8. The measured water level at Chenglingji station in 1996.

Table 5. The operating water level of the gate in East Dongting Lake (unit: m).

Gate Case 1 Case 2 Case 3 #a 33.10 * 32.90 32.70 #b 32.50 * 32.20 32.00 *: the safe water level near the gate. Appl. Sci. 2019, 9, x 11 of 16

Appl. Sci. 2019, 9, 2465 11 of 16 Appl. Sci. 2019, 9, x 11 of 16

Figure 9. The layout of the IFCS in East Dongting Lake.

Table 5. The operating water level of the gate in East Dongting Lake (unit: m).

Gate Case1 Case 2 Case 3 #a 33.10* 32.90 32.70 #b 32.50* 32.20 32.00 FigureFigure 9. 9.The The layoutlayout ofof thethe IFCS in East East Dongting Dongting Lake. Lake. *: the safe water level near the gate Figure 10 presentsTable 5. the The simulated operating water fluctuation level of the of thegate water in East level Dongting at Chenglingji Lake (unit: m). station to show the effect ofFigure the operation 10 presents on the the simulated water level fluctuation reduction of during the water the level flood at period. Chenglingji Before station the gate to show is opened, the effect of the operation on theGate water level Case1 reduction during Case 2the flood Case period. 3 Before the gate is opened, there is a period (11–18 July) when the calculated water level is higher than that in the natural condition there is a period (11–18 July)#a when the33.10* calculated 32.90water level is 32.70 higher than that in the natural as the floodwater is restricted in the main channel after building the dike in the EDL. The water condition as the floodwater is#b restricted 32.50* in the main channel32.20 after building 32.00 the dike in the EDL. The level begins to decline after the gate opens due to the excess floodwater flowing into the inner lake. water level begins to decline after*: the the gate safe openswater duelevel to near the theexcess gate floodwater flowing into the inner For case 1, the operation water level was set as equal to the safe water level, with a corresponding peak lake. For case 1, the operation water level was set as equal to the safe water level, with a water level reduction of 0.22 m. When the operation water level of both gates decreases by 0.02 m correspondingFigure 10 presentspeak water the simulatedlevel reduction fluctuation of 0.22 of m. the When water the level operation at Chenglingji water levelstation of to both show gates the (Casedecreaseseffect 2), of the the by reduction operation 0.02 m (Case of on the the 2), water water the reduction levellevel reduction is more of the e waffi duringcient,ter level the and floodisthe more peakperiod. efficient, value Before and is decreasedthe the gate peak is opened,value by 0.28 is m. However,decreasedthere is instead,a byperiod 0.28 the m.(11–18 reduction However, July) of wheninstead, peak the water the calculated reduction level decreases water of peak level to water 0.19 is higher mlevel when decreases than we that continue to in 0.19 the tom natural decreasewhen thewecondition operation continue as water tothe decrease floodwater level bythe 0.02 operationis restricted m (Case water in 3). the level main by channel0.02 m (Case after 3).building the dike in the EDL. The water level begins to decline after the gate opens due to the excess floodwater flowing into the inner lake. For case 1, the operation water level was set as equal to the safe water level, with a corresponding peak water level reduction of 0.22 m. When the operation water level of both gates decreases by 0.02 m (Case 2), the reduction of the water level is more efficient, and the peak value is decreased by 0.28 m. However, instead, the reduction of peak water level decreases to 0.19 m when we continue to decrease the operation water level by 0.02 m (Case 3).

FigureFigure 10. 10.Simulation Simulation ofof fluctuationfluctuation of water level level at at Chenglingji Chenglingji station. station.

4. Discussion

4.1. 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 ofFigure the IFCS. 10. Simulation We compared of fluctuation the eff ofective water floodlevel at control Chenglingji capacity station. and the difference of Appl. Sci. 2019, 9, x 12 of 16

4. Discussion

4.1. 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 maximum water levels of the same with/without IFCS, and the 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 m3 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 m3, which is about nine times larger than that in the natural condition, asAppl. the Sci. EDL2019 is, 9 ,nearly 2465 empty before the first flood wave arrives (see Figure 11a). During the second12 of 16 flood wave, the natural EFC of EDL increases to 1.54 billion m3, while it becomes 7.28 billion m3 with themaximum IFCS. It wateris about levels 4.7 times of the larger same hydrographthan that in the with natural/without condition, IFCS, and because the hydrographs the water level in 1998 in EDL were ischosen still very as an low example. before the second flood wave arrives (see Figure 11b). As for the third flood wave, 3 the floodIn the control natural capacity conditions, of Dongting the eff ectiveLake could flood still control be increased capacity to (EFC) 2.16 isbillion limited, m , becausewhich is the nearly safe 2.8floodwater times larger occupies than mostthat under of the the capacity natural of theconditio lake priorn due to to the the arrival fact that of the the flood water wave. level Takein the the inner East lakeDongting is still lakelower as than an example, that in the the main EFC channel, is only about which 0.71 indicated billion mthat3 during there was the firststill floodextra wavefloodwater under storagethe natural capacity condition in the (see EDL Table (see3 Figure). However, 11c). the EFC could be greatly increased after the application of theThe IFCS, Chinese as the Central safe floodwater Government is not proposed allowed to a enter polic they known inner lake. as ‘‘return Therefore, land the to EFClake’’ increases after the to 19986.37 billionflood to m control3, which the is aboutwater ninelevel times in Dongting larger than Lake that [5]. in The the lake natural embankment condition, would as the EDLbe removed is nearly andempty the beforeland surrounding the first flood the wave Dongting arrives Lake (see Figure would 11 bea). converted During the into second a lake flood area wave, to increasethe natural the capacityEFC of EDLof the increases lake. The to 1.54“return billion land m to3, whilelake” itpoli becomescy has a 7.28 positive billion effect m3 with on the the flood IFCS. control It is about in Dongting4.7 times Lakelarger area, than which that in was the investigated natural condition, by Jiahu because (2017) [18] the while water using level ina numerical EDL is still simulation very low method.before the The second water floodsurface wave area arrives has been (see gradually Figure 11 increasedb). As for in the recent third years flood [23], wave, but the it is flood proceeding control verycapacity slowly, of Dongting because it Lake has coulddramatic still effect be increased on the topeople 2.16 billionwho live m3 ,in which that area, is nearly since 2.8 they times have larger to movethan thatout underof that the place. natural However, condition the due IFCS to theincreases fact that the the flood water control level in capacity the inner by lake exploring is still lower and developingthan that in the the potential main channel, flood which control indicated capacity that of the there lake was itself, still extraand thus floodwater it has little storage effect capacity on the in peoplethe EDL who (see live Figure in the 11 lake’sc). surrounding area.

(a) (b) (c)

FigureFigure 11. 11. WaterWater level level distribution distribution in in Dongting Dongting Lake Lake before before the the flood flood wave wave arrives arrives in in 1998. 1998. (a (a) )3 3 July; July; ((bb)) 22 22 July; July; and, and, ( (cc)) 16 16 August. August.

TheThe operation Chinese Central of the Governmenthydraulic gates proposed in the aIFCS policy has known a significant as “return effect land on to flood lake” mitigation. after the 1998 If theflood operating to control water the level; water (i.e., level the in gate Dongting will be Lake open [5ed]. when The lake the embankmentwater level is wouldover this be level removed near andthe gate)the land is too surrounding high, as a consequence, the Dongting the Lake gate would would be open converted relatively into alate, lake which area toindicated increase that the capacitythere is 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 Jiahu (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 Appl. Sci. 2019, 9, x 13 of 16 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 comparingAppl. Sci. 2019 Case, 9, 2465 1 with Case 2. If the operating water level is too low, the floodwater would enter13 the of 16 inner lake too early. This part of the floodwater would occupy some part of the lake storage capacity, resultingthere not in being there enough not being effective enough capacity effective to store capaci thety excess to store floodwater the excess when floodwater the peak when flood dischargethe peak floodarrives. Hence, thearrives. reduction Hence, of thethe peak reduction water levelof the would peak decrease water level when would comparing decrease Case 3when with comparingCase 2. Thus, Case the 3 operatingwith Case water 2. Thus, level the of operating the gate plays water a level very importantof the gate roleplays in a floodwater very important mitigation, role inwhich floodwater still needs mitigation, furtherstudy which of still the needs operating further water study level of for the a operating particular gatewater to level obtain for the a particular optimized gatewater to levelobtain reduction, the optimized especially water in level a complex reduction, system especially that includes in a complex several gates system to bethat required includes in severalthe IFCS. gates Additionally, to be required some in otherthe IFCS. factors Additional of the gately, some (e.g., theother width, factors height, of the and gate gate (e.g., opening, the width, etc.) height,that aff andect thegate operation opening, of etc.) the that gate affect were notthe incorporatedoperation of the in the gate current were model,not incorporated since the currentin the currentstudy focuses model, on since applying the current and estimating study focuses the feasibility on applying for flood and mitigationestimating of the the feasibility IFCS in a largefor flood lake . mitigationTherefore, of future the IFCS study in mighta large be lake. necessary Therefore, to consider future study the abovementioned might be necessary factors to consider to obtain the the abovementionedoptimized water factors level reduction. to obtain the optimized water level reduction.

4.2.4.2. Design Design of of the the Dike Dike and and its its Cost Cost HowHow to to design design and and build build the the huge huge dike dike in in the the larg largee lake lake is is the the key key point point for for the the IFCS. IFCS. The The huge huge dikedike could could combine combine a a solid solid base base dam dam and and a aflexible flexible soft soft dam dam (e.g., (e.g., rubber rubber dam, dam, see see Figure Figure 12) 12 )on on it. it. TakingTaking the #1 dikedike inin thethe West West Dongting Dongting Lake Lake (see (see Figure Figure5a) 5a) with with a 50-year a 50-year flood flood standard standard for the for IFCS, the IFCS,for which for which the water the water level is level 35.36 is m 35.36 (see m Figure (see 13Figure) at Nanzui 13) at Nanzui station asstation an example, as an example, the top elevation the top elevationof the inner of the dike inner should dike be should 35.36 m be with 35.36 level m with of 33.0 level m ofof the 33.0 solid m of base the damsolid and base a dam height and of 2.36a height m of ofthe 2.36 flexible m of softthe flexible dam. For soft a normaldam. For flood, a normal if the flood, water levelif the reacheswater level the controlreaches level the control (i.e., 34.2 level m in(i.e., the 34.2current m in study) the current at Nanzui study) station, at Nanzui the #1 gate station, would the be #1 opened gate towould store thebe opened excess floodwater. to store the However, excess floodwater.for an extreme However, huge flood for an (e.g., extreme 50-year huge flood), flood the (e.g., flexible 50-year soft flood), dam should the flexible be inflated soft dam by air should or water, be inflatedand the by top air height or water, could and be the lowered top height to nearly could 33.0 be m lowered if the soft to nearly dam is 33.0 deflated m if the and soft the dam main is channel deflated is andhigher the thanmain 35.36channel m andis higher it is still than increasing 35.36 m and at Nanzuiit is still station.increasing Therefore, at Nanzui the station. water levelTherefore, is at least the water2.3 m levelhigher is at than least the 2.3 dike. m higher We can than assume the dike. that We the can lake assume with IFCSthat the isback lake towith the IFCS natural is back condition to the naturaland the condition dike has and little the eff diectke on has the little flood effect propagation. on the flood However, propagation. more However, experiments more and experiments numerical andsimulations numerical should simulations be conducted should to estimatebe conducted the eff ectto estimate of the dike the on effect the flood of the propagation dike on inthe the flood lake propagationin the future. in the lake in the future.

(a) (b)

FigureFigure 12. 12. TheThe schematic schematic of of the the combined combined dike dike for for IFCS. IFCS. (a (a) )The The water water block block by by the the combined combined dike; dike; ((bb)) sections sections of of the the dike. dike.

Based on our assumption of the dike in Dongting Lake, the whole length of the dike is about 160 km and the average unit cost of the dike is about US$1.635 million per kilometer [11]. Hence, the total costs are about 1.633 160 = US$ 261.28 million. If we take three times as the estimated value × Appl. Sci. 2019, 9, 2465 14 of 16 for a conservative estimation, the total cost is about US$ 783.84 million, which only accounts for 32.4% of the direct economic losses (i.e., US$ 2.42 billion) that are caused by the 1998 flood disaster in the

DongtingAppl. Sci. 2019 Lake, 9, x area. 14 of 16

Figure 13.13.The The probability probability of theof maximumthe maximum water water level at level Nanzui at stationbyNanzui stationby fiting Peason fiting III distributionPeason III (1950–2014).distribution (1950–2014).

4.3. TheBased Feasibility on our Analysisassumption of the of IFCS the di forke Other in Dongting Large Lakes Lake, the whole length of the dike is about 160 km andThe the feasibility average of unit a similar cost of scheme the dike was is about analyzed US$1.635 in Taihu million (i.e., theper thirdkilometer largest [11]. fresh Hence, water the lake total in China)costs are by about Yang 1.633 (2010). × 160 It was = US$ found 261.28 that million. there was If we only take 14.6% three of times the total as the storage estimated for flood value control for a underconservative the natural estimation, condition the for total the cost 1997 is type about flood, US$ while 783.84 the million, effective which flood only control accounts capacity for would 32.4% beof increasedthe direct toeconomic 90% of thelosses total (i.e., storage US$ with2.42 thebillion) new th scheme.at are caused The IFCS by canthe also1998 beflood applied disaster into in other the flood-proneDongting Lake lake area. areas, such as Tonle Sap Lake (Cambodian) and Poyang Lake (China). The water depth in Tonle Sap fluctuates from 1.5 to 10 m with the corresponding surface area from 2600 to 15,0004.3. The km Feasibility2 [24]. We Analysis assumed of the the IFCS minimum for Other lake Large area Lakes as the inner lake (i.e., 2600 km2). After the applicationThe feasibility of IFCS intoof a Tonlesimilar Sap scheme Lake, thewas water analyzed level in di Taihufference (i.e., is 8.5 the m third (from largest 1.5 m fresh to 10.0 water m) in lake the 3 2 innerin China) lake by resulting Yang (2010). in the totalIt was flood found control that capacitythere was being only increased 14.6% of the by 22.1total km storage(= 8.5 for m flood2600 control km ), 3 × whichunder the is 27.6% natural of condition the total storage for the (801997 km type, Yu flood, (2019) while [25 ]).the For effective the Poyang flood control Lake, the capacity surface would area 2 rangesbe increased from 714to 90% to 3164 of the km totalduring storage 2001–2010 with the [26 ne].w We scheme. assumed The that IFCS the can surface also be area applied of the innerinto other lake 2 isflood-prone about 64% lake of the areas, maximum such as lake Tonle area Sap (i.e., Lake about (Cambodian) 2025 km ), and according Poyang to Lake the preliminary(China). The design water ofdepth the IFCSin Tonle in Dongting Sap fluctuates Lake. Thefrom safe 1.5 waterto 10 m level with is 19.50the corresponding m and the highest area level from recorded 2600 to in15,000 history km is2 [24]. 22.50 We m (2assumed Aug. 1998) the minimum in Poyang lake Lake area [27 ].as Hence, the inner the lake water (i.e., level 2600 diff erencekm2). After is 3.0 the m, with the corresponding flood control capacity of 9.49 km3 (= 3.0 m 3164 km2). The ecological water application of IFCS into Tonle Sap Lake, the water level difference× is 8.5 m (from 1.5 m to 10.0 m) in levelthe inner is 12.07 lake m resulting in Poyang in Lake the total [28], flood and thus control the dicapacityfference being between increased the maximum by 22.1 km water3 (= level8.5 m and × 2600 the ecologicalkm2), which water is 27.6% level isof 10.52the total m in storage the lake. (80 Therefore, km3, Yu the(2019) flood [25]). control For capacitythe Poyang could Lake, beincreased the surface to 3 2 21.30 km (= 10.52 m 2025 km )2 with the IFCS, which is 2.24 times larger than that under natural area ranges from 714 ×to 3164 km during 2001–2010 [26]. We assumed that the surface area of the condition.inner lake is Based abouton 64% the of abovethe maxi analysis,mum lake the area IFCS (i.e., has about great 2025 potential km2), toaccording increase to the the flood preliminary control capacitydesign of of the the IFCS lake, in and Dongting thereby toLake. mitigate The safe the floodwater disaster level is in 19.50 the flood-prone m and the lakehighest area. water level recorded in history is 22.50 m (2 Aug. 1998) in Poyang Lake [27]. Hence, the water level difference is 4.4. Limitation of the IFCS and the Current Model 3.0 m, with the corresponding flood control capacity of 9.49 km3 (= 3.0 m × 3164 km2). The ecological waterThe level flow is 12.07 field in m the in Poyang lake and Lake the water [28], and exchange thus the between difference the lake between and river the wouldmaximum be dramatically water level changedand the ecological after building water several level is dikes 10.52 in m the in lakethe lake. although Therefore, the IFCS the couldflood significantlycontrol capacity mitigate could the be excessincreased floodwater to 21.30 km during3 (= 10.52 the floodm × 2025 season. km2) with Thus, the the IFCS, effect which of the is IFCS2.24 times on the larger sediment than that transport under andnatural water condition. environment Based should on the be above further analysis, studied. th Somee IFCS factors has great (e.g., potential wind forcing, to increase drag coe theffi cient,flood control capacity of the lake, and thereby to mitigate the flood disaster in the flood-prone lake area.

4.4. Limitation of the IFCS and the Current Model The flow field in the lake and the water exchange between the lake and river would be dramatically changed after building several dikes in the lake although the IFCS could significantly Appl. Sci. 2019, 9, 2465 15 of 16 and precipitation-evaporation, etc.) that affect the hydrodynamic process were not incorporated in the current model, due to the lack of these data. Hence, future study may be necessary to consider the abovementioned factors for a more accurate simulation of the hydrodynamic process.

5. Conclusions To further mitigate flood disasters, there is a need to regulate the downstream flood, as dams can only control the flood from upstream of the dam. In this study, we examined the feasibility of an innovative flood control scheme (IFCS) to mitigate the excess floodwater in Dongting Lake, which is located downstream of the Three Gorges Dam. The highest water level in a major flood is specially modelled and compared while using a 2D depth-averaged hydrodynamic model, i.e., MIKE 21 FM, which was applied to model the Dongting Lake. The following conclusions can be drawn from the current study:

1. The numerical simulations agree reasonably with the observations overall with the NSE over 0.87, the maximum water level difference between the measured and simulated data is within 0.16 m at all selected stations, which indicates that the current MIKE 21FM model can dynamically simulate the flooding process in Dongting Lake. 2. The effective flood control capacity could be greatly increased by using the IFCS due to the fact that the safe floodwater (small/middle scale flood) is not allowed to enter the inner lake. Therefore, the effective flood control capacity increased by 4.7 times, 2.8 times, and two times during the first, second, and third flood wave, respectively, after the application of the IFCS in Dongting Lake for the 1998 type flood, which is one of the largest floods in the 20th century. 3. The operation of the IFCS plays a very important role in floodwater mitigation, which still needs further study to achieve the optimized water level reduction, especially in a complex system, which includes several gates in the flood control scheme (IFCS). 4. The IFCS also has great potential to increase the flood control capacity of the lake and thereby to mitigate the flood disaster in the flood-prone lake area according to the feasibility analysis for another large lake.

Author Contributions: Conceptualization, S.-Q.Y.; Formal analysis, Y.L. (Yizhuang Liu) and U.K.; Supervision, C.J., S.-Q.Y., and M.S.; Validation, Y.L. (Yuannan Long), B.D., and L.Y.; Writing—Original draft, Y.L. (Yizhuang Liu); Writing—Review & editing, S.-Q.Y., C.J., M.S., and K.E. Funding: This research was funded by the China Scholarship Council, the National Natural Science Foundation of China (Grant No. 51839002) and the Natural Science Foundation of Hunan Province, China (Grant No. 2018JJ3535). Acknowledgments: We would like to thank Dr. Ma Yuan and Yan Shixiong for providing the Figure2. Conflicts of Interest: The authors declare no conflict of interest.

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