Research Into the Eutrophication of an Artificial Playground Lake Near The

sustainability

Article Research into the Eutrophication of an Artiﬁcial Playground Lake near the Yangtze River

Min Pang 1, Weiwei Song 2,3,* ID , Peng Zhang 2 ID , Yongxu Shao 2, Lanyimin Li 2, Yong Pang 2,4,* ID , Jianjian Wang 5,* and Qing Xu 6

1 College of Engineering, Cornell University, Ithaca, NY 14850, USA; [email protected] 2 College of Environment, Hohai University, Nanjing 210098, China; [email protected] (P.Z.); [email protected] (Y.S.); [email protected] (L.L.) 3 College of Hydrology and Water Resources, Hohai University, Nanjing 210098, China 4 Key Laboratory of Integrated Regulation and Resources Development on Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, China 5 School of Hydrology and Water Resources, Nanjing University of Information Science and Technology, 219 Ninliu Rd., Nanjing 210044, China 6 School of Hydraulic Energy and Power Engineering, Yangzhou University, Yangzhou 225009, China; [email protected] * Correspondence: [email protected] (W.S.); [email protected] (Y.P.); [email protected] (J.W.)

Received: 5 January 2018; Accepted: 16 March 2018; Published: 19 March 2018

Abstract: Water pollution in urban rivers is serious in China. Eutrophication and other issues are prominent. Taking the artificial Playground Lake in Zhenjiang as an example, a numerical model combining particle tracing, hydrodynamics, water quality and eutrophication was constructed to simulate the water quality improvement in Playground Lake with or without water diversion by pump and sluice. Simulation results using particle tracking showed that the water residence time depended on wind direction: east wind, 125 h; southeast wind, 115 h; south wind, 95 h. With no water diversion, the lower the flow velocity of Playground Lake under three wind fields, the more serious the eutrophication. Under pump diversion, the water body in Playground Lake can be entirely replaced by water diversion for 30 h. When the temperature is lower than 15 ◦C, from 15 ◦C to 25 ◦C and higher than 25 ◦C, the water quality can be maintained for 15 d, 10 d and 7 d, respectively. During high tide periods of spring tides in the Yangtze River from June to August, the water can be diverted into the lake through sluices. The greater the ∆h (the water head between the Yangtze River and Playground Lake), the more the water quality will improve. Overall, the good-to-bad order of water quality improvements for Playground Lake is as follows: pumping 30 h > sluice diversion > no water diversion. This article is relevant for the environmental management of the artificial Playground Lake, and similar lakes elsewhere.

Keywords: hydrodynamic; numerical simulation; eutrophication; water diversion

1. Introduction Eutrophication is one of the global water environment problems. The main sign of eutrophication is the abnormal proliferation of algae in water, while dynamic changes of algae in water are affected by their internal physiological characteristics and external factors. The growth of algae is affected not only by external factors such as sunlight, nutrients, transparency, water temperature and pH value, but also by hydrodynamic conditions in the water body, such as ﬂow velocity [1,2], ﬂow rate [3] and water disturbance [4,5]. On the other hand, the eutrophication of lakes is closely related to human activities in the basin. Industrial, agricultural and urban domestic sewage is continuously diverted

Sustainability 2018, 10, 867; doi:10.3390/su10030867 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 867 2 of 19 into lakes. Humans damage the natural ecological environment through lake reclamation, lakeshore construction and aquaculture, thus increasing the import of nutrients [6–9]. Although the Chinese government has devoted a great deal of manpower and material resources, some control measures have not yet achieved the desired results in some areas with frequent eutrophication of lakes. Overall regional control should be implemented to control the eutrophication of lakes [10]. In the face of agricultural non-point-source pollution, it is relevant to carry out pollution source management, nutrient transfer management and soil erosion control [11]. Some scholars have proposed the use of sediment dredging and pollution source interception methods to improve water quality [12,13]. Eutrophication control of the water body is beneficial not only to the environment but also to the survival and development of human beings [14–16]. A common phenomenon associated with the eutrophication of lakes is the abundance of phytoplankton [17]. Algae usually overgrow, form algal bloom and result in the deterioration of water quality and a series of serious water environment problems. In the formation of algal bloom, the Chl-a concentration is generally above 10 mg/m3. Due to the widespread presence of algal bloom in freshwater ecosystems, microcystis bloom has received the highest attention as the most studied algal bloom [18,19]. At present, most researchers agree that the formation of cyanobacteria bloom is generally caused by the physiological characteristics of cyanobacteria and environmental factors such as temperature, sunlight, nutrients, and other organisms [20]. With the eutrophication of lakes, especially the increase of phosphorus concentration, the species composition of phytoplankton usually leads to the formation of algal bloom. The ratio of total nitrogen (TN) to total phosphorus (TP) in water also significantly affects the phytoplankton species composition. Generally, cyanobacterial blooms are dominant when TN:TP < 29 [21]. In the early stage of eutrophication, phosphorus is the limiting factor for algae growth, and its increased concentration can lead to the massive growth of algae. With the rapid growth of the urban population and the rapid development of industry in China, the eutrophication of urban water bodies has become a serious ecological problem in the urban environment [22–24]. Eutrophication increases the amount of organic matter in the water, multiplies pathogens, and produces harmful algal toxins, which threatens the safety of drinking water. Large numbers of algae spread over the water and block out the sunlight to the bottom of the water so that the photosynthesis of the bottom of the water is prevented, which reduces oxygen release. When algae multiply rapidly and nutrition depletes, large-scale death occurs in underwater plants. Plants are decomposed by microorganisms, which consumes a lot of oxygen. Therefore, the concentration of dissolved oxygen in the water decreases, which can cause the death of aquatic animal, especially fish [25–28]. Some algae emit noxious odors and this can prevent a water body from being used by the public. Ecological models are important tools for lake eutrophication research and lake ecosystem management [29–32]. The development of lake eutrophication models has developed from simple models of single layer, single component, and zero dimensions to complex, multi-layer, multi-component models with three dimensions [33–36]. According to the characteristics of complexity, lake eutrophication models are divided into simple regression models, simple nutrient balance models, water quality, ecologically and hydrodynamically complex models, as well as ecologically dynamic models [37–39]. With the advances in computing, monitoring and communication technologies, simulation and forecasting technologies for water environments are also constantly improving [40,41]. At present, there are many models for simulating hydrodynamic processes, such as QUAL2 K (River and Stream Water Quality Model), MIKE11 (Modelling System for Rivers and Channels), WASP (Water Quality Analysis Simulation Program) and EFDC (Environmental Fluid Dynamic Code). For example, Zhenjiang Magic Ocean World, a typical playground in Zhenjiang City, carried out playground hydrodynamic and water quality improvement measures, mainly using a two-dimensional, hydrodynamic/aquatic ecological model. Sustainability 2018, 10, x FOR PEER REVIEW 3 of 19

2. Study Area and Methods

2.1. Study Area Located at 119°28’ E and 32°15’ N, Zhenjiang City has a monsoon climate with a transition from Sustainability 2018, 10, 867 3 of 19 a warm temperate zone to the northern subtropical zone, belonging to a semi-humid zone. The average annual precipitation is 1082.7 mm and the average annual evaporation is 894.6 mm. The daily maximum2. Study Areaevaporation and Methods during the year generally occurs in July and August [42], with the minimum in January. Over the years, the average temperature is 15.4 °C, with a highest temperature of 40.9 °C 2.1. Study Area and a lowest temperature of −12 °C. The sunshine is sufficient, with an average annual sunshine h of 2073.8 Locatedh [43]. atThe 119 annual◦280 E andsunshine 32◦150 N,percentage Zhenjiang is City47~49%. has a According monsoon climate to statistics, with a transitionthe annual prevailingfrom a warm winds temperate are E, ENE zone and to ESE the (9% northern each)—ES subtropicalE in summer zone, belonging(13%) and toENE a semi-humid in winter (9%). zone. The prevailingThe average wind annual of Zhenjiang precipitation City is 1082.7northeast mm to and east the by average south. The annual average evaporation annual iswind 894.6 speed mm. is 3.4The m/s daily [44], maximum which is high. evaporation during the year generally occurs in July and August [42], with the ◦ minimumThe Zhenjiang in January. Magic Over Ocean the years, World the project average is surrounded temperature isby 15.4 waterC, on with three a highest sides. The temperature north side ◦ − ◦ is ofa diversion 40.9 C and channel, a lowest the temperature south sideof is the12 originC. Theal sunshinepilot diversion is sufficient, river withand the an averagesoutheast annual side is Neijiangsunshine Lake, h of 2073.8which his [ 43about]. The 372 annual km sunshineaway from percentage the downriver is 47~49%. estuary According and is to not statistics, affected the by annual prevailing winds are E, ENE and ESE (9% each)—ESE in summer (13%) and ENE in winter saltwater intrusion. Neijiang Lake is connected to the Yangtze River by the diversion channel and (9%). The prevailing wind of Zhenjiang City is northeast to east by south. The average annual wind joins the Yangtze River downstream at the Jiaonan Sluice. The exchanged water volume per year speed is 3.4 m/s [44], which is high. between Neijiang and the Yangtze River is typically between 4.5 × 109 m3 and 1.5 × 1010 m3. More than The Zhenjiang Magic Ocean World project is surrounded by water on three sides. The north 85% of the exchanged water volume is concentrated in the flood season. The tides in the Yangtze side is a diversion channel, the south side is the original pilot diversion river and the southeast side Riveris Neijiang result in Lake, high whichand low is abouttides twice 372 km a day away in Ne fromijiang the Lake, downriver with the estuary rising and tide is lasting not affected 3.42 h by and thesaltwater ebbing intrusion.tide lasting Neijiang 9.25 h Lake on average is connected [36,37]. to the The Yangtze water River system by theof Playground diversion channel Lake, and the joins water conservationthe Yangtze project River downstream and the surrounding at the Jiaonan land Sluice. use situation The exchanged are shown water in volume Figure per1. year between NeijiangThe total and construction the Yangtze River area isof typically the Zhenji betweenang Magic 4.5 × 10Ocean9 m3 andWorld 1.5 project× 1010 m is3 .101,050 More than m2 85%[45]. ofThe mainthe exchangedconstruction water items volume includ is concentratede Playground in Lake, the flood Ocean season. World, The tidesthe Water in the YangtzeTourism River Center result and otherin high tourism and lowfacilities, tides twiceamong a daywhich in NeijiangPlayground Lake, Lake with is the made rising up tideof #1, lasting #2 and 3.42 #3 hsections and the of ebbing Shuijie, thetide central lasting landscape 9.25 h on lake average and Moya, [36,37]. which The water carries system out flood of Playground control and Lake, ecological the water dispatch conservation through threeproject sluices and and the surroundingone culvert [46]. land use situation are shown in Figure1. TheThe concentrations total construction of COD area (Chemical of the Zhenjiang Oxygen MagicDemand), Ocean ammonia World projectnitrogen, is 101,050TP, TN mand2 [ 45Chl-a]. of ThePlayground main construction Lake are items3.03 mg/L, include 0.15 Playground mg/L, 0.039 Lake, mg/L, Ocean 1.182 World, mg/L the and Water 0.016 Tourism mg/L Center on average, and respectively.other tourism According facilities, amongto Surface which Water Playground Class LakeIII standards is made up [47,48], of #1, #2the and concentrations #3 sections of Shuijie,of COD, ammoniathe central nitrogen landscape and lake Chl-a and can Moya, meet which the standards; carries out the flood exceeding control and standard ecological rates dispatch of TP and through TN are 27.3%three and sluices 66.7%, and respectively. one culvert [ 46].

FigureFigure 1. Study area.area. Sustainability 2018, 10, 867 4 of 19

The concentrations of COD (Chemical Oxygen Demand), ammonia nitrogen, TP, TN and Chl-a of Playground Lake are 3.03 mg/L, 0.15 mg/L, 0.039 mg/L, 1.182 mg/L and 0.016 mg/L on average, respectively. According to Surface Water Class III standards [47,48], the concentrations of COD, ammonia nitrogen and Chl-a can meet the standards; the exceeding standard rates of TP and TN are 27.3% and 66.7%, respectively.

2.2. Study Methods

2.2.1. Two-Dimensional Hydrodynamic Model The two-dimensional hydrodynamic governing equations in a Cartesian coordinate system are the continuity equation and momentum equations for the integral of the three-dimensional Renault Navier–Stokes equations [30,31] of the incompressible ﬂuid along the direction of water depth, which can be expressed as follows: Continuity equation: ∂h ∂hu ∂hv + + = hQ (1) ∂t ∂x ∂y Momentum equation:

∂hu ∂hu2 ∂hvu ∂t + ∂x + ∂y 2 ∂S = f hv − gh ∂η − h ∂Pa − gh ∂ρ + τsx − τbx − 1 ∂Sxx + xy (2) ∂x ρ0 ∂x 2ρ0 ∂x ρ0 ρ0 ρ0 ∂x ∂y ∂ ∂ + ∂x (hTxx) + ∂y hTxy + husQ

∂hv ∂hv2 ∂hvu ∂t + ∂y + ∂x 2 τ τ ∂S ∂S = − f hu − gh ∂η − h ∂Pa − gh ∂ρ + sy − by − 1 yx + yy (3) ∂y ρ0 ∂y 2ρ0 ∂y ρ0 ρ0 ρ0 ∂x ∂y ∂ ∂ + ∂x hTxy + ∂y hTyy + hvsQ where t represents time; x and y represent Cartesian coordinates; h represents total water depth; η represents water level; ρ represents water density; u and v represent average water depth; f = 2Ωsinϕ denotes the Coriolis factor (Ω is the angular velocity of the Earth’s rotation, ϕ is the geographical latitude); Sxx, Sxy and Syy are the radiation stress tensors; Pa is the atmospheric pressure; Q is the point source emissions; g is the gravitational acceleration;

Z η Z η hu = udz, hv = vdz (4) −d −d where ρ0 represents the relative density of water; and (us, vs) represents the rate at which the outside world is released into the water body. Transverse stress Tij includes viscous resistance, turbulent frictional resistance, and differential advection frictional resistance, which can be calculated using the eddy viscosity equation with the mean vertical velocity:

∂u ∂u ∂v ∂v T = 2A , T = A + , T = 2A (5) xx ∂x xy ∂y ∂x yy ∂x

2.2.2. Two-Dimensional Water Quality and Eutrophication Model (1) Basic equations of the water quality model The water quality equation is based on the mass balance equation. The three-dimensional water quality transport equation contains a lot of uncertain parameters. Under the existing conditions, the veriﬁcation of this model is difﬁcult. Considering the factors such as data and model Sustainability 2018, 10, 867 5 of 19 calculation workload, the vertically average two-dimensional water quality model is adopted [49,50]. The rwo-dimensional water quality transport equation is:

∂C ∂C ∂C ∂ ∂C ∂ ∂C i + U i + V i = E i + E i + K C + S (6) ∂t ∂x ∂y ∂x x ∂x ∂y y ∂y i i i where: Ci is the pollutant concentration; u and v are the flow velocity in the x and y directions; Ex and Ey are the diffusion coefficients in the x and y directions; Ki is the pollutant degradation coefficient; and Si is the sediment release of pollutant. In order to introduce a quantitative relationship between sediment resuspension flux and hydrodynamic conditions in the model and reflect the change of sediment resuspension flux of pollutant with the flow velocity, the sediment resuspension flux is calculated using the relationship obtained from sediment resuspension experiments when the mathematical model is established [49,50]. This mainly reflects the handling of the source sink term Si, as follows: α S = i (7) i H

2 where αi is the sediment resuspension ﬂux (g/(m ·d)), αi = ζi·βi exp(ξi·P); H represents water depth√ (m); βi is the proportion of sediment pollutants in SS (%); P represents co-velocity (cm/s), 2 2 P = u + v ; and ζi, ξi are the sediment resuspension parameters. (2) Basic equations of the Ecolab eutrophication model The content of Chl-a in lakes is the major parameter for evaluating the water trophic status. There are many factors affecting Chl-a content in lakes. It is generally acknowledged that sunlight, temperature, precipitation, nutrients and pH can affect it. In this paper, the impact of the nutritive salt (total nitrogen, total phosphorus) concentration on algae growth was investigated. Chl-a concentration was regarded as the evaluation index. According to the principle of mass conservation, the basic equation of eutrophication is [36–41]:

∂C ∂C ∂C ∂ ∂C ∂ ∂C chl−a + U chl−a + V chl−a = E chl−a + E chl−a + S (8) ∂t ∂x ∂y ∂x x ∂x ∂y y ∂y chl−a where: V S = G (t) − D (t) − s (8a) chl−a PI PI D

GPI = K1·Phtsy·F(N, P) (8b)

DPI = µ·F(N, P) (8c)

In the equation, Cchl-a represents the concentration of Chl-a; U and V respectively represent the flow velocity in the x and y directions, which can be calculated from the water volume model; Ex and Ey represent the lateral and longitudinal diffusion coefficients of algae; Schl-a represents the conversion of Chl-a; GPI represents algae growth; DPI represents algae death; Vs represents algae sedimentation ; D represents water depth; K1 refers to the correlation coefficient between Chl-a content and photosynthesis of phytoplankton; Phtsyn refers to the photosynthesis of plants in unit water volume; µ refers to the mortality rate under optimal nutrition conditions; and F (N, P) indicates the nutrient limit function, whose model conceptual diagram is shown in Figure2.

2 F(N, P) = (8d) 1 + 1 F(N) F(P)

PN − PNmin F(N) = PC (8e) PNmax − PPmin Sustainability 2018, 10, 867 6 of 19

PP PC − PPmin ·(KC + PPmax − PPmin) F(P) = (8f) (PPmax − PP)·(KC + PP/PC − PPmin) where PNmin and PNmax are respectively the minimum and maximum nitrogen content of algae (gN/gC). PPmin and PPmax are the minimum and maximum phosphorus content of algae, (gP/gC). KCSustainabilityis the half-saturation 2018, 10, x FOR phosphorusPEER REVIEW content of phytoplankton (gP/gC). 6 of 19

O2 N2 CO2 Atmosphere Reaeration

Water Degradation Oxidative decomposition Death DIC POC DOC DO

Resuspension Settlement Sediment release SOD Photosynthesis Nitrification Mineralization Sustainability 2018, 10, x FOR PEER REVIEW 6 of 19 Death Degradation Denitrification PON DON NH4 NO3

O2 NRespiration2 CO Sediment Resuspension Settlement Sediment 2 release N absorption Atmosphere release Reaeration Phytoplankton Water Degradation Oxidative decomposition Death MineralizationDIC Death DegradationPOC DOC C absorptionDO POP DOP PO4 P absorption Resuspension Settlement Sediment release SOD Photosynthesis Suspension Sediment Resuspension Settlement Sediment Nitrification / settlement Mineralization release release Death Degradation Denitrification NH NO PON DON C, N, P cycle 4 3 Sediment Respiration Resuspension Settlement Sediment release N absorption release Phytoplankton Figure 2. Model conceptual diagram. Death FigureDegradation 2. ModelMineralization conceptual diagram.C absorption POP DOP PO4 P absorption Suspension 2.3. Model Setup and ParameterResuspension SelectionSettlement Sediment Sediment / settlement release release 2.3. Model Setup and Parameter Selection C, N, P cycle

2.3.1. Model Setup 2.3.1. Model Setup Figure 2. Model conceptual diagram. In this paper, the study area was divided into 3812 grids by using mixed grids of three or four In this paper,2.3. Model the Setup study and areaParameter was Selection divided into 3812 grids by using mixed grids of three or four polygons. The grid spacing was about 8~10 m [30,31]. The lake was supposed to be stationary and polygons.have no Thedisturbance grid2.3.1. Model spacing at Setup the wasinitial about time. 8~10The time m [30 step,31 ].was The 60 lake s. Figures was supposed 3 and 4 show to be the stationary model grid and haveand no bathymetry disturbance Inof this at85 thepaper,elevation. initial the study time. area was The divided time into step 3812 was grids 60 by s.using Figures mixed grids3 and of 4three show or four the model grid and bathymetrypolygons. of 85elevation. The grid spacing was about 8~10 m [30,31]. The lake was supposed to be stationary and 85 elevation:have no National disturbance Vertical at the initial Datum time. The 1985, time step wh wasere 60 the s. Figures average 3 and sea 4 show level the modelof the grid Yellow Sea (in Qingdao)85 elevation: wasand established bathymetry National of as Vertical85 theelevation. unified Datum base 1985, in 1956. where the average sea level of the Yellow Sea (in Qingdao) was established85 elevation: as National the uniﬁed Vertical baseDatumin 1985, 1956. where the average sea level of the Yellow Sea (in Qingdao) was established as the unified base in 1956.

Figure 3. Model grid. Figure 3. Model grid.

Figure 3. Model grid. Sustainability 2018, 10, 867 7 of 19

Sustainability 2018, 10, x FOR PEER REVIEW 7 of 19

Figure 4. Model bathymetry of 85 elevation. Figure 4. Model bathymetry of 85 elevation. 2.3.2. Parameter Selection 2.3.2. ParameterDue Selection to the current under-planning state and no excavation operations at Playground Lake, the results of the model calculation were checked to ensure that they meet stronger relations between the Due toreal the status current of the under-planningpark and the model stateto estimate and the no effect excavation of changes, operations according to at Wang Playground Hua’s in Lake, the results of thesitu modeldata and calculation routine monitoring were checked data of Neijiang to ensure Lake that and theydiversion meet channel stronger [22–25]. relations The main between the real statuswater of the quality park and and eutrophication the model toparameters estimate in the the effectmodel are of changes,shown in Table according 1. to Wang Hua’s in situ data and routine monitoringTable data 1. ofMain Neijiang water quality Lake and eutrophication and diversion parameters. channel [22–25]. The main water quality and eutrophication parameters in the model are shown in Table1. Number Parameters Value [22–25] Unit 1 Chl-a growth rate 1.8 per day 2 Table 1. Main Chl-a sedimentation water quality rate and eutrophication 0.11 parameters. per day 3 Sediment oxygen consumption 0.5 per day Number4 Nitrification oxygen Parameters demand of ammonia nitrogen Value 3.42 [22–25] g O2/g NH4-N Unit 5 Denitrification oxygen demand of nitrite 1.14 g O2/g NO2-N 16 Chl-aDenitrification growth rate rate 0.1 1.8per day per day 27 Chl-a Phosphate sedimentation degradation rate rate 0.06 0.11 g P/m3/day per day 3 Sediment oxygen consumption 0.5 per day 2.4. Calculation Programs 4 Nitrification oxygen demand of ammonia nitrogen 3.42 g O2/g NH4-N 5In order Denitrification to ensure the water oxygen quality demand in the ofstud nitritey area and to meet the 1.14 water use requirement g O2/g of NO 2-N 6landscape and recreation, Denitrification two kinds of ratewater diversion schemes were proposed 0.1 to control water per day 7eutrophication, as follows: Phosphate degradation rate 0.06 g P/m3/day

2.4.1. Water Diversion through Sluice 2.4. Calculation Programs During high tide periods of the spring tides in the Yangtze River, if the water level of the Yangtze In orderRiver to is ensure0.3 m higher the than water that qualityof Neijiang in Lake the studywith sluice, area water and will to be meet diverted the into water Playground use requirement of landscapeLake and by gravity recreation, to improve two water kinds quality of water and eutr diversionophication schemes status by the were water proposed head between to control the water #1 inlet, #2 inlet and outlet (Figure 5). eutrophication,Model as follows: boundary conditions: The initial water level was set at 2.67 m. The temperature was 28 °C. The initial flow rate was set to zero [42–44]. The initial water head Δh (the water head between 2.4.1. Waterthe Diversion Yangtze River through and Playground Sluice Lake) was 0.2–0.3 m. The flow rate of water diversion through sluice at #1 inlet and #2 inlet were 8.27 m3/s and 1.82 m3/s, respectively. During high tide periods of the spring tides in the Yangtze River, if the water level of the Yangtze River is 0.3 m higher than that of Neijiang Lake with sluice, water will be diverted into Playground Lake by gravity to improve water quality and eutrophication status by the water head between the #1 inlet, #2 inlet and outlet (Figure5). Model boundary conditions: The initial water level was set at 2.67 m. The temperature was 28 ◦C. The initial ﬂow rate was set to zero [42–44]. The initial water head ∆h (the water head between the Yangtze River and Playground Lake) was 0.2–0.3 m. The ﬂow rate of water diversion through sluice at #1 inlet and #2 inlet were 8.27 m3/s and 1.82 m3/s, respectively. Sustainability 2018, 10, 867 8 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 8 of 19

◦ Weather conditions:conditions: TheThe measurementmeasurement data data show show that that the the average average temperature temperature in 2016in 2016 was was 16.8 16.8C; ◦ °C;the the coldest coldest month month was was January January when when the average the average monthly monthly temperature temperature was 2.97 wasC; 2.97 the °C; hottest the hottest month ◦ monthwas August was whenAugust the when average the monthlyaverage temperaturemonthly temperature was 29.39 wasC. The 29.39 average °C. The annual average precipitation annual precipitationin the last two in years the last was two 1082.7 years mm, was which 1082.7 wasmm, unevenly which was distributed. unevenly distributed. The precipitation The precipitation was mostly wasconcentrated mostly concentrated in the spring, in summer the spring, and autumnsummer [ 45and,46 autumn]. In particular, [45,46]. the In precipitationparticular, the was precipitation the highest wasin the the summer, highest exceedingin the summer, 45% of exceeding the total annual45% of precipitation.the total annual The precipitation. average annual The windaverage speed annual was wind3.4 m/s. speed was 3.4 m/s. Initial pollution source load: The The amount amount of of emissions emissions by by tourists tourists and and sewage produced by tourism facilities was was calculated as as the initial pollution source load, according to the measurement data and index establishment method [[48].48].

Figure 5. Route of water diversion through three sluices (#1-3) and one culvert (#4). Figure 5. Route of water diversion through three sluices (#1-3) and one culvert (#4). 2.4.2. Water Diversion through Pump: 2.4.2. Water Diversion through Pump Through the #1 pumping station, the diversion water will enter Playground Lake to improve its waterThrough quality and the #1eutrophication pumping station, status. the The diversion positions water of the will three enter sluices Playground and one Lake culvert to improve are shown its inwater Figure quality 1. The and specification eutrophication parameters status. Theare provided positions ofin theTable three 2. sluices and one culvert are shown in Figure1. The specification parameters are provided in Table2. Table 2. Specification parameters of three sluices and one culvert. Table 2. Specification parameters of three sluices and one culvert. Number Size Numbersluice Net width of 9 m, bottom Size elevation of 1.2 m #1 pump Single pump flow rate of 1.85 m3/s with a total of two sluice Net width of 9 m, bottom elevation of 1.2 m #1 #2 sluicepump Single Net width pump of flow 10 m, rate bottom of 1.85 elevation m3/s with of a1.0 total m of two #3 sluice Net width of 9 m, bottom elevation of 1.5 m #2 sluice Net width of 10 m, bottom elevation of 1.0 m #4 culvert Net size of 2 m × 2 m #3 sluice Net width of 9 m, bottom elevation of 1.5 m Playground Lake#4 culvert has a storage capacity of about Net size 350,000 of 2 m ×m³.2 mThe design flow rate of a single pump was 1.85 m3/s. Water can all be replaced within 27 h by double pump diversion, according to the initialPlayground calculation. Lake The has pump a storage diversion capacity will of aboutnot affect 350,000 recreational m3. The activities design flow such rate as ofwatercraft a single duepump to wasits low 1.85 flow m3/s. rate, Water so that can continuous all be replaced pump within diversion 27 h bycan double be used. pump In order diversion, to ensure according that the to waterthe initial is completely calculation. replaced, The pump the diversion pump diversion will not was affect proposed recreational to continue activities for such 30 ash. If watercraft the #1 sluice due turnedto its low off flow and rate,the #1 so pump that continuous turned on, pump the #2 diversion sluice, #3 can sluice be used. and #4 In orderculver tot will ensure divert that the the water, water withis completely most of the replaced, water flowing the pump out diversion of the #2 sluice, was proposed so that water to continue in the Moya for 30 area h. If cannot the #1 be sluice effectively turned changed.off and the Therefore, #1 pump it turned is necessary on, the to #2 properly sluice, #3 deploy sluice andthe three #4 culvert sluices will and divert one culvert the water, to make with mostsure thatof the the water water flowing is completely out of the replac #2 sluice,ed. As so the that ratio water of inthe the storage Moya ca areapacity cannot of Shuijie be effectively and the changed. central landscape lake area for the Moya area is about 3:1, the pump diversion was designed from 0:00 to 22:00 on the first day (lasting 22 h) to replace the water in Shuijie and the central landscape lake area, Sustainability 2018, 10, 867 9 of 19

Therefore, it is necessary to properly deploy the three sluices and one culvert to make sure that the water is completely replaced. As the ratio of the storage capacity of Shuijie and the central landscape lake area for the Moya area is about 3:1, the pump diversion was designed from 0:00 to 22:00 on the ﬁrst day (lasting 22 h) to replace the water in Shuijie and the central landscape lake area, and from 22:00 on the ﬁrst day to 6:00 on the second day (lasting 8 h) to replace the water in the Moya area. Three sluices and one culvert scheduling plan during pump diversion is shown in Table3.

Table 3. Three sluices and one culvert scheduling plan during pump diversion.

Time (24 h) Pump #1 Sluice #2 Sluice #3 Sluice #4 Sluice Remarks Before pumping Close Open Open Open Open / Shuijie and central landscape 0:00~22:00 Open Close Open Close Close lake area water diversion 22:00~6:00+1 Open Close Close Open Open All areas water diversion Close #1 sluice for 1 h to 6:00+1~7:00+1 Close Close Open Open Open prevent backwater After pumping Close Open Open Open Open Open the ﬂow pump Note: time+1 means the next day.

Zhenjiang Magic Ocean World is affected by a wind field. However, due to the large range of variation of wind speed and direction, it is hard to form a stable flow field and the lake flow does not have a very high degree of regularity. In order to better reflect the influence of the wind field on the flow field in Playground Lake, three kinds of wind speed and direction with high frequency were used in the model simulation. The wind directions were E, SE and S, and the wind speed was 3.4 m/s. The flow field, water quality and eutrophication status under three water diversion modes in three wind fields were simulated. Owing to the small size of Playground Lake and the large diversion flow, the flow field in Playground Lake is mainly affected by water diversion. Therefore, the impact of wind direction on the flow field is almost negligible [35–39]. Thus only the flow field, water quality and eutrophication status of the water diversion through the pump and sluice under SE wind with the highest wind frequency were simulated [51,52]. The model calculation scheme is presented in Table4.

Table 4. Model calculation scheme.

Program Wind Direction Wind Speed Temperature Water Diversion 1 E, SE, S No 2 SE3.4 m/s 28 ◦C Pump 3 SE Sluice

3. Results and Discussion

3.1. Particle Tracking Four particles were located at the #2 entrance of the river. The movements and positions of each particle in the flow field [53–57] are provided in Figure6, which shows that: 1 Water residence time depended on wind direction: east wind, 125 h; southeast wind, 115 h; south wind, 95 h. 2 Particles did not pass the Moya area in all three wind directions, and almost all particles entered the Shuijie area through the central landscape lake, and then entered the Yangtze River diversion channel. Only one particle reached to the east bank of Playground Lake in the east wind. 3 Particles in the central landscape lake experienced a backflow phenomenon and the water residence time was longer. However, eventually the water still flowed out through the Shuijie area to achieve water exchange. Sustainability 2018, 10, 867 10 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 10 of 19

Sustainability 2018, 10, x FOR PEER REVIEW 10 of 19

Figure 6. Particle tracking pathway and water residence time in three wind directions. Figure 6. Particle tracking pathway and water residence time in three wind directions.

3.2. Flow Field Calculation and Analysis 3.2. Flow Field CalculationFigure 6. Particle and Analysistracking pathway and water residence time in three wind directions. 3.2.1. Calculation and Analysis of Flow Field with No Water Diversion 3.2.1.3.2. Calculation Flow Field andCalculation Analysis and ofAnalysis Flow Field with No Water Diversion Using the established hydrodynamic model for the artificial Playground Lake [44–46] and Using3.2.1.depending Calculation the on established the and above Analysis hydrodynamiccalculation of Flow scheme, Field model with the flowNo for Water field the Diversion artificialin different Playground wind fields Lakewith no [44 water–46] and diversion was calculated (Figure 7). In all kinds of wind fields [49–51], the model can simulate for ten dependingUsing on thethe aboveestablished calculation hydrodynamic scheme, model the flow for the field artificial in different Playground wind Lake fields [44–46] with noand water days to form a stable flow field. Figure 7 showed that: ① The flow velocity of the central landscape diversiondepending was calculated on the above (Figure calculation7). In all scheme, kinds the of windflow field fields in [ different49–51], the wind model fields can with simulate no water for ten lake in three wind fields is low, and the shallow depth of shore water is greatly affected by wind daysdiversion to form a was stable calculated flow field. (Figure Figure 7). In7 all showed kinds of that: wind fields1 The [49–51], flow velocitythe model of can the simulate central for landscape ten speed, showing slightly higher flow velocity in different wind directions. ② In three wind fields, the lake indays three to form wind a fieldsstable isflow low, field. and Figure the shallow 7 showed depth that: of ① shore The flow water velocity is greatly of the affected central by landscape wind speed, flow velocity in the Moya area is very low, only the surface flow moves slightly with the wind lake in three wind fields is low, and the shallow depth of shore water2 is greatly affected by wind showingdirection. slightly ③ higherUnder the flow east velocity wind, the in differentflow velocity wind is directions.almost zero becauseIn three the windwind direction fields, the is flow speed, showing slightly higher flow velocity in different wind directions. ② In three wind fields, the velocityperpendicular in the Moya to the area #1, is#2 very and #3 low, sections only of the Shij surfaceie. In the flow southeast moves and slightly south wind, with the the flow wind velocity direction. flow velocity in the Moya area is very low, only the surface flow moves slightly with the wind 3 Underincreases theeast in the wind, #1 and the #2 flow sections velocity of Shijie. is almost zero because the wind direction is perpendicular to ③ the #1,direction. #2 and #3 sectionsUnder the of east Shijie. wind, In thethe southeastflow velocity and is south almost wind, zero thebecause flow the velocity wind increasesdirection is in the perpendicular to the #1, #2 and #3 sections of Shijie. In the southeast and south wind, the flow velocity #1 and #2 sections of Shijie. increases in the #1 and #2 sections of Shijie.

Figure 7. Velocity distribution in different wind directions without water diversion.

3.2.2. Calculation and Analysis of Flow Field with Water Diversion through Pump Figure 7. Velocity distribution in different wind directions without water diversion. CombiningFigure 7. theVelocity wind distributiondata over the in years, different the windflow field directions of Playground without water Lake diversion.after pumping for 3.2.2.22 h and Calculation 30 h was and simulated Analysis in theof Flow most Field prevailing with Water southeast Diversion wind [50–54].through Figure Pump 8a,b showed that: 3.2.2.① Calculation After water and diversion Analysis for of22 Flowh, due Field to the with sudden Water widening Diversion at the through junction Pump of Playground Lake Combining the wind data over the years, the flow field of Playground Lake after pumping for and the west side of central landscape lake, the water flow changed from moving wave to diffusion 22 h and 30 h was simulated in the most prevailing southeast wind [50–54]. Figure 8a,b showed that: Combiningwave. Along thethe direction wind data of the over di theffusion years, wave, the the flow flow field velocity of Playground decreased. In Lake the central after pumpingand east for ① After water diversion for 22 h, due to the sudden widening at the junction of Playground Lake 22 h andareas 30 of hthe was central simulated landscape in thelake, most as the prevailing water flowsoutheast gradually stabilized, wind [50– the54 ].flow Figure velocity8a,b increased. showed that: and the west side of central landscape lake, the water flow changed from moving wave to diffusion 1 After② After water water diversion diversion for for 22 h,30 dueh, the to #2 the sluice sudden was closed widening and the at the#3 sluice junction and #4 of culvert Playground were open. Lake and wave. Along the direction of the diffusion wave, the flow velocity decreased. In the central and east the westThe sideflow ofvelocity central of landscape the Moya lake,area theand waterexit area flow became changed higher from and moving the water wave body to diffusion was soon wave. areas of the central landscape lake, as the water flow gradually stabilized, the flow velocity increased. Along the direction of the diffusion wave, the flow velocity decreased. In the central and east areas of ② After water diversion for 30 h, the #2 sluice was closed and the #3 sluice and #4 culvert were open. the centralThe flow landscape velocity lake,of the as Moya the water area flowand exit gradually area became stabilized, higher the and flow the velocity water body increased. was soon 2 After water diversion for 30 h, the #2 sluice was closed and the #3 sluice and #4 culvert were open. The flow Sustainability 2018, 10, 867 11 of 19

velocitySustainability of the Moya2018, 10, areax FOR andPEER exitREVIEW area became higher and the water body was soon replaced. 11 of 19 In the southeastSustainability of Playground 2018, 10, x FOR Lake PEER REVIEW and in the middle of the Moya area, the ﬂow velocity was 11 relatively of 19 replaced. In the southeast of Playground Lake and in the middle of the Moya area, the flow velocity slow due to the width being wider than the average wide of the channel. Due to the closure of #2 was relatively slow due to the width being wider than the average wide of the channel. Due to the sluice,replaced. there were In the stagnating southeast of ﬂow Playground areas. Lake and in the middle of the Moya area, the flow velocity wasclosure relatively of #2 sluice, slow theredue to we there widthstagnating being flow wider areas. than the average wide of the channel. Due to the closure of #2 sluice, there were stagnating flow areas.

(a) (b)

Figure 8. (a) Flow field with(a) water diversion through pump for 22 h.(b ()sb ) Flow field with water Figure 8. (a) Flow ﬁeld with water diversion through pump for 22 h. (b) Flow ﬁeld with water diversion through pump for 30 h. diversionFigure through 8. (a) Flow pump field for with 30 h. water diversion through pump for 22 h. (b) Flow field with water diversion through pump for 30 h. 3.2.3. Calculation and Analysis of Flow Field with Water Diversion through Sluice 3.2.3. Calculation and Analysis of Flow Field with Water Diversion through Sluice 3.2.3.Combining Calculation the and historical Analysis wind of Flow field Field data with [42 –Water44], the Diversion water head through between Sluice the #1 inlet, #2 inlet Combiningand outletCombining (Δ theh) was historicalthe simulatedhistorical wind windto be field 0.3field datam datain [the42 [42–44],– most44], theprevailing the water water head southeasthead between between wind the the[35 – #1#137]. inlet, The #2steady inlet inlet and outletandflow (∆ h)outlet field was in (Δsimulated Figureh) was 9. simulated showed to be0.3 that: to mbe ① in0.3 The them in flow most the velority most prevailing prevailing of Moya southeast southeastarea was wind the wind highest [35 [35–37].–37 ].in ThePlayground The steadysteady flow Lake, followed by the Shuijie area. The flow velocity in central landscape lake was relatively low, and fieldflow in Figure field 9in. Figure showed 9. showed that: 1 Thethat: flow① The velority flow velority of Moya of Moya area area was was the the highest highest in in Playground Playground Lake, the flow velocity in some areas was lower than 0.01 m/s. There were some stagnating flow areas. ② followedLake, by followed the Shuijie by the area. Shuijie The area. flow The velocity flow velocity in central in central landscape landscape lake was lake relativelywas relatively low, low, and and the flow For Shuijie area, the water flow entered through #1 inlet, and most water flowed into the central velocitythe inflow some velocity areas inwas some lower areas thanwas lower 0.01 m/s. than 0. There01 m/s. were There some were stagnating some stagnating flow areas. flow areas. 2 For ② Shuijie landscape lake through the #1 and #2 sections of Shuijie. A small part of the water flowed through area,For the Shuijie water flowarea, enteredthe water through flow entered #1 inlet, through and most#1 inlet, water andflowed most water into flowed the central into landscapethe central lake landscapethe #3 section lake of through Shuijie. theIt can #1 beand known #2 sections that the of flShuijie.ow rate A in small the #1 part and of #2 the sections water flowedwas larger through than through the #1 and #2 sections of Shuijie. A small part of the water flowed through the #3 section of thethat #3 of sectionthe #3 section. of Shuijie. It can be known that the flow rate in the #1 and #2 sections was larger than Shuijie. It can be known that the flow rate in the #1 and #2 sections was larger than that of the #3 section. that of the #3 section.

Figure 9. Flow field with water diversion through sluice. Figure 9. Flow field with water diversion through sluice. Figure 9. Flow ﬁeld with water diversion through sluice. Sustainability 2018, 10, 867 12 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 12 of 19

3.3. Water Quality and EutrophicationEutrophication

3.3.1. Calculation and Analysis of the WaterWater QualityQuality andand EutrophicationEutrophication withwith NoNo WaterWater Diversion Diversion The distributiondistribution ofof Chl-a,Chl-a, TNTN andand TP TP in in Playground Playground Lake Lake were were obtained obtained after after the the model model reached reached a fullya fully stable stable state state [38– 41[38–41],], as shown as shown in the Figure in the 10 a–c.Figure The 10a–c. comprehensive The comprehensive score of the eutrophication score of the assessmenteutrophication is shown assessment in Figure is shown10d. The in resultsFigure showed10d. The that: results 1 The showed concentrations that: ① The of Chl-a,concentrations TN and TP of andChl-a, the TN comprehensive and TP and the score comprehensive of eutrophication score assessmentof eutrophication in three assessment wind directions in three were wind 0.014 directions mg/L, 1.49were mg/L, 0.014 mg/L, 0.11 mg/L 1.49 mg/L, and 58.0, 0.11 respectively.mg/L and 58.0, With respectively. no water diversionWith no water and indiversion three kinds and ofin windthree field,kinds the of wind water field, of the the Shuijie water area, of the Moya Shuijie area area and, the Moya outlet area area and connected the outlet with area the connected culvert cannot with the be effectivelyculvert cannot replaced, be effectively which was replaced, consistent which with was the cons flowistent field. with Water the retention flow field. led Water to higher retention Chl-a, TNled andto higher TP contents. Chl-a, TN 2 andIn the TP case contents. of no water② In the diversion case of and no water the east diversion wind, as and the the wind east direction wind, as was the perpendicularwind direction towas the perpendicular #1, #2 and #3 sectionsto the #1, of #2 Shuijie, and #3 the sections contents of Shuijie, of Chl-a, the TN contents and TP of in Chl-a, the water TN bodyand TP in in this the area water were body higher in this than area those were under high theer than other those two under wind directions.the other two Under wind the directions. east and southeastUnder the winds, east and the southeast contents ofwinds, Chl-a, the TN contents and TP of in Chl-a, the water TN bodyand TP in in the the central water landscape body in the lake central were higherlandscape than lake that were under higher the souththan that wind, under because the south the water wind, was because more the affected water by was the more inflow affected from #2by sluicethe inflow in the from first two#2 sluice wind in directions, the first two which wind improved directions, the waterwhich quality improved of the the central water landscape quality of lake. the Whilecentral affected landscape by thelake. inflow While of affected the outlet, by the the inflow water qualityof the outlet, in the the outlet water area quality was the in bestthe outlet under area the southwas the wind, best followedunder the by south the southeast wind, followed wind and by the finally southeast the east wind wind. and finally the east wind.

(a)

(b)

Figure 10. Cont. Sustainability 2018, 10, 867 13 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 13 of 19

(c)

(d)

Figure 10. Chl-a distribution (a), TN distribution (b), TP distribution (c) and comprehensive score of Figure 10. Chl-a distribution (a), TN distribution (b), TP distribution (c) and comprehensive score of eutrophication assessment ((dd)) withwith nono waterwater diversiondiversion underunder differentdifferent windwind conditions.conditions.

3.3.2. Calculation and Analysis of Water Quality and Eutrophication with Water Diversion through 3.3.2. Calculation and Analysis of Water Quality and Eutrophication with Water Diversion throughPump Pump After water diversion for 22 h,h, thethe Chl-a,Chl-a, TN andand TPTP concentrationsconcentrations andand eutrophicationeutrophication comprehensive score of Playground Lake were presentedpresented in Figure 1111a.a. The overall water quality in the easterneastern lakelake areaarea hadhad beenbeen significantlysignificantly improved, improved, except except the the local local shore shore stagnant stagnant area. area. The The Chl-a, Chl- TNa, TN and and TP TP concentrations concentrations of of the the Shuijie Shuijie and and central central landscape landscape lake lake werewere significantlysignificantly reduced,reduced, and were 0.010.01 mg/L,mg/L, 1.42 mg/L and and 0.06 0.06 mg/L, respectively, respectively, and and the eutrophication score significantlysignificantly decreased to 51.4, showing mildmild eutrophication.eutrophication. After waterwater diversiondiversion for for 30 30 h, h, the the remaining remaining water water in in the the Moya Moya area area was was replaced. replaced. The The Chl-a, Chl-a, TN andTN TPand concentrations TP concentrations and eutrophication and eutrophication comprehensive comprehensive score of Playgroundscore of Playground Lake are presented Lake are in Figurepresented 11b. in The Figure water 11b. quality The in water the Moya quality area in improved. the Moya The area Chl-a, improved. TN and TPThe concentrations Chl-a, TN and in theTP Shuijieconcentrations and central in the landscape Shuijie lakeand reducedcentral landscape to 0.009 mg/L, lake reduced 1.36 mg/L to and0.009 0.02 mg/L, mg/L, 1.36 respectively. mg/L and 0.02 The eutrophicationmg/L, respectively. comprehensive The eutrophication score decreased comprehens to 47.7.ive Thescore results decreased showed to 47.7. that waterThe results diversion showed had anthat obvious water diversion effect on waterhad an quality obvious improvement effect on water in Playground quality improvement Lake. in Playground Lake. Sustainability 2018, 10, 867 14 of 19 Sustainability 2018, 10, x FOR PEER REVIEW 14 of 19

score

(a)

score

(b)

Figure 11. (a) Chl-a, TN and TP concentrations and eutrophication comprehensive score after water Figure 11. (a) Chl-a, TN and TP concentrations and eutrophication comprehensive score after water diversion for 22 h. (b) Chl-a, TN and TP concentrations and eutrophication comprehensive score after diversion for 22 h. (b) Chl-a, TN and TP concentrations and eutrophication comprehensive score after water diversion for 30 h. water diversion for 30 h.

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3.3.3. Calculation and Analysis of Water Quality andand EutrophicationEutrophication withwith WaterWater DiversionDiversion through throughSluice Sluice Under water diversion through the sluice, the Chl-a,Chl-a, TN, TP concentrations and eutrophication 1 comprehensive score presented in Figure 12 show that: ① The improvement of water quality with sluice diversion was was very very rapid rapid and and obvious. obvious. The The wa waterter quality quality was was consistent consistent with with the the regulation regulation of ofthe the flow flow field. field. The The greater greater the Δ theh, the∆h, higher the higher the flow the flowvelocity. velocity. The better The better the water the waterquality, quality, the lower the 2 lowerthe eutrophication the eutrophication evaluation evaluation score. score. ② For Forthe thewhole whole Playground Playground Lake, Lake, the the Chl-a, Chl-a, TN TN and and TP concentrations and eutrophicationeutrophication comprehensivecomprehensive scorescore were were 0.01 0.01 mg/L, mg/L, 1.571.57 mg/L,mg/L, 0.030.03 mg/Lmg/L and 49.8, respectively.respectively. TheThe water water quality quality of of central central landscape landscape lake lake and and the the Shuijie Shuijie area area were were relatively relatively poor andpoor prone and prone to eutrophication. to eutrophication.

score

Figure 12. Chl-a, TN and TP concentrations and eutrophication comprehensive score with water Figure 12. Chl-a, TN and TP concentrations and eutrophication comprehensive score with water diversion through sluice. diversion through sluice.

4. Water Ecological Protection Measures 4. Water Ecological Protection Measures Priority 1—Water diversion by sluice: When the tide level of the Yangtze River is higher than Priority 1—Water diversion by sluice: When the tide level of the Yangtze River is higher than that of Neijiang Lake (3.9~4.1 m), the sluice in diversion channel and three sluices and one culvert are that of Neijiang Lake (3.9~4.1 m), the sluice in diversion channel and three sluices and one culvert are open to replace water in Playground Lake. open to replace water in Playground Lake. Priority 2—Water diversion by pump: According to the temperature and the corresponding Priority 2—Water diversion by pump: According to the temperature and the corresponding interval days, when the temperature higher than 15 °C and there is no water diversion by pump, in interval days, when the temperature higher than 15 ◦C and there is no water diversion by pump, in order to speed up the water replacement in stagnating and slow flow areas, the ecological scheduling order to speed up the water replacement in stagnating and slow ﬂow areas, the ecological scheduling scheme should be implemented, as shown in Table 5. scheme should be implemented, as shown in Table5. Table 5. Ecological scheduling scheme.

Sluice Diversion Water Temperature Pumping Open the sluice in the diversion channel 1 # pump diversion Note: Sluice diversion is generally carried out in <15 °C 15 d/time, each time 30 h June, July and August (temperature above 23 °C), 15 °C~25 °C 10 d/time, each time 30 h twice per month. >25 °C 7 d/time, each time 30 h Sustainability 2018, 10, 867 16 of 19

Table 5. Ecological scheduling scheme.

Sluice Diversion Water Temperature Pumping Open the sluice in the diversion channel 1 # pump diversion Note: Sluice diversion is generally carried <15 ◦C 15 d/time, each time 30 h out in June, July and August (temperature 15 ◦C~25 ◦C 10 d/time, each time 30 h above 23 ◦C), twice per month. >25 ◦C 7 d/time, each time 30 h

According to the model calculations, the following ecological operation was planned: During high tide periods of spring tides in the Yangtze River from June to August, the water level of the Yangtze River is higher than that of Playground Lake. In other time, the water is pumped by #1 pump. Under pump diversion, the water body in Playground Lake can be replaced by water diversion for 30 h. When the temperature is lower than 15 ◦C, from 15 ◦C to 25 ◦C and higher than 25 ◦C, the water quality can be maintained for 15 d, 10 d and 7 d, respectively.

5. Conclusions The purpose of this paper was to predict the eutrophication of the planned Playground Lake. Some targeted water quality protection measures were put forward according to possible eutrophication or poor water quality. The eutrophication model presented in this paper was used to simulate the scenarios under different water diversions and wind directions. According to the simulation results and numerical analysis, the main conclusions are as follows:

(1) Simulation results using particle tracking showed that the water residence time depended on wind direction: east wind, 125 h; southeast wind, 115 h; south wind, 95 h. Particles did not pass the Moya area under all three wind directions. Particles in central landscape lake experienced a backflow phenomenon and the water residence time was longer. However, eventually the water still flowed out to achieve water replacement. With no water diversion, the flow velocity in Playground Lake under the three wind fields was low, and the shallow depth of shore water was greatly affected by wind speed. (2) The Chl-a, TN, TP concentrations and eutrophication comprehensive score under the three wind directions were 0.014 mg/L, 1.49 mg/L, 0.11 mg/L and 58.0, respectively. In conformity with the flow field, the water retention caused the Chl-a, TN, and TP contents to be higher. Under pump diversion, the water replacement result of water diversion for 30 h was better than that of water diversion for 22 h. Following water diversion for 22 h, the eutrophication comprehensive score was 51.4, showing mild eutrophication. Following water diversion for 30 h, the eutrophication comprehensive score was 47.7 points, so the water quality improvement effect was more obvious. Under sluice diversion, the flow field scope of the Moya area was the largest in Playground Lake, followed by the Shuijie area. The flow velocity in central landscape lake was low, and in some areas it was lower than 0.01 m/s. The improvement of water quality with sluice diversion was very rapid and obvious. The water quality was consistent with the regulation of the flow field. The greater the ∆h, the higher the flow velocity. The better the water quality, the lower the eutrophication evaluation score. The central landscape lake and three sections of Shuijie had relatively poor water quality leading to eutrophication. Overall, the good-to-bad order of water quality improvements for Playground Lake is as follows: pumping 30 h > sluice diversion > no water diversion. (3) According to the model calculations, the following ecological operation was planned: During high tide periods of spring tides in the Yangtze River from June to August, the water can be diverted into the lake through sluices. At other time, the water is pumped by the #1 pump. Under pump diversion, the water body in Playground Lake can be replaced by water diversion for 30 h. When the temperature is less than 15 ◦C, from 15 ◦C to 25 ◦C and higher than 25 ◦C, Sustainability 2018, 10, 867 17 of 19

the water quality can be maintained for 15 d, 10 d and 7 d, respectively. These water quality improvement measures can effectively control the occurrence of eutrophication.

Acknowledgments: The research was supported by Priority Academic Program Development of Jiangsu Higher Education Institutions, National Water Pollution Control and Treatment Science and Technology Major Project (Grants No. 2014ZX07405-002), Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grants No. KYCX17_0417), National Natural Science Foundation of China (Number: 51609116) and Natural Science Foundation of Jiangsu province (Number: BK20160961). Author Contributions: Min Pang drafted the manuscript. Weiwei Song and Peng Zhang carried on the model calculation. Yongxu Shao and Lanyimin Li made the contributions on the data analyses and manuscript revisions. Qing Xu collected the research status at home and abroad. Yong Pang and Jianjian Wang carried out the design of the outline. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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