Numerical study of hydro-environmental processes of Poyang Lake subject to engineering control

Poyang Lake, the largest freshwater lake in China, plays a key role in regulating the hydrology, water quality and ecosystem in the middle reaches of the Yangtze River. Recent industrialization and urbanization in Jiangxi province have led to rapid increase in water consumption and water quality deterioration. In this research, a numerical model is developed to simulate the hydrodynamics and water quality evolutions in Poyang Lake and its surrounding river network. The study links an in-house one-dimensional river network model with a MIKE multi-layered three-dimensional hydro-environmental model. The validated model is used to investigate the impact of a proposed downstream barrage on Poyang Lake ’ s ﬂ ow and ecosystem. All of the ﬁ ve considered barrage operation schemes demonstrate the capability of increasing the water level and inundation area during the dry period, especially in the lake ’ s downstream region. The barrage schemes help reduce the lake ’ s nutrient concentrations, due to the enhanced dilution at higher water levels. Nonetheless, the barrage leads to DO depletion which could pose a threat to the aquatic wildlife and habitats. This study ’ s ﬁ ndings contribute to a better understanding of the hydro-environmental in ﬂ uence of the proposed barrage and thus the optimisation of its operation to mitigate the adverse in ﬂ uence.


INTRODUCTION
Located in Jiangxi province in southeastern China, Poyang Lake is the largest freshwater lake of the country. It sits on the southern bank of the middle and lower reaches of Yangtze River, and has an area of 16.2 × 10 4 km 2 , accounting for 9% of Yangtze River basin (Zhang et al. b).
Poyang Lake connects with the Yangtze River through a channel in the north and also receives water from a complex river system in the south, consisting of five major rivers namely Ganjiang, Fuhe, Xiushui, Xinjiang and Raohe. The lake region serves as a natural hydrological moderator for the Yangtze River, providing huge volume for water retention and pollutant dilution and an irreplaceable habitat for numerous aquatic organisms along the Yangtze River basin. The lake also supports agriculture, tourism and is the key freshwater source for around 12.4 million people in nearby cities of Nanchang, Jingdezhen and Jiujiang (Zhang et al. ).
Since the completion of the Three Gorges Dam (TGD) and its associated reservoir in 2003, the flow characteristics of Yangtze River have recorded significant changes which created knock-on effects on its surrounding water bodies (Lai et al. b). More outflows from Poyang Lake to the Yangtze River reduced the water storage capability of Poyang Lake (Guo et al. a). The lake's water levels have witnessed a declining trend in the 21st century. Monitoring stations around Poyang Lake all logged the lowest historic water levels over the last two decades and the average water level drop ranges from 14.0 to 13.3 m over the last 60 years (Guo et al. b). The main reasons behind the water level drop include decreasing rainfall and influx from upstream rivers, leading to a prolonged dry period. As a result, the self-purification ability of the lake reduces and hence the water quality worsens, resulting in the deterioration of ecology. Within the period of 2000-2007, the river length with the level of water quality no better than Class III (poor quality) increased from 170 to 221 km, while the number of polluted river sections increased from 6 to 22 (Gao et al. ). The main pollutants in the region include TN, TP, NH 3 -N and COD (Luo et al. ), which can be directly tracked to the five major rivers as a result of increased industrial activities, municipal wastes and agricultural fertilizers and poultry farming (Chen et al. ).
The decline in water quantity and quality in Poyang Lake has sparked concerns over freshwater security for the humans and wildlife in the region. The Poyang Lake barrage project was therefore proposed to prevent the lake's water level from further declining because of climate change, urbanization and hydraulic control on the Yangtze River (Hu & Ruan ). The proposed project involves a series of sluice gates being installed along the narrowest section of the channel at the northern mouth of the lake ( Figure 1). The main goal of the project is to manage the water level in Poyang Lake by closing the sluice gates during dry seasons. In the wet season, the sluice gates are fully opened allowing free exchange of water, energy and biology between the lake and Yangtze River (Lai et al. ). In support of the proposed barrage to be built, local government offices and hydrological authorities have designed various operational schemes for the project (Zhang et al. ). These schemes aim to manage water resources and mitigate potential flooding hazards in the Yangtze River basin. Nonetheless, these operations could alter the natural processes in the hydrological, hydrodynamic, sediment transport, water quality and ecological aspects and weaken the existing interaction and natural connection between the Yangtze River and Poyang Lake. Sudden changes in the water level and flow velocity of the hydro-environment could affect the pollutant transport due to the disturbance on the flow driven material convection.
Large difference in water level between the Yangtze River and Poyang Lake could also hinder the daily movement and migration of aquatic animals in the region. Serious degradation of water quality and outburst of algae may result in eutrophication, disappearance of wetlands and damage to wildlife habitat for endangered birds, fish and mammals. Therefore, it is crucial that quantitative analysis is carried out to evaluate the impacts of these schemes. The potential environmental effects of the barrage project in Poyang Lake have rarely been explored via numerical modelling. In this study, we aim to utilise a hybrid multi-dimensional hydrodynamic and water quality model to investigate the impact of the various operation where (x, y, z) represent the Cartesian coordinates and (u, v, w) are the three corresponding velocity components, t is time, g is the gravitational acceleration, η is the free surface elevation, d is the still water depth, S is the discharge magnitude of the point sources, f ¼ 2ωsinφ is the Coriolis parameter (in which ω is the Earth's angular rate of revolution and φ is the geographic latitude), ρ is the water density, ρ 0 is the reference density of water, p a is the atmospheric pressure and (u s , v s ) are the discharge velocities of inflow water, ν t is the vertical turbulent viscosity. (F u , F v ) are the horizontal stress terms relating to the horizontal eddy viscosity. The total water depth, h ¼ η þ d, can be obtained from the kinematic boundary condition at the surface once the velocity field is known from the momentum and continuity equations. The surface and bottom boundary condition for u, v and w are: where (τ sx , τ sy ) are the x and y components of the surface wind stress, (τ bx , τ by ) are the x and y components of the bottom friction stresses.
The 3-D governing equations for the transports of heat (T ), salinity (s) and scalar quantity of concentration (c) are based on the general advection-diffusion-reaction equations (DHI b) as follows.
@T @t þ @uT @x þ @vT @y þ @wT @z where D v is the vertical turbulent diffusion coefficient, Ĥis a source term due to atmospheric heat exchange, (T s , s s , c s ) are the temperature, salinity and pollutant concentration of the source inflow, respectively, k p is the first-order decay rate of the scalar quantity, (F T , F s , F c ) are the horizontal diffusion terms for the temperature, salinity and concentration respectively, which are functions of horizontal diffusion coefficients and eddy viscosity.
The finite volume method is used to discretize the sol- where A is the wetted cross sectional area of river channel, t is time, Q is the flow discharge, x is the distance of river channel, q is the lateral discharge per unit channel length, g is the gravitational acceleration, Z is the water surface elevation above datum, s e is the slope due to local head loss and s f is the friction slope, approximated by s f ¼ n 2

Q|Q|/A(A/B) 4/3 in which n is the Manning coefficient and
B is the wetted cross-sectional width, and L is the momen- where c is the concentration of a water quality state variable, D x is the longitudinal dispersion coefficient, S c is the external bacterial decay term that equals to cK d A, with K d being the decay factor, and S k is the kinetic source term due to chemical reaction. The general mass balance equation is then solved for each state variable (Ambrose et al. ).
In multi-layered 3-D water quality models, the simulation is performed using the ECO Lab module as part of the software package MIKE21/3 Flow Model FM. The ECO Lab module is a numerical unit designed specifically for ecological and water quality modelling. It is used as a platform for customizing models that describe water quality, eutrophication, heavy metals and ecology in aquatic ecosystems using process-oriented formulations. The biochemical processes involved in this research include the dissolved oxygen process, the nitrogen cycle and the phosphorus cycle.

1-D Numerical models
In the 1-D model, the study area covers the majority of the middle and lower reach of Yangtze River from Yichang to Datong, together with Dongting Lake and Poyang Lake, as shown by the grey region in Figure
in which N is the number of observed points,T i is the model- were computed and compared with the measured data.

RESULTS AND DISCUSSIONS Proposed barrage scenarios
The proposal to construct the Poyang Lake barrage was first     (2) storage period, (3) recharging period, (4) supply period, and (5) outflow period. The numerical model developed in this study is applied to examine the impact of different barrage schemes on the Poyang Lake's hydro-environment. The operational patterns of the schemes in Table 1   The magnitude of the water level differences between the schemes, however, depends on locations. The difference between water levels generated by different schemes in the winter season at Kangshan is smaller than those in Wucheng and Xingzi. Kangshan is at a southern location further away from the barrage and with a higher elevation than the other two monitoring points, so the barrage's influence decreases with the distance. In contrast, locations in the northern and eastern regions will be influenced by the barrage in a greater degree, as at Xingzi and Wucheng.
The rise in water levels due to barrage control will lead to the increase of water volumes and the expansion of water  Table 2 below.
As expected, S5 gives the largest increase in both the water surface area and volume among all the schemes. S4 has the smallest impact on the surface areas and volume.
In general, the influence of barrage on the water volume is relatively small with the biggest impact coming from S5 at 14.5 × 10 8 m 3 , which is still less than 20% of the lake capacity. The increase in volume in S4 is only at 2.1 × 10 8 m 3 , which is less than 3% of the lake capacity. Because the Poyang Lake is broad but shallow, the barrage has led to the significant increase in the surface areas through the rise of water levels.
The increase in water levels under the barrage control can bring subsequent environment concerns. Reduction in water level fluctuation in the lake will diminish the flow exchange between the Yangtze River and Poyang Lake, hence decreases the water flow rate and velocities. Lower water turnover rate could result in longer retention time and may cause eutrophication. Moreover, rise in water level will inundate lake regions that potentially include wetland areas and wildlife habitats, in which temporary dry condition and sun radiation are essential. Further investigation is needed on the impact of the barrage scheme on these aspects.
The effect of the barrage schemes on the Poyang Lake water environment was also analysed. Detailed simulation temporally and spatially across the Poyang Lake are shown in the form of subplots for the water quality indicators DO, NH 3 -N and PO 4 -P. Given the shallow nature of Poyang Lake with water depth generally below 10.0 m, temperature stratification is expected to be very weak even after the construction of the barrage. Temperature is therefore excluded from the detailed discussions due to its small variation. In the submaps, the colour spectrum ranges from purple (low concentration) to red (high concentration) with white region representing dry out region where water level is low and the flow is negligible. Most of the white areas belong to the shallow sand shoals at relatively high elevations.

Barrage impact on DO concentration of Poyang Lake
In Figures 11 and 12, the DO concentrations across the whole of Poyang Lake domain are present. Figure 11 represents DO concentrations in its natural state in 2007.   Under the influence of the barrage, the NH 3 -N concentrations in S1 are reduced in the dry season due to the increase in water levels in this period. As shown in Figure 14, the very high NH 3 -N concentrations along the narrow water causeways have disappeared. Boundary inflows with high NH 3 -N concentrations can still be seen in the east and northwest regions (Figure 14(b)), however these influxes are soon diluted once they enter the lake area. With high water levels maintained throughout the year, the dilution effect is apparent throughout the flooding period (Figure 14(d) and 14(f)), prior the winter months when the concentration slightly rises again. The increased water level and water volume in Poyang Lake thus enhance the dilution of NH 3 -N and hence result in a reduction in the NH 3 -N concentration.

Barrage impact on PO 4 -P concentration of Poyang Lake
The spatial distributions of the PO 4 -P concentration in Poyang Lake are shown in Figures 15 and 16. In Figure 15, no barrage is included and the PO 4 -P distribution shown is at its natural state in the Poyang Lake environment. In the early months of the year, the PO 4 -P concentration is high along the main channels, as the low water levels restrict the spread of the input pollutants out of the channels. Concentrations in the range of 0.015-0.075 mg/L can be observed (Figure 15(a) and 15(b)). When the water levels rise in April, the PO 4 -P concentration spreads out to a larger region as indicated by the increase in blue color in Figure 15(c). The influx of PO 4 -P from boundaries remains high in June at 0.060-0.065 mg/L as seen in Figure 15(d).
As the flooding season arrives, the increase in water volume dilutes the PO 4 -P concentration. Although inflows with high PO 4 -P concentration can still be observed in Figure 15(e) and 15(f), the overall concentration in the majority of the lake is below 0.005 mg/L. In the later months of the year, drop in water levels in the lake triggers the increase of PO 4 -P concentration along the lake's main channel, as seen in Figure 15(h).
The PO 4 -P distributions under the influence of S1 are shown in Figure 16. The increase in water levels has alleviated the PO 4 -P concentration in the middle of the lake domain during the dry season (Figure 16(b)). As the water levels rise further due to flooding, the PO 4 -P distribution becomes similar to that in S0, including the outburst of PO 4 -P concentration in June (Figure 16(d)), which is due to the inflow of a few highly concentrated pollutant streams from Raohe and Xinjiang in the east. The dilution effect on

CONCLUSIONS
The aim of this study is to develop a numerical model that can simulate the hydro-environmental evolution in the Poyang Lake, which has complicated geometry and experiences large water level fluctuations. The model has been used to assess the hydraulic and water-quality changes as a result of the different operations of a proposed barrage project, which is designed to control the lake water level and water exchange between the Lake and Yangtze River.
The combined 1-D and multi-layered 3-D hydro-environmental models have been successfully calibrated and validated.
In terms of hydrodynamic impact of the barrage, the schemes can effectively raise water levels in the lake in winter months, although the water level rise becomes insignificant in the southern part of the lake, which has a large distance from the barrage. A drawback of the water level increase is the inundation of wetlands and wildlife habitats, as well as the reduction of water exchange inside the lake and thus the local accumulation of contaminants in the long term. In terms of the impact on water quality, all five operation schemes are able to reduce the nutrient concentrations in Poyang Lake during the dry season because of the enhanced dilution effect associated with the increased water level and water volume. The concentrations of both NH 3 -N and PO 4 -P are below the present values with no barrage in place. The different degrees of reduction depend on the designed water level increments in different schemes.
The greater the increase in water levels, the lower the nutrient concentrations. This is a welcoming change, as the barrage can mitigate the pollution issues in the Poyang Lake during dry seasons. However, the rise in water levels also triggers a reduction in DO concentration due to a lower level of water exchange and the reduced re-aeration rate as a result of the reduced flow speed. The oxygen depletion may be hazardous to aquatic wildlife and ecosystem. Schemes 1 and 5 considered in the paper give the largest increase in water levels and thus the greatest impact on the lake hydro-environment.
Apart from the positive and negative impacts of the barrage on the Poyang Lake environment, further investigations will be carried out concerning the barrage influence on downstream regions along the Yangtze River. It should also be noted that our present study does not consider sediment transport, which can be justified as the sediment concentration is low in the lake and the simulation time is relatively short. For long-term predictions, the influence of the sediment transport and other broad climate, hydrological and environmental changes shall also be considered.
The barrage should incorporate a flexible design so that its operation can be easily altered to adapt to the new situation in the future.