CUADRO DE USO DE SUELO ZONA U-V2' (USO VIVIENDA Nº 2 PRIMA)

Lake Taihu, located at one of the most economically developed areas in China, plays an important role of intense land and water use in the development of the region. Over the past 30 years, due to the increasing population and rapid economic growth in the basin, Lake Taihu has suffered dramatic environmental degradation. Hence, the numerical modelling of Lake Taihu including hydrodynamics modelling and ecological modelling has been carried out for several decades to illuminate the property of flow mechanisms, water quality, eutrophication and ecosystems of Lake Taihu.

2.4.1 Hydrodynamic modelling

Both two-dimensional and three-dimensional mathematical models have been employed to simulate hydrodynamic of Lake Taihu by researchers since 1980s. Because Lake Taihu is a typical shallow lake with huge areas, vertical physical processes such as seasonal stratification are less important compared to horizontal transport processes for many applications. Therefore, two-dimensional models are widely used to model horizontal water currents which are greatly driven by unstable wind forcing and lightly driven by inflow-outflow forcing.

Wang (1987) uses a two-dimensional finite difference model to simulate the wind-driven currents of Lake Taihu with constant wind conditions. Subsequently, Wu and Pu (1989) simulate the wind-induced currents and study the effect of wind on water levels in Lake Taihu by using an irregular-grid based model with an exact fit function for the lake shore and bottom topography. Thereafter, Wang et al. (1992) used a nested 2D hydrodynamic grid model to simulate wind-driven currents in Lake Taihu and accurately predicted flow direction. Liu (1993) simulated the currents in Meiliang Bay within a finite element method based model. Jiahu and Qun (1997) discuss application of the method of numerical inlay to set up an inlay-mesh model of the characteristics of East Taihu Lake's wind-driven currents in various wind regimes. Ma and Cai (1999) apply a two-dimensional upwinding finite element numerical model to investigate the distribution of total phosphorous in Lake Taihu. Xu and Liu (2009) shows that wind-induced currents in Taihu Lake are formed by the wind, the lake boundary and the landform of the lake bottom as well. There are obvious different laws of the current and hydrodynamics in different lake regions, and the reason caused the difference is analyzed with the fluid flow mechanism.

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The aforementioned studies were all based on the hypothesis that there is no vertical shear stress and vertical variation of lake currents is excluded. However, in some areas of Lake Taihu, strong shear and mixing occur in the vertical direction and environmental factors such as dissolved oxygen, chlorophyll, light intensity and turbidity have an obvious vertical gradient. Thus, three-dimensional hydrodynamic models are required to model these vertical processes and gradients.

Pang et al. (1994) simulate the wind driven currents and wind-induced water setup with the influence of local wind field of Lake Taihu. He pointed out that the long narrow path between Xishan Island and Dongshan Island was most important for the formulation of global circulation flow in Lake Taihu. Zhu and Cai (1998b) developed a 3D hydrodynamic model for current simulation in Meiliang Bay. The lake was divided into five layers, and the differential equation was solved by the frog leap scheme (Zhu & Cai, 1998a). The results showed the horizontal current distribution in different layers, with the surface current being consistent with the wind direction, and the bottom current opposite to that at the surface.

Although these models separate lake currents into several layers, it is difficult for them to capture the vertical structure of the lake currents, because there are more layers in deep areas compared to shallow areas when water level changes dramatically and layer currents in deep areas cannot be computed. To deal with this issue, Liang and Zhong (1994) introduced a model with vertical σ-coordinate into the numerical simulation of Lake Taihu. Hu et al. (1998b) develop a three-dimensional model with multiple σ-layers and simulated the influence of No.3 Typhoon in 11th, Oct, 1997 on water level in Lake Taihu and water flow in the lake. (Hu et al., 1998a) also analyzed the difference between the depth-mean current field and each individual depth current field and calculated the wind driven flow field with southeast, northeast and southwest wind. Li and Pang (2004) build a model with nested grids in horizontal dimension and σ-coordinate grid in vertical dimension to simulate the currents in Meiliang Bay and Gonghu Bay.

Change of relevant coefficients in three-dimensional models may greatly influence the accuracy of model outputs.The influence of the variation of vertical eddy viscosity coefficients on water depth, velocity profile and bed shear stresses are studied byWang et al. (2001)and Song et al. (2003). Zhou et al. (2009) studied the effect of several proposed formulae of the drag coefficient varying with the wind speed on the numerical simulations of wind-induced flow.

24 2.4.2 Water quality modelling

The study of water quality by simulating the physical, geochemical and biological processes associated with the eco-environmental issues within the lake is regards as a significant method to investigate the influence of eutrophication on local environment and ecological systems. Numerical modelling of water quality also could be applied to evaluate the efforts such as water transfer project have been made to control the process of eutrophication and water quality deterioration.

Many water quality models combine an advection-diffusion model with a lake hydrodynamic model to investigate transport of materials and concentration of objective chemical/biological variables such as total nitrogen (TN), total phosphorous (TP), suspended substance (SS), chemical oxygen demand (COD) and biochemical oxygen demand (BOD) in Lake Taihu (Ma and Cai, 1997; Zhu and Cai, 1998a; Hong and Pang, 2005; Pang et al., 2006; Li et al., 2011c). In addition to advection and diffusion, some models also consider the processes of sediments re-suspension and matter cycling (Pang et al., 2008; Li et al., 2009; Tang et al., 2011a; Weiping et al., 2011). Moreover, to study the changes in water quality and whole lake ecosystem, many ecological processes such as variation of aquatic animal, macrophytes, and phytoplankton are also required in some environmental applications. Thus, integrated water quality model like ecological models including more state variables are developed for lake environment control and research for examining the processes and determining the water quality (Rufeng, 1997; Li et al., 2004; Hu et al., 2006; Mao et al., 2008; Han and Hu, 2012).

An integrated water transfer project named “Yin Jiang Ji Tai”, which is regards as an emergency approach for pollutants dilution and lake water refreshment, has been utilized since the year 2002. This project introduce freshwater from Yangtze River into Lake Taihu to improve the water quality of the lake. The effects of “Yin Jiang Ji Tai” on water quality in Lake Taihu have been studied by many researchers. Hu et al. (2008) analyzed the environmental effect of water transfer in 2002 and 2003 by using both the observed data and estimated nutrient concentration with the elimination of effect from natural factors to evaluate the variations of space and time factors of water quality improvement induced by the two transfers. Zhai et al. (2010) use both ecological indicators and affiliated indicators to evaluated the water quality in Lake Taihu after water transfers from 2002 to 2007 with respect to the reference situation of years from 1998 to 2001 when the project of 'Yin Jiang Ji Tai' was not conducted. The conclusion showed the water transfers altered the ecosystem status and had positive effects on Lake Taihu and most of its sub-zones. Li et al. (2011c) used EFDC model to calculate water age and explored the time

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required for the water flowing from Wangyu River into the lake to exchange. Later, Li et al. (2013a) test the effect of two pump stations set around Meiliang Bay using water age as indication and he also evaluate the effects of different transfer routes on water quality by using numerical simulation (Li et al., 2013b). Ma et al. (2015) investigated the temporal and spatial distributions of water quality changes throughout the Lixia River watershed, by evaluating the effects of experimental water transfers from the Yangtze River on major water quality parameters. Notable improvements in water quality are found in the Lixia River watershed and the magnitude of water quality improvement was dependent on the quality of transferred water and associated flow rates, as well as subzones and fluctuations in transfer duration.