Development And Application Of A Three Dimensional Hydrodynamic And Water Quality Lake Model

Lakes are resplendent jewels of the earth. The hydrological process and ecologicalsystem structure in lakes are undergoing deep change due to the dual influence of natureand human behaviors. Exploring evolution procedures and patterns of lakehydro-ecological system, as well as revealing interaction rules between air, water andsediment of the system, are important and necessary research topics. However, intensiveexchanges of substance and energy between atmosphere, hydrosphere and biosphere takeplace in the lake system, resulting in extremely complex mixture of physical, chemical andbiologic procedures. This poses a great challenge on researchers who try to simulate thelake system with numerical models, because a variety of methods and knowledgebelonging to diverse research fields like chemistry and biology have to be integrated intothe hydrodynamics framework, which is the basis for the simulation. In order to meet thischallenge, this paper is focused on the development and application of a three-dimensionalhydrodynamics and water quality lake model using unstructured grid and finite volumemethod (FVM), aiming to provide a general tool for modeling the flow field, transport,and eutrophication processes in lakes. The main contributions and innovations of thiswork are outlined below:(1) In order to simulate the complicated circulation and mixture schema of lakes, amathematic model consisting of continuity, momentum, temperature and transportationequations was established. The standard k-ε vertical turbulent model and Smagorinskyhorizontal turbulent model were used to close the model physically and mathematically.Two-dimensional vertically integrated equations were also deducted to satisfy requirementof numerical computation. In particular, this work provides a method for estimating heatflux on the free surface of lakes where the flux is determined based on the balance ofincoming solar radiation, net back radiation flux, sensible heat exchange, and evaporativeheat. This method was liveraged in the temperature model and was proved to be a simpleand practical way for simulating lake temperature since it minimized requirements ofmetrological data.(2) Based on principles of WASP, a3D water quality model was constructed. The model is able to simulate spatial and temporal variations of eight water quality variablesincluding dissolved oxygen, carbon biological oxygen demand, nitrogen andphytoplankton. Specially, the original WASP model was improved by providing methodto calculate nutrient fluxes from the bottom sediment layer, allowing for better simulationof interactions between water column and bottom sediment.(3) A3D finite volume method for lake hydrodynamics on unstructured grid wasdeveloped. The horizontal computational domain are represented using non-overlappingunstructured triangular meshes, while irregular bottom elevations are represented with astretched or sigma vertical coordinate. The model’s time integration employs aninternal-external mode splitting procedure to separate the internal shear mode from theexternal free surface gravity wave. The external mode solution uses a modifiedfourth-order Runge-Kutta time-stepping scheme to calculate the two-dimensional surfaceelevation field and the depth averaged velocities. The model’s internal momentumequation solution is solved numerically using a simple combined explicit and implicitscheme. Specially, the model uses Osher’s scheme to compute numerical flux across theboundary of triangular elements in horizontal direction and use TVD scheme to solvevertical advection.(4) The3D model was applied to the Great Lake Donghu area in Wuhan city, Chinato evaluate its effectiveness. To provide better boundary and condition data for drivingand validating the model, RS and GIS techniques were employed to extract water depth,temperature and water quality information from RS images including MODIS, LandsatTM and Landsat ETM+. Furthermore, GIS technique was fully leveraged to aidpre-and-post processing involved in the modeling task. With this methodology,hydrodynamics and temperature field of six major lakes (i.e., Lake Donghu, Lake Shahu,Lake Yangchunhu, Lake Beihu, Lake Yanxihu and Lake Yandonghu) located in the studyarea were carefully researched by considering different metrological conditions. Specialfocus was put on simulation of water quality in Lake Donghu. The modeling resultsshowed that the simulated currents, temperature and water quality variables in watercolumn of the six lakes agreed with the observed or estimated data well. The presentedmethodology that integrates GIS, RS into hydrodynamics modeling provides a reliable, easy-to-use and one-stop solution for modeling lake system.(5) A project aimed to reestablish hydraulic connectivity between lakes in the GreatLake Donghu area and the Yangtze River is being implemented in Wuhan at present. Withthe purpose of offering a scientific tool for investigating water quality restoration effectsfollowing alternative designs of the project and determining suitable operation strategies,the3D model was utilized once again. After proposing three water network connectivityschemes and operation solutions as well as an assessment indicator system used toevaluate and compare different solutions, the model was applied to analyze watermovement patterns and possible water quality changes responded to different solutions.Advantages and shortcomings of the solutions were compared under the same roof of theindicator system to determine the best one for restoration of the water quality in the lakes.