In this thesis we develop a model for the long-time horizontal circulation in a shallow lake. The goal is to have a model that can capture the large-scale features of the circulation yet can be run quickly and cheaply. We start with shallow water models and add relevant physical terms: Coriolis force, wind shear, bottom drag, viscosity, and nonhomogeneous boundary conditions. The resulting equations are similar to the two-dimensional Navier Stokes equations and can be analyzed with similar methods. We pose the equations in a weak form and show that they are well-posed. We then discretize the equations. We use the finite element method for the spatial discretization and show that our choice of elements satisfies stability criteria. Next we test our model. We first consider problems with analytically tractable behavior and verify that our model produces correct results. Then we model Lake Erie, both with no wind and with a steady wind. We compare the results of our model to experimentally obtained measurements of the currents. Our results compare well under conditions of no wind or of steady wind, but not as well when the wind is variable.

Bibliography: leaves 231-240 / vi, 240 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1986

Bibliography: leaves 169-178.

Bibliography: leaves 169-178 / iv, 201 leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, 1989

Modal analysis for strongly stratified lakes was extended to obtain a better understanding of the dynamics of the basin-scale motions. By viewing the basin-scale motions as a superposition of modes, that have distinct periods and three-dimensional structures, the method provides a conceptual understanding for the excitation, evolution, and damping of the basin-scale motions. Once the motion has been decomposed into modes, their evolution and energetics may be extracted from hydrodynamic simulation results and field data. The method was applied to Lake Biwa, Japan, and Lake Kinneret, Israel, and used for a theoretical study. The real lake applications showed that winds excited basin-scale motions that had a surface layer velocity structure similar to the wind stress pattern. Three-dimensional hydrodynamics simulations of Lake Biwa indicated that most of the energy input from winds was partitioned into the internal waves that decayed within a few days. The gyres, on the other hand, received much less energy but dominated the dynamics during calm periods due to their slow damping. Analyses of field data from Lake Kinneret suggested that the internal waves, excited by the strong winds every afternoon, were damped over a few days primarily due to bottom friction. Theoretical investigations of damping mechanisms of internal waves revealed that bottom friction induced a velocity anomaly at the top of the boundary layer that drained energy from the nearly inviscid interior by a combination of internal wave cancelling and spin-down. These results indicate that gyres induce long-term horizontal transport near the surface and internal waves transfer energy from winds to near-bottom mixing. Modal structure of dominant basin-scale internal waves can induce large heterogeneity of nearbottom mass transfer processes. The method presented here provides a tool to determine how basin-scale motions impact on biogeochemical processes in stratified lakes.

The onset of summer stratification in temperate lakes and reservoirs forces a decoupling of the hypolimnion from the epilimnion that is sustained by strong density gradients in the metalimnion. These strong gradients act as a barrier to the vertical transport of mass and scalars leading to bottom anoxia and subsequent nutrient release from the sediments. The stratification is intermittently overcome by turbulent mixing events that redistribute mass, heat, dissolved parameters and particulates in the vertical. The redistribution of ecological parameters then exerts some control over the ecological response of the lake. This dissertation is focused on the physics of deep vertical mixing that occurs beneath the well-mixed surface layer in stratified lakes and reservoirs. The overall aim is to improve the ability of numerical models to reproduce deep vertical mixing, thus providing better tools for water quality prediction and management. In the first part of this research the framework of a one-dimensional mixed-layer hydrodynamic model was used to construct a pseudo two-dimensional model that computes vertical fluxes generated by deep mixing processes. The parameterisations developed for the model were based on the relationship found between lake-wide vertical buoyancy flux and the first-order internal wave response of the lake to surface wind forcing. The ability of the model to reproduce the observed thermal structure in a range of lakes and reservoirs was greatly improved by incorporating an explicit turbulent benthic boundary layer routine. Although laterally-integrated models reproduce the net effect of turbulent mixing in a vertical sense, they fail to resolve the transient distribution of turbulent mixing events triggered by local flow properties defined at far smaller scales. Importantly, the distribution of events may promote tertiary motions and ecological niches. In the second part of the study a large body of microstructure data collected in Lake Kinneret, Israel, was used to show that the nature of turbulent mixing events varied considerably between the epilimnion, metalimnion, hypolimnion and benthic boundary layer, yet the turbulent scales of the events and the buoyancy flux they produced collapsed into functions of the local gradient Richardson number. It was found that the most intense events in the metalimnion were triggered by high-frequency waves generated near the surface that grew and imparted a strain on the metalimnion density field, which led to secondary instabilities with low gradient Richardson numbers. The microstructure observations suggest that the local gradient Richardson number could be used to parameterise vertical mixing in coarse-grid numerical models of lakes and reservoirs. However, any effort to incorporate such parameterisations becomes meaningless without measures to reduce numerical diffusion, which often dominates over parameterised physical mixing. As a third part of the research, an explicit filtering tool was developed to negate numerical diffusion in a threedimensional hydrodynamic model. The adaptive filter ensured that temperature gradients in the metalimnion remained within bounds of the measured values and so the computation preserved the spectrum of internal wave motions that trigger diapycnal mixing events in the deeper reaches of a lake. The results showed that the ratio of physical to numerical diffusion is dictated by the character of the dominant internal wave motions.

This thesis examines geomorphic processes that impact thermal stratification within floodplain lakes. Previously, thermal stratification was not thought prevalent in these numerous small water bodies, despite being found in small reservoirs. Thermal stratification leads to long term anoxia in the bottom waters and the release of metals and nutrients from the sediments. This can result in water quality problems such as algal blooms (including potentially toxic cyanobacteria), loss of fish habitat and deterioration of wetland habitat. Four floodplain lakes of the Macdonald River Valley NSW were profiled and tested over 3?? years to assess to what extent, under what conditions, and with what chemical and physical effects, they thermally stratified and destratified. The morphometry, bed facies, vegetation, hydrology and local weather patterns for each lake were also investigated. Physical and chemical analyses found elevated nutrients in all four lakes and higher concentrations of total phosphorous and total nitrogen in the bottom waters. These results were not dependant on stable thermal stratification and long-term anoxia in the bottom waters. Inflow samples had much higher concentrations of nutrients than the lake samples, so the catchment is the likely source. Samples were analysed for phytoplankton and significant populations of potentially toxic cyanobacteria were present. Stratification depended upon whether the lake was "sheltered" from wind stirring or from incident radiation, either topographically by steep slopes surrounding the lakes, or by submerged and emergent vegetation. Aquatic vegetation limits the amount of transfer of turbulent kinetic energy from wind shear and the amount of incident radiation received. At one lake, sheltered areas reached temperatures much higher than less sheltered areas. Topographic sheltering can make a difference of up to three degrees Celsius across the lake surface. Destratification depended on wind speed, wind direction and orientation of the lake to the dominant wind direction. An extension to the classification of fluvial lakes (from Timms 1992) is presented, with further subdivisions to describe the lakes as "sheltered" or "open" and the type of sheltering (vegetation or topography) present.

This thesis examines geomorphic processes that impact thermal stratification within floodplain lakes. Previously, thermal stratification was not thought prevalent in these numerous small water bodies, despite being found in small reservoirs. Thermal stratification leads to long term anoxia in the bottom waters and the release of metals and nutrients from the sediments. This can result in water quality problems such as algal blooms (including potentially toxic cyanobacteria), loss of fish habitat and deterioration of wetland habitat. Four floodplain lakes of the Macdonald River Valley NSW were profiled and tested over 3?? years to assess to what extent, under what conditions, and with what chemical and physical effects, they thermally stratified and destratified. The morphometry, bed facies, vegetation, hydrology and local weather patterns for each lake were also investigated. Physical and chemical analyses found elevated nutrients in all four lakes and higher concentrations of total phosphorous and total nitrogen in the bottom waters. These results were not dependant on stable thermal stratification and long-term anoxia in the bottom waters. Inflow samples had much higher concentrations of nutrients than the lake samples, so the catchment is the likely source. Samples were analysed for phytoplankton and significant populations of potentially toxic cyanobacteria were present. Stratification depended upon whether the lake was "sheltered" from wind stirring or from incident radiation, either topographically by steep slopes surrounding the lakes, or by submerged and emergent vegetation. Aquatic vegetation limits the amount of transfer of turbulent kinetic energy from wind shear and the amount of incident radiation received. At one lake, sheltered areas reached temperatures much higher than less sheltered areas. Topographic sheltering can make a difference of up to three degrees Celsius across the lake surface. Destratification depended on wind speed, wind direction and orientation of the lake to the dominant wind direction. An extension to the classification of fluvial lakes (from Timms 1992) is presented, with further subdivisions to describe the lakes as "sheltered" or "open" and the type of sheltering (vegetation or topography) present.

[Truncated abstract] The lake boundaries are an important source of sediment, nutrients and chemicals. For life inside the lake, the exchange between the lake boundaries (littoral) and lake interior (pelagic) is of central importance to Limnology as the net flux of nutrients into the water column is both the driving force and limiting factor for most algae blooms found during the stratification period. Consequently, the understanding of the relevant processes defining such an exchange is a further step toward a sound basis for future decisions by lake managers in order to ensure high water quality. The objective of this research was to investigate the physical processes responsible for the exchange of water and particles between the lake boundaries and the lake interior. An integrated approach using field experiments and 3D modelling as applied to Lake Kinneret (Israel) is presented. The field data revealed large-scale metalimnion oscillations with amplitudes up to 10 m in response to westerly diurnal winds, the existence of a well-defined suspended particle intrusion into the metalimnion of the lake, characterized by high concentrations of organic matter, and a well-mixed benthic boundary layer (BBL). The changes in the thermal structure explained the observed vertical and horizontal movements of the suspended particle intrusion. The horizontal advective transport via the metalimnion, associated with the velocities induced by the basin-scale mode-two Poincare wave, controlled the exchange between the lake boundaries and lake interior on daily time scales. The observed BBL over the lake slope varied markedly with time and space. Detailed comparison of simulation results with field data revealed that the model captured the lake hydrodynamics for time scales from hours to days. The model could then be used to extract the residual motions in the various regions of the lake. The residual motions below the surface layer were predominantly forced by the basin-scale internal wave motions, but the residual motion in the surface layer was found to be very sensitive to the curl of the wind field. The residual circulation was responsible for redistributing mass throughout the lake basin on time scales from days to weeks. A clear connection of dynamics of the BBL with the large-scale features of the flow was addressed. The time history of the mixing in the BBL and the resulting cross-shore flux was shown to vary with the phase of the basin-scale internal waves.