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This thesis is primarily focused on flow-field upstream of hydropower intakes, with emphasis on the use of temperature control curtains and predicting the flow acceleration zone. By reviewing the available literature, it is concluded that the flow-field upstream of hydropower intake systems can be modeled by potential flow theory. The understanding of near intake flow-field can be useful in fish entrainment studies and in designing fish repulsion systems. To control downstream river temperatures, a flexible curtain was installed upstream of several dams in California. Flow downstream of the curtain was analyzed using a Computational Fluid Dynamic (CFD) solver with rigorous validation by experimental data. The experiment was conducted with a 4 beam Acoustic Doppler Velocimeter (ADV) probe. The study shows that wall jet properties downstream of the curtain are affected by the water depth and the inlet Reynolds number. Empirical expressions were developed to predict jet properties and the wall shear stress. Flow upstream of the curtain was analyzed using potential flow theories with validation by the CFD solver. In this part, a theory based on Schwarz-Christoffel transformation was developed to predict the flow-field upstream of the curtain without accounting for any density stratification in the water body. It is observed that the acceleration zone upstream of the curtain can be affected by sink opening size, its location and water depth. The effect of boundaries on flow upstream of a line sink and the interaction of multiple sinks were analyzed. The effect of stratification on a line sink is also analyzed. A theory is developed to predict the incipient withdrawal condition when a sink is located on the horizontal bottom. The theory is also extended to a tilted bottom. The effect of boundaries on the incipient withdrawal condition is analyzed. When only one layer is being withdrawn, it is shown that a homogenous equation can be applied to a stratified condition by assuming an upper layer boundary at the interface. In addition to these works, a despike algorithm for ADV data is developed, and a numerical analysis on central difference scheme is presented. / Water Resources Engineering

[Truncated abstract] The process of selective withdrawal has, over many years, been used as an effective tool for extraction of water of particular quality from stratifed reservoirs. While the formation and steady-state theory of selective withdrawal in a stratifed fluid at rest has been extensively studied, little is known how vertical displacements of stratifcation due to long internal waves affect the water quality of the outflows. The first part of this study investigates the effect of basin-scale internal waves on the water quality parameters in Lake Burragorang, a large water supply reservoir for the city of Sydney, Australia. It is shown from field observations how the steady-state formulation of selective withdrawal can be used to predict the outflow water quality in reservoirs where internal waves are present, with a temperature prediction accuracy within 0.2 oC. . . In order to explain fluctuations in water quality parameters of the outflows, such as turbidity, it is important to know not only the stratifcation conditions in front to the offtake, but also to understand the dynamics of suspended particles in the upper reaches of the reservoir. In the third part of this study, transport and settling of suspended particles was investigated in the Wollondilly arm of Lake Burragorang by combination of direct and inverse methods. The inverse method was modifed to enable the separation of advective and diffusive transport of suspended particles from Stokes settling controlled by gravity, yielding twodimensional fields of particle velocities and settling fluxes in the upper reaches of the reservoir. These estimates are compared to the direct measurements of sedimentation fluxes made by the sediment traps and LISST-100.

In this doctoral thesis I analyzed the response of stratification in dimictic waters to climate change and management stratigies, by using one, two and three dimensional hydrodynamic models.:List of Tables v List of Figures vi List of Abbreviations x 1. Introduction 1 1.1. Thermal stratification 2 1.2. Recent studies on stratification and current research gaps 3 1.3. Main research questions and hypotheses 6 1.3.1. Assessing vertical diffusion in a stratified lake using a 3D hydrodynamic model 6 1.3.2. Stratification dynamics in Rappbode Reservoir under different wind conditions 7 1.3.3. Variable withdrawal elevations as a management tool to counter the effects of climate warming in Rappbode Reservoir 7 1.3.4. Ensemble warming projections in Rappbode Reservoir and potential adaptation strategies 8 2. Assessing vertical diffusion in a stratified lake using a 3D hydrodynamic model 9 2.1. Abstract 9 2.2. Introduction 9 2.3. Materials and Methods 11 2.3.1. Study site 11 2.3.2. Numerical model 12 2.3.3. Model setup and parameters 15 2.3.4. Calculation of vertical turbulent diffusion coefficient Kz 16 2.3.5. Model performance and uncertainty analysis 17 2.3.6. Scenarios 17 2.4. Results 18 2.4.1. Model performance and validity 18 2.4.2. Mixing conditions in the metalimnion 22 2.4.3. Effects from changing temperature and wind conditions 23 2.5. Discussion 25 2.5.1. Evaluation of the modelling approach 25 2.5.2. Opportunities and limits of the 3D hydrodynamic model 26 2.5.3. Limnological processes and lake ecosystem dynamics 27 2.6. Conclusion 28 3. Stratification dynamics in a reservoir under different wind conditions 30 3.1. Abstract 30 3.2. Introduction 30 3.3. Methods 32 3.3.1. Study site 32 3.3.2. Numerical model 33 3.3.3. Model setup and input data 33 3.3.4. Model calibration 35 3.3.5. Wind scenarios 35 3.3.6. Evaluation of simulations 37 3.4. Results 38 3.4.1. Calibration results 38 3.4.2. Statistical properties of wind velocity at Rappbode Reservoir 39 3.4.3. Scenario S1: Stratification phenology in 2015 at different wind conditions 40 3.4.4. Scenario S2: Sensitivity to wind speed under averaged meteorological conditions 41 3.4.5. Scenario S3: Effects of short-term wind events on stratification 42 3.4.6. Scenario S4: Effects of short-term wind events on stratification (seasonally varying) 45 3.5. Discussion 46 4. Variable withdrawal elevations as a management tool to counter the effects of climate warming in Germany’s largest drinking water reservoir 49 4.1. Abstract 49 4.2. Introduction 49 4.3. Methods 51 4.3.1. Study site 51 4.3.2. Numerical models 52 4.3.3. Model setup and input data 54 4.3.4. Model calibration 56 4.3.5. Scenarios 57 4.4. Results 57 4.4.1. Model calibration 57 4.4.2. Scenario S1: Influencing ice dynamics by withdrawal regime 60 4.4.3. Scenario S2: Mixing regimes under different withdrawal and warming regimes 61 4.5. Discussion 63 4.6. Conclusions 67 5. Ensemble warming projections in Germany's largest drinking water reservoir and potential adaptation strategies 68 5.1. Abstract 68 5.2. Introduction 68 5.3. Methods 70 5.3.1. Study site 70 5.3.2. Hydrodynamic model 71 5.3.3. Model setup and calibration 72 5.3.4. Response of thermal structure to climate change 74 5.3.5. Alternative management scenarios using different withdrawal strategies 75 5.3.6. Thermal indices and statistics 75 5.4. Results 76 5.4.1. Model calibration 76 5.4.2. Climate projections 78 5.4.3. Response of thermal structure to climate change 79 5.4.4. Interaction between climate warming and alternative withdrawal scenarios 83 5.5. Discussion and conclusions 85 6. General discussion 88 6.1. Evaluating performance of the applied models 88 6.2. The response of thermal dynamics and vertical diffusion to climate change 89 6.2.1. The response of thermal structure and vertical exchange to variations in wind speed 89 6.2.2. The response of winter stratification to increase in air temperature 90 6.2.3. The response of thermal dynamics to different future climate projections 92 6.3. The effect of selective withdrawal strategy on modifying the thermal structure in Rappbode Reservoir under climate change 93 6.4. Additional perspectives 95 6.4.1. Modelling in connective systems 95 6.4.2. Future changes of vertical diffusion 95 6.4.3. Near-term forecast 96 7. References 97 References to own articles used in this thesis 110 Other papers accepted or under review during the PhD study 111 Appendix 2 112 Appendix 3 114 Appendix 4 120