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Investigation of the effects of Thermal Stratification on Hydrodynamics of a Reservoir

Final Activity Report Summary - RESTRAT (Investigation of the effects of Thermal Stratification on Hydrodynamics of a Reservoir)

In a reservoir, the wind induced currents and the structure of the thermocline mainly control the vertical distribution of heat, dissolved substances and nutrients in the water column. Understanding the lake hydrodynamics is important for the management of water resources and thermal stratification is of great importance for the pattern of mixing in lakes and reservoirs.

Stratification during the summer acts as a barrier restraining the mixing of the water column. The warm water in the epilimnion is unable to drive through the cold, dense water of the hypolimnion. As a result of the incomplete mixing of the water column and lack of light for the photosynthesis at the hypolimnion, water column can become anoxic.

This study was motivated by the degradation of water quality in summer due to thermal stratification as observed in many reservoirs and lakes around the world. The objective of this study is to investigate the structure of the thermal stratification, its relation to wind and flow conditions, and its effect on water quality parameters including dissolved oxygen and suspended sediment concentrations. Within the scope of this study, monthly measurements were conducted and the impact of these factors on stratification and on water quality parameters was investigated.

It was observed that the water quality parameters are correlated to temperature profiles in the vertical except at the thermocline. Thermocline behaved as a barrier for dissolved oxygen, which dropped well below the standard limit of 5 mg/l at the thermocline leading to the development of anoxia. Widely used nondimensional parameters in the literature were not sufficient to assess the behaviour of DO in the water column, and variations in the air temperature and humidity must be included in the analysis. Turbidity profiles were highly affected with stratification where suspended sediment concentrations increased at the thermocline. Other parameters leading to changes in concentrations of dissolved oxygen and sediment concentration were investigated through statistical analysis of observed data.

Multivariate analysis investigated the impact of the different variables including air temperature, wind speed, wind direction, lagged wind speed on each particular water quality parameter. Wind direction and evaporation were found non influential on water quality parameters. Although rainfall data were excluded in the statistical analysis, due to lack of rainfall, it is recommended to include rainfall data in the models predicting turbidity. Results of the statistical analysis showed that air temperature, lagged wind speed and humidity are the influential parameters affecting variations in water quality parameters.

The complexities of the hydrodynamic processes in a reservoir suggest the use of numerical modelling approaches to provide a description of circulation, mixing and density stratification. A three-dimensional hydrodynamic model; Environmental fluid dynamics code (EFDC) has been applied to the study site and results are compared to the observations.

The effects of selective withdrawal from different outlets located along the water intake structure on hydrodynamics of a stratified reservoir was investigated through numerical modelling and analytical analysis. Withdrawal of the water at the bottom outlet was found to be the most effective choice encouraging the mixing of the water column and thus reducing anoxia.

Among various water quality management techniques, artificial destratification is recommended to be implemented in the studied reservoir (Tahtali, Turkey) using small diameter diffuser ports creating high velocity water jets since this type of treatment would mix hypolimnetic and epilimnetic water and would prevent anoxia.

Impacts of possible climate change were also modelled numerically based on projections of global climate change models. The results indicated that the thermocline depths were lowered in the water column causing the deterioration of water quality. The results of this study can be used to guide the further investigations in the studied site.