The Effect of Temperature Differentials on the Hydrodynamics of Sediment Retention Ponds

Abstract

This thesis describes an experimental investigation on the effects of temperature differentials on the performance of a laboratory-scaled sediment retention pond (SRP). The thesis aims to evaluate the effects of temperature as an environmental parameter on the pond’s hydraulics, as the temperature aspect is not currently among the guidelines for designing an SRP. The experiments were conducted using a geometric scale of 1:10 (model: prototype) of an SRP. Several temperature differentials were created by designing a state-of-the-art system comprising two separate recirculating systems. Each system included one heat pump, in which 11 kW was used to change the temperature of water in the pond, while 21 kW was used to change the temperature of water in the inflow tank. To investigate the temperature effects on the hydrodynamics of the model pond, tracer experiments were conducted, and the results were compared to the different temperature configurations. The hydraulic residence time of the inflow particles was also analysed by producing the residence time distribution (RTD) curves resulting from the tracer experiments. The RTD curves were used to indicate the overall performance of the pond as several hydraulic parameters were inferred from them, such as short-circuiting (SC) indices, hydraulic efficiency indices and mixing indices. Velocity field measurements were performed in the middle cross-section and longitudinal section of the pond to show the differences in velocity profiles on various temperature configurations. The velocity measurements were obtained using a three-dimensional velocimeter that showed dissimilarities in the RTD curves obtained from tracer studies. A computational fluid dynamic (CFD) model was used for numerical investigations to observe the flow pattern, density differentials and velocity profiles, which are critical when investigating temperature differentials. The results of the experiments showed that the hydraulic indices decreased when temperature differences existed between the inflow and water in the pond. When analysing the flow patterns, a higher tracer velocity in hot and cold influent test cases was observed compared to the isothermal test case, while velocity measurements showed the significant effects of buoyancy in changing velocity contours. The effect of inlet width was also tested, and the results showed that the widest inlet provided the best hydraulic performance when inflow was colder than the water in the pond. Conversely, the shortest inlet width successfully improved the performance of the pond in hot influent test cases. When analysing the effect of baffles, solid baffles in both of the tested flow rates and temperature configurations enabled the highest pond hydraulic efficiency as the solid baffles had significantly lower velocity magnitudes compared with others. Using medium-mesh led to the highest SC index value, as well as the highest mixing-index value. Numerical modelling showed that the differences between the temperature of water in the pond and the inflow were critical, regardless of the value of temperature in the pond or inflow. The study comprehensively investigated the effects of temperature differentials between the inflow and water in the pond on the performance of the pond. The results can provide guidelines for accounting for temperature as a useful parameter when designing an SRP. Through the analysis of temperature differences between influent and ambient water in the pond, more useful configurations and baffle types are identified. Moreover, the most appropriate inlet width is identified, which may help to improve the performance of ponds during the design phase.