Three-Dimensional Simulation and Physical Modelling of Flows in Detention Tanks - Studies of Flow Pattern, Residence Time and Sedimentation

Doctoral thesis, 2004

Improving effluent water quality to receiving waters using stormwater ponds and storage tanks is acknowledged to be an effective method. The performance of the method depends however on flow conditions such as velocity distribution, short-circuiting currents and the volume through which the influent water flows, the so called effective volume. Enhancing performance, e.g. increasing the settling of suspended solids in stormwater ponds, demands better design and a deeper understanding of fluid dynamics. The overall objectives of the work reported in this thesis are to study the possibilities of using a generic Computational Fluid Dynamics (CFD) software and three-dimensional flow simulations as a design-engineering tool and to enhance knowledge of fluid dynamics in stormwater ponds and detention tanks. To be able to use three-dimensional simulations as a design tool, it is important that the simulations can predict the important flow features of ponds and tanks. Comparative studies were made using numerical and physical modelling in a large-scale model of a detention tank and a small-scale model of a storage tank. The flow pattern and residence time were analysed in the physical model of the detention tank using drogue tracking and impulse tracer tests. Sedimentation efficiency and distribution of sediment on the tank bottom were studied in the storage tank. Good agreement is found between measured and simulated flow patterns, residence time and particle removal for high inflow velocities. For low flow rates, difficulties were encountered in both the measurements and the simulations. Simulations show that an island placed in front of the inlet, so that the influent water hits the island perpendicularly, is most efficient for avoiding short-circuiting currents and extending the residence time in a rectangularly shaped pond. A subsurface berm placed in front of the inlet does not have the same impacts on short-circuiting. Moving the inlet so that only one circulation is created in the tank, extends the time needed for the initial tracer to reach the outlet, but the effective volume decreases. Denser influent water may change the flow pattern and decrease the effective volume. The suggested bed shear stress boundary condition for particle contact with the bottom is simple but gives reasonable results and can be used to estimate removal efficiencies and possible location of sediment.