Evaluation of effectiveness of spillways using computational fluid dynamics.

Abstract

Climate change remains one of the key contributing factors to the increased inter annual extreme rainfall, which may produce conditions outside the current design criteria for hydropower dams. Hydropower plants are designed with spillways to safely carry away excess water from the reservoir when the water levels exceed the desired levels, and partly because of flooding. The study focused on the CFD evaluation of the effectiveness and performance of the flap spillway gates in controlling water levels in the reservoir. ANSYS Fluent was used. It uses the Reynolds Averaged Navier-Stokes equations with VOF turbulence model to solve flow over an open channel. Turbulent kinetic energy, momentum equation, and turbulent dissipation rate were discretized using second-orderr upwind scheme while volume fraction and pressure using PRESTO and compressive schemes respectively. SIMPLE algorithm was used for pressurevelocity coupling. A generalized rectangular weir equation was used to determine the maximum discharge through the inflatable rubber spillway gates (02) and the flap spillway gates (03). The results obtained were 1,088.56 m3/s and 248.28 m3/s respectively. Using Unsteady Reynolds Averaged Navier Stokes model based on Shallow Water Equations (SWEs), a numerical model for controlling water levels in the reservoir was developed. CFD simulation of the reservoir was performed to evaluate the capability of the flap gates to discharge excess water when fully open. Three flood Scenarios involving operational and non-operational turbines were considered at design inflow of 94.15 m3/s and 698.89 m3/s when the initial water level in the reservoir was 5.2 m. CFD and mathematical model results were compared and a good agreement was found with a maximum variation of ±3.10%. At a flood load of 698.89 m3/s, the reservoir fills up in less than 3.5s while at a design load of 94.15 m3/s, the reservoir level drops to the bottom level of the three gates in 25s. The study demonstrates successful application of CFD model in validating reservoirs and spillway gates designs. It gives hydraulic engineers and CFD modelers more information on integrating CFD and mathematical models into design process of hydraulic structures. The study proposes modification of the flap gates to increase their overall capacity to match the maximum flood load to prevent overtopping of the reservoir. It also proposes a study of the discharge over dynamic flap gates considering Achwa HPP I dam break upstream of Achwa HPP II.