Numerical Investigation of Different Stepped Spillway Geometries over a Mild Slope for Safe Operation Using Multi-Phase Model

Abstract

The appropriate design and operation of spillways are critical for dam safety. To enhance design practices and gain insights into flow hydraulics, both experimental and numerical modeling are commonly employed. In this study, we conducted a numerical investigation of flow over a mildly sloping (1V:3H) stepped spillway with various step geometries using a multi-phase mixture model with dispersed interface tracking in ANSYS Fluent. The model was validated against experimental data from Utah State University, focusing on water surface profiles over the crest, velocities, and air concentrations. The validated numerical model was used to simulate flow over different step geometries (i.e., 0.2 m H uniform Step, 0.1 m H uniform step, non-uniform steps, adverse slope steps, and stepped pool) for a range of discharges from 0.285 m3/s/m to 1.265 m3/s/m. While flow depths over the crest and velocities in the chute compared well with experimental results, air concentrations exhibited some deviation, indicating numerical limitations of the solver. The shift in the location of the inception point was found to be mainly influenced by a higher flow rate than the different design configurations over an identical mild slope. The downstream non-linear flow velocity curve with different flow rates indicated less effectiveness of the step roughness over a high flow rate as a result of the reduction in relative roughness. The theoretical velocity ratio indicated the least reduction in downstream velocity with the stepped pooled spillway due to the formation of a “stagnant pool”. A higher negative-pressure region due to flow separation at the vertical face of the steps was obtained by adverse slope steps, which shows that the risk of cavitation is higher over the adverse slope step spillway. Turbulent kinetic energy (TKE) was found to be higher for uniform 0.2 m H steps due to the strong mixing of flow over the steps. The least TKE was found at the steps of the stepped pool spillway due to the formation of a “stagnant pool”. Uniform 0.2 m H steps achieved the maximum energy dissipation efficiency, whereas the stepped pool spillway obtained the least energy dissipation efficiency, introducing higher flow velocity at the stilling basin with a higher residual head. The adverse slope and non-uniform steps were found to be more effective than the uniform 0.1 m H steps and stepped pool spillway. The application of uniform steps of higher drop height and length could achieve higher TKE over the steps, reducing the directional flow velocity, which reduces the risk of potential damage.