Effects of Step Pool Porosity upon Flow Aeration and Energy Dissipation on Pooled Stepped Spillways

The hydraulics of stepped spillways with flat steps has been studied for the last three decades, including for embankment dam slopes, but studies of alternative stepped designs are limited. In this study, a pooled stepped spillway was investigated in a relatively large-size facility, and three different pool wall porosities were tested. The flow patterns, the macro- and microscopic air-water flow properties, and the energy dissipation performances were recorded; the results were compared with the flat stepped spillway design for the same chute slope ($\theta =26.6°$). The investigations highlighted a close agreement between air-water flow properties on the configurations in terms of void fraction, turbulence levels, bubble count rate, and chord sizes. The interfacial velocity distributions showed larger interfacial velocity on the pooled step configurations of approximately 5–10% linked with a reduced flow depth. On the porous pooled stepped spillways, the interfacial velocities within the cavity highlighted the flow through the pores and the reduction in cavity recirculation. The porous pooled weir reduced the form drag, and the residual energy was approximately 1.5–2 times larger on the porous pooled stepped chute with $Po=31\%$ and approximately 1.3 times larger on the porous steps with $Po=5\%$ compared with the flat stepped chute. The flat step design appeared to be the most advantageous in terms of flow stability and energy dissipation performance.