Effect of water cushion on dynamic pressures at impingement area

Abstract

A free-fall water jet released from a spillway may cause significant scour on the downstream river reaches due to high dynamic pressures. Energy dissipators are generally used at the downstream of a spillway to disperse the energy of the water jet. In case of low air concentration, the jet may hit the river bed with high dynamic pressure heads that can cause significant scour adjacent to the dam body. The scour can cause significant stability problems on the dam body. To prevent the phenomenon, water cushion can be a practical tool to reduce the jet impact at the impingement area. In this study, the effectiveness of the water cushion is inspected by measuring the dynamic pressure variations against different discharges and varying water cushion levels. The results of the experimental study show that dynamic pressure reduction is reversely proportional to the water cushion level until a certain discharge. Then, water cushion level changes become meaningless, especially for small discharges.