Effect of pool confinement on pressures around a block impacted by plunging aerated jets

The erosion caused by jets issued from hydraulic structures progressively develops a confined scour-hole on the riverbed. A realistic scour assessment must consider both the influence of the air entrained when the jet plunges into the pool and the flow patterns induced by bottom geometry. This experimental study systematically analyzes the combined influence of jet aeration and pool confinement on the dynamic pressures affecting the water-rock interface and inside 3D fissures around a block. The results show that confinement reduces mean pressures and pressure fluctuations when the pool is relatively deep, but almost no influence is found when the pool is shallow, while air entrainment has an opposite effect. Three mechanisms are identified, two of them depend on the pool depth. Furthermore, when a block is mobile, pressures are attenuated inside the surrounding joints. The consequent block vibrations and the presence of air reduce pressure waves celerity inside the fissures.