The influence of plunge pool air entrainment on the presence of free air in rock joints

High-velocity plunging water jets, appearing at the downstream end of dam weirs and spillways, can create scour of the rock. The prediction of this scour is necessary to ensure the safety of the toe of the dam as well as the stability of its abutments. A physically based engineering model has been developed at the Laboratory of Hydraulic Constructions for evaluation of the ultimate scour depth. This model is based on experimental measurements of water pressures at plunge pool bottoms and inside underlying rock joints. The pressures inside the joints revealed to be of highly transient nature and governed by the presence of free air. The amount of free air is a function of the instantaneous pressure inside the joint and the instanta-neous air content in the plunge pool. This statement is based on an experimental and numerical determination of the governing “wave celerity-pressure” relationships. These relationships are defined by the ideal gas law and Henry’s law.