Effect of jet air content on plunge pool scour

The effect of air discharge on plunge pool scour was investigated by using a simplified experimental configuration. Instead of considering the complete arrangement involving chute and deflector resulting in an air-water jet impinging on a sediment surface, the mixture flow was produced with a circular pipe for which the air concentration and the jet diameter close to impact on the free water surface are known. The results of this study were primarily directed to the definition of a three-phase Froude number that accounts for the combined effects of an air-water mixture jet on scour. The analysis of data allows simple estimates of the scour geometry including a generalized scour profile, the width of scour, and the temporal advance of the extreme scour depths. It was pointed out that for a certain water velocity and selected grain characteristics, the addition of air to the jet results in an increase of scour depth. However, if the reference would be the air-water mixture velocity, scour depth decreases significantly by the addition of air to the jet.