Local scour due to free overfall jet

Abstract

A study was conducted to determine how the tailwater depth beneath a free-falling water jet affects the dimensions of local scour. Available data on scouring in uniform bed material due to circular and rectangular outflow jets were compiled and analysed. It was found that the scour hole geometry (i.e. maximum depth of scour, width of scour hole, length of scour hole and height of ridge formed downstream of the scour hole) is a function of the densimetric Froude number and the ratio of tailwater depth to drop height. The effect of tailwater on scour was investigated and limiting values for tailwater are introduced. It was observed that at low values of tailwater depth, increasing tailwater depth is associated with increasing scour hole dimensions, while at high values of tailwater depth the reverse trend occurs. For low tailwater conditions, the densimetric Froude number has little or no effect on the ridge height downstream of the scour hole, which is affected only by tailwater depth. New empirical equations for the prediction of scour hole geometry and ridge height are presented.