The temporal scour progress downstream of low-head stilling basins under unsteady flow

This paper deals with the temporal evolution of local scour downstream of low-head stilling basins. In comparison to previous companion papers the most real conditions of unsteady approach flow are considered here. In this aim, experiments were carried out in a 1 m wide and 20 m long rectangular channel. Free overfall jets plunging into a horizontal apron were caused by an ogee-crested spillway. A nearly-uniform sediment with median grain size d50=9 mm was used as mobile bed. The temporal effect of six different hydrographs on local scour depth was investigated under clear-water scour condition. Based on the data collected and the previous results for steady flow conditions a predictive model is proposed to simulate the temporal progress of the scour depth along the channel axis as function of tailwater densimetric Froude number, relative submergence, and dimensionless time.