Local Scouring in Low and High Gradient Streams at Bed Sills

The main characteristics of local scouring downstream of bed sills, forming a staircase-like system in high-gradient streams with non-uniform alluvium, have been investigated through 13 clear-water laboratory runs. Three initial longitudinal slopes and different flow rates were considered, keeping the same distance between the baffles. The grain size distribution of the sediment is that of a real alpine torrent scaled to the model dimensions. The measured scour depth, length and shape are compared to previous results concerning low gradient and uniform sediment gradings. A dimensional analysis approach appears to remain valid; nevertheless some simplifications cannot be made, since the jet regime plays an important role both for the depth and the length of the scour, and consequently affects the scour shape. Two semi-empirical equations obtained joining the previous low-gradient with the new high-gradient data are proposed for the prediction of the maximum scour depth and length; the best correlation is shown to be achieved by adopting the D95, but the results are very little sensitive with regard to the grain size in the experimental range tested. A numerical example is also presented.