Laboratory Study of Scour Downstream of Stepped-labyrinth Weirs

One of the important issues in rivers planning is the stabilization of erosive reaches. For this purpose, grade control structures that can be made of stone and masonry or concrete materials are used. Stepped weirs are among the grade control structures for modification of rivers bed slopes. Paying attention to the geometry of scour depth at the downstream of these structures is important in order to design structures safely and stably. In the present study, the effects of labyrinths with different geometries on the scour depth variation at downstream of the stepped weirs with slopes of 1:1, 1:2 and 1:3 were investigated experimentally. The experiments were performed for various conditions of discharge, geometry of labyrinths and tail water depths. Analyses of the results showed at the best case of labyrinth geometry for the 1:1 stepped slope, the maximum scour depth corresponding to the minimum and maximum tail water depths reduced 30.1% and 65%, respectively as compared to the flat step. By reducing the steps slopeto 1:2, considered geometries of labyrinth did not have significant effect on reduction of the maximum scour depth. Further reduction of the steps slope to 1:3 at the best case of geometry reduced the maximum scour depth corresponding to the minimum and maximum tail water depths, 25.2% and 24.5% respectively, as compared to the flat step.