Experimental investigation of apron installation level on variation of local scour at the downstream of block ramps

Maintaining the level of rivers bed, especially in a reach where their’s slope increased due to many reasons specifically sand and gravel mining is an essential issue to the conservation of rivers. Grade control structures are eco-friendly structures that are common to stabilize river’s beds and banks. Control and reduction of scour hole at the downstream basin is an important issue in the prevention of undercut and failure. In the current research, the effect of installation level of the apron on the variation of scour depth at downstream of the block ramp with slopes of 1:3 and 1:5 was considered experimentally under a range of flow discharges, the block ramp's surface roughness size and configuration. The comparison of the results indicated that there is a direct relationship between the level of the apron level and the scour depth so that by reducing the level of the apron to 1/10 and 1/6 of the block ramp height, the maximum scour depth increases in an average 36 and 41 for slope of 1:3 and 32 and 35 for slope of 1:5, respectively. By increasing the size of surface roughness such that the overpass flow regime changes to nappe flow, the reduction of installation of the level of block ramp does not noticeable effect on the increase of the scour depth.