Experimental investigation of effect of the apron installation on local scour at the downstream of block ramp

River bed control is a critical issue that river engineers facing with it today. Block ramps are grade control structures, implementing as a countermeasure to overcome successive erosion in rivers bed, having suitable environmental conditions in terms of aquatic migration to upstream reaches, and also increasing the dissolved oxygen in the water. One of the important issues in designing and safetying these structures is to predict and reduce the magnitude of scour depth in the downstream pools. In the present study, the effect of apron installation on the reduction of scour in the downstream of the block rams under the smooth and rough surface of the structure was investigated, experimentally. The experiments were performed for slopes of 1:3 and 1:5, the ratio of the critical depth to the height of the structure 0.14-0.2, roughness elements height of 1.15-5.2 cm with different arrangements; as well as apron installation with a length of equal and half of the height of the structure at the hydraulic and physical hydraulic lab of University of Guilan in 2020 and 2021. The comparison of the results of all slopes of the structures showed that the apron installation is directly related to the reduction of the maximum scour depth, so that by installing an apron with length of equal to the structure height, the maximum scour depth for slopes 1:3 and 1:5 reduced by an average of 58 and 43%, respectively, compared to the smooth block ramp. By increasing the roughness height, the maximum scour depth over the range of minimum and maximum flow discharges and the structures slopes reduced by an average of 39 to 62 % compared to the corresponding smooth block ramp. A comparison of the results shows that for block ramp with the slope of 1:3, changing the roughness arrangement from staggered to compact in case of large roughness, had no effect on reduction of the maximum scour depth, but in the slope of 1:5, reduced the maximum scour depth in the range of 18 to 42 %.