Experimental study of flows over Block Ramps on stability of non-cohesive beds

Abstract

Block ramps are ecofriendly drop structures, which ensure more stability in the downstream riverbed. The scouring in the stilling basin bed material generated by the rapidly varied flow transition affects the stability of the ramp structure. In the present study, a modified stilling basin bed configuration is tested along with the uniform conventional stilling basin bed arrangements to check the stability of non-cohesive beds downstream to block ramps. Uniform non-cohesive horizontal bed configurations and bed with sill arrangement are tested for varying block ramp slopes (20% ≤ i ≥ 30%). The critical particle densimetric Froude number (Fc*) is selected as the criteria to predict the hydraulic conditions for incipient sediment motion in the uniform beds and it is compared with the established literature formulations to estimate the stability of the beds under macroroughness conditions. Scour volumes from each experimental run is quantified and the intensity of sediment motion is determined and related with the critical particle densimetric Froude number. The findings of the paper also describe the effect of Fc* on dimensionless scour depth. Spatial shift in the maximum scour depth formation from the toe of the block ramp for the tested flows over the bed sill arrangement has been observed. Experimental observations and comparison of results show the effect of sill gradation pronounced for non-uniform bed configuration, which effectively reduce the scour depth henceforth it stabilizes the upstream block ramp structure relative to the uniform beds selected.