Experimental Study of Submergence Ratio on Local Scour Around a Square Pile in Steady Flow

Scour around a submerged square pile was realized experimentally in a steady flow to study the effects of flow depth on local scour. Flow depth to pile height ratios ranging from 1.5 to 5 in uniform sand and 2 to 5 in non-uniform sand were tested in the approaching flow velocity to critical velocity (larger than which the sediment particle is motivated) ratios of 0.56 and 1.03, respectively. The influences of flow depth were investigated on the basis of analysis of the three-dimensional topography, temporal maximum scour depth, bed profile development, and equilibrium scour depth. Results showed that the maximum scour depth was at the upstream corners of the pile other than at the stagnation point. The evolutions of the maximum scour depth data in non-uniform sand were well fitted with a recent exponential function, which characterized the initial, developing, and equilibrium stages of scour depth. The scour hole slopes upstream of the pile were found to be parallel to each other in the process of each test and were mainly governed by the sediment repose underwater. The equilibrium scour depth varied slightly with flow depth when the submergence ratio was larger than 1 in uniform sand while it was 2 in non-uniform sand. The armoring effects of coarse sediment particles markedly reduced the sediment transport in non-uniform sand despite the 0.34 increment in non-uniformity.