Effect of Sediment Gradation on Clear Water Scour

A comparison of the published experimental data on scour depths at bridge piers shows a large range of values at what appears to be identical conditions. It was therefore decided to investigate whether or not this variation could arise from different grain-size distributions of the bed material used. The experiments were carried out in a reecirculating flume 38 m long, 1.5 m wide, and 1.2 m deep, using a 100 mm-diam cylindrical pier. In the experiments, the d₅₀ grain size and the grain-size distribution were systematically varied. Trial runs showed that for a given shear velocity, depth of flow greater than about two pier diameters had no observable effect on the depth of scour, and thereafter all tests were run at a flow depth of 0.60m, or six pier diameters. The velocity of the approach flow and the shear velocities were determined from the slope of the velocity distribution. Details of the study have been reported by Ettema together with a detailed evaluation of the development of the scour hole.