Experimental study of local scour around a compound pile under steady current

Local scour around a compound pile in steady current is investigated experimentally under clear water condition. The compound pile comprises a circular pile with a circular foundation that is exposed to the water. Experiments are conducted for pile-to foundation diameter ratios d/D = 0.33, 0.5 and 0.71 and wide range of foundation height-to-diameter ratios (H/D) under a Shields parameter that is 0.92 times the critical Shields parameter for scour. Based on experimental data, a new empirical formula is developed for predicting the scour depth, which increases with the increase of H/D. Compared with previous formulae, this formula does not involve equivalent diameter or equivalent foundation height. When H/D is very small or zero, scour is initially very slow and accelerates when the horseshoe vortex becomes strong due to the increase in the exposed foundation height. Particle Image Velocity (PIV) measurement was conducted to measure horseshoe vortex on a smooth boundary. The scale and the shear stress of horseshoe vortex measured by PIV are found to vary with the foundation height following a similar trend as the scour depth of sand bed.