Characterization of horseshoe vortex in a developing scour hole at a cylindrical bridge pier

Since local scour at bridge piers in rivers and estuaries is a major cause of bridge failure,estimation of the maximum local scour depth is of great importance to hydraulic and coastal engineers.Although numerous studies that focus on scour-depth prediction have been done and published,understanding of the flow and turbulence characteristics of the horseshoe vortex that drives the scour mechanism in a developing scour hole still is immature.This study aims to quantify the detailed turbulent flow field in a developing clear-water scour hole at a circular pier using Particle Image Velocimetry(PIV).The distributions of velocity fields,turbulence intensities,and Reynolds shear stresses of the horseshoe vortex that form in front of the pier at different scour stages(t = 0,0.5,1,12,24,and 48 h)are presented in this paper.During scour development,the horseshoe vortex system was found to evolve from one initially small vortex to three vortices.The strength and size of the main vortex are found to increase with increasing scour depth.The regions of both the maximum turbulence intensity and Reynolds shear stress are found to form at a location upstream of the main vortex,where the large turbulent eddies have the highest possibility of occurrence.Results from this study not only provide new insight into the complex flow-sediment interaction at bridge piers,but also provide valuable experimental databases for advanced numerical simulations.