Local Scour Around Bridge Piers

Local scour caused by the horseshoe vortex system which forms at the base of the piers is considered. Piers which induce a pressure field strong enough to cause the formulation of the horseshoe vortex system are termed blunt nosed; all others being classed as sharp-nosed piers. The condition of the sediment transported into and out of the scour hole forms the basis for the further classification of the scour process into clear-water scour and scour with continuous sediment motion. The pier Reynolds number is shown to be an important variable describing the strength of the horseshoe vortex system. Further, the equilibrium scour depth depends on the initial sediment transport condition for a steady uniform flow with fully developed bed material transport. Design criteria are proposed for blunt-nosed piers under the conditions of clear-water scour and scour with continuous sediment motion. Several deviations from the above ideal design conditions are discussed; methods for design in these cases are suggested.