Time scale for local scour at bridge piers

The temporal development of clear-water local scour depth at cylindrical br idge piers in uniform sand beds is considered. New data are presented and u sed to quantify the influence of flow duration on the depth of local scour. An equilibrium time scale (t*) is defined. The data show that both t* and the equilibrium scour depth (d(se)) are subject to similar influences of fl ow and sediment parameters, as might be expected because they are inherentl y interdependent. A method is given for determination of the time for devel opment of d(se) for a given pier, sediment, and approach flow velocity and the concomitant estimation of the scour depth at any stage during developme nt of the equilibrium scour hole. The results show that the scour depth aft er 10% of the time to equilibrium is between about 50% and 80% of the equil ibrium scour depth, depending on the approach flow velocity.