Velocity modification at the bridge abutments because scour

At the corner of the abutment were streamline concentration, sharp water level drop and rapid increase of velocity. Formula for calculation local velocity at the abutments is elaborated. Local velocity is depending on open flow parameters, backwater value, rate of contraction by bridge crossings and depth of scour. Relative local flow modification at the bridge abutment at plain river bed, relative local and critical velocities changes during scour at steady and unsteady flow, uniform bed and at bed layering is studied. Comparison of the local velocity with approach flow and mean velocity under the bridge opening was made. The local velocity V_lt is reducing and the critical velocity V0t is increasing because of the scour within the time step at steady or at unsteady flow. When relative local velocity βV_0t/V_lt becomes equal to one the scour stops. For unsteady flow local and critical velocities are different at any steps of hydrograph because of the increased flow discharge and the scour depth. Dependence of the relative local and critical velocities at steady and unsteady flow, development in time and because of the scour, impact layering of the river bed is presented in paper in Figures and compared with tests results. (paper)