Scours Evolution at Bridge Abutments Under Unsteady Flow Events

Extreme floods become more frequent and is the reason rising load on structures in rivers and the risk to be damaged. The methods of scour calculation at bridge abutments are based on that design flood up to 1% probability (Arneson et al. 2012) accepted by many authors is constant during the maintenance period of the bridge and the equilibrium depth of scour often used for foundation design. Inspite of that equilibrium depth of scour sometimes can be overestimated, because in nature scour is formed under unsteady flow. The bridge abutments usually are constructed on the floodplain, where is no sediment movement, here we considering scour at clear water conditions and estimating scour evolution when sediments are removing out of scour hole. As a basis, the equation of the bed sediment movement was used and a semi-analytical model for computing depth of scour evolution under unsteady flow at the abutments is elaborated. The tests were made in flume for free surface flow conditions, for rigid and sand bed of the bridge crossing model with different contraction rate, depth, discharge, Froude number, steady and unsteady flow, with uniform or stratified bed. To study different parameters impact on scour at the abutment, More than 50 tests were made. Tests results are in good agreement with calculated data by model presented. In the paper is presented the relative local and critical velocities evolution, impact the bed material, flow contraction and unsteady flow on scour depth under clear water conditions at vertical wall abutment.