Scour Evolution Around Bridge Piers Under Hydrographs with High Unsteadiness

Abstract

The temporal development of scour at both the side and the front of circular bridge piers is studied under clear-water conditions with artificially generated, linearly rising and falling, asymmetric triangular-shaped hydrographs categorized as highly unsteady with relatively short rising durations. The base flow conditions are kept well below the conditions for scour inception so as to investigate the hysteresis in initialization of the scour. A conceptual model consisting of an S-shaped time–depth relationship is generated, and actual and effective time of scour inception, duration and finalization parameters are defined. Both the effective and the actual time of scour inception decreased with increasing unsteadiness, either at the side or at the front nose of the piers; however, scour finalization does not depend on the hydrograph unsteadiness The effect of flow deceleration during the falling phase of the hydrograph on scouring was weaker than the effect of flow acceleration during the rising phase. A five-step procedure is proposed involving the calculation of (1) the densimetric Froude number corresponding to effective scour inception at piers side, (2) the final scour depth at piers side, (3) and at the front nose, (4) the exponent n and finally (5) the scour depth time evolution for both side and front nose of the piers. The method is verified both by experimental and literature data.