Experimental study on the scour due to a water jet subjected to lateral confinement

Abstract

The effect of lateral confinement due to very close side walls, on erosion caused by a water jet issuing from a rectangular sluice aperture, has been studied experimentally in a narrow laboratory flume. Velocimetry (PIV) in presence of a smooth, no erodible bottom, revealed self-similar velocity profiles with the structure of a wall jet, but upward-shifted with respect to the classical two-dimensional profile. The classical definition of the Shields number was modified, to include the influence of the relative roughness and the inlet aspect ratio. This last coefficient was also relevant in the definition of a suitable length scale to account for the effect of the lateral confinement. Scour patterns in the narrow flume have also shown differences with respect to erosion by two-dimensional wall jets: the downstream mound undergoes a morphological transition, from triangular to trapezoidal, at the early stage of the scour development, and grains move downhill driven by gravity inside the granular wedge, in a regime of steady recirculation (SR). Above this regime, a digging–refilling (DR) cycle, with periods of the order of seconds, triggers at a well-defined Shields number.