Scour and flow around submerged structures

Abstract

The results of an experimental investigation on scour of non-cohesive uniform and non-uniform sediment beds and flow around fully submerged structures over a fixed plane bed are discussed. In the study, the submergence factor, Froude number and cylinder Reynolds number for all the experiments were kept in the ranges of 0·25–0·75, 0·23–0·70 and 13 500–58 650 respectively. The free surface profile over the submerged structures consists of distinct upper and lower layers; the lower layer is characterised by continuous mixing with the generated downstream wake whereas the characteristics of the upper layer depend on the submergence factor and the Froude number. Using observed scour depths at different times, the time variation of scour depth is scaled by an exponential law. The non-dimensional time scale decreases following a power-law relationship with an increase in densimetric Froude number. It is evident that there is a considerable reduction in the non-dimensional scour depth with an increase in the submergence ratio, mainly due to the decrease in the intensity of the horseshoe vortex. It is also evident that the normalised equilibrium scour depth decreases with an increase in the ratio of cylinder diameter to sediment size. A significant reduction of 40% of the scour depth in uniform sediment was observed in experiments with non-uniform sediments. The three-dimensional flow fields around the submerged structure over a fixed plane bed were measured with an acoustic Doppler velocimeter. The vertical velocity distributions indicate deceleration of the approaching flow, acceleration around the submerged body and the approach to the original state of open-channel flow downstream of the submerged structure.