Modeling and measurement of sediment transport by waves in the vortex ripple regime

Abstract

Above steep, wave‐induced sand ripples, which occur extensively in shallow sea areas, the momentum transfer and suspended sediment dynamics are dominated by the formation and shedding at flow reversal of lee wake vortices. Since two‐dimensional models of this process are unduly complex for practical application, a simple, one‐dimensional vertical (1DV), two‐layer, model is presented here for the flow and transport above such ripples. In the lower layer of thickness equal to two ripple heights, vortex shedding is represented by a time‐varying eddy viscosity with peak values at flow reversal while, in the upper layer, a standard turbulence‐closure formulation is used. Suspended sediment is introduced at the ripple crest by a time‐varying pick‐up function. The ripple dimensions and suspended grain size are also predicted. The model results are compared with data obtained beneath weakly asymmetrical waves in a large‐scale flume. Intrawave measurements of suspended concentration were obtained using an acoustic backscatter system, and sediment profiles obtained above different locations on a moving rippled‐bed profile are used to provide intraripple and ripple‐averaged descriptions of the intrawave concentration field for comparison with the model. The results of a harmonic analysis suggest that the mean component and second harmonic (two‐peak symmetry term) of the concentration are well predicted, particularly near the bed. The modeled wave‐related component dominates the net sand transport rate; near‐bed transport is in the onshore direction, while transport in the outer boundary layer is offshore. The new 1DV formulation provides a simple, but realistic, modeling approach for the rippled regime.