Wave-induced Eulerian drift above vortex ripples

Progressive surface waves induce a near-bed Eulerian drift due to the effect of vertical orbital velocities in the bottom boundary layer (see Longuet-Higgins, 1953) and asymmetry in the turbulence intensity in successive wave half cycles (see Trowbridge and Madsen, 1984). Although this wave-induced current is small relative to the horizontal orbital velocities, it is potentially important for the net sand transport since most of the sand is present in the few centimeters directly above the bed.

Davies and Villaret (1999) developed a practical model of the drift induced by weakly asymmetrical progressive waves in the bottom boundary layer above rippled and very rough beds. This model is based on velocity data from small wave flume experiments with relatively short wave periods (T < 3 s). Recently, van der Werf et al. (2007) carried out detailed velocity measurements above mobile ripples in full-scale oscillatory flows (T ≥ 5 s). For fixed period, orbital velocity amplitude and grain size the degree of orbital flow asymmetry was varied. In this paper we use these data to study the effect of orbital flow asymmetry on the Eulerian drift above vortex ripples. We adjust the model of Davies and Villaret (1999) for the oscillatory flow tunnel case where the flow is purely horizontal and uniform in the horizontal direction, and compare the original and adjusted model with the oscillatory flow tunnel data.