Detailed measurements of velocities and suspended sand concentrations over full‐scale ripples in regular oscillatory flow

Abstract

The knowledge and modeling of wave‐induced sand transport over rippled beds still has significant shortcomings, which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments. We have carried out new measurements of the detailed time‐dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions. The fact that the ripples were mobile and the flow conditions were full‐scale makes these measurements unique. We made velocity measurements for 14 different flows and concentration measurements for three of these flows. The velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal. Vortex formation results in near‐ripple flow reversals ahead of free‐stream reversals and velocity maxima near the ripple crest that are much higher than the free‐stream maxima. Asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow. The time‐ and bed‐averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow. The vortices are responsible for three main concentration peaks: one just after on‐offshore flow reversal associated with the passage of a sand‐laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples. The sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripple's lee slope around the time of on‐offshore flow reversal. The net (time‐averaged) current‐related and wave‐related horizontal sand fluxes are generally offshore directed and mostly contained within 1.5 ripple heights above the ripple crest. The wave‐related suspended transport component is larger, but the contribution of the current‐related suspended sand transport cannot be neglected. In addition to the measured offshore net transport of suspended sand, there is an onshore‐directed transport very close to the ripple surface. The total net transport is in the offshore direction for this specific asymmetric flow condition.