Observations of near-bed orbital velocities and small-scale bedforms on the Dutch lower shoreface

Abstract

The lower shoreface, with water depths between about 8 and 20 m, forms the transition between the inner shelf and upper shoreface. Knowledge of the lower shoreface is essential, as it is – in many cases – the sediment source for the upper shoreface and beach. This paper presents new data of near-bed orbital velocities and small-scale bedforms at various depths and locations on the Dutch lower shoreface. Near-bed orbital velocities were beyond 1 m/s during high-energetic wave conditions. They increase with wave height and decrease with water depth, and can be reasonably well described by linear wave theory. Ripple heights range between 0.01−0.03 m and ripple lengths between 0.08−0.20 m. Ripple dimensions are controlled by wave mobility, with lower and shorter ripples for higher waves, and not so much by the currents. The Van Rijn (2007) formula generally overpredicts the ripple heights, and the variation with tidal currents in time. The measurements clearly indicate significant sediment mobility at the lower shoreface under higher wave events. It is yet unclear what this means for the net sand transport. This will depend on the subtle timing of sediment suspension, wave-mean currents and near-bed orbital velocities. It requires detailed modeling to determine lower shoreface net transport rates, and to unravel the controlling sand transport mechanisms.