Cross-shore sand transport in shallow water

Abstract

The temporal and spatial variations in the relative importance of three primary suspended transport components (high frequency and low frequency oscillatory, and steady current driven transport) were examined, in an attempt to explain onshore sandbar migration. The data came from a three week field campaign carried out in the inner surf zone of Vejers beach, Denmark, during which periods of low energetic swell and mild energetic wave conditions (resulting in onshore migration), and a storm event (causing offshore sandbar migration) were experienced. Almost exclusively net offshore directed transport, dominated by undertow driven transport, was recorded during high tide throughout the field campaign. For (non-breaking) swell conditions it was found that the importance of this steady transport component on net transport reduces in comparison to both oscillatory transport components, with increasing water depth and increased distance from the shoreline. Adversely, for mild and high energetic wave conditions the steady component rather increased in importance with increasing water depth and increased distance from the shoreline. A non-dimensional parameter was analysed on its ability to predict net onshore directed transport. It was found that it was a reliable indicator for the relative importance of the oscillatory components compared to the steady component, but not their direction. Only in shallow water (< 0.8 m), close to the shoreline (< 30 m), was there a tendency for both oscillatory currents to be onshore directed, which occasionally allowed for net onshore directed transport. Since the data gathered during mid to high tide was almost solely offshore directed, it is thought that enough onshore directed transport must have taken place during the shallow water and sheet flow conditions of low tide to compensate for this predominance of net offshore directed transport and allow for the onshore migration of sandbars.