The effect of shoreward propagating accretionary waves on the intertidal beach

Abstract

The horn of a bar can separate and migrate onshore as an individual coherent feature. These features are named Shoreward Propagating Accretionary Waves (SPAWs). To better understand SPAWs, the effect of these features on the intertidal beach was determined for the Egmond coast by making use of Argus imagery. For all selected days the shoreline was mapped for all Argus images available on that day with the Intertidal Beach Mapper (IBM). With a loess interpolation a bathymetry was determined from these shorelines. In addition, barlines which were classified for each day of the study period with the BarLine Intesity Mapper (BLIM) were used. From these data the width, volume, alongshore variability and morphology of the intertidal area were determined. After the SPAW emerged from the inner bar the alongshore variability of the shoreline increased. A seaward protrusion was already present shoreward of the SPAW. Until the period the SPAW welded to the beach the seaward protrusion grew and the volume of the intertidal beach increased with ~4000 m3. When the SPAW welded to the beach the volume of the intertidal area increased with ~6000 m3 in just two weeks, which corresponds to an onshore sediment flux of ~2.0 m3/m/day, and the shoreline moved even more seaward. In the following period the sediment was transported in alongshore and cross-shore direction. The alongshore variability decreased from ~30 m to ~15 m for the low tide line in just two months because the sand dispersed alongshore, forming a new intertidal bar. The mean location of the high tide line moved seaward and its alongshore variability increased. Sediment from the lower intertidal area was transported to the upper intertidal area to even further shoreward because of the increase in intertidal beach width. Because there was a large data gap the exact period the intertidal beach was affected by the SPAW event is unknown, but it could be said that it ranged between 2 and 6 months.