| dc.description.abstract | Sandy shoals that originate from the ebb tidal delta supply the downdrift barrier island with large volumes of sand. This sand is partly
being redistributed over the downdrift island and the other part erodes away. My thesis was about studying the effect that shoal attachments to Ameland have on the downdrift sand volumes and where this sand is being redistributed to. Therefore the JarKus and Vaklodingen bathymetric datasets were analyzed for the period 1971-2023. Volume changes were obtained by subtracting yearly bathymetries from the 1971 reference bathymetry. By summing these volumes changes in the cross-shore dimension, the 4D data was collapsed to 3D data [time,distance alongshore and volume change]. Collapsing this to 2D data by dividing the alonshore dimension into 5 areas and summing the volumes per area allowed for comparing volume changes against time for these areas. The results show that these volume changes showed a peak in sand volume which occurred after shoal attachment (west of Ameland) in 1985 (Bornrif Strandhaak). This peak in volume declined alongshore over time as the attached shoal started to migrate alongshore in the form of a longshore sandwave and lost its volume over time (e.g. Figure 1). The volume loss alongshore amounted to about 4% per year and the alongshore migrating shoal/sandwave disappeared/decayed about 8 km downdrift (to the east) of where it initially attached. It took at least 25 years for this alongshore migrating shoal to disappear. The speed of the alongshore migrating shoal over the whole elevation range was on average 325 m/yr. The largest response in volume change was visible in the zone where waves are active, which is the +1 to -5 elevation range. In addition, the beach (+1 to +5) and the deeper depths (-5 to -12.5) showed a relative steady increase in sand volumes in the west but this switched to a decline in downdrift direction. Furthermore, the feeding effect of the shoal was also visible in the volume changes halfway the island (further to the east), where before sand by the shoal was supplied, the volume changes were declining with respect to 1971, but after the supplying of sand by the shoal, the volume changes switched from a decline to an increase. In addition, just east of the shoal attachment zone, sand volumes have been increasing since 1971 with the largest increase for elevation range -5 to -12.5 m. In contrast, the results show that before the shoal delivered sand roughly halfway the island, the sand volumes were declining with the largest decline happening for elevation class -5 to -12.5 m. Analysis of the center of mass of the migrating shoal alongshore revealed that sand arrived in the deeper elevation range first and arrived later in shallower depths with a time-delay in this range of 0.5-2 meter elevation/yr. | |
