Slope Changes as Indicator for the Shoreface Erosion in Ameland

By Pinelopi Trapezanlidou (Greece)

Abstract
This research was conducted to determine the coastal geomorphological dynamics of the Ameland island in the Netherlands for the period 1965 to 2010. Ameland is a barrier island and it has a crucial role to protect the mainland from erosion. However, Ameland is facing erosion events, mainly because of the tidal inlet currents, the wave activity and the storm events. The objective of this thesis is to analyze temporal changes of the slope gradient, as this has been proven to be a good measurement of coastal vulnerability.
The study defines and visualizes the shoreface changes by analysing the slope angle and aspect. These variables have been derived from the JARKUS data (bathymetry measurements collected along cross-shore transects), to show the slope evolution with a layer resolution of twenty meters in both dimensions. While the maps of the statistical slope analysis (mean, STD, maximum) and the Beta coefficient linear regression gave an overview of the slope steepness at a pixel level. Then, a trend analysis using the BFAST method was performed in order to decompose the slope data into trends and to detect abrupt changes. Subsequently, it was examined if these abrupt changes could be explained by either storm or nourishment events.
The findings revealed that the coastline has a higher slope angle than the offshore area, likely due to the wave action and the ebb-tidal currents. However, a sand bar (Bornrif) which attached to the north–west part of Ameland, during the course of the study period, showed to intensify the sand transportation by causing changes in the coastal area. Also, the overall slope angle trend showed an upward movement indicating a corrosive tendency. Finally, despite the nourishments that have been applied in certain areas it was found that the slope is still increasing, which may indicate that Ameland needs more effective measures (e.g. groins, seawalls, breakwaters and coastal barrages) in order to protect the coastal area from the natural processes.

Keywords: slope angle; slope aspect; shoreface; erosion; sedimentation; coastline; Ameland; JARKUS; nourishment