Dune cliff erosion and beach width change at the northern and southern spits of Sylt detected with multi-temporal Lidar

Since the end of Pleistocene the west coast of Sylt (Germany) has been a very dynamic geomorphic system. At present especially the northern and southern spits (Ellenbogen and Hِrnum-Odde) are changing rapidly. Airborne Lidar (light detection and ranging) has been used since 1999 to measure the coast inter- and intra-annually. The aim of this study is to exploit the potential of the laser data to describe and explain the changes at the west coast of the northern and southern spits of Sylt. Lidar accuracy verification approves the value of the data. After offset correction a horizontal difference of 0.5 m and a maximal vertical difference of 0.12 m remain. Dune cliff changes are monitored by applying semi-automatic breakline extraction procedures to multi-temporal Lidar data. The breakline extraction is based on filter methods derived from digital image analysis. Furthermore beach width changes are analyzed. At the Hِrnum-Odde and at the Ellenbogen a maximal dune retreat of 140 m and 70 m respectively was measured. Different annual as well as seasonal coastal change patterns could be revealed. Dunes are only eroded during winter due to storm surges, while the beach width varies during all seasons due to generally high shoreline dynamics. Beach width influences dune cliff erosion differently at the two study sites. At the west coast of the northern spit backshore dunes are eroded suddenly after an enduring period of general beach width decrease. Afterwards beach width increases and cliff position stabilizes again. Beach nourishment can protect the dunes due to beach width preservation. At the west coast of the southern spit local interactions dominate. Expanded beach segments cause lee effects and hence dune erosion. Different beach nourishment projects could not avoid dune erosion.