Monitoring of Coastal Dynamics at the Island of Langeoog, Germany by Means of Multi-Sensor Satellite Data

Kurzfassung

Langeoog is one of the East Frisian Islands in the German Wadden Sea, with topographical and bathymetric characteristics significantly influenced by environmental factors such as the tides and currents. Coastal protection is of great importance for the preservation of the islands in order to counteract periodically occurring sand deficits caused by erosion processes such as seasonal storm surges and the resulting strong disturbance of the sediment balance. Langeoog is a special case in this context, as it is the only one of the barrier island chain that largely is managed without the use of coastal protection structures such as groynes or revetments due to the prevailing current and sedimentation processes. The only measures to preserve the sand deposits on the north side of the island are carried out at regular intervals in the form of sand replenishment. In order to carry out such measures effectively and efficiently, detailed knowledge of the development of the highly dynamic coastal area is required. In this study, we investigate the suitability of optical multi-sensor satellite data for a comprehensive identification and analysis of coastal dynamics on the Island of Langeoog in the period 2018 to 2023. For this purpose, very high resolution (VHR) and high resolution (HR) optical satellite data were processed to estimate individual components of coastal dynamics such as the coastline, sediment accumulations such as sand flats, the location of the dune base and the extent of permanently dry sand areas under normal tidal conditions. Both index-based threshold methods and a random forest-based classification algorithm were used to extract these parameters. The results indicate that the data are very well suited to derive and analyse individual components of coastal dynamics. The derivation of the Normalized Difference Water Index (NDWI) from VHR and HR optical data showed a sand plate approaching the northwestern tip of the island for the period 2021 to 2023, which landed on the coast during 2023, resulting in a seaward shift of the coastline. A further histogram analysis was performed to extract the beach areas of the island and thus the landward boundary of the beach, the dune foot, from the NDWI information. Results showed that the position of the dune base remained stable over the study period. By applying the classification algorithm, dry sand areas (above the high tide water line) could additionally be extracted from the image data, which particularly show the influence of erosion and accumulation events. This study demonstrates that the use of multi-sensor optical satellite data is suitable for the holistic analysis of coastal dynamics, as the findings on the individual components can be linked to each other as well as to specific erosion (storm surges) and accumulation events (sand accumulation). The results thus show great potential for supporting coastal monitoring in applied coastal protection.