Assessing Sand Deposit Dynamics at the Island of Langeoog, Germany by Means of Multi-Sensor Remote Sensing Data

Kurzfassung

Langeoog is one of the East Frisian Islands located in the German Wadden Sea. The island's coastal morphodynamics are strongly influenced by environmental factors such as tides and currents. As natural sediment supplies cannot always compensate for coastal erosion of sand deposits, coastal protection measures are crucial for preserving the island. Langeoog is a special case in this context, as it is the only island in the barrier island chain that is mostly managed without the use of coastal protection structures such as groynes or revetments, due to its prevailing current and sedimentation processes. The island’s settlement and infrastructure are surrounded by a protective dune and adjacent sandy beach areas to the west and north. Conservation measures to preserve the dune are only necessary in the northern part, in front of the Pirolatal, where sand replenishments are carried out regularly to counteract ongoing beach erosion by restocking the sand deposits in front of the protective dune. To initiate necessary measures and estimate the required sediment volumes, knowledge about the development of this beach section is essential for local coastal protection authorities. In this study, we investigate the suitability of optical multi-sensor remote sensing data to analyse changes in the sand deposits and their effects on the condition of the protective dune in front of Pirolatal on Langeoog Island from 2018 to 2023. For this purpose, we processed high-resolution (HR) and medium-resolution (MR) optical satellite data, applying index-based threshold methods to estimate several proxies of coastal dynamics, such as the instantaneous waterline, the location and state of the protective dune, and the extent of permanently dry sand areas under regular tidal conditions. We compare the results to elevation data to assess the potential of 2D remote sensing data for monitoring this coastal section. The results show that the state of the beach and the height of the dune’s break-off are strongly influenced by accretion events (sand replenishments) and ongoing erosion, particularly during storm surges in the winter season. The condition of the sand deposit is also crucial for determining the position of the instantaneous waterline. This study demonstrates the benefit of a multi-sensor optical satellite data approach to support coastal monitoring and applied coastal protection efforts.