Long-term Effects of Coastal Protection Measures on Sedimentology in a Dynamic Tidal Inlet: Spiekeroog, East Frisian Islands

Abstract

Coastal erosion and flood protection are significant global challenges and will become more important in the future. Hard coastal protection structures, such as groynes are implemented in high-risk areas to provide immediate results. However, their specific impacts on current dynamics and sediment properties, especially in tidal inlets, are inadequately understood. This study investigates the long-term effects of coastal protection measures on sedimentology and morphology, focusing on a tidal inlet in the East Frisian Islands, Northern Germany. The erosive eastward littoral drift on the Southern North Sea coast sets an ideal backdrop. To protect the East-Frisian islands from storm surges groynes were built. A groyne on Wangerooge Island restricts the North Sea - Wadden Sea exchange, hypothesized to alter flow dynamics and sediment transport within the inlet “Harle”. Sedimentological analysis, including grain size and component analysis, reveals seven distinct sedimentological facies. Integrated multibeam data creates a facies map indicating higher grain size in main channels and the formation of a sandbar segregating the main tidal channels in the Harle. Erosion and deposition areas highlight altered current patterns, including circulation eddies and mixing shear zones. Comparisons with an almost unaffected tidal inlet between the islands of Langeoog and Spiekeroog emphasize the enduring impacts of the coastal protection measures on sedimentology in the "Harle". This study illuminates the complex interplay between coastal protection and sedimentology in a high-dynamic tidal inlet system, providing insights coastal management amid global challenges.