Facies Mapping and Seasonal Sediment Shift of Tidal Inlets: Impacts of Coastal Protection Measures

Coastal areas play a vital role in the face of climate change, especially concerning sea level rise, coastal protection, and ecosystem preservation. This study investigates the impacts of hard coastal protection on mesotidal inlets, focusing also on seasonal sediment shift and comparing natural versus modified systems. A surface facies map was compiled for the almost natural tidal inlet of the East Frisian Islands Otzumer Balje to contrast to the Harle inlet, which is influenced by long-term coastal protection measures. Sediment samples were collected after storm season and fair-weather season to analyze grain size and composition. Results show that both inlets experience seasonal changes in sediment characteristics due to biological factors like benthos growth and physical factors such as storm induced sediment transport. The Otzumer Balje exhibits seasonal sediment shift, with coarser materials prevailing in summer and allochthonous mudflat sediments accumulating during winter storms. In contrast, the Harle features a higher diversity of bioclastic materials, including bryozoan-encrusted shell fragments, indicative of more stable sediment conditions. Geomorphologically, the Otzumer Balje is characterized by a single-channel system with active subaquatic dunes, whereas the Harle consists of two main channels influenced by groyne-induced scouring and sediment deposition. Biological activity differs between the inlets, with the Harle displaying more diverse benthic communities and pronounced bryozoan colonization due to lower sedimentation rates. The study underscores the role of coastal protection measures in shaping these tidal inlets and highlights the necessity of coastal management strategies that account for the interaction between human interventions and natural dynamics.