Assessing sediment accumulation at inundated anthropogenic marshland in the southern North Sea: Using turbidity measurements and particle tracking

New approaches to coastal protection measures become increasingly important to protect for or to mitigate sea level rise (SLR) worldwide. Measures that involve only the heightening of dykes or revetments are prone to disturb the natural adaptation capacity especially of shallow marine tidal systems and adjacent marshlands. This applies to ten island-like marsh areas called Halligen in the southern North Sea, Germany. Here, the adaptation potential depends on the accumulation of suspended particulate matter (SPM) during annually occurring inundations that generates relevant vertical accretion rates. According to previous studies, todays accretion rates are too low to keep pace with SLR. To invoke appropriate strategies to mitigate the imbalance it is crucial to know whether the particle accumulation is material limited (i.e. there is not sufficient SPM available in the water column around the Halligen) or transport-limited (i.e. SPM is available but not transported efficiently onto the Halligen). In this study we assessed the spatial and temporal distribution of SPM at the largest Hallig Langeness with an autonomous working turbidity measurement system. Recorded inundation events covered a wide range of water levels resulting from moderate wind conditions to storm surges. We were able to quantify the impact of the existing coastal protection measures on present sediment accumulation rates. A detailed numerical sediment transport model of the Hallig and its surrounding tidal flats was used to simulate the effect of various height adjustments of revetments in different hydrodynamic scenarios.