Suspended particulate matter for sediment accumulation on inundated anthropogenic marshland in the southern North Sea – Potential, thresholds and limitations

Abstract

Determining the availability of suspension load, i.e. suspended particulate matter (SPM), in shallow marine tidal systems is necessary to estimate whether coastal lowlands may experience vertical accretion rates that keep pace with global sea level rise. In particular anthropogenic marshland areas are affected by limitations of SPM accumulation due to artificial structures such as dikes and revetments that act as transport barriers.With the use of an autonomously operating turbidity measuring network installed at and around the Hallig Langeness in the North Frisian Wadden Sea, we have analyzed eight individual inundations on this island-like marshland during harsh weather conditions. The results reveal that the majority of the available SPM could not accumulate on the marshland due to adjacent coastal protection measures. This reduction in SPM affects especially grain sizes within the very fine to fine sand range between 63 and 200 μm that can account for up to 70% of the sampled SPM during an inundation. Furthermore, we have determined a tidal average SPM concentration threshold of 0.15 g/l should be available on the tidal flat in order to allow sufficient vertical marshland accretion. Although this threshold is exceeded by the determined SPM on the tidal flats around Langeness, the available SPM on the marshland after passing the coastal protection measures is at most 20%. Hence, an optimized coastal management strategy has the potential to increase the sediment accumulation rate on the marshlands by a factor of five, which might allow for keeping pace with local sea level rise.