Storm surge induced sediment accumulation on marsh islands in the southeastern North Sea: Implications for coastal protection

Abstract

Coastal marshlands in tide-dominated shallow marine systems are prone to erosion and vulnerable to long term sea level rise. The natural adaptability of such areas mainly depends on sufficient sediment supply delivered during inundation events and protection against erosion processes. Artificial structures such as dykes, revetments and tidal gates ensure protection against erosion but influence the natural adaptability of the marshland. Establishing a reasonable balance between natural adaptation processes and coastal protection measures forms a challenging task. With the use of an autonomously operating turbidity sensor network and sediment traps installed at and around a marsh island in the North Frisian Wadden Sea (Hallig Langeness, Germany), we measured suspended particulate matter (SPM) concentrations and vertical accretion adjacent to artificial revetments and breakwater constructions. Measurements included seven storm surges in the winter season 2019/20 with peak water levels of 2.94–3.65 m above sea level. We found clear relations between SPM on the tidal flats and SPM on the marshland. While SPM concentrations in the tidal flats (SPMi) during storm surges range from 0.5 ± 0.014 to 1.14 ± 0.014 g/l, those in the marshland behind breakwaters (SPMrs) only reach on average 28%–47% of SPMi depending on the specific location. SPMrs/SPMi and peak water levels shows positive correlation, while SPMrs and the duration needed to fully inundate the marshland exhibits a negative correlation. Vertical accretion of the marshland is 7.4 ± 4.6 mm for the winter season 2019/20 and therefore about 2.6 times higher than the average for four years (2016–2020) at 2.8 ± 2 mm/a. The average vertical accretion per inundation increases from 0.2 mm at moderate inundations to 0.84 mm at storm surges. The hampering effect of breakwaters on sediment accumulation seems to diminish at storm surges compared to moderate inundations.