Assessing sediment accumulation at inundated anthropogenic marshland in the southeastern North Sea: Using particle tracking on modified coastal protection structures

Abstract

Adaptation of protection structures to the continuously rising sea level is a major topic at many coastal areas all over the world. In shallow marine environments like tidal flats these structures often aim at mitigation rather than prevention of inundations. These anthropogenic interventions, however, are prone to disturb sediment transport onto regularly inundated marshlands. This applies to ten island-like marsh areas called Halligen in the southeastern North Sea, off the coast of Schleswig-Holstein, Germany. The capability of these marsh surfaces to keep pace with sea level rise depends on vertical accretion rates due to the accumulation of suspended sediments that reach the Hallig surface during annually occurring inundations. According to latest findings, the current coastal protection structures such as revetments, dikes and breakwaters represent an obstacle for sediment transport, which intensifies the imbalance between sea level rise and vertical accretion rates on the Halligen.A detailed hydrodynamic model of the largest Hallig Langeness and its surrounding tidal flats is applied to investigate adjustments to impermeable breakwater constructions for different inundation scenarios. The sediment transport from the tidal flats towards the marshland and the subsequent accumulation is modelled by Lagrangian particle tracking. The particle tracking results indicate almost doubling of accumulated particles by decreasing the breakwater heights from 2.56 ± 0.3 m to a constant height of 1.92 m in the moderate inundation scenario. In comparison, storm surges conditions show an increase of particle accumulation by a factor of 6.9. The results further delineate a potential increase of the recently measured vertical accretion rates by almost an order of magnitude, given an appropriate design and management of the protection structures.