Land reclamation as a cause of fine-grained sediment depletion in backbarrier tidal flats (Southern North Sea)

Abstract

Grain size distribution patterns along the East Frisian coast (German Wadden Sea) were evafuated with the aim of identifying potential effects of man-made structures (e.g. dikes) on the textural composition and sediment budget of the area. It was found that the distribution of various grain size fractions and settling velocities revealed a well-defined cross-shore energy gradient. Adjacent to the mainland dike the sediment is dominated by the 3.0–3.5 phi sand fraction (>70% by weight). In comparison to the sedimentary sequences normally observed in unobstructed backbarrier depositional environments, the sediments along the land-ward margin are in this case conspicuously depleted in grain sizes <3.5 phi, the corresponding size fractions contributing <20% by weight. This suggests that the dike interrupts the normal energy gradient, thereby inhibiting the deposition of sediments with settling velocities <0.5 cm s?1. This interpretation is confirmed when comparing individual cross-shore transects. The shorter the transect between the barrier island shore and the dike, the greater the fine sediment depletion. Since many faunal assemblages are grain-size specific, it is further suggested that the backbarrier ecosystem must have undergone significant modifications as a result of the substantial reduction in mud flats since the onset of dike construction some 1000 years BP. It is predicted that in the wake of the persisting and possibly accelerating sealevel rise this process of fine sediment depletion will continue and also affect progressively coarser sediments.