The missing mud flats of the Wadden Sea: a reconstruction of sediments and accommodation space lost in the wake of land reclamation

Abstract

Over past centuries, the combined effect of land reclamation and sea-level rise has resulted in higher average energy levels along the mainland coast of the West and East Frisian Wadden Sea because catchment areas have been greatly reduced in size. This is reflected in a conspicuous depletion of fine-grained sediments in modern back-barrier tidal flats, pure mud flats being found only in a few sheltered coastal embayments. However, the widespread occurrence of clay-rich soils on the reclaimed lands suggests that large mud flats and salt marshes must have existed in the past along most of the southern North Sea coast. In this study, the results of a modelling exercise are presented in which the composition and the accommodation space of fine-grained sediments lost in the course of land reclamation have been reconstructed for two East Frisian tidal basins. This has been achieved by extending the shorenormal gradients documented in the mean settling velocities of Wadden Sea sediments landwards into the reclaimed areas by numerical procedures. The modelling result confirm that large intertidal flats occupied these areas, and that the sediments were predominantly composed of grain sizes < 63 μm. The results suggest that land reclamation is the main reason for the missing mud flats in the Wadden Sea. Since sea-level rise is accompanied by increasing energy levels in back-barrier tidal basins, fine-grained sediments will continues to be eliminated from the landward margin of the Wadden Sea. Any acceleration in sea-level rise will therefore aggravate the situation.