Effects of Climate and Human Interventions on the Evolution of the Wadden Sea Depositional System (Southern North Sea)

Abstract

The most important constraints on barrier island genesis are a low coastal gradient, a low to moderate wave climate, low to intermediate tidal ranges, and a sufficient supply of sediment to construct and maintain the barrier and its shoreface. Sediment sources can be remote (river, updrift coast), local (upper shoreface) or a combination of both. With the exception of tidal range, the constraining factors are directly or indirectly controlled by climatic factors. Wave height and tidal range are loosely correlated and define the lower and the upper limits of barrier island stability within the mixed tide/wave energy zones. Since sea-level rise creates a sediment deficit in the tidal basin, the sediment budget ultimately controls the morphodynamic response and the resulting stratigraphy: progradational (remote supply > deficit), aggradational (remote supply = deficit), or transgressive (remote supply <-<< deficit). In the case of the southern North Sea, the western section from Den Helder (Netherlands) to the mouth of the Elbe river (Germany) the system is transgressive. From the Elbe river to St. Peter-Ording (Germany) the tidal flat system is aggradationalprogradational. From St. Peter-Ording to the island of Sylt the tidal basins, which were originally created by coastal erosion (storm scouring) during the late Middel Ages, are aggradationalprogradational. The island of Sylt itself is strongly transgressive. From Sylt to Skallingen (Denmark) the barrier system is again aggradational-progradational. Morphodynamic changes recorded in historical times (since about AD 1000) mostly reflect responses to human interventions in the western sector of the Wadden Sea (Den Helder to the Elbe river), whereas north of the Elbe river they are dominated by natural adjustment processes (erosion/accretion) which, in most places, incorporate a variable anthropogenic overprint. In the wake of a potentially accelerating sea-level rise, it is predicted that shore protection schemes will increasingly inhibit natural island dynamics and thereby threaten island stability, while storm-barriers will eventually have to be constructed in the lower reaches of all major rivers along the coast. In addition, the depletion of fine-grained sediments will continue, the system becoming increasingly more sand dominated and experiencing a further loss of mud flats and salt marshes.