Sea level rise and the sediment budget of an eroding barrier on the Danish North Sea coast

Abstract

Aagaard, T. and Sørensen, P., 2013. Sea level rise and the sediment budget of an eroding barrier on the Danish North Sea coast.The barrier spit Skallingen on the Danish North Sea coast accreted significantly for a period of three centuries prior to 1970 but it is now strongly eroding and the average rate of shoreline recession is about 4.2 m/yr. The causes for the erosion are examined through a sediment budget approach and it is found that although mean sea level is presently rising at a rate of about 3.3 mm/yr, the impact of longshore sediment transport divergence is far more important to barrier erosion. Sediment losses from longshore transport gradients are partly offset by minor gains through onshore transport of sand across the shoreface as well as by beach nourishment. The results illustrate that the Bruun model (Bruun, 1962) is insufficient to predict coastal erosion and even though sea level rise may cause erosion, far larger erosion rates can be induced by indirect human impact. Moreover, since the barrier was formerly in an accretive state, the absolute magnitude of (some of) the sediment budget terms have clearly changed significantly during recent decades, and using a numerical model it is found that longshore sediment losses have increased by a factor 3–4 over the past century. This increased rate/gradient of longshore sediment transport has been due mainly to inlet channel dredging which degraded the ebb tidal delta that earlier sheltered the downdrift end of the spit.