Storms, shoreface morphodynamics, sand supply, and the accretion and erosion of coastal dune barriers in the southern North Sea

The coast of the southern North Sea is bound by dune barriers that have developed adjacent to a shallow storm- and tide-dominated shoreface comprising numerous shore-parallel to sub-shore-parallel tidal sand banks. The banks evolve under the joint control of tide-, wave- and wind-induced shore-parallel currents, which tend to 'stretch' them, eventually leading to bank division, and to shoaling and breaking storm waves, which tend to drive them ashore. The banks, thus, modulate the delivery of storm wave energy to the coast, redirect currents alongshore and are the sand sources for the accretion of coastal dunes. Foredune accretion occurs where major sand banks have migrated shoreward over the last centuries to be finally driven ashore and weld under the impact of storm waves. Morphological changes in the bank field can impact on shoreline stability through dissipation or enhanced shoreward transmission of storm wave energy and effects on radiation stress, particularly when waves are breaking over the banks. Where banks are close to the shore, mitigation of offshore sediment transport, especially during storms, can occur because of gradients in radiation stress generated by the complex 3D bank structure. These macro-scale mechanisms involve embedded meso-scale interactions that revolve around the mobility of sand waves, mobility of beach bars and troughs and foredune mobility, and micro-scale processes of bedform mobility in the subaqueous and intertidal domains, and of swash and aeolian beach-dune sand transport. These embedded interactions and the morphodynamic feedback loops illustrate the importance of synchroneity of sand transport from shoreface to dune on this coast. Large stretches of the foredunes show either signs of stability, or mild but chronic erosion. Furthermore, a demonstrated lack of a clear relationship occurs between storminess and coastal response over the second half of the 20th century. The present situation may be indicative of conditions of rather limited sand supply from offshore, notwithstanding the abundance of sand on the nearby shallow shoreface, except in areas where a nearshore storm-driven tidal sand bank has become shore-attached. Apart from the important influence of shoreface sand banks and of wave-bank interactions, foredune accretion and erosion also depend on various context controls that include individual storm characteristics, wind speed and incidence relative to the shore, tidal stage during storms, and direct human intervention on the shore through foredune and beach management. The bewildering variability inherent in these intricately related parameters may also explain the poor relationship between storminess and barrier shoreline change and will still continue to render unpredictable the response of shores to individual storms. (C) 2012 Elsevier B.V. All rights reserved.