Sediment supply from shoreface to dunes: linking sediment transport measurements and long-term morphological evolution

A significant problem in current coastal research is the understanding of linkages between morphological phenomena occurring on different temporal and spatial scales. Morphodynamic processes in the nearshore typically exhibit nonlinear behaviour and consequently, phenomena which occur on short temporal (event) scales as, e.g. observed during field experiments have generally been difficult to upscale to provide an understanding of the long-term behaviour of the coast on the time scale of seasons or decades. This paper documents a persistent onshore migration of nearshore bar features over a period of 3 decades using a set of cross-shore profile surveys from a barrier on the Danish North Sea coast. The observed behaviour is consistent with experimental data obtained during weeklong field campaigns when sediment transport and bar migration were found to be landward directed during high-energy dissipative conditions. Hydrodynamic and sediment transport processes leading to onshore bar migration and eventually welding of the bars are documented to provide an explanation for the observed long-term evolutionary trends. In addition, bar welding and the resulting intertidal bar formation represent an onshore sediment storage which is subsequently available for aeolian transport. Observations on aeolian sediment transport indicate that transport rates and sediment supply to the foredunes critically depend on the presence of dry sand sources as represented by well-drained intertidal bars. Measurements of long-term accretion rates in foredunes indicate that volumes deposited on the crest and lee slope of the dunes are in reasonable correspondence with volumes supplied to the subaerial beach through the process of bar welding. Hence, the data suggest that a persistent onshore-directed sediment supply occurs from the shoreface, across the surf and intertidal zones and into the dunes of the investigated barrier.