Sediment transfer from beach to shoreface: The sediment budget of an accreting beach on the Danish North Sea Coast

The sediment budget along the southern part of the exposed Danish North Sea coast was assessed through a combination of cross-shore profile analysis, numerical modeling and field measurements of cross-shore and longshore sediment transport at the boundary between the upper and the lower shorefaces. The beaches along this coast are accreting but observed sediment storage is far smaller than sediment supplied by southerly directed wave-driven currents. The excess sediment volumes are transferred seaward across the shoreface by systematically offshore-migrating nearshore bars that deliver sediment to the lower shoreface from where it is eventually removed by northward-directed wind-generated currents. In the geological past, sediment storage rates were much larger than at present and it is likely that the rate of storage along this coastline decreased as shoreface geometry adjusted towards equilibrium associated with shoreface steepening.

► Longshore supply of sediment is very large at this beach but the beach does not prograde significantly.
► This sediment supply is due to wave-induced currents on the upper shoreface.
► The longshore sediment supply is balanced by offshore transport by seaward migrating bars.
► Ocean currents remove the sediment from the lower shoreface.