Intertidal beach change during storm conditions; Egmond, The Netherlands

The processes responsible for sediment transport and morphological change in the intertidal zone of natural beaches are not well known. Consequently, factors determining whether the intertidal beach erodes/accretes or remains stable in the face of significant wave impact, and the mechanisms behind the elusive concept of ‘equilibrium beach slope’ are poorly understood. A field experiment was designed to measure suspended sediment transport in the swash and inner surf zone through successive tidal cycles in the intertidal zone at Egmond Beach, The Netherlands, using a stack of fiberoptic backscatter sensors (UFOBS-7). The experimental period comprised three large storm events, including an extreme storm with offshore significant wave heights in excess of 7 m. Surveys spanning a >1 km long section of the beach as well as observations from an ARGUS video camera system showed that gently sloping shoreline salients (megacusps) were highly stable in position and displayed only minor slope adjustments through the storm period, whereas steeply sloping embayments were eroded significantly. The sediment transport measurements at a shoreline salient indicated that the subdued net beach response was because offshore sediment transport occurring under surf zone conditions at high tide was compensated by onshore sediment transport at low tide when swash zone processes dominated. Such temporally varying, tidally modulated sediment transport rates and directions are suggested to be able to maintain a quasi-equilibrium intertidal beach slope and the results indicate that a negative feedback existed between initial beach slope and net sediment transport patterns.