Beach and Nearshore Systems

Abstract

Synopsis Sandy beach and nearshore zones extend from the limits of wave action on the beach backshore offshore to the limit of the movement of sand on the bed by waves. These are the most dynamic of coastal systems and there is a continuous exchange of sediment between the two, driven by alternations between storm and fairweather conditions. Because even quite small waves are capable of moving sand, the continued presence of beaches reflects a dynamic equilibrium between processes that act to move sediments offshore and those that tend to move them onshore. On sandy coasts the profile normal to the coast tends towards an equilibrium form with slope decreasing exponentially offshore. The actual steepness of the profile is controlled primarily by grain size, with the slope decreasing with decreasing grain size. Repeated measurements over a period of years shows that the elevation of the bed can fluctuate over one or two metres or more close to shore, but that the amount of fluctuation or reworking by waves decreases into deeper water. This is a response to decreasing wave orbital motion with water depth and the decreasing frequency of storm events with waves large enough to move sediment in deeper water. The beach and nearshore profile may be smooth, or more often it is characterised by the presence of sand bars and associated troughs landward of them.