Abstract

Large cusps are common along many shorelines, sometimes isolated but at times forming a rhythmic series of such forms with a uniform spacing. The role of the cell circulation, rip currents and associated longshore currents, in producing such cusps is examined. A series of laboratory wave basin experiments is presented in which the cell circulation of water modifies an initially smooth and straight sand beach. In all cases, it is found that cusps develop in the lee of the rip currents so that a series of cusps is formed with the same spacing as the rip currents. These are compared with giant cusps and beach cusps observed on natural beaches. An equilibrium cusp development is found in the experiments in which, having produced the cusps, all cell circulation and other longshore currents suddenly cease to exist; the rip currents disappear. It is demonstrated that the equilibrium state consists of a balance between the forces that tend to drive a longshore current from an oblique wave approach to the flanks of the cusps, and the forces that normally produce the cell circulation. Cusps, having been produced by rip currents, can therefore be observed on beaches although the rips are no longer present.

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