BEACH CUSP MORPHODYNAMICS

Detailed measurements of three-dimensional beach cusp morphology were made on a steep gradient, low energy, microtidal beach in Perth, Weste rn Australia. During the held campaign a variety of wave conditions an d tidal ranges were experienced, and these differing hydrodynamic cond itions were reflected in a consistent pattern of morphological changes to the beach cusp system. A useful parameter to delineate between tre nds of cusp destruction and re-formation appeared to be the surf simil arity parameter xi=tan beta/root H-0/L-0, where H-0 is offshore wave h eight, L-0 is deep water wave length and tan beta is beach gradient. F or xi<1.2 the beach cusps were planed off, whereas cusp morphology was enhanced when xi>1.2. A small storm was experienced at the start of t he field campaign period and resulted in considerable erosion of the b each face. The cusp morphology across the lower beachface was destroye d, but a subtle remnant of the pre-storm cusp morphology was preserved on the upper beachface. When cusps reformed after the storm, under th e influence of declining wave conditions, they appeared at the same lo cation and with the same dimensions as the pre-storm cusp morphology. Hence, it is considered that the cusp re-formation was controlled more by the antecedent morphology than the hydrodynamic conditions. This i ndicates that positive feedback between swash hydrodynamics and beachf ace morphology, necessary to form beach cusps, does not require a larg e variation in relief. (C) 1997 John Wiley & Sons, Ltd.