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.