Rhythmic beach cusp formation: A conceptual synthesis

Abstract

Observations of a sandy beach at Santa Barbara, California, during February 1980 documented the sequence of beach cusp formation, replacement by a second set of rhythmic cusps obliterating the first set, and finally complete planation of all rhythmic cusps during a major storm. Although neither cusp-building event was observed during its initiation, the wave and tide conditions during formation were well documented by in-situ instrumentation. Based on these field observations and those by other investigators, a coherent conceptual theory of beach cusp formation is proposed, including formative, maintenance and destructive phases. This theory proposes rhythmic cusp formation during relative stillstands of sea level of sufficient duration to permit initial perturbation of beach geometry; regularly spaced cusps acquire their initial spacing from subharmonic edge waves (such that spacing derives from the incident wave period); growth of the initial perturbation continues until cusps reach a limiting value, not necessarily with continuation of edge wave motion; the cusp spacing persists until a change in wave conditions either destroys the cusps, or establishes a radically different spacing; and finally, that large storms will destroy the rhythmic cusp field, leaving either a planar beach or a beach with arhythmic cusp-like features.