BEACH STEPS - AN EVOLUTIONARY PERSPECTIVE

Field observation of contrasting beach-step behavior at Canaveral Nati onal Seashore on two subsequent days when incident-wave conditions in the inner surf zone were similar prompted this re-examination of our c onceptual and quantitative understanding of beach steps. These lower-f oreshore features are more complex than previously assumed, evolving t hrough erosional as well as accretional phases, and displaying equifin ality in geometric form but not necessarily internal sedimentary struc ture. Past and recent evidence is reviewed that links beach steps to i ncident waves at the surging-plunging transition and to the action of a backwash vortex. Tides and low-frequency waves likely play no direct role in beach-step initiation, although their presence can have prono unced influences on modulating nearshore hydrodynamics, and thus, on b each-step maintenance and evolution. A generalized, conceptual model c apturing these aspects of beach-step dynamics is presented. Beach-step initiation proceeds via step ''carving'', ''excavation'', or ''buildi ng'' depending on the erosional-accretional character of the beach-for eshore system. Subsequent evolution of the step form may take one of s everal alternative morphodynamic pathways including step-face ''retrea t'', step ''drag down'' or step ''infilling/elimination'' depending on tidal stage/range or wave set-up/set-down. Additional data on equilib rium beach-step forms and associated morphodynamic and hydrodynamic co nditions in the field are necessary before quantitative models of beac h-step existence and evolution can be formulated with realistic result s.