BEACH STEPS - AN EVOLUTIONARY PERSPECTIVE

Field observation of contrasting beach-step behavior at Canaveral National Seashore on two subsequent days when incident-wave conditions in the inner surf zone were similar prompted this re-examination of our conceptual and quantitative understanding of beach steps. These lower-foreshore features are more complex than previously assumed, evolving through erosional as well as accretional phases, and displaying equifinality in geometric form but not necessarily internal sedimentary structure. Past and recent evidence is reviewed that links beach steps to incident 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 pronounced influences on modulating nearshore hydrodynamics, and thus, on beach-step maintenance and evolution. A generalized, conceptual model capturing these aspects of beach-step dynamics is presented. Beach-step initiation proceeds via step ''carving'', ''excavation'', or ''building'' depending on the erosional-accretional character of the beach-foreshore system. Subsequent evolution of the step form may take one of several alternative morphodynamic pathways including step-face ''retreat'', step ''drag down'' or step ''infilling/elimination'' depending on tidal stage/range or wave set-up/set-down. Additional data on equilibrium beach-step forms and associated morphodynamic and hydrodynamic conditions in the field are necessary before quantitative models of beach-step existence and evolution can be formulated with realistic results.