MORPHOLOGICAL EVOLUTION OF BEACH CUSPS AND ASSOCIATED SWASH CIRCULATION PATTERNS

A numerical model capable of simulating the motion of water particles on beach cusp morphology under the influence of an initial velocity an d gravity is presented. The model indicates that the typical swash pat tern on beach cusps is three-dimensional, with wave uprush diverging a t the cusp horns resulting in concentrated backwash streams in the emb ayment. The degree of horn divergence is an increasing function of the parameter epsilon(S/lambda)(2), where epsilon quantifies the prominen ce of the beach cusps, S is the horizontal swash excursion length and lambda is the cusp spacing. The numerical experiments are supplemented with detailed field measurements of beach cusp morphological change. The field data include three types of morphological response, each cha racterised by a particular pattern of swash circulation, that can be d elineated using epsilon(S/lambda)(2). When epsilon(S/lambda)(2) < 0.01 5, beach cusp morphology is large and/or subdued in relation to the sw ash length. Swash circulation is essentially two-dimensional (oscillat ory flow) and results in steepening of the beachface and infilling of the cusp embayments. For epsilon(S/lambda)(2) = 0.015 to 0.15, wave up rush is deflected from the cusp horns and flows into the embayments wh ere it exits in a concentrated backwash stream (horn divergent flow). The ensuing swash/backwash inequality reinforces cusp development and maintains existing cusp morphology. When epsilon(S/lambda(2) > 0.15, b each cusps are small and/or pronounced in relation to the length of th e swash. Overtopping and pending of the cusp horn takes place and swas h circulation is from the embayment to the horn thorn convergent flow) . As a result, cusp horns are eroded and accretion occurs in the embay ments. (C) 1998 Elsevier Science B.V. All rights reserved.