Morphological evolution of beach cusps and associated swash circulation patterns

Abstract

A numerical model capable of simulating the motion of water particles on beach cusp morphology under the influence of an initial velocity and gravity is presented. The model indicates that the typical swash pattern on beach cusps is three-dimensional, with wave uprush diverging at the cusp horns resulting in concentrated backwash streams in the embayment. The degree of horn divergence is an increasing function of the parameter ε S/λ 2 , where ε quantifies the prominence of the beach cusps, S is the horizontal swash excursion length and λ 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 characterised by a particular pattern of swash circulation, that can be delineated using ε S/λ 2 . When ε S/λ 2<0.015 , beach cusp morphology is large and/or subdued in relation to the swash length. Swash circulation is essentially two-dimensional (oscillatory flow) and results in steepening of the beachface and infilling of the cusp embayments. For ε S/λ 2=0.015 to 0.15, wave uprush is deflected from the cusp horns and flows into the embayments where it exits in a concentrated backwash stream (horn divergent flow). The ensuing swash/backwash inequality reinforces cusp development and maintains existing cusp morphology. When ε S/λ 2>0.15 , beach cusps are small and/or pronounced in relation to the length of the swash. Overtopping and ponding of the cusp horn takes place and swash circulation is from the embayment to the horn (horn convergent flow). As a result, cusp horns are eroded and accretion occurs in the embayments.