Beachface morphology and surf beat sediment transport in laboratory scale surf and swash zones

Abstract

Predicting the evolution of the beachface is important in terms of coastal protection, recreation and environmental sustainability. Swash zone sediment transport plays a major role in this morphological response. However, despite recent advances in swash zone modelling and data collection, specific details are lacking, notably the cross-shore variation in net transport rates, the influence of surf beat and wave groups on swash sediment transport and the effects of the feedback between the surf and swash zones. This paper provides novel data from carefully controlled small-scale laboratory experiments investigating surf beat sediment transport. The data include very detailed observations of the cross-shore distribution of net sediment transport in the surf and swash zones for both eroding and accreting beaches. Further, these distributions are contrasted for monochromatic waves, monochromatic waves with surf beat and bichromatic wave groups with surf beat. In this case, free surf beat is shown to reduce nearshore wave heights and reduce short wave velocity amplitudes. Free surf beat is found to promote onshore sediment transport under accretionary conditions and reduce offshore transport under erosive conditions. Hence, the free surf beat tends to transform the beach toward a reflective state and possibly protects the beach. However, a bichromatic wave group shows the opposite effect. Additional data show the influence of the beach face evolution on the onshore migration of longshore bars, illustrating coupling between swash and surf zone morphology.