Extreme erosion on high-energy embayed beaches: Influence of megarips and storm grouping

Abstract

Megarips have long been recognised as an important, yet poorly documented, mechanism of beach erosion on high-energy embayed beaches. The persistence and cumulative effect of megarips during storm groups, are described from three embayed beaches (Arrifana, Mt. Clérigo and Amoreira) exposed to high-energy wave conditions in the mesotidal, bedrock-dominated southwestern coast of Portugal. Morphological changes, determined by topographic monitoring and supplemented by interpretation of digital imagery over two years, revealed the development of storm-induced megarips, which exerted a major influence on beach erosion. Differences in megarip influence within and between beaches are related to embayment geometry and orientation,which significantly influence the nearshore wave field. Rip location is topographically controlled, being determined by alongshore variations in breaking wave height and obliquity, along with interaction of wave-driven circulation patterns and embayment nearshore topography. Moderate beach sand loss during individual storms was linked to the development of megarips with associated rip-neck and feeder channels. Extreme erosion, however, occurred when megarips and feeder channels persisted during successive storms, promoting continued erosion and seaward sediment export. Observations show that once initiated, megarip channels persist for several months and continue to act as conduits for offshore sediment transport under non-storm conditions. The maintenance of such rip circulation systems, driven by morphodynamic feedback, reduces beach recovery ability until the rip-neck and feeder channels are infilled.