Shoreline dynamics and beach rotation of artificial embayed beaches

摘要

Artificially embayed beaches are an increasingly common environment in retreating and urban coasts. The study of embayed beaches usually focuses on the concept of some equilibrium configuration of the shoreline caused by the wave approach and the location of protection structures, but detailed studies of their shoreline dynamics are scarce. The study reported herein monitored the shoreline dynamics of three artificial embayed beaches in Barcelona City during a three-year period using an Argus video system to examine the behaviour of the emerged beach in order to assess the main factors affecting the shoreline, and to analyze the processes causing beach rotation at different time-scales. The length and degree of protection of the three beaches were different and so were their morphological behaviours. Two of the beaches presented a general retreating trend with rapid/abrupt displacements of the shoreline caused by oblique wave incidence during storm events and to the effects of beach nourishment. After nourishment, slower changes at the beaches determined beach evolution which was also influenced by the formation of long-lasting megacusps. The third beach, the most protected one, showed an accretionary trend, no formation of megacusps and episodes of beach rotation related to the gradual recovery of the beach after storm events. It is concluded that factors controlling the evolution of Barcelona City beaches are associated with both natural processes and man-induced actions, resulting in a more complex pattern than would be expected for small embayed beaches. Beach rotation appears as a key process for understanding the morphodynamics of embayed beaches. It occurs as a fast response to storms but also as a medium-term adaptation of the shoreline configuration to morphological changes in the beach or to the recovery of a more stable orientation after storm events and man-made changes in the beach orientation resulting from nourishment or sand relocation.