Short-Term Beach Rotation Processes in Distinct Headland Bay Beach Systems

This paper investigates morphological changes in headland bay beaches with emphasis on short-term beach rotation processes, elucidating how it is affected by the planform/degree of curvature of the beach, and by the different morphodynamic characteristics of the beach systems monitored. The beaches monitored in the present study were Balneario Camboriu, Brava and Taquaras/Taquarinhas beaches. They have different lengths, degrees of curvature, and levels of exposure to the incident waves, and represent different beach types. Indentation ratio and the SL/CL ratio were measured, and beach profile surveys every 15 days were made in order to measure variations of beach volume and width for each beach. Visual wave and beach observations were recorded daily. Results indicate that morphological changes in headland bay beaches are influenced mainly by beach planform and indentation ratios, presence of rip currents and submerged bars, shoreline length, and beach type. The beach volume and with variations demonstrated that headland bay beaches have defined sectors with different behaviour, as influenced by headland impact on incident waves and longshore currents. Short-term beach rotation is manifested as out of phase variation of beach volume and width between opposite ends of a headland bay beach. Rotation amplitude of about 20 meters was observed at a dissipative beach (Balneario Carnboriu), and on the reflective beach of Taquaras/Taquarinhas. Brava beach did not show clear patterns of short-term beach rotation, but there was a subdivision of the beach into two sectors with different magnitudes of sediment removal and behaviour. The occurrence of short-term beach rotation processes in some of the beaches indicates that, erosive events are often caused by a realignment of the beach shoreline in response to a shift in incident wave direction. In these cases the sediment eroded is not lost from the beach system but deposited elsewhere along the beach, and often returning to the initial location in response to a new shift in wave direction.