Medium timescale beach rotation; gale climate and offshore island influences

Beach profile surveys, gale climate and atmospheric variations were utilized to assess medium timescale morphological change at South Sands, Tenby, West Wales. Due to beach aspect in relation to offshore islands, gale wave height decreased as wave direction rotated eastwards (r = 0.83) and westwards (r = 0.88). Similarly, wave heights were in attuned to variations in positive (r = 0.68) and negative (r = − 0.72) NAO Index, showing a wave height reduction occurred during weakly negative/positive or transitory phases; morphological change was attuned to atmospheric variation at a 2-year timelag. Shelter from offshore islands is given to waves from the predominant southwesterly direction and was confirmed by negligible correlation with South Sands morphology. However, outside the shelter of these offshore islands, correlation was found between south-eastward rotating wave directions (135°–180°) and morphological change, which resulted in southern and central beach erosion and accretion to the north. With a southwesterly rotation (243°–256°) the opposite was true. Beach rotation expressed by volume change within the sub-aerial zone had a negative phased relationship between beach extremities (r = − 0.94) and a timelagged association within the intertidal zone (r = 0.55). Analyses resulted in the development of two medium timescale rotation models based on incident wave direction and climatic variability. Results have global implications for headland bays in the lee of offshore islands, as well as macro-tidal beach areas; and consequently similar models could inform local, regional and national beach management strategies

► Using beach profiles and environmental forcing we assess beach rotation.
► We examine how beach morphology is influenced by offshore islands.
► NAO variations were established as a key driver for beach rotation.
► Gale wave approach modified by offshore islands also had influence.
► Knowledge enabled two conceptual models to be proposed.