Non-uniformity of storm impacts on three high-energy embayed beaches

Coastal storms are highly significant and destructive events and important natural hazards that have major impacts upon the physical and human-cultural coastline environments. Exceptional coastal storms in the last decades have emphasized the urgent need to reinforce scientific knowledge regarding the frequency, impact and role of these extreme events. This is particularly relevant for embayed beaches, where geomorphological control promotes characteristic morphodynamic responses that are distinct from the better studied long-linear beaches. This paper analyses storm response from datasets of morphological change obtained from three embayments located in three distinct wave environments. Analysis of the morphological response to extreme storm events, particularly the 2007 March Equinox storm in eastern South Africa, the January-February 2009 storm group in southwestern Portugal and the December 2011-January 2012 storm group in northwestern Ireland, demonstrate that storm-induced morphological change in the Durban Bight, Arrifana beach and Portstewart Strand is highly variable alongshore. Retreat values for the mean high water spring shoreline are shown to differ significantly within each embayment. This alongshore non-uniform response to storm-events is attributed to boundary effects triggered by the presence of natural headlands or outcrops and/or artificial engineering structures. Megarip development drives variability in storm response in Arrifana, while wave shadowing and interruption of longshore sediment movement under south-easterly/west-northwesterly storm waves promotes the northward/westward increase in shoreline retreat in the Durban Bight and Portstewart Strand, respectively. Boundary effects significantly influence short-term morphodynamic behaviour during storms and, consequently, determine alongshore non-uniform morphologic response to storm events, irrespective of embayment dimensions and event magnitude.