Medium timescale beach rotation; gale climate and offshore island influences

Abstract

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

Introduction

Headlands have a significant influence on planform, sediment transport and morphodynamics of embayed beaches (Short and Masselink, 1999). Complexities of headland impact in altering beach morphodynamic behaviour often mean that embayed beaches receive less attention than unconstrained beaches (Klein et al., 2002). Most research has concentrated on small groups of embayed beaches (Sedrati and Anthony, 2007, Dehouck et al., 2009, Loureiro et al., 2009) and many works have focused at decadal, multi-decadal, and to a lesser extent, at short-term timescales. In the former, detailed studies have been made by Short et al., 2000, Short and Trembanis, 2004, Ranasinghe et al., 2004, Anthony and Dolique, 2004, Dolique and Anthony, 2005, Ojeda and Guillen, 2008, Pinto et al., 2009, and in the latter by Ruiz de Alegria-Arzaburu and Masselink, 2010, Thomas et al., 2011a.

On a regional basis, Klein and Menezes (2001) used several parameters (i.e. wave and sedimentary statistics, fall velocity) in conjunction with beach profiling and planform characteristics to typify 17 differently exposed pocket beaches in Brazil. Bowman et al. (2009) carried out the largest detailed study along the Catalan coast, studying 72 pocket beaches to determine indices based on standard planform parameters that classified embayments by degrees of indentation.

Beach rotation is a phenomenon in embayed beaches that are exposed to variable directional wave climate and short-term beach rotation is sometimes caused by shoreline realignment in response to a shift in incident wave direction (Klein et al., 2002, Thomas et al., 2011a). These shifts have been associated with changing climatic conditions, for example, in a detailed study of Narrabeen Beach, New South Wales, Australia, Ranasinghe et al. (2004), showed that variations during strong positive/negative Southern Oscillation Index phases (El Niño/La Niña) influenced beach rotation. Other variables also influence rotation, as shown by Anthony et al. (2002) at Montjoly Beach, Cayenne, French Guiana, where patterns of nearshore mudbank migration induced rotation. Similarly, Thomas et al. (2011b) linked beach diminution and migration, associated with spit collapse, to multi-century shoreline rotation within the current area of study.

This study assesses shoreline behavioural patterns between April 1996 and April 2007 at South Sands, Tenby, South Pembrokeshire West Wales [GR 212200, 198599, Fig. 1]. Extensive research has been carried out in headland embayed areas. However, this work investigates morphological change of an embayed beach and this differs from most other studies for two important reasons; (1) morphological variations of this littoral are influenced by the position of offshore islands, and (2) the inter-survey-averaged NAO Index is used irrespective of phase or strength. Together with these new findings, this research will make a further contribution to macro-tidal beach studies for which information is extremely sparse. In addition to major climatic variation (NAO) several forcing variables related to wind and wave regimes, are used to evaluate how offshore Caldey and St Margaret's islands influence morphological changes and beach rotation.