Medium timescale beach rotation; gale climate and offshore island influences
Highlights
► 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.
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.
Section snippets
Physical and geological background
Carmarthen Bay is a relatively wide (30 km), shallow (10 km deep) embayment, the bay is largely controlled by geology, being cut into softer Carboniferous rocks. The Tenby Peninsula, on the western side of the bay, is characterised mainly by rocky cliffs and small embayment's that contain Pocket beaches that have formed as a result of differential erosion of the softer mudstone-rich Carboniferous Coal Measures and Millstone Grit (Halcrow, 2010).
Tenby's landscape and its environs were formed by
Methodology
In the area of study, the active beach profile was characterised by three profiles spaced at 580 m centres, representative of southern (T1), central (T2) and northern (T3) beach sectors (Fig. 1c), extending from the dune system control point to low water (approximately 250 m). The profile locations enabled analysis of beach rotation by detailing the relationship between beach extremities.
The profiles were truncated to the high spring tidal level; sectional volumes, i.e. the morphological
Volume changes
Fig. 2 shows individual cross-shore profile envelopes between 1996 and 2007; all three profiles are concave and, according to the classification of Short and Masselink (1999), indicative of two beach states: a dissipative/intermediate mid to low tidal zone and intermediate/reflective high tidal zone. The greatest variance in beach level occurs within the high tidal zone where the standard deviation (σ) is at its maximum value for all three profiles. (σ = 0.27 m, 0.8 m and 0.55 m respectively). The
Discussion
The study has examined available information on wind, waves and volume change over an 11-year period to establish the influence of offshore islands on morphology changes on South Sands beach, Tenby Pembrokeshire. Sub-aerial beach erosion in southern and central areas (r = − 0.722 and − 0.88, respectively) resulted in annual losses of circa 0.9 m3 m−1 and 0.5 m3 m−1, respectively and northern area accretion (r = 0.85) resulting in an annual gain of circa 1.9 m3 m−1. Intertidal beach erosion was widespread
Conclusions
Headland embayed beaches in the lee of offshore islands react differently to those on open coastlines, particularly when sheltered from predominant wave directions. This research showed that wave rotation in either direction from directly behind offshore islands, has inverse effects on beach morphology, promoting beach rotation. Wave rotations were directly linked to gale wave height, while variations in major atmospheric conditions, through timelag associations, were established as key drivers
Acknowledgement
ATW wishes to acknowledge that this paper is as a result of an affiliation with the e-GEO, Research Centre for Geography and Regional Planning, FCSH Universidade Nova de Lisboa, Lisbon, Portugal.
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