Regionally-Coherent Embayment Rotation: Behavioural Response to Bi-Directional Waves and Atmospheric Forcing

Abstract

Bi-directional wave climates often drive beach rotation, increasing erosional risk at semi-sheltered locations. Identification of rotation and forcing mechanisms is vital to future coastal defence. In this study, regional investigation of modelled wave data revealed strong bi-directionality between dominant south-westerly and sub-dominant easterly waves for 14 offshore locations along the length of the south coast of England, U.K. South-westerly wave power was well correlated to positive phases of the West Europe Pressure Anomaly (WEPA), whilst easterly wave power was well correlated with negative phases of the North Atlantic Oscillation (NAO). Additionally, decadal records of beach morphological change and associated wave forcing, were investigated for 22 coastal sites across the same region. Significant rotational behaviour was identified at 11 sites, leading to the creation of a rotation index. Beach rotation was attributed to shoreline angle, with the strongest rotation occurring at south-east-facing beaches, with high obliquity to dominant south-westerly waves. The beach rotation index was well correlated with the normalized balance of wave power from opposing south-westerly and easterly directions. Direct correlations between beach rotation and WEPA at two sites showed that future forecasts of atmospheric indices may allow prediction of rotational beach state, at seasonal scales.

Atmospheric controls and long range predictability of directional waves in the United Kingdom & Ireland

“…All nodes in the southern Ireland, English Channel and southern North Sea coasts (southfacing) are strongly directionally bimodal (Figure 3), with coastal orientation suggesting dominance of alongshore sediment transport processes, which is supported by observations along this coast by Wiggins et al (2019b). Analysis shows that WEPA (and EA to a lesser extent) significantly explains variability in winter-averaged wave power for all southwesterly orientated…”

“…It is well established that climate indices like ENSO in the Pacific and AO/NAO in the Atlantic are leading modes of atmospheric variability and strongly affect winter wave energy (Dodet et al, 2010;Bromirski et al, 2013;Castelle et al, 2017), and recent studies have also shown how extreme phases can lead to large-scale coastal erosion and shoreline change (Masselink et al, 2016;Barnard et al, 2017;Dodet et al, 2018). But, there is now a growing base of evidence highlighting the role leading modes of climate variability also have in controlling wave direction and associated longshore sediment re-distribution and shoreline rotation at the coast (e.g., Silva et al, 2012;Splinter et al, 2014;Goodwin et al, 2016;Wiggins et al, 2019b). In extreme cases, -3 as shown by Wiggins et al (2019a) when studying the impacts of the extreme storm wave events of the 2013/14 winter in Northwest Europe, resultant embayment rotation in semi-sheltered regions can lead to extreme coastal vulnerability and infrastructural failure.…”

“…Where the wave climate is significantly directionally bimodal, a wave power directionality index (WDI) is computed following Wiggins et al (2019a), which represents the winter-averaged standardized wave power balance between two opposing wave directional modes, shown by Wiggins et al (2019b) to correlate with observed beach rotation. At each node, an index of the relative balance between winter wave power contributions from the two modal directions is computed, using the equation:…”

“…Further to this, modelling work by Splinter et al (2012) and field observations by Harley et al (2017) havehighlighted, not only the relationship between climate indices on wave direction and the subsequent impact on shoreline dynamics along the east coast of Australia, but also the impact of storm wave direction on coastal vulnerability along embayed coasts in general. In northwest Europe, research by Wiggins et al (2019a) shows that winter-averaged variability in NAO and WEPA has significant skill in explaining wave directional balance in regions where wave climate is strongly bi-directional, as well as driving beach rotation in these regions (Wiggins et al, 2019b).…”

The Mediterranean Coast of Andalusia (Spain): Medium-Term Evolution and Impacts of Coastal Structures

“…The analysis of beach plan form and its temporal variation, which depends on wave climate [75,82] and natural and human structures characteristics [73], can be used to determine coastal sectors with drift-aligned or swash-aligned shoreline trends. Drift-aligned shoreline sectors are the result of a clear unidirectional transport while swash-aligned shoreline sectors are the result of a bidirectional longshore transport and/or a cross-shore transport [31].…”