Studies of coastal vulnerability due to climate change tend to focus on the consequences of sea level rise, rather than the complex coastal responses resulting from changes to the extreme wave climate. Here we investigate the 2013/2014 winter wave conditions that severely impacted the Atlantic coast of Europe and demonstrate that this winter was the most energetic along most of the Atlantic coast of Europe since at least 1948. Along exposed open‐coast sites, extensive beach and dune erosion occurred due to offshore sediment transport. More sheltered sites experienced less erosion and one of the sites even experienced accretion due to beach rotation induced by alongshore sediment transport. Storm wave conditions such as were encountered during the 2013/2014 winter have the potential to dramatically change the equilibrium state (beach gradient, coastal alignment, and nearshore bar position) of beaches along the Atlantic coast of Europe.

The southwest coast of England was subjected to an unusually energetic sequence of Atlantic storms during the 2013/2014 winter, with the 8-week period from mid-December to mid-February representing the most energetic period since at least 1953. A regional analysis of the hydrodynamic forcing and morphological response of these storms along the SW coast of England highlighted the importance of both storm-and site-specific conditions.The key factor that controls the Atlantic storm wave conditions along the south coast of southwest England is the storm track. Energetic inshore wave conditions along this coast require a relatively southward storm track which enables offshore waves to propagate up the English Channel relatively unimpeded. The timing of the storm in relation to the tidal stage is also important, and coastal impacts along the macro-tidal southwest coast of England are maximised when the peak storm waves coincide with spring high tide. The role of storm surge is limited and rarely exceeds 1 m.The geomorphic storm response along the southwest coast of England displayed considerable spatial variability; this is mainly attributed to the embayed nature of the coastline and the associated variability in coastal orientation. On west-facing beaches typical of the north coast, the westerly Atlantic storm waves approached the coastline shore-parallel, and the prevailing storm response was offshore sediment transport. Many of these north coast beaches experienced extensive beach and dune erosion, and some of the beaches were completely stripped of sediment, exposing a rocky shore platform. On the south coast, the westerly Atlantic storm waves refract and diffract to become southerly inshore storm waves and for the southeast-facing beaches this results in large incident wave angles and strong eastward littoral drift. Many south coast beaches exhibited rotation, with the western part of the beaches eroding and the eastern part accreting.

Field measurements of vertical pore‐pressure gradients within the bed are used to quantify instantaneous (8 Hz) rates of swash infiltration‐exfiltration across the beach face. Cyclic infiltration‐exfiltration is associated with individual swash events, with observed vertical flow rates O(10−3) m/s. Rates of net swash‐groundwater exchange (i.e., through‐bed flow integrated over several swash cycles) are two orders of magnitude smaller. At the timescale of individual swashes, vertical pore‐pressure gradients within the beach face are much greater than horizontal pore‐pressure gradients. This permits application of the numerical solution of Darcy's law for one‐dimensional vertical flow to model fluctuating pore pressures (and hence vertical through‐bed flow). Vertical flow through a porous bed modifies sediment mobility in (at least) two ways: (1) Seepage forces change the effective weight of surficial sediments, and (2) boundary layer “thickening” or “thinning” result in altered bed shear stresses. By considering these two (opposing) effects separately, a new Shields parameter is derived that incorporates terms for through‐bed flow. Simulation of time‐varying seepage force and bed stress effects over an uprush‐backwash cycle suggests that the effect of altered bed stresses dominates over the change in effective weight and that infiltration‐exfiltration effects are most important during uprush. Simulated transport rates are increased by up to 40% of the peak transport rate during uprush and reduced by 10% during backrush. In summary, swash infiltration‐exfiltration across a saturated beach face enhances the net upslope transport of sediment.

A pioneering and replicable method based on a 66‐year numerical weather and wave hindcast is developed to optimize a climate index based on the sea level pressure (SLP) that best explains winter wave height variability along the coast of western Europe, from Portugal to UK (36–52°N). The resulting so‐called Western Europe Pressure Anomaly (WEPA) is based on the sea level pressure gradient between the stations Valentia (Ireland) and Santa Cruz de Tenerife (Canary Islands). The WEPA positive phase reflects an intensified and southward shifted SLP difference between the Icelandic low and the Azores high, driving severe storms that funnel high‐energy waves toward western Europe southward of 52°N. WEPA outscores by 25–150% the other leading atmospheric modes in explaining winter‐averaged significant wave height, and even by a largest amount the winter‐averaged extreme wave heights. WEPA is also the only index capturing the 2013/2014 extreme winter that caused widespread coastal erosion and flooding in western Europe.

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