Abstract
Abstract. The extreme sea levels and waves experienced around the UK's coast during the 2013/14 winter caused extensive coastal flooding and damage. Coastal managers seek to place such extremes in relation to the anticipated standards of flood protection, and the long-term recovery of the natural system. In this context, return periods are often used as a form of guidance. This paper provides these levels for the winter storms, and discusses their application to the given data sets for two UK case study sites: Sefton, northwest England, and Suffolk, east England. Tide gauge records and wave buoy data were used to compare the 2013/14 storms with return periods from a national data set, and also joint probabilities of sea level and wave heights were generated, incorporating the recent events. The 2013/14 high waters and waves were extreme due to the number of events, as well as the extremity of the 5 December 2013 "Xaver" storm, which had a high return period at both case study sites. The national-scale impact of this event was due to its coincidence with spring high tide at multiple locations. Given that this event is such an outlier in the joint probability analyses of these observed data sets, and that the season saw several events in close succession, coastal defences appear to have provided a good level of protection. This type of assessment could in the future be recorded alongside defence performance and upgrade. Ideally other variables (e.g. river levels at estuarine locations) would also be included, and with appropriate offsetting for local trends (e.g. mean sea-level rise) so that the storm-driven component of coastal flood events can be determined. This could allow long-term comparison of storm severity, and an assessment of how sea-level rise influences return levels over time, which is important for consideration of coastal resilience in strategic management plans.
Highlights
Storm surges and flooding are a threat to low-lying coastal zones, with risks increasing due to sea-level rise (Haigh et al, 2010; Menéndez and Woodworth, 2010; Wahl et al, 2011) and floodplain development (Hanson et al, 2011; Stevens et al, 2014, 2015)
The extremity of the 2013/14 “storm season” was assessed for two case studies (Sefton, northwest coast; Suffolk, east coast), from a perspective that could, for example, help to inform coastal managers who have to plan for defence upgrades
The second objective was to assess temporal clustering of extremes, as determined by counting high water (HW), HS and joint level (JL) above an annual return period threshold, and how many of these events fell within annual storm seasons across the data sets
Summary
Storm surges and flooding are a threat to low-lying coastal zones, with risks increasing due to sea-level rise (Haigh et al, 2010; Menéndez and Woodworth, 2010; Wahl et al, 2011) and floodplain development (Hanson et al, 2011; Stevens et al, 2014, 2015). Wadey et al.: Assessment and comparison of extreme sea levels and waves (Matthews et al, 2014), and combined with large tides, caused an unusually clustered sequence of extreme coastal sea-level events (Wadey et al, 2014). The magnitude of these high waters and lack of time for recovery between events caused coastal erosion, damage to defences and flooding. This unusual “season” began with the St. Jude’s Day Storm (27 October 2013), followed by extreme sea levels in early November.
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