Coastal Storms: Detailed Analysis of Observed Sea Level and Wave Events in the SCOPAC Region (Southern England)
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SCOPAC RESEARCH PROJECT Coastal storms: detailed analysis of observed sea level and wave events in the SCOPAC region (southern England) Debris at Milford-on-Sea after the “Valentines Storm” February 2014. Copyright New Forest District Council. Date: December 2020 Version: 1.1 BCP - SCOPAC 2020 Rev 1.1 Document history SCOPAC Storm Analysis Study: Coastal storms: detailed analysis of observed sea level and wave events in the SCOPAC region (southern England) Project partners: • Bournemouth Christchurch Poole (BCP) Council / Dorset Coastal Engineering Partnership • Ocean & Earth Science, University of Southampton (UoS) • Coastal Partners (formerly Eastern Solent Coastal Partnership (ESCP)) Project Manager: Matthew Wadey (BCP Council) Funded: Standing Conference on Problems Associated with the Coastline (SCOPAC) Data analysis: Addina Inayatillah (UoS), Matthew Wadey (BCP/DCEP), Ivan Haigh (UoS), Emily Last (Coastal Partners) This document has been issued and amended as follows: Version Date Description Created by Verified by Approved by 1.0 16.11.20 SCOPAC Storm MW, AI, IH, SC Analysis Study EL 1.1 30.12.20 SCOPAC Storm MW, AI, IH, SC SCOPAC Analysis Study EL RSG BCP - SCOPAC 2020 Rev 1.1 SCOPAC Storm Analysis Study PROLOGUE Dear SCOPAC members, Our coastline is exposed to storm surges and swell waves from the Atlantic that as we know can result in flooding and erosion. Changing extreme sea levels and waves over time need to be assessed so risks can be understood; as both one-off events and as a consequence of successive events (“storm clustering”). The notable winter of 2013/14 saw repeated medium to high magnitude events prevailing over a relatively short time period. Many beaches were stripped of material, resulting in extreme overtopping and undermining of sea defences. In response to the storms regarded as unprecedented, the Environment Agency released £270 million funds for emergency works. The impacts of the 2013/14 winter were severe, with £130 million in damages to residential properties and £170 million in damages to businesses situated on the coast (Environment Agency, 2016). The consequences would have been much worse however, if flood forecasting and warning systems and coastal management practices were not in place. Since the winter of 2013/14, it has been noted that the SCOPAC region has continued to experience draw down of beaches and localised sea defence failures. We know that sea levels are rising, as updated recent information from the UKCP18 projections is an integral part of our FCERM planning. However, the unknown question is whether storm magnitude and frequency are also increasing, signifying a climate change influence? As part of the National Network of Regional Monitoring Programmes, nearshore wave buoys have been deployed since 2003, providing a valuable dataset for analysis to help answer this question. At a SCOPAC meeting on the 27th January 2017, the officers and councillors were interested in this subject following another challenging winter in 2015/16 and Storm Angus causing damage to assets at the start of the 2016/17 storm season. Subsequently, SCOPAC commissioned this research project to put the recent winter storm seasons into context with longer datasets. Dr Ivan Haigh and Dr Matthew Wadey, experts in the analysis of sea level, storm surge and wave data sets, led on the investigation. It was decided that this research would form a first phase, focusing on the hydro- dynamic forcing factors, with a potential second phase focusing on the impacts of storms on SCOPAC’s beaches and sea defences. This technical report summarises the work undertaken by the project team, for which there is a supporting info-graphic for SCOPAC members and the public, highlighting the key findings. We hope you find this research as interesting as we do, Dr Samantha Cope SCOPAC Research Chair 3 SCOPAC Storm Analysis Study EXECUTIVE SUMMARY Prompted by concerns over climate change and recent stormy winters, in particular that of 2013/14 (W2013/14), this study assesses sea level and wave data along the south coast of the UK to determine context and extremity of events and winters within the spans of these data sets. These data are available as 15-30 minutely spaced time series since the early 20th century for sea level and since 2003 for waves. In terms of the conceptual flood system known as the “source-pathway- receptor-consequence” model, this study focuses upon coastal flood “sources”. “Consequences” are briefly reviewed to extend the context but future focus on the other components is recommended. This formulation of regional and local scale results within the SCOPAC region is quite unique amongst other studies and gives up to date guidance on possible indicators of climate change and winter extremities for a range of audiences (coastal practitioners, academics, politicians). In terms of TRENDS AND PATTERNS, 9 sea level (tide gauge) sites were assessed across the English Channel. At all sites, mean sea level is rising and the rate of rise has accelerated in recent years - this is the most certain finding to take from this analysis. The most reliable long- term rate relevant to the English Channel (and the SCOPAC region) at Newlyn indicates a rise in mean sea level of 1.86 mm/yr. between 1915 and 2019, increasing to 3.8 mm/yr. between 1990 and 2019. The analysis suggests changes to tidal characteristics such as mean high water and tidal range, although as yet with no consistent spatial trend evident across the region. Statistically significant increases in storm surges are not found, consistent with findings from other parts of the world. All the 9 wave buoy sites (except the furthest east, Folkestone) have an increasing trend in wave height. There is a signal that wave period has increased at most sites but with trends that are not statistically significant due to the short data span (2003 onwards) – hence the exceedance analysis in the next section is pertinent to understand this. Combining wave height and period into a wave power (indicative of the energy dispensed onto the shoreline) and run up (indicative of overtopping or flood potential), there is an upward trend with time (the report explains how the statistical significance varies). The W2013/14 had notably higher wave power across all sites. Combining waves and sea level into time series of total water level suggests a statistically significant annual increase for the Solent and eastern SCOPAC region across the entirety of the datasets, with the W2013/14 an outlier for clustering and extremes. Events (peaks in the respective data) were assessed against known limits (e.g. return periods) to count THRESHOLD EXCEEDANCES from 2003-2019 at four “site pairings” (sea level and wave recording sites), as follows: (1) Weymouth-Chesil; (2) Poole Bay, (3) The Solent and (4) Newhaven-Rustington. A consistent statistical trend cannot be found, mostly likely because of the relatively short data lengths. However, comparing winters since 2003 highlights how recent years have seen growing “clusters” of extreme events: in the western-central SCOPAC area, W2013/14 definitively produced the highest count of extreme sea level and wave height events; in the east SCOPAC region W2013/14 and W2015/16 were comparably extreme. 4 SCOPAC Storm Analysis Study In terms of STORM EVENTS AND EXTREMES from the four site pairings, in the west of the region W2013/14 recorded the highest surge, wave height, wave run-up, and most powerful waves (all on the 14th February 2014 “Valentines Storm”) since 2003. In the west and central SCOPAC area, the highest water level was on 10th March 2008 although with missing sea level data at Bournemouth considered, the 2014 Valentines Storm is probably the most extreme event in terms of physical loading upon coastal defences and flood potential. The 6th January 2014 is also noteworthy for the prolonged, region-wide, high wave run up and power. In the Solent: W2013/14 had the highest sea level event since 1961 (on the 5th-6th December 2013 when a North Sea surge propagated from the east during calm wave conditions in the Channel). Meanwhile the 5th February 2014 and 14th February 2014 produced the most powerful waves and highest run up since the records began in 2003. The highest waves on record in the eastern part of the region were on the 24th December 2013. The highest total water levels were more recently during Storm Eleanor in January 2018. Again, this is indicative of the past recent decade containing more extreme events than other comparable intervals of time during the past half century. The next area of investigation was into BIMODAL WAVES AND SWELL. Such conditions are generally considered as unusual and energetic wave conditions that can cause more damage at the coast. For each winter season, this part of the analysis highlighted that the W2013/14 and W2015/16 stand out as the most bimodal since data sets began in 2003; with Jan-Feb 2014 and Dec-2015 being the most extreme months. It seems given events that overlap with these months, that persistent bimodal wave activity could be, as previous studies have alluded to, directly linked to beach drawdown and structural failures. A linear trend through the data suggests the possibility of increasingly bimodal seas over time – further work is needed to assess the cause and nature of this at a regional and local scale, since these findings are consistent with broader scale global studies that suggest a climate change link. A brief look at CONSEQUENCES was the final step to provide a broader historical perspective including examining events before good quality sea level and wave data sets are available. This is using a longer systematically assessed ‘qualitative’ data set. We found evidence of 187 distinct coastal flood events within the SCOPAC region in a 318-year period from 1703 to present.