SCOPAC Storm Analysis Study

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.

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. This again highlights that W2013/14 has seen the most events of any season in this data set (although availability recently of data may bias this). However, it also highlights the role of modern risk management because W2013/14 does not even rank in the top 10 winters for the most severe flooding (even though it is possible that it was the most extreme winter from a waves and sea level perspective). The most severe coastal flood events occurred on 15th January 1918, 13th December 1989, 26th November 1954, 26th December 1912 and 1st January 1877. It is possible that an event in November 1824 was the most severe in every regard (sources to consequences) but a lack of information means this cannot be verified. Further studies should continue to monitor events and winters in context with the ongoing data sets, and take the opportunity to extend the wave data analysis back further in time from numerical model hindcasts.