Storm surges

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Past trends

Producing a clear picture of either past changes or future projections of storm surges for the entire European coast line is a challenging task because of the impact of local topographical features on the surge events. Whilst there are numerous studies for the North Sea coastline, fewer are available for the Mediterranean and Baltic Seas, although this situation is starting to improve.

The most comprehensive global studies of trends in extreme coastal sea level and storm surges examined trends from hourly tide gauge records at least for the period since 1970, and for earlier periods of the 20th century for some locations [i]. The results show that changes in extreme water levels tend to be dominated by the change in the time mean local sea level. In the north-west European region there is clear evidence of widespread increase in sea level extremes since 1970, but much less evidence of such a trend over the entire 20th century. When the contribution from time mean local sea level changes and variations in tide are removed from the recent trends, the remaining signals due to changes in storminess are much smaller or even no longer detectable.  

Additional studies are available for some European coastal locations, but typically focus on more limited spatial scales. A study that examined the trend in water levels at 18 sites around the English Channel found that the rates of change in extreme water levels were similar to the rates observed for mean sea level change [ii]. However, the study also noted sizeable variations in storm surge heights, with the largest surge intensity occurring in the late 1950s. This large natural variability makes it difficult to detect changes in the rate of change in water level extremes. A similar conclusion, that the change in annual maximum sea levels are increasing at a rate not significantly different from the observed increase in mean sea level, was found in separate analyses for Newlyn in the UK for the period 1915–2005 [iii] and for 73 tide gauges along the Atlantic and Mediterranean coastlines in southern Europe [iv]. In contrast, significant increases in storm surge height during the 20th century were found along the Estonian coast of the Baltic Sea [v].

In conclusion, whilst there have been detectable changes in extreme water levels around the European coast line, most of these are dominated by changes in time mean local sea level. The contribution from changes in storminess is currently small in most European locations and there is little evidence that any trends can be separated from long-period natural variability [vi].

Projections

Future projections in storm surges can be made using either dynamic or statistical modelling of storm surge behaviour driven by the output of general circulation climate models [vii]. The uncertainty in future projections of storm surges remains high and is ultimately linked to the uncertainty in future mid-latitude storminess changes. This is an area where current scientific understanding is advancing quickly as climate model representations of aspects of northern hemisphere storm track behaviour are showing improvements associated with, for instance, greater ocean and atmosphere resolution [viii]. However, the newest global climate models have not yet, typically, been downscaled to suitably fine scales and used in studies of future storm surges. 

Several climate modelling studies have projected changes in storm surge height and frequency for the 21^st century. The limited number of studies that separate out any long-term climate change signal from multi-decadal climate variability suggests that changes in atmospheric storminess are likely to be less important than increases in mean local sea level. Based on the RCP4.5 scenario, recent studies estimate increases in the frequency of flooding events by more than a factor of 10 at many European locations and reaching between 100 and 1000 at some European locations [ix]. These numbers should be considered with caution because in many cases, and from a protection perspective, it is more instructive to look at the change in the height of a given return period event rather than the change in frequency of a given height of event. Large changes in flood frequency mean that what is an extreme event today may become the norm by the end of the century at some locations. However, for any particular location it is important to look in detail at the change in protection height that might be required. Where the flood frequency curve is very flat the increases might be modest. Where the flood frequency curve is steeper larger increases in protection height or alternative adaptations might be needed.

Two recent studies on future changes in surge magnitude in the North Sea region addressed some of the deficiencies in earlier studies by using ensemble simulations of climate models to drive a surge model of the North Sea for the period 1950–2100. One study found no significant change in the 1 in 10 000 year return values of storm surges along the Dutch coastline during the 21st century [x]. The other study projected small changes in storm surge heights for the 21st century around much of the UK coastline. Most of these changes were positive but they were typically much less than the expected increase in time mean local sea level over the same time period [xi]. However, larger increases in storm surge for this region during the 21st century cannot yet be ruled out.

A study on the Mediterranean region projected a reduction in both the number and frequency of storm surge events during the 21st century [xii]. A study on the Baltic Sea projected increases in extreme sea levels over the 21st century that were larger than the time mean local sea-level rise for some future scenarios simulated by some of the climate models used [xiii]. The largest changes in storm surge height were in the Gulf of Finland, Gulf of Riga and the north-eastern Bothnian Bay. A study on storm surges around the coast of Ireland projected an increase in surge events on the west and east coasts but not along the southern coast [xiv]. However, not all of the changes were found to have a high statistical significance.

At some locations, such as Hamburg, local changes in bathymetry caused by erosion, sedimentation and waterworks can have a much larger impact than climate change [xv]. Finally, recent work has shown that sea-level rise may also change extreme water levels by altering the tidal range. The tidal behaviour is particularly responsive in resonant areas of the Bristol Channel and Gulf of St. Malo (with large amplitude decreases) and in the southeastern German Bight and Dutch Wadden Sea (with large amplitude increases). These substantial future changes in the tides could have implications, for instance, for estimating requirements for flood defences [xvi].