Extreme sea levels in the English Channel 1900 to 2006

Abstract

Coastal populations are growing at a rapid pace and this is being accompanied by an increased investment in infrastructure at the coastal zone. Combined with this is the concern of enhanced coastal flooding due to rising sea levels and climate change. Hence, it is of utmost practical importance that probabilities of current and future extreme sea level are accurately evaluated so that the changing flood risk can be assessed and defences upgraded where appropriate. This thesis tests the hypothesis that changes in extreme still water level can be approximated by just adding changes in mean sea level to current return levels estimated from measured data, for the English Channel region.

A data archaeology exercise has been undertaken to extend the sea level records along the UK south coast. This exercise increased the sea level data set for this region by 173 years. These new records have been analysed along with existing data to determine rates of change in both mean and extreme sea level, and to estimate probabilities of extreme sea level using four statistical methods: (i) the annual maxima method; (ii) its extension to the rlargest
annual events method; (iii) the joint probabilities method; and (iv) the revised joint probabilities method.

Relative mean sea-level trends vary by between 0.8 and 2.3 mm/yr around the Channel over the 20th century. These trends have been estimated using a new approach, in which the coherent part of the sea level variability around the UK is defined as a single index. This is then subtracted from the sea level records prior to fitting trends. The recent high rates of mean sea-level rise observed over the last decade are not unusual on a century scale context. The tidal and non-tidal components of sea level, along with tide-surge interaction, have been separately analysed for trends before analysing variations in extreme sea levels. There is evidence for an increase in extreme sea levels during the 20th century, but at rates not significantly different to that of mean sea level. There is no evidence of a longterm increase in storm count, duration or intensity. The revised joint probabilities method is found to out perform the other statistical methods, in terms of prediction errors.

Results confirm that changes in extreme sea levels during the 20th century can be estimated, to an accuracy of 0.1 m, by simply adding mean sea level changes to return levels estimated from measured data. The return levels should be estimated using the revised joint probabilities method wherever possible.

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Identifiers

ORCID for Ivan David Haigh: orcid.org/0000-0002-9722-3061

ORCID for Robert Nicholls: orcid.org/0000-0002-9715-1109

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