Estimating probabilities of extreme sea-levels

A key problem in the design of sea defences is the estimation of quantiles of the distribution of annual maximum hourly sea-levels. Traditional statistical analyses fail to exploit the considerable knowledge of the astronomical tidal component of the sea; consequently the corresponding results are highly site specific. Using results from extreme value theory an ad hoc method developed by oceanographers to overcome this problem is revised. The method is illustrated with data from three sites on the east coast of England which exhibit widely differing characteristics.