Changes in extreme high waters at Liverpool since 1768

Changes in values of annual maximum high water (AMxHW), annual maximum surge-at-high-water (AMxSHW) and surge at annual maximum high water (SAMxHW) have been investigated using tide gauge data from Liverpool for the period 1768-1999. AMxHW and SAMxHW (measured with respect to mean high water) were found to vary considerably front year to year, but to exhibit no long-term change over the 232 years. On the other hand, values of AMxSHW were found to be larger in the late-18th, late-19th and late-20th centuries than for most of the 20th century, qualitatively consistent with knowledge of temporal variations in storminess in the region based on meteorological data and anecdotal information. The generalized extreme value method was used to present the available data on AMxHW and other annual extreme parameters in the 'return period' form most often employed by coastal engineers, with conclusions on the differences between each set of parameters in each epoch consistent with those obtained from the original time series. Finally, changes in the statistical distribution of surge-at-high-water (SHW), demonstrated by investigation of variations of percentile levels of SHW values, provided additional information on the temporal variations in extreme surges to that provided by AMxSHW values, pointing in particular to increased storminess during the late-18th and late-20th centuries, with a suggested secular trend in distribution shape from the late-18th century until recent decades. Copyright (C) 2002 Royal Meteorological Society.