Research has found that both flood magnitude and frequency in the UK may
have increased over the last five decades. However, evaluating whether
or not this is a systematic trend is difficult because of the lack of
longer records. Here we compile and consider an extreme flood record
that extends back to 1770. Since 1770, there have been 137 recorded
extreme floods. However, over this period, there is not a unidirectional
trend of rising extreme flood risk over time. Instead, there are clear
flood-rich and flood-poor periods. Three main flood-rich periods were
identified: 18731904, 19231933, and 1994 onwards. To provide a first
analysis of what is driving these periods, and given the paucity of more
sophisticated datasets that extend back to the 18th century, objective
Lamb weather types were used. Of the 27 objective Lamb weather types,
only 11 could be associated with the extreme floods during the gauged
period, and only 5 of these accounted for > 80% of recorded extreme
floods The importance of these five weather types over a longer
timescale for flood risk in Carlisle was assessed, through calculating
the proportion of each hydrological year classified as being associated
with these flood-generating weather types. Two periods clearly had more
than the average proportions of the year classified as one of the flood
causing weather types; 19001940 and 19832007; and these two periods both
contained flood-rich hydrological records. Thus, the analysis suggests
that systematic organisation of the North Atlantic climate system may be
manifest as periods of elevated and reduced flood risk, an observation
that has major implications for analyses that assume that climatic
drivers of flood risk can be either statistically stationary or are
following a simple trend. Copyright (c) 2011 Royal Meteorological
Society