From the GWPF Observatory

This new paper presents the first coherent large-scale national analysis undertaken on historical flood chronologies in Britain, providing an unparalleled network of sites (Fig. 1), permitting analysis of the spatial and temporal distribution of high-magnitude flood patterns and the potential mechanisms driving periods of increased flooding at a national scale (Britain) since AD 1750.

The apparent increase in flooding witnessed over the last decade appears in consideration to the long-term flood record not to be unprecedented; whilst the period since 2000 has been considered as flood-rich, the period 1970–2000 is “flood poor”, which may partly explain why recent floods are often perceived as extreme events. The much publicised (popular media) apparent change in flood frequency since 2000 may reflect natural variability, as there appears to be no shift in long-term flood frequency.

Abstract

The last decade has witnessed severe flooding across much of the globe, but have these floods really been exceptional? Globally, relatively few instrumental river flow series extend beyond 50 years, with short records presenting significant challenges in determining flood risk from high-magnitude floods. A perceived increase in extreme floods in recent years has decreased public confidence in conventional flood risk estimates; the results affect society (insurance costs), individuals (personal vulnerability) and companies (e.g. water resource managers). Here, we show how historical records from Britain have improved understanding of high-magnitude floods, by examining past spatial and temporal variability. The findings identify that whilst recent floods are notable, several comparable periods of increased flooding are identifiable historically, with periods of greater frequency (flood-rich periods). Statistically significant relationships between the British flood index, the Atlantic Meridional Oscillation and the North Atlantic Oscillation Index are identified. The use of historical records identifies that the largest floods often transcend single catchments affecting regions and that the current flood-rich period is not unprecedented. […]

click on image to enlarge

Summary

The apparent increase in flooding witnessed over the last decade appears in consideration to the long-term flood record not to be unprecedented; whilst the period since 2000 has been considered as flood-rich, the period 1970–2000 is “flood poor”, which may partly explain why recent floods are often perceived as extreme events. The much publicised (popular media) apparent change in flood frequency since 2000 may reflect natural variability, as there appears to be no shift in long-term flood frequency (Fig. 5). In reviewing the flood series for European systems for which long flood series have been reconstructed, a complex picture is identified; whilst flood-rich phases appear synchronous across many systems (1765–1780) others show less synchronicity (1920s), whereas a number of prominent floodrich phases at a European scale appear subdued or are not evident in the British FI (1750s).

The principal findings of this work are that of the strong correlations between flood-rich/flood-poor phases and solar magnetic activity, AMO and NAOI, indicating a clear driver for flooding patterns across Britain. The specific mechanisms that govern the relationship between the spatial/temporal distribution of flood clusters and solar activity remain unclear. This work suggests that high-magnitude flood-rich periods relate to negative NAOI across much of the country, in western catchments with a stronger westerly airflow signal significantly correlating to positive NAOI, with reasonable correspondence with previously diagnosed periods of climatic variability identified from…