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. […]

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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 individual series from across Europe. It also identifies the importance of the Atlantic Multidecadal Oscillation as a clear correlation is shown between higher North Atlantic sea temperatures and increased severe flood events across much of Britain. It is worth noting that when the threshold is reduced to the 0.8 percentile of events (Fig. 5), significant correlations remain between the British FI and summer, winter, annual AMO (1850) and NAOI (Trouet et al., 2009). The inclusion of historical flood information provides a better understanding of long-term flood patterns. The detection of flood-rich periods and attribution to periods of climatic change are tentative. The historical records still hold a wealth of untapped information for which specific discharges cannot be estimated, but from which indices could be extracted in the future (Barriendos and Coeur, 2004). The wealth of information presented by the historical records presents valuable new information for flood risk assessment and management (Kjeldsen et al., 2014); as new flood chronologies become available, more detailed and complete indices-based chronologies will improve the resolution and enhance understanding of flood-rich and flood-poor periods, presenting a more complete depiction of the role of climate and extreme floods. Extending the records back to a millennial time frame is possible, providing valuable insights into long-term trends and patterns of flood frequency and potential climatic drivers of flooding.

Neil Macdonald and Heather Sangster (2017), Hydrology and Earth System Sciences 21: 1631-1650.
Full paper