Sunday, 9 February 2014

Record breakers? Climate change, statistics and the recent UK floods

This morning the Met Office and the Centre for Ecology & Hydrology released a report on the recent storms and floods in the UK.

You can read the full report here, and a short summary of the hydrological analysis on our website here

The report documents the recent record-breaking weather and flooding episode, and discusses whether climate change contributed to the severity of the weather and its impacts. Climatology, meteorology and hydrology are covered in the report. 

In this blog post we highlight some of the hydrological conclusions that came from analysis of the long-term datasets held at CEH and our long history of research into floods.

With regard to the record-breaking nature of the flooding the report makes the following points:

  • The full gamut of flood manifestations – tidal, pluvial (flash), fluvial and groundwater – has been experienced over the last eight weeks.
  • The exceptional run of severe winter storms over December 2013 and January 2014 carried with them large amounts of rain which has led to very serious flooding across southern England. 
  • Estimated outflows (river flows) from Great Britain remained close to the highest ever recorded during late December and, subsequently, throughout most of January across large parts of England and Wales.
  • In a series from 1883, flows on the River Thames at Kingston (close to the tidal limit) remained above 275 m3 s-1 for longer than in any previous flood episode, and continue to exceed this threshold into early February. 
  • A preliminary analysis suggest that outflows aggregated over six weeks were the greatest since the 1947 floods – the most extensive in England & Wales during the 20th century.
  • In December and January, a few rivers (including the Mole, Wey and Medway, which, on the basis of preliminary data, recorded their highest flows since the extreme floods of September 1968) registered outstanding maximum flows. 
  • Generally, however, the peak flows registered during the recent flooding were not extreme. On the Thames the highest flow in 2014 has been exceeded during 14 earlier floods (most prior to 1950). 
  • The floodplain inundations across the UK caused major disruption to transport, agriculture and restricted sporting and recreational activities, and resulted in severe difficulties for some low-lying hamlets (most notably in the Somerset Levels). However, given the overall volume of runoff, the amount of property flooding at the national scale was relatively modest; a tribute to the general effectiveness of flood defences.

Figure 1: Combined flows (m3 s-1; black) for the Thames, Severn, Trent, Yorkshire Ouse and Usk in 2013 and January 2014, plotted against period of record (1969-2013) daily maximum flows (blue), daily minimum flows (pink) and daily mean flows (grey)

With regard to whether climate change has been a contributing factor to the current flooding the report states:

  • The cluster of drought and flood events through the early years of the 21st century and the recent runoff and recharge patterns, are near to the extreme range of historical variability, and therefore also raise the question that they may reflect anthropogenic climate change. It is important to note, however, that differing flood types may be expected to respond differently to increasing temperatures. Tidal flood risk is increasing as sea levels rise but the outlook is more complex in relation to fluvial flooding.
  • Published studies have observed increased river flows in the winter half-year and a tendency for higher flows to occur more frequently, and this has been reinforced in recent years. Importantly, however, such a trend may not be accompanied by any increase in magnitude of major flood events. In the UK, no positive trend in water-year maxima was found in the 130-year series for the Thames.
  • Enhanced groundwater flood risk may be expected if average winter rainfall in the UK increases. Flash flooding, which can be exacerbated by land management and land use practices (particularly the extension of impermeable areas), may also increase if the recent intensification in rainfall translates into an enduring trend. 
Much more detail can be found in the full report. More information about the report's conclusions is available on the Met Office website here.

Journalists can call our Press Office if more information is needed on the hydrological conclusions.

Barnaby Smith, CEH Media Relations Manager

Further links

Jamie Hannaford from the Centre for Ecology & Hydrology contributed a chapter to the Living with Environmental Change climate change report card for water entitled, "Observed long-term changes in UK river flow patterns: a review"

An interim update on the UK's hydrological situation - issued by CEH on 6 January 2014

Rainfall, UK floods and the potential impacts of climate change? - blog from 6 January 2014

NB: the Met Office updated the report on 11 February to give a more detailed assessment of sea level rise

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