Monday, 7 December 2015

Cumbria Flooding 2015 – Uncertainty in Flood Risk Management

By Ian Joyner
Associate Director, Flood Risk at CBRE

Storm Desmond resulted in significant flooding across the north-west of England over the weekend.  Our flood risk lead, Ian Joyner, considers the questions this major event poses for flood risk management experts and, ultimately, those relying on their advice with respect to their homes and businesses.

The extraordinary flooding in Cumbria brought into sharp focus some critical issues in UK flood risk management.

  • We saw the on the ground impacts of an extreme flood which some have used to criticise authorities’ previous predictions
  • A number of new flood defence systems were overwhelmed raising uncomfortable questions about the public funding and benefit of these schemes
  • Although one flood event cannot be attributed to climate change, the frequency of extreme floods appears to be increasing.  This is something climate change experts have been warning about for some time, not least the UK’s Committee on Climate Change [1]

Understandably the focus for the authorities is now on response and recovery. Once the dust has settled, the data from the Environment Agency’s rainfall and river gauging network will help to indicate the rarity of the event we’ve just witnessed. Initial estimates [2] suggest a 24-hour rainfall total of around 340mm was reached, surpassing previous highs. 

In addition, the storm tracked across some of the most irregular topography in England, adding significantly to the variability of rainfall experienced. Moist air was forced to rise above the Lake District fells where rainfall meets steep rocky slopes and thin soils, running off rapidly into fast-flowing ghylls and becks.  Flood estimation (emphasis on ‘estimation’) is a complex science, borne out of years of fastidious data collection on the country’s rivers and streams and decades of academic and industry research. Estimating the impact of an extreme storm is subject to considerable uncertainty but it’s a reasonable assumption, had Storm Desmond not occurred following a thoroughly wet November in the North West, for example, that its impacts would have been less severe.

Much media attention has been wide of the mark, exclaiming that flood defences ‘failed’ or were ‘breached’. In fact, defences in the affected towns appear largely to have held but been ‘overtopped’.  This means river levels were higher than the defences were designed for, allowing floodwater to cascade over and affect properties protected behind. This distinction, while considered pedantic to those in the media, is of crucial importance to those managing the response to an event, where the effective management of a flood can be interrupted by the need to respond to a media narrative.  

Thankfully, a breach, which could very well have resulted in loss of life, appears to have been avoided.  It is also likely that defences delayed the onset of flooding, providing valuable time for people to heed warnings and take action. Ultimately, with flood defences being extremely costly to construct, a line has to be drawn and the need to balance protection against environmental, landscape and visual impact is even more critical in landscapes such as the Lake District. It seems, on this occasion, that homes and businesses flooded are unfortunate victims of an event so large it would not be reasonable, or even possible, to fully prevent. Future effort is perhaps better spent raising awareness of the possibility of these events.

As commentators and the industry come to terms with yet another ‘extreme’ flood hitting the headlines, the evidence for climate change impacts on the frequency of extreme weather gets all the more compelling. When assessing flood risk to property, there is a real need to consider both the sensitivity of flood risk to climate change and the residual risk faced in those places apparently protected behind flood defences.

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