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How extreme were the Carlisle floods?

How extreme were the Carlisle floods?

The festive month didn't get off to the best of starts for the people of Carlisle when Storm Desmond paid the north western town an unwelcome visit. Over a period of less than 24 hours the usually picturesque streets were transformed into a landscape more suitable for kayaking than the morning commute. Over the ensuing days there has been much conjecture about what was to blame for the unexpected disaster which all but brought the town to a standstill. One theory making the rounds is that the flood defences had failed on an epic scale, but is that the real story?

Hydrological Facts

Cumbria is no stranger to floods, with the last major one occurring as recently as 2009. Storm Desmond has gone into the record books, however, as the official rain gauge located at Honister Pass in the Lake District National Park recorded 341mm of rainfall in the 24 hours ending at 1800 GMT on the 5th December 2015. A second rain gauge at Thirlmere also recorded a record rainfall, this time of 405 mm falling in just 28 hours. The estimate of return period for this magnitude of rainfall is about 1 in 1300 years, meaning there is a 1 in 1300 chance of it occurring in any one year! Interestingly, the previous 24hr rainfall record was also measured in Cumbria (Seathwaite, 2009) and came in at 316mm.

Is this because of climate change?

Well, the Met Office report named “Changes in the frequency of extreme rainfall events for selected towns and cities (2010)" surmises that "all winter rainfall events are projected to become more frequent". Carlisle was one of the cities which was included in this report: "For Carlisle... the changes for winter rainfall in the 2040s are extremely significant". Following the Northern Hemisphere's hottest summer on record and the UK's hottest day it is clear that the climate is changing. How much, however, is open to debate.

The sheer magnitude of the storm may not have been the only contributory factor to the floods.

Some commentators have suggested that upland catchment management may reduced the ability of the catchment to absorb the rainfall. Carlisle sits on a rise at the confluence of 3 rivers; the Eden, the Caldew and the Petteril. Following the last great storm to hit Cumbria in 2009 the River Liza remained clear whilst the others roared in fury under the deluge. This was due to it being 'rewilded' following the floods of 2005 and consisted of natural barriers being allowed to fall into place. These obstructions held back the water, filtering it through before releasing it slowly so the outlying catchment was able to absorb it. Had all the rivers undergone this process there mightn't have been any floods, either in 2009 or this month.

Did the flood defences fail?

In a word: no. Flood defences are built on predictions as nobody actually knows how much rainfall will fall at any given time in the long term future. Those currently in place in Cumbria were designed after the floods of 2005 to withstand a flood with return period of 1 in 200 years, based on the storm data available then. When we get rainfall of the magnitude of Storm Desmond (1 in 1300 years) , which exceeds the design standard of the defence, the water rises to such an extent that it simply pours over the top of the defence which is designed to withstand a certain pre- defined level. In normal circumstances they would have done the job they were intended for, but they were no match for Desmond.

Assessments on flood defences are made by using a range of evaluations. They are chiefly taken by NaFRA, National Flood Risk Assessment, and allow the Environment Agency analysts to spot current and predict future trends, determine the effects climate change will have in the future, track the development on flood plains and also stay up to date on the conditions of flood defences right across the UK.

The economic impact on the area

While an emergency fund has been set up to help those households which have lost most of their possessions just a few weeks before Christmas, the outlook for the local economy in the aftermath of the floods is bleak to say the least. Many smaller shops who rely on the Christmas trading assessing the costs of the damage to both their properties and income and whether they open their doors again remains to be seen. The latest estimate is that there are around 8,000 jobs in jeopardy as it is rumoured that several large employees who have now suffered twice due to severe floods will move elsewhere rather than rebuild in Cumbria.

So what happens next?

A total of £2.3bn has been pledged by the government to improve the UK's flood defences over the next 6 years. Individual schemes will range from tens of thousands to hundreds of millions of pounds. What measures will be taken in and around Carlisle remains to be seen but to those currently without power or homeless it will sadly be too late for those treasured possessions lost forever.

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