The Great Flood of London?

The life of London goes back over 2,000 years. So how much sense does it make to spend upwards of £10 million to reduce the risk of an event that would have happened only once had London existed five times as long?
The City of London, guardian of Hampstead Heath, plans to build a ten-foot clay mound between some of the Highgate ponds to prevent a once in 10,000 year event, a flood caused by 5.3 inches of rain falling in four hours. If this were to happen, water would cascade off the Heath and inundate homes from Finsbury Park to Kilburn.
That’s not difficult to believe – if it were to happen. But it is a remarkably unlikely event. The only comparable flood in recent years occurred in 1975 when 6.7 inches of rain fell in 19 hours and some homes were flooded in Gospel Oak.
The City’s hands are tied by legislation passed last year, the Flood and Water Management Act of 2010, and by the rules laid down in the 1975 Reservoirs Act. The earlier act specifies that where a breach would endanger lives in a community, the minimum standard is a 10,000 year flood – a stretch of time that encompasses virtually the entire history of cities.
The 1975 act covered reservoirs of greater than 25,000 cubic metres – equivalent to a football pitch 14 feet deep in water. But the 2010 Act extended this to any reservoir of over 10,000 cubic metres, which means that all the ponds in the Hampstead and Highgate chain are included.
These are merely the most visible of many smaller bodies of water classified as reservoirs that will be caught as the 2010 legislation comes into force. Across the country remedial action to meet the standard is likely to cost plenty, but as the mandatory regulatory assessment of the impact of the regulations has yet to be carried out, we don’t know just how much. The engineering survey on Hampstead Heath alone cost £250,000, the works will be £10 million plus.
Yet when was the last time anybody was killed in the UK by a reservoir failure? 1925. 
The fact that an event is very rare does not mean that it couldn’t happen tomorrow, and the consequences if it did happen would be large – up to 1,500 residents of Gospel Oak, Dartmouth Park and Parliament Hill could die, records Camden New Journal.  “Up to” is generally journalistic code for “this is the top estimate”, but the City of London has yet to publish the report it commissioned from Dr Andy Hughes of Atkins Ltd, so we do not know its central estimate. A summary of the report is available here.
Given the many risks to life that are far more likely to occur – a fully-laden jumbo jet crashing on central London, for example – is it proportionate to spend so much to reduce a risk that is so small? This is, incidentally, nothing to do with climate change, since the reservoir legislation has been in place since 1975, when climatologists were arguing that we were heading for an Ice Age rather than global warming.
Indeed, if climate change were to increase the risk of such catastrophic downpours, then meeting the one in 10,000 year guarantee would become even more difficult, short of draining the ponds completely or building huge dams and culverts.
During the passage of the bill, Anne McIntosh, then Conservative MP for the Vale of York, said: “We remain unconvinced and would have liked the time to look at the reservoir provisions more carefully. I am sure that the Minister and the House will recognise that the provisions have serious cost implications, especially for those who have small reservoirs on farms and golf courses, and other third-party users. The increased cost of the obligations imposed is causing serious concern and alarm.”
In reply, Huw Irranca-Davies, the minister responsible, said “We have settled on this position as a result of consistent engineering advice based on a risk assessment approach”. But the comprehensive Pitt Review into the 2007 floods made 92 recommendations, none of them involving changes in the regulation of reservoirs.
The Act was rushed through Parliament in the dying days of the Labour Government. It lays on ministers the obligation to review the burden of regulation on reservoirs, but not until 12 months after the relevant parts of the Act come into force. That might be a good opportunity to look again at whether these provisions are indeed justified, when compared with other, much greater, hazards we face every day.