Freak storms are now normal

Observations on the weather front

Defending the government this month after a rainstorm flushed 600,000 tonnes of sewage into the Thames, the environment minister, Elliot Morley, remarked: "You have got to bear in mind that the recent overflow we have had is a one-in-60-year event. It wasn't normal." It is hard not to feel at least slightly reassured by such words - most of us won't be around in another 60 years, after all - but this is a mistake, and not just because we should care about those who come after us.

These "return periods", as they are known - one in 60 years, one in 100 and so on - have been used by engineers as a way of expressing flood risk since Victorian times, and underpin the design of much of the infrastructure that surrounds us. Increasingly, however, they are regarded as misleading if not downright dangerous.

One reason for this was given by Morley himself, who admitted in the same BBC interview: "With global warming, and climate change, it may well be that such events are more frequent in the future."

Yes indeed. While the prospect of global warming in Britain can evoke balmy notions of bountiful vineyards and pavement dining, the best available evidence - from the computer models of the Hadley Centre for Climate Prediction and Research - says that its most noticeable effect in the coming decades will be more downpours. This makes calculating flood risk and return periods extremely difficult.

Well into the 1980s, when the global climate was still thought to be in a "steady state", young engineers were taught that they could find all they needed to know about rainfall dangers in 30 years of data; now it seems that a couple of millennia of data would not be enough to give a reliable picture, even if such data existed.

In effect, we are in the dark, and so to give the impression that the capital's mainly Victorian sewers should be able to cope with the rain 59 years out of every 60 would be at best optimistic and at worst reckless. Another good reason to beware of return periods is that the formula of a "one-in-60-year event" reassures disproportionately. While most people tend to infer that this leaves ample time to prepare for a recurrence, in fact there is no mathematical reason why the same event should not be repeated tomorrow, and indeed, again the following day.

Ask the people of Lewes in East Sussex, who, after flooding in 1960 that was measured as a once-in-a-century event, constructed new defences capable of withstanding anything on a similar scale. In September 2000, the whole lower town was under water.

There is more at stake here than a dodgy form of words. If the downpours this summer have a moral for us, it is that we should stop thinking of them as rare events and start viewing them as something like normality.

Instead of focusing, as we do now, on ever faster ways of flushing rainwater off towards the rivers and the sea - thus increasing the risk of disasters when bottlenecks occur - we should give more thought to slowing things down, to trapping more of the rain for longer near the point where it fell to earth.

This is called rainwater storage, and we will be hearing a lot about it. Big domestic water butts, porous pavements and floodable car parks and playing fields will become part of our flood defences. Even the unloved puddle in the street and the park, which at present, by reflex, we want to see drained and filled in, can do its bit.

Brian Cathcart is Director of Hacked Off. He tweets as @BrianCathcart.