The extensive flooding in the UK has been widely reported. It's been a great time for the news media... some real news that people actually want to hear about! Even in areas not affected, journalists are jumping on the bandwagon. I live in Switzerland but this is a risk story I can't resist!

Cardiff, on the South Wales coast, lies at the confluence of 2 rivers, the Taff and the Ely, which flow into Cardiff Bay on the River Severn estuary. So, it's not surprising to hear that the city has been affected by severe flooding every 20 years or so. This happened whenever there was a combination of unusually heavy rains and high tides.

But, while nearby Gloucester and Tewkesbury have been seriously hit this week, Cardiff has not been troubled by flooding. The last big flood in Cardiff was in 1979. Since then, flood defences have been constructed along the Rivers Taff and Ely... and the Cardiff Bay Barrage has been built across the mouth of the bay.

The flooding this week in Gloucestershire is said to be the worst for 60 years. One of the places seriously affected was Tewkesbury, where the River Avon joins the River Severn. The town was inaccessible by road and Tewkesbury Abbey found itself on an island. Two men died while trying to pump floodwater out of a cellar at Tewkesbury Rugby Club.

The location of Tewkesbury in the river system, and its lack of adequate flood defences, means that it is liable to flooding from time to time. This should be no surprise to anybody. Even the names of some of the Gloucestershire villages featuring in recent BBC news reports warn of their susceptibility to flooding... names like Moreton-in-Marsh and Bourton-on-the-Water!

What's the risk?

You don't have to rely on giveaway placenames. The UK Environment Agency publishes detailed, interactive maps of flood risk on their website. These maps show that large parts of Tewkesbury have a 'significant' risk of flooding. They estimate the probability to be 1.3%, or a serious flood about once every 75 years. Sounds about right.

For the Cardiff Bay area, the Environment Agency says the flood risk is 'low'... probability 0.5%, or once in 200 years. This low estimate takes account of the flood defences that are in place. So, whereas Cardiff used to experience serious flooding every 20 years, the flood defences have reduced the risk to once in 200 years.

What can we do about it?

Why do they not build similar flood defences in Tewkesbury and all the other flood-prone places? The simple answer is cost. And the not-so-simple answer is cost-benefit. The flood defences along the River Taff at Cardiff were built at a cost of £3m. The Cardiff Bay Barrage cost a massive £220m (although that was justified on the grounds of economic regeneration as well as flood defence). And all this for a relatively-frequent 20-year flood cycle. Such costs are difficult to justify for a 75-year flood cycle. As so often the case with risk assessment, there are trade-offs to be made. We find there are other things that we need to spend our money on.

So, in many cases, we have to resign ourselves to the fact that there will be floods from time to time and focus our efforts on minimizing the impact of the flooding. However, it can be difficult to pursuade people to modify their behaviour, especially when the floods occur so rarely. It's not difficult to keep wedding photos and important documents in a waterproof container if you live in a flood-risk area. But most people just don't.

Research in Switzerland

A recent study (Siegrist and Gutscher, 2006) looked at people's perceptions of flood risk in Switzerland. The Swiss authorities also publish detailed maps showing the risks for different areas. The researchers asked people to give their own assessment of the flood risk in their area and these perceptions were compared to the experts' estimates. The results were similar to findings in other risk perception studies; people at high risk tended to underestimate the risk while those at low risk tended to overestimate. Most of them didn't know about the official flood-risk maps. Nevertheless, people in general agreed with the experts as to whether the flood risk was above or below average.

What's really interesting about the Swiss study is that they asked people about their preparedness for flooding. In particular, they asked whether people kept important documents in their cellars. They found that people's behaviour was much the same, whether or not they lived in a high-risk or a low-risk area. In other words, people still kept important stuff in their cellar even though they lived in a high-risk area... and they themselves recognised that they lived in a high-risk area!

Hope springs eternal!