What is it about surface water flooding that means they seem to come out of nowhere? A few thoughts about this 'other' sort of flood

At the time of writing, the big clean-up is under way in parts of the North of England after flooding caused by the recent exceptionally heavy rain. The damage and disruption is extensive and serious. Fortunately, nobody has lost their life, but many will have lost they life they had. Recovery will be long, difficult and painful.

Some places experienced close to a month’s worth of rain in just a few hours and that is a perfect recipe for surface water flooding in some places. So, how do we understand this risk and why does it often take us nearly by surprise? We seem to get very precise and reliable forecasts and warnings when it comes to the risk of flooding from rivers and the sea. So, what’s different about this risk?

It’s generally caused by a pattern of multiple convective rainstorms (also known as thunderstorms!). They usually depend on a warm earth to force hot air upwards, where it cools rapidly and creates big, heavy clouds.  This destabilises the atmosphere and can lead to exceptionally heavy rain. This falls at such a rate that it simply overwhelms the ground’s normal holding and drainage capacity. That’s why they tend to happen in summer.

It’s also why surface water flooding events like this are less predictable than other types of flooding, like from rivers or the sea. The most you can reasonably expect is between 24 and 6 hours of advanced warning. Even then, predictions are generally less reliable and usable. It is possible to predict the amount of water that is likely to fall, but it is very difficult to predict exactly where it will fall – even if you know it’s coming to a certain region. So, it is a very localised and short-lived event – at least as far as the immediate impact is concerned. They also have a very rapid (even sudden) onset – hence the popular expression 'flash flood'. The severity of that will depend on very local physical geography.

Other factors complicate the response and recovery picture. Rescue needs specialised equipment and vehicles – which may have to get there through the floods and over roads and bridges that may have been washed out. Other vital infrastructure will be trashed, like electricity sub-stations, water and sewage plants and telecommunications. You will have seen these images in the news over the last few days. This can delay both the response and recovery effort – extending the possible time period to a year or more if substantial rebuilding must be done. 

It seems safe to say that people, communities, infrastructure operators and businesses are less aware of, and less prepared for, this type of flooding. This is not because it’s rare: it’s common enough at a national scale.  But it seems to be more evenly distributed and doesn’t have a pattern of recurring in the same, familiar sorts of places. So, it tends to be a 'freak' event - not a visible feature of the local 'risk landscape' – until it happens. For example, if you live by the river in York or on the Somerset Levels, you will probably understand a fair amount about fluvial (river) flooding! But if you live or work on relatively high ground, comfortably far from any rivers, there is little reason to suppose you are vulnerable to this risk. And in some respects you aren’t actually more vulnerable than anyone else.

This seems to leave us with a risk that is expected to happen moderately frequently, can happen almost anywhere and is very difficult to predict with any useful accuracy. Add to that the acknowledgement that climate change will probably increase both its frequency and its severity.

We have a dedicated warning service for both riverine and coastal flooding, but not one for surface water flooding. That may be due to a recognition that we simply can’t do it with enough accuracy and reliability to make it worthwhile? Nevertheless, a few such incidents over the same summer might very well lead to calls for setting up some form of surface water flood warning service.