Flooding devastates communities and takes billions of pounds out of the UK economy. In 2015 Storm Desmond broke all the records with 341mm of rainfall in 24 hours in the wettest Lake District December since records began in 1788. The Association of British Insurers has estimated that Desmond cost £1.3 billion, but when you add on the uninsured losses and costs to the public purse for repairing critical infrastructure like flood defences, roads, bridges and railways, the total bill reaches more than £5 billion.
Historical evidence suggests that whilst the Cumbria floods are record-breaking compared with living memory, they are not completely unprecedented: periods of severe flooding hit the UK in 1947 and 1929, the latter half of the 19th century was particularly flood-rich, and geological evidence shows there were even bigger floods in the 17th century. Meteorologists say the spate of floods since the mid-1990s is down to an unusually warm North Atlantic bringing an intermittent stream of strong storms across the UK; and while the blame for the recent floods can’t be pinned for certain on global warming, climate change looks likely to increase flood risk in the future, with more intense summer storms and wetter winters in the uplands of the west.
Faced with the prospect of severe flooding continuing, governments and flood-hit communities are searching for solutions. Natural flood management – restoring the landscape to reverse or mitigate the impacts of agriculture and urbanisation - has hit the headlines regularly in recent years as a possible answer. Its advocates point to benefits for wildlife, biodiversity and landscapes; and the fact that it can be much more cost effective to manage flood risk at source than to deal with the aftermath of flooding when it occurs. But for policymakers, natural flood management presents a complex and delicate balancing act between the needs of different groups, including the public, farmers and landowners. Added to that is the more fundamental issue that its effectiveness at both small and large scales has been unclear.
An 18-month, comprehensive review of the scientific evidence on natural flood management, commissioned by the Oxford Martin School and aimed at helping to navigate these murky waters, has just published its results.
Carried out by experts from across the country, it found that the evidence for natural flood management is strongest when targeted at local problems with minor flooding. A good example of this is the community-led Pontbren Farmers Project in which tree ‘shelterbelts’ have been planted in carefully-chosen places on the hillsides. The local benefits to the farmers themselves are very clear, during small storms at least: the presence of trees (and the absence of livestock) increases the rate at which water soaks into the soil. The shelterbelts also stop soil silting up rivers and limit the spread of agricultural pollution downstream. But in spells of very wet weather the power of the trees to prevent flooding diminishes, leading to only a 2-11% reduction in flood flows. This reduction is not to be sniffed at, and of course there are the added benefits for biodiversity, soil conservation and the landscape. But ultimately it does not make for a reliable defence against the worst floods.
Another conundrum is that, in studies up to now, the collective effects of small-scale measures don’t seem to add up to a significant flood risk reduction over all. In one of the biggest studies of natural flood management, in the Hodder River catchment in Lancashire, the local water utility company tried initiatives such as reducing sheep numbers on farmland, and whilst a local impact was detectable, these measures had little effect in the main river. The project scientists later established that the maximum flood level was heavily influenced by several critical zones in the river basin, offering hope that these measures can be targeted in the future.
So do these issues mean natural flood management won’t be able to fulfil the promise many hope it holds? Reviewing the evidence shows it’s certainly not a silver bullet, and won’t protect against the effects of the worst storms, but the news isn’t all bad.
It’s likely to be best put into service where there’s a combination of risk of minor floods, and where there will be benefits for wildlife and the landscape. Natural measures can be implemented alongside traditional flood risk management too: in Pickering in North Yorkshire, the community has come together to provide protection against a one-in-25 year flood, although the jury is still out on how much of this is down to woodland measures in the uplands versus an engineered offline storage reservoir downstream.
There are more than 170 natural flood management schemes in operation in the UK: each put in place to tackle a pressing local flood problem, and often driven by the enthusiasm of community groups and responsible land managers. What we need now is proper monitoring and evaluation of these schemes, and the Environment Agency should fund them where they work and where they give value for money.
Dr Simon Dadson is Associate Professor in Physical Geography at Oxford University’s School of Geography and the Environment.
- Full paper - A restatement of the natural science evidence concerning catchment-based ‘natural’ flood management in the UK