Floods and Trees – an upstream solution?

Another week of exceptional rainfall and flooding has brought damage and misery to large parts of northern England, Scotland and Ireland. First and foremost, help must be given to the communities and businesses badly affected. But as the floods waters recede, we need to ask whether we can help to prevent floods on this scale in the future.

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One option that has started to gain widespread attention (Independent, Guardian, Mail on Sunday, Financial Times) is Natural Flood Management, which uses natural habitats such as wetlands, flood plains and woodlands to slow the flow of water, and so reduce flooding.

Under this system, wetlands and bogs would be restored, rivers would be allowed to flow more naturally onto flood plains, and new woodlands would be created. All these measures increase the ability of the land to store water and slow the flow of water downstream, thus helping to reduce the peak levels of water in our rivers and consequent risk of flooding.

At UBoC we are particularly interested in the creation of new woodlands in appropriate locations, because these will store carbon and promote biodiversity as well as help to reduce flooding.

One of the most important differences between woodland and agricultural land is the rate at which water infiltrates the soil. Agricultural soils lack deep-rooted vegetation and suffer compaction from animals and machinery, and so tend to inhibit water infiltration. This means that water flows quickly over farmland into the river, where it causes flood peaks further downstream. In wooded areas, by contrast, tree roots help to create porous soils which, together with leaf litter, act like a sponge – allowing water to percolate slowly into the water system, rather than running over the surface directly into streams and rivers.  Also, trees and vegetation, by effectively increasing the surface area upon which the rain falls (and then drips to the ground), themselves act as a “brake” on water, further slowing its journey downstream.

So increasing the amount of woodland clearly can help to reduce downstream flooding, but quantifying these benefits has proved more challenging.

Increased woodland cover would appear to have the greatest benefit for relatively small catchments (less than 100 km2) and for smaller flood events, where a reduction in the flood peak of more than 10% is possible. Restoring woodland close to rivers and on flood plains may have a particular positive benefit for reducing floods. Even very small areas of floodplain woodland may be able to reduce downstream flooding by 1-2%, with greater reductions when more woodland is planted. Even a 1% reduction in flood peak would have considerably reduced the damage caused by the recent floods. However, analysis of the largest and most damaging flood events has found limited evidence that these flood events are altered by increased woodland cover.

Not all forests are equal, and there is an important distinction between natural and plantation forests. Plantation forests with a dense canopy often have little ground vegetation, and forestry activities such as drainage, logging and road construction all have detrimental effects on downstream flooding. Forest plantations with extensive drainage can even increase flood flows by 20-30%; while logging roads and clear felling of trees can increase downstream floods by an additional 10%. To help prevent some of these negative impacts, modern forest management practices are now used across the UK. Nevertheless, restoration of some of our plantation forests back to semi-natural woodland could play an important role in mitigating flood risk in some areas. Restoration may be particularly relevant in the uplands, where rainfall is highest and timber production from forests is more limited because poor access makes extraction of timber less economic.

More extensive calls to “rewild” some of our uplands may offer even greater potential to reduce flood risk (it’s even been suggested that beavers could play a role), and FRMRC research suggests that restoring natural woodlands onto 5% of our uplands could reduce flood peaks by 29%.

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Anecdotal evidence from Pickering in North Yorkshire suggests that natural flood management can work (reported in the Independent), although rainfall here was less than in other regions, so the role of recent management work is still uncertain. Future schemes will need to be intensively monitored to ensure they have the desired impacts.

A wood debris dam, as used in Pickering’s Slowing The Flow project – from Forestry Commission.

By contrast, other land uses in the uplands could be exacerbating flood risk. Large areas of moorland are currently managed for the production of grouse for shooting. A particularly controversial issue is the controlled burning of heather, widely practised to improve conditions for red grouse, and so maximise the number of birds that can be shot. However, burning upland areas may have very negative implications for downstream flooding. Research carried out at Leeds shows that for the largest 20% of rainstorms, burnt catchments have a significantly more intense flood response compared to unburnt catchments. These largest storms cause the most damaging floods, meaning that widespread burning of our uplands may be a contributor to catastrophic flooding. To reduce future floods, increased restrictions on heather burning may be needed, with monitoring and penalties for offenders that consistently break rules. Some have gone further and called on grouse moors to close.

Several decades of research have informed our understanding of the interactions between land management and flooding. However, the potential for land-use management to reduce flooding is still an active area of research. Any future action needs to be based on the best available scientific knowledge, and where our understanding is lacking there is an urgent need for new research. All actions need to be carefully considered, including the potential for changed management to cause negative impacts. Even engineered flood defences, which can help protect specific communities, can lead to an unintended increase in flooding problems downstream.

Turning science into policy and widespread action will be even more difficult. Natural flood management and increased tree cover can conflict with other land-uses, in particular agricultural production. Farmers and other land managers are understandably resistant to any change that may threaten the economics of their business. Farmers spend considerable time and effort improving productivity of their land through improved drainage. Increased woodland cover and reversing historical drainage schemes will certainly reduce agricultural production. Farmers are also weary of repeated U-turns in policy and financial incentives they are offered by the government. If we expect land managers to make widespread changes that reduce the threat of flooding we need joined-up and long-term policy for natural flood management. Actions to reduce flooding can also help improve biodiversity as well as increasing carbon storage so helping to mitigate climate change. With long-term policy and financial incentives in place, farmers could see flood management, alongside food production, as an important service they provide to the country. Crucially we need to ensure that any policy changes to reduce flood threat to our urban centres also maintain and improve the economics of rural Britain.

Of course, flooding is not a problem confined to the UK. Across the world, floods leave 3 million people homeless and affect more than 60 million people each year. At the same time as floods hit Northern England, 100 000 people were forced from their homes across Argentina, Brazil, Uruguay and Paraguay. Forests may also be able to play an important role in helping reduce flooding in the tropics, although the extent that forests can reduce the largest and most damaging flood events is still being argued. A simplistic message that “forests prevent floods” is misleading and it is unrealistic to expect that restoring forests and woodlands will prevent all floods. However, changes to the way we manage our land, including more woodlands and forests, could lead to important reductions in flooding that require further attention. A petition to the UK Government is calling for debate on this issue in Parliament.

Dr Dominick Spracklen (edited)