Trees and flooding

Recent flooding has highlighted the importance of land use in either contributing to or mitigating flood risk. In particular the suggestion that trees might play an important role in helping reduce flood risk.

Reports from the Woodland Trust on water and farming and water in towns provide a review of the evidence for the role of trees. Forest Research has also undertaken a significant amount of work looking at the role of trees in delivering better water quality as well as modelling the impacts of increased tree cover on flood risk. These show that trees can make an important contribution both to mitigating flooding and improving water quality.

Healthy river with clean water and good plant life - Pierre Terre/Wikimedia Commons

Healthy river with clean water and good plant life – Pierre Terre/Wikimedia Commons

Lessons from Pontbren

In 2012 The Woodland Trust and Coed Cymru, on behalf of the Pontbren farmers, commissioned a review of the scientific evidence and the story of how, over the last 15 years, Pontbren farmers have transformed their farms.  There are key lessons from Pontbren. The hedgerow restoration, new hedgerows and shelter belts have created a financially and biologically more sustainable farmed landscape. The trees have provided shelter for the sheep, helped in excluding areas prone to the ever increasing menace of liver fluke, and reduced surface water runoff from fields which carried with it nutrients and soil.

The studies at Pontbren showed that planting tree belts across the slopes led to increased infiltration of water into the soil – more than 60 times that of neighbouring sheep grazed pasture without tree belts.  This is as a result of the improved soil structure and effect of tree roots. When this effect was modelled across the catchment the result was a potential reduction in peak stream flows of as much as 40%. This is clear evidence that integrating trees into our upland farms will play a part in reducing flood risk downstream.

This farmer led initiative also showed the importance of an intimate knowledge of the land in designing and siting tree belts. The farmers knew where shelter was needed, but they also knew where runoff was a problem, where land lay wet for much of the year and those areas prone to erosion. They have managed simultaneously to improve the resilience and sustainability of their farms whilst delivering public benefits of improved water quality and flood mitigation. In addition, the new planting and restored hedgerows and woodland have delivered biodiversity benefits and helped store carbon.

There are undoubtedly opportunities in the uplands for more extensive areas of new woodland, but the answer is not to remove farmers. Farmers in the uplands are a crucial element to the survival of communities in these areas. In Wales 80% of farmland is classed as upland – that’s most of Wales.  These farms are not massive agribusinesses, they are generally small family farms. They are central to the survival of communities and an essential element of Welsh culture and language. But with more trees and hedges on them, these farms can also be a part of the nation’s flood defences.

Cooperation in Cumbria

In Cumbria the Woodland Trust has been working with Natural England and the Rivers Trusts, bringing together people and resources to look at how to manage trees and woodlands in the uplands to benefit rivers. A particular feature of Cumbria is over 112,000 ha of Common Land.  Common land is owned by one person but may be managed by several others with rights.

On Tebay Common, over 1,000ha at the western end of the Howgills north of Kendal, the Trust has been working with Natural England, the Tebay Common Grazing Association and the landowner. The intensity of past grazing has given Howgills an almost treeless landscape. Over the next two winters around 65,000 trees and shrubs will be planted on 110ha (270 acres). The trees will be planted to create an open habitat suitable for future wood pasture grazing and perfect for threatened species such as the Linnet, Lesser Redpoll and Tree Pippit.

The negotiations have taken three years and involved many parties – the Tebay Grazing Association alone has over 20 farmers with rights to graze the common. Many others have an interest including local residents, the Open Spaces Society, archaeologists and others concerned with landscape and ecology.

By reducing grazing and allowing taller vegetation and tree planting more water will be retained in boggy areas and percolate deep into the soil.  This will mean slower run off and less sediment. The trees should also help river shading which will moderate water temperature, important in successful spawning of fish and for other river wildlife.

Downstream

Modeling undertaken by Forest Research shows that woodland strategically located on floodplains can mitigate large flood events by absorbing and delaying the release of flood flows. Research based on the River Cary in south-west England suggested that a 2.2km reach of floodplain woodland could increase flood storage by as much as 71%, delaying the flood peak progressing downstream. Further work at Pickering as part of a project called ‘Slowing the Flow’ has shown how tree planting and other natural measures can help reduce flooding. This suggests there are opportunities for creating strategically placed floodplain woodland to alleviate downstream flooding, particularly the increased risk associated with climate change.

Balancing act

Water management is undoubtedly an important ‘ecosystem service’ of upland areas. But the UK National Ecosystem Assessment recognises four categories of ecosystem service; supporting, regulating, provisioning and cultural. It is essential that the cultural value of upland farming is not lost in the response to recent flooding. Pontbren shows that a balance can be met for maintaining farming in upland landscapes with the wider needs of society.

Planting trees and woodland will not stop all flooding. Engineered flood defences will continue to be needed to prevent inundation of many places. But it is clear that strategically located trees and woodland can play an important part in reducing flood risk, whilst also improving water quality, contributing to biodiversity conservation, storing carbon and helping support more sustainable farming practices. It is these multiple benefits which make it such a compelling option – for farmers as well as Government, its agencies and the many communities impacted by flooding

Trees and other aspects of land management have a critical role to play in protecting property and farmland. It is vital the government responds positively to this opportunity by putting in place support for creation and management of trees and woodland which will ensure their full role in flood defence can be realised.

Mike Townsend, Senior Conservation Advisor

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About Kay Haw

Assistant Conservation Adviser, Woodland Trust. Nature is my passion, especially woods and trees which are just amazing elements of life. One day (soon) I hope we humans learn to work in harmony with Mother Earth.
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7 Responses to Trees and flooding

  1. Beverley Phillips says:

    There’s so much common sense in this article that I think everyone should read it. We need to reverse the mistakes of the past, and as quickly as possible.

  2. Roderick Leslie says:

    Mike, you are right to say that trees won’t stop all flooding. However, until very recently the conventional wisdom is that heavy engineering will and the debate now is between a single focus ‘control nature by force’ versus using a range of measures together, of which trees are one – and engineering like dredging still has a, but not the only, place.

    You pick up one very important point which many commentators are missing: slowing the flow in the lower reaches of the catchment may in fact be more important in moderating peak flows (as opposed to total flow) than what goes on in the headwaters. Whilst wetlands – and especially grassland for the water to spread over – is vital trees create more ‘friction’ and slow the flow more effectively. A further example that makes this point very clearly was the LIFE programme which restored straightened rivers in the New Forest – with the trees so dramatically slowing the flow that concerns for downstream flooding switched to water backing up !

    Finally, we must bear in mind that well designed woods planted primarily for flood control can do much more – open access countryside, providing energy and timber from wood and also influencing siltation and water quality.

    .

    • Rod
      Thanks. Yes, it is the multifunctional value of trees and woodland which makes them such an attractive option. Farmers at Pintbren are using wood from the shelter belts for firewood and for woodchip bedding as a substitue for straw, which is an expensive bedding material in the west of the UK becasue of haulage costs.

      Mike

  3. Rwthless says:

    I’ve always loved trees and it is obvious that belts of trees in upland areas reduce the chances of landslip and avalanches. If they reduce the leaching off of nutrients and water to lower areas, it slows down the movement of water as well. I am moving to a place where if flooding has not happened yet, it could do so in future if something is not done to encourage take up of water.

    I’m enthusiastic about returning the hedges that were rooted out in the 1950s, even by my father, though I only remember the one hedge. Maybe the encouragement of hedging and ditching could be a new feature in landscape management far more productive in the long term than pointless and counter productive dredging that would deepens channels to below sea level, preventing them draining into the sea. Indeed, it could do the reverse and salinate good grazing land.

  4. June McCarthy says:

    Peat bogs, wet meadows, marshy land used to hold a lot of water.
    What have we done in recent years?
    Drained so much of this type of landscape, and also got rid of dips and hollows, making so many large characterless open flat fields that don’t hold water, but allow quick run off of rain to rivers and streams This, together with the destruction of hedgerows and felling off many mature trees is a recipe for …. FLOODING Add in all the concrete that has been poured, all the bricks built, tarmacadam laid, so many gardens lost to patios and hard standing for cars. so that rain can not permeate or sink into the ground, but instead runs straight to drains and then to rivers …. and you have a recipe for ….EVEN GREATER FLOODING. Mix these factors with signicantly higher rain fall and you have a recipe for DISASTROUS FLOODING! We have to reinstate the bogs, marshes, wet meadows, plant hedgrows and trees, do away with vast tracts of flat open fields in order to help to reduce fast run off of heavy rains, to help reduce the burden on rivers and so help to avoid disastrous flooding.

  5. gaw001 says:

    Here here . . . . good stuff . . . . . don’t forget that engineered (ie dense) floodplain hedgerows also potentially have a role to play in diffusing (slowing down) flood peaks. Refer http://ukrivers.wordpress.com/

  6. June McCarthy says:

    Just going on the evidence from our own back garden , which was always waterlogged in winter when we first moved here,- the soil was heavy and clay like and the rain water used to sit in puddles on the surface of the lawn. We planted a number of trees and shrubs around the lawn and now , even with after all this heavy rain, the water drains away and we don’t get puddling on the lawn. Besides taking up water, trees also create channels for water to flow deep into the ground via tree root systems. We should be planting belts of treees, three rows dep at least and digging double ditches at the sides of rivers where floods are occuring.

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