6 October 2017

New report reviews role of trees in flood alleviation

Severe floods that hit Cumbria in December 2015 led to an increase in calls for natural solutions to be included in flood alleviation schemes. Acknowledging ongoing debates around the evidence for tree cover as an effective flood mitigation measure, the UK’s Centre for Ecology & Hydrology has published results of a systematic review of the current evidence. Its findings will inform policy and planning decisions, and identify knowledge gaps and areas for priority research.

The review, conducted in collaboration with Forest Research, the National Farmers’ Union (NFU), National Trust, Woodland Trust, WWF-UK, Environment Agency, Coed Cymru and Lancaster University Environment Centre, looked specifically at the influence of trees in a catchment on flood peak (the maximum river discharge recorded during a flood event). Based on the 71 studies examined by the authors, there is broad support for the conclusion that increased tree cover in catchments results in decreasing flood peaks, while decreased tree cover results in increasing flood peaks.

Considering just observational studies (approximately half of the total number of studies), the authors note that the difference between the numbers of studies reporting an influence and those reporting no influence of trees on flood peak becomes less clear. Analysis of the small number of observational studies that differentiate on the basis of flood magnitude suggests that whilst there is strong evidence of an influence during small floods, the majority of observational studies relating to large floods report that trees have no observable influence on flood peak.

The authors call for an examination of the role of key factors including those associated with characteristics of the forest, catchment and climate, which might explain the more mixed results from observational studies. There is also a need for more empirical data and improved measurement of high flows, to better quantify the effects of woodland creation and evaluate flood risk model outputs.

Lead author, Charlie Stratford, cautions against “the expectation that tree planting is the panacea to all flooding and recommends further research to better understand optimal deployment of natural solutions, their likely downstream impacts on flows, and the role they play in an integrated approach to flood risk management.”

Dr Nick Chappell, Senior Lecturer at Lancaster Environment Centre, said: “The interaction between forests and floods has always been a much debated topic. While the review shows that we now have much more scientific evidence to draw upon, quality observations collected as part of robust experimental designs that are supported by equally robust analysis of hydrological change remain limited for large flood events. Consequently, the exact role of trees in mitigating large flood events in the UK remains uncertain. It is hoped that this new review stimulates new experimental studies to quantify the role of tree planting in the mitigation of large floods.

Dr Stewart Clarke, Freshwater & Estuaries National Specialist, National Trust, said, “This review reinforces the need to consider different options when it comes to land management, and to plan for change. There is no singular method for managing flooding and it is important to think holistically to find practical, sustainable solutions. Natural solutions, including planting trees in upstream catchments, should be considered as one of those options.”

Dr David Tickner, Chief Freshwater adviser at WWF-UK said, “Although scientific questions remain, this research suggests that measure to improve the health of our river catchments, specifically maintaining or restoring tree cover, can help to reduce flood risks.

“Given that rivers and forests provide a wide range of other benefit - including improvements to health, water quality and wildlife – WWF calls on UK governments, regulators and water companies to increase investment in “green infrastructure” as a tool for addressing flooding and other challenges.”

Martin Rogers, NFU flood adviser said, “The NFU supports that natural flood management techniques, in the right location, have their place, but as this review outlines they are not the universal panacea and should be used as part of a cohesive and carefully planned package of measures, looking at upstream attenuation and downstream conveyance to address shorter and longer term flood risk.

“Natural flood management implementation also requires active and full engagement and agreement with land managers, especially farmers, to ensure schemes can work alongside other land uses, including agriculture and food production.”

The report, ‘Do trees in UK-relevant river catchments influence fluvial flood peaks?’, is available for download at http://nora.nerc.ac.uk/id/eprint/517804.