Forests ‘make minimal difference’ in reducing impact of major floods – study

Forestry should not be relied upon to reduce risk from major flood events, that are likely to be more frequent and more extreme due to global warming, according to a study by researchers in Ireland and the UK.

Forests can mitigate flood risk “but may offer less protection against major events than had been hoped”, they conclude.

"It would be unwise to assume a newly-planted forest could protect homes on at-risk land," suggest the researchers from Trinity College Dublin; NUI Galway and the Centre for Ecology and Hydrology in the UK.

Their study published in Science of the Total Environment shows forests can suppress small storm flows but “they are likely to make minimal difference in reducing the devastating impacts of major flood events”.

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The recent Cop26 global climate summit strongly endorsed the environmental benefits of large-scale forest planting and reforestation in addressing the climate and biodiversity crises affecting the Planet. It also backed scaling up of adaptation measures to protect against the inevitable consequences of global heating, notably flooding.

Senior author Prof Liwen Xiao of TCD School of Engineering said it was true forests could lock up large amounts of carbon and aid environmental rehabilitation and biological diversity, while “the common perception among many natural resource managers and the public that forests can mitigate large flood events hasn’t been well examined” up to now.

He added: “Our work showed forests reduced storm flows but only when the peaks of the floods were well below the average peaks seen in a typical year. When flood events increased in size and in larger catchment areas forest impacts were far less notable, and these two scenarios are the most important for us to build knowledge in seeing as they are linked to the greatest incidences of flood damage and loss of life.”

While forests bring many environmental benefits to habitats, their research “raises red flags to any developers and land managers working on the basis that they can protect against serious flood damage. It would be unwise, for example, to assume a newly planted forest could protect homes on at-risk land,” he said.

The scientists evaluated data spanning flow measurement over 66 years and 610 "storm events" at three sites in Ireland and the UK. The sites were based in Nephin Forest, Co Mayo; Kielder Forest on the England-Scotland border and Hafren Forest, Wales. These sites have similar climates and are typical of forests in their respective countries, which suggests the findings are likely to be widely applicable.