27 April 2017

Extensive natural flood management such as tree planting could reduce flooding even in extreme events like Storm Desmond, according to a new modelling study.

Natural Flood Management (NFM) schemes - which include tree planting, pond creation and peatland restoration - have been proposed as a potential way to offset inland flooding but as yet none of these schemes have been implemented extensively enough to directly demonstrate their effectiveness.  

However, a new modelling study of recently flooded rivers and their tributaries in Cumbria has shown that extensive and appropriately located NFM could have made a significant difference to flood flows even during an extreme event such as Storm Desmond in 2015 which caused an estimated £500m of damage.

This reduction in flood risk would complement traditional engineering measures, according to a new report published by river charity, The Rivers Trust working with JBA Consulting,  Lancaster University and United Utilities. 

A critical part of the modelling work – which looked in detail at data collected from the catchments of the rivers Eden, Kent and Derwent in the run up to the floods of 2015 – was to understand how much confidence we can place in the likely benefits of NFM interventions.

To do this, researchers used powerful computers to combine existing knowledge about the effectiveness of NFM measures and tested this against the extent of the rainwater flowing into catchments during storm Desmond.

They then produced a series of maps to quantify the potential flood-reducing benefits of NFM. These maps are already being used to inform planned delivery of £3M of NFM investment by the Rivers Trust and other organisations. 

Dr Nick Chappell of Lancaster University said: “Measures such as tree planting could have a double benefit: not only do the trees remove surface flows by soil drying and enhanced permeability but reanalysis of old studies shows even leafless deciduous woods can also throw 10 to 20 percent of rainfall back up to the sky through evaporation from wetted branches. 

“Our modelling techniques demonstrated that by adding a range of NFM measures at many key locations in the landscape, significant reductions to flood peaks on quite large rivers may be produced.  The research showed these benefits could potentially be produced, even for an event as extreme as Desmond, though our work highlighted the need for new experimental evidence addressing incremental changes in rainfall reaching the woodland floor through such extreme storms.”

David Johnson from The Rivers Trust said: “Catchment management of flood risk needs a long term plan which includes both traditional engineering and extensive NFM delivery. The Rivers Trust approach is to understand, maintain and add to a catchment’s natural capacity to reduce flooding, using the available evidence.”