The Role of Trees in Combating Flood Risks in Greater Manchester
As Greater Manchester welcomed in the new year, many of our communities have once again felt the destructive effects of flooding. The flash storms on New Year’s Eve overwhelmed rivers and canals, leading to widespread damage across Bolton, Manchester, Stockport and Wigan. This flooding caused significant disruption to businesses and left communities starting the new year grappling with the aftermath.
There is a powerful yet sometimes overlooked ally in tackling flooding: trees. Planting more trees and looking after our existing woodlands offers a natural, sustainable solution to mitigating flood risks, providing a much-needed addition to engineered flood defence infrastructure.
Trees are multifunctional, living tools that help make our urban areas more resilient to the challenges of a rapidly changing environment. They can play a critical role in managing water flow, reducing surface runoff and helping to protect vulnerable areas from flooding.
As Greater Manchester continues to grow, managing its urban and rural landscapes becomes more pressing than ever. Rather, using data from our tree and woodlands strategy, All Our Trees, Greater Manchester’s Forest Plan, we have explored the relationship between trees and flood prevention, the role of woodland management and how City of Trees supports this through our work across Greater Manchester.
A Region Under Threat
Greater Manchester faces significant challenges when it comes to flooding. Nestled in a natural basin surrounded by the Pennies, the city region is home to many densely populated urban areas that are particularly vulnerable. This geography combined with the overloaded drainage systems and increasing amounts of impermeable surfaces driven by urban expansion, places the region at a higher risk of frequent flooding.
The current climate crisis further intensifies this risk. Rising temperatures are driving more extreme weather events with flash floods and prolonged rainfall becoming more frequent and more severe. This increased risk combined with our unique geography, requires urgent action and sustainable or natural flood management solutions have a role to play in managing flood risk.
How Trees Help Mitigate Flood Risks
Natural flood management solutions, like tree planting and woodland management, must be considered and invested in as part of the wider flood defence response.
How does it work?
Flooding occurs when rainfall runs off land, buildings and impermeable surfaces like roads and car parks at a rate that it is unable to drain away in streams, rivers, drains or sewers. If the volume of rainwater ‘runoff’ entering combined surface water and sewerage systems exceeds their capacity this can result in the sewers discharging directly into nearby watercourses.
Trees play a key role in intercepting this rainwater to reduce or slow down the rate of runoff. They have the potential to intercept water during rainfall events, which is held in the canopies of the trees
After these rainfall events, this moisture is then re-evaporated into the atmosphere. The cycle may repeat many times, diverting water and preventing it from entering combined sewers.
Some of the rainfall will also be directed down the trees’ network of branches and stem directly into the soil via the spaces created by the trees’ roots. In these two ways trees reduce runoff and replenish groundwater supplies.
Greater Manchester has an estimated total tree population of 11,321,386 trees with a leaf area of approximately 859.1 km² – equivalent to 120,300 Old Trafford football grounds. The effect of this leaf area is to produce an avoided runoff of some1,644,415 m3 per year. This is the equivalent of more than 657 Olympic-sized swimming pools.
Shaping Sustainable Landscapes
Strategically planted trees along riverbanks and in flood prone areas act as natural barriers, slowing down water flow and reducing erosion that leads to riverbank collapse. Trees need to be planted in appropriate places in the uplands to intercept the water to slow down or reduce the amount of water entering the rivers at their source. Trees further downstream can stabilise riverbanks and prevent soil displacement which can worsen flooding downriver.
Data shows that targeted planting initiatives in Greater Manchester’s river valleys can significantly reduce localised flood risks, protecting vulnerable communities. The map below shows priority areas for riparian woodland planting and wider catchment planting. Using various data points, the map highlights the areas with the greatest need for the specific benefits that additional trees could provide related to natural flood management, and the physical possibility to plant these trees.
The Bigger Picture
Our Role in Flood Mitigation – Beyond Planting
City of Trees is on a mission to plant and look after the urban forest in Greater Manchester. Through tree planting and protecting and managing existing woodlands to support rainwater management functions, we hope to tackle widespread issues like flooding, whilst creating a more climate resilient environment for nature and our communities.
This multi beneficial approach has been demonstrated at Dales Brow in Salford, where a Sustainable Urban Drainage scheme has shown natural solutions can address issues of localised flooding whilst also providing an enhanced green space for local people and wildlife.
This scheme co-developed between the Environment Agency, Salford City Council, United Utilities and City of Trees has transformed the landscape with the installation of two bio-swales (sunken, marshy channels), the creation of a new 64sqm wetland area, a 40m long beech hedge as well as hundreds of new plants and trees.
Rainwater running off the road is now diverted away from the sewers into the swales. In heavy rainfall events, the rainwater will travel along the swales and into a temporary wetland area.
We know that tree planting alone is insufficient without long term management plans that invo. Growing and maintaining healthy woodlands ensures their continued effectiveness in flood prevention.
Co-benefits of Trees
Trees deliver multiple benefits that stretch far beyond flood management. In Greater Manchester, trees filter 847 tonnes of air pollutants each year, improving air quality and reducing health risks.
On top of this, they absorb 56,530 tonnes of carbon each year. This contributes to the region’s climate resilience and supports local net-zero goals, as CO2 is the main greenhouse gas contributing to global heating and perpetuating more extreme weather events.
Beyond the environmental impact, trees and woodlands have been shown to improve both physical and mental wellbeing. Individuals that have access to high quality green space within their local area are likely to report lower levels of stress and higher overall wellbeing.
Data also shows the undeniable economic benefit of our urban forest. The total annual economic value of air pollution filtration, stormwater attenuation and carbon sequestration in Greater Manchester’s trees is £33,298,891.
What Can Be Done Today?
The scenes from the start of the year highlight one thing clearly, the threat is very much a reality, and immediate action must be taken in Greater Manchester. Trees and woodlands offer a proven, sustainable solution to mitigating flood risk whilst delivering a host of co benefits for the environment, health, the economy and our communities.
Greater Manchester has an opportunity to lead by example, showcasing how natural flood management can combat climate challenges, whilst creating a region for our communities to thrive.
Creating a flood resilient Greater Manchester requires collaboration across all sectors. City of Trees has committed to tree planting and woodland management targets, however these efforts must be complemented by partnerships with local authorities, businesses and community groups. This partnership is already represented across Greater Manchester in the successes of the Irwell Catchment Partnership and the Upper Mersey Catchment Partnership.
With collaboration and community support, the region can build resilience, protect its communities and potentially inspire other regions to follow suit.