4 minute read
Smithills
Smithills, England, United Kingdom
Planting 140,000 trees for the Northern Forest initiative. Dean Brook in Smithills, Bolton, in the northwest of England rises 456 meters onto the peatlands of Winter Hill. At its rural source waters, Dean Brook flows off the West Pennine Moor Site of Special Scientific Interest, then through woodlands and agricultural channels before flowing through the urban areas of Smithills. Dean Brook poses a flood risk to Smithills, spilling 12,282 cubic meters of water in a 100-year event, affecting a total of 53 properties and more downstream. In 2015, the Woodland Trust acquired 6 square kilometer flowing down to Smithills; and together with the Environment Agency, Mersey Forest, and the University of Liverpool set to restoring the site through natural flood management. The team began by planting 30,000 new trees; then installed thirteen engineered logjams to function as leaky dams, which intercept flood flows but allow lower flows to pass through; and reconnected parts of Dean Brook with its floodplain by uncovering 150 meters of buried watercourse. The project is planting 140,000 trees on the Woodland Trust’s estate as a pilot site through the Northern Forest initiative with support from the Heritage Lottery Fund. Together, these efforts will slow, store, disconnect and hold-up overland flows, in effect holding floodwaters in the landscape.
Article cover: The cascading series of engineered log jams naturally slows the water flowing downstream. (Photo by Woodland Trust, Environment Agency, and Mersey Forest)
Producing Efficiencies
The project team used existing one- and twodimensional models to determine the exact volume of water spilling onto the floodplain during a 100year flood event for Smithills. Using that volume as a guide, the team—along with volunteers and specialist contractors—planted willows; built various water retention measures, including log jams, clay-core piped bunds (small drained embankments), and detention basins; and took sections of rivers out of constrained buried pipes. Comparing pre- and postconstruction data showed the leaky dams delay and reduce peak flows by around 20%, depending on catchment conditions and the nature of the rainfall event.
Using Natural Processes
The engineered logjams are living barriers that will grow into place over time. As the timber jams degrade and decompose, the dense willows planted behind take root and grow, not only securing the structure but also filtering polluted water. Elsewhere, the team built shallow, piped clay-core embankments to back up flood flow at certain points. Where the team removed streams from pipes (called daylighting), they restored floodplains by lowering the land and landscaping. These measures make space for water, particularly for floodwater upstream that would have otherwise passed downstream to the community at flood risk.
Broadening Benefits
Alleviating flood risk in Smithills represents one of the major successes of this project, but the tens of thousands of planted trees and reconnection of Dean Brook with its floodplain have also led to the reappearance of the keeled skimmer dragonfly (Orthetrum coerulescens), unseen in the region for many years. Wading birds and fish will benefit from the new refugia and foraging grounds, and the protection of the peatlands and moorlands will sequester carbon dioxide and methane, greenhouse gases that directly contributes to global heating and climate change—all without lowering the agricultural value of the land.
Promoting Collaboration
As a demonstrator site for the Northern Forest initiative and the Woodland Trust’s largest English site (receiving EU LIFE IP funding under project number LIFE14 IPE/UK/027), the Smithills project represents an important opportunity to show local landowners, farmers, and community stakeholders how natural flood management projects can work. Natural Course funded in part the project’s communications output. This engagement allowed the project to gain momentum and traction. Through the partnership of the Woodland Trust, the Environment Agency, and the Mersey Forest, thousands of volunteers have supported the project, making the large-scale planting efforts possible.
