4 minute read
Low Leighton
New Mills, England, United Kingdom
Alleviating flood risk on the Peak District Pennine fringe. Leighton Brook is a degraded ordinary watercourse that rises in the rural Peak District National Park headwaters and joins with the River Sett in the town of New Mills, Derbyshire. Historical maps of the river-catchment show that the landscape has lost more than 20 hectares of woodlands and trees in the last 100 years. The brook’s straightening and entrenchment along with overgrazed, hardened, and treeless hillsides speed up the rate at which rain reaches the watercourse, heightening flood risk in the built-up environment downstream, where no space exists for excess flood water during periods of high flow. This has resulted in 17 floods in the last 10 years. Therefore, in 2017, the Environment Agency, in partnership with Mersey Forest and the Derbyshire County Council conducted a catchment investigation. Piece Farm, a mixed livestock farm under a high-value government agricultural and environmental agreement for wading birds, manages the source waters and had already created two large attenuation ponds. To further mitigate flooding concerns, the project team tested a natural flood management scheme, planting a thousand willow trees and installing engineered logjams in 2019. These solutions create long-term, secure features in the landscape, delivering 3,000 cubic meters of attenuation.
Article Cover: Water fills the containment pond. (Photo by Derbyshire County Council, Environment Agency, Mersey Forest, and Transition New Mills)
Producing Efficiencies
While the land covers only 1.6 square kilometers of headwaters, the drainage of the area is complex, with many forks and confluences. Therefore, the team used a detailed overland flow routing map created with the Arc Map tool to identify which watercourse put Low Leighton most at risk for flooding. Additionally, the trees cleared in the nearby town of Bury, to make way for a conventional flood defense, became the timbers in the engineered logjams, recycling material and avoiding the need to fell other trees. Volunteers from Transition New Mills, a community sustainability and carbon reduction initiative, planted the supporting locally sourced willows.
Using Natural Processes
Putting dead wood in a river channel alone to naturally manage floods seldom functions as a longterm solution; but these engineered logjams with large, long timbers held in place by stakes at the front and back will naturally slow the runoff from Pierce Farm. As the logs decompose, the dense willows planted downstream take root and grow, securing the structure and filtering the water through the complex system of root fibers. The willows will eventually grow across the stream, providing resilience to the living logjam and the surrounding wet woodlands habitat.
Broadening Benefits
The community has noticed that slowing the flow at the top of the catchment, where stream runoff begins, has resulted in a less-flashy watercourse. With reduced flood risk comes reduced costs for flood recovery, but this project also provides numerous ecological benefits. As the willow trees grow, they will provide new habitat for willow warblers (Phylloscopus trochilus) and willow tits (Poecile montanus). The small woodlands and the willow timber in the logjams will benefit hedgehogs (Erinaceus europaeus) and benefit badgers (Meles meles). Furthermore, the project’s partnership with Salford University has led to much academic research as students monitor the site as part of their coursework.
Promoting Collaboration
The Environment Agency, Mersey Forest, and Derbyshire County Council’s partnership, and the landowner’s enthusiastic support, allowed the team to draw on multiple funding sources to complete the project. Additionally, by working with the University of Liverpool and Salford University, the team was able to secure monitoring equipment and data processing, saving time and money while creating educational opportunities for students. The project’s collaborations have been so successful that they will continue for the next 10 to 20 years, leading to a better understanding of natural flood management’s effects across a catchment.
