4 minute read
Sankey Natural Flood Management Initiative
Sankey Valley, St. Helens, England, United Kingdom
A series of Natural Flood Management (NFM) strategies were implemented in 2012 in the Sankey Valley in Northwest England. The NFM initiative was spearheaded by St. Helens Council but was ultimately embraced by the broad partnership of organizations that comprise The Mersey Forest. A monitoring study conducted by the Universities of Liverpool and Newcastle found that four engineered log jams—commonly referred to as “leaky dams,” constructed by Groundwork Charity as part of the initiative—improved water quality, trapped sediment, and attenuated floodwater in the catchments. The water attenuation, in particular, contributes in a small way to the protection of Blackbrook, a downstream community that has flooded three times since 2000, with a 5 percent chance of flooding in any given year. The Environment Agency of England (EA) then led three environmental outcome days, building seven more leaky dams. The initiative was profiled by the EA in the Working with Natural Processes-Evidence Directory; by Economics for the Environment (Eftec) consultants and the Green Alliance in their Natural Infrastructure Schemes in Practice publication; and by the Institution of Civil Engineers, in an engineering sustainability-themed paper.
Article cover: Willow-arch shelter belt across a modelled area of sheet overland flow. (Photo by Carl Smethurst, The Mersey Forest)
Producing Efficiencies
According to a study completed by the two local universities, the four leaky dams improved water quality by reducing phosphate by 94 percent. The study also found the dams trapped sediment: 792 cubic meters of sediment were projected to accumulate in the reservoirs by 2035, which would have resulted in the loss of a third of the reservoirs’ capacity. Additionally, the structures were found to have attenuated 2,500 cubic meters of floodwater in the catchment.
Using Natural Processes
Natural leaky dams are typically created from pieces of wood—although living vegetation may be added to the structure— that accumulate in rivers themselves, and on river banks and floodplains. These dams usually occur naturally along rivers when trees fall into the water and get snagged in an area; the dams are also sometimes the result of beaver activity, although these tend to be more air-tight. Leaky dams can be engineered and built through human effort to restore rivers and floodplains, and to slow and store floodwater. The woody barriers provide a number of benefits, such as water quality improvement, habitat provision, and climate regulation.
Broadening Benefits
In the community of Blackbrook, there are 18 properties at flood risk, including three businesses and a major commercial traffic lane; the leaky dams delivered so far offer a small, incremental reductions in flood risk. With more attenuation targeting flood peaks, the greater the reduction of risk for Blackbrook. Leaky dams—both natural and engineered—provide habitat diversity by creating pools and varied channel morphology while also improving water quality. The structures support fish and macroinvertebrate life cycles and provide nutrients for aquatic organisms; the dams also provide basking and perching sites for reptiles and birds. The barrier of willow shelter belt intercepts sheet overland flow. A footpath tunnel was also created to enhance recreational opportunities and the aesthetics of the area.
Promoting Collaboration
The first four leaky dams were funded by Natural England and built by the Groundwork Charity. Detailed hydraulic modeling to understand the effect of NFM features was funded through the EA. The EA then built seven more leaky dams through three environment outcome days. The initiative was brought about by a wide-ranging partnership of multiple stakeholders: public, private, and charitable organizations.
The Mersey Forest is a partnership of seven local authorities (Cheshire West and Chester, Halton, Knowsley, Liverpool, Sefton, St. Helens and Warrington); landowners; the Forestry Commission; Natural England; EA; businesses, including United Utilities; and most importantly, local communities.
