A 520-acre complex of ten operational reservoirs in north-east London, Walthamstow Wetlands is Europe’s largest urban wetland nature reserve while also supplying around 3.5 million households in and around the capital with water.
In 2017 the reserve officially opened, bringing much greater visibility to the 150-year old site, with improved visitor facilities and free entry to the wider public in an area of London that has historically been deprived and underdeveloped.
A rural escape for residents and visitors
Plans have been underway since 2012 to transform the site into a wildlife-rich reserve and rural escape for residents and visitors.
As well as a nature reserve with footpaths and cycle tracks twisting between the fully operational reservoirs, the multi-million-pound development, funded by the Heritage Lottery Fund, Thames Water, the London Borough of Waltham Forest and the London Wildlife Trust, encompasses the redesigned Engine House with a visitor centre, café and re-built tower for nesting swifts and roosting bats. The Grade II listed Coppermill Tower meanwhile offers panoramic views of the city.
Design for a sustainable future
Ramboll was employed by the London Borough of Waltham Forest to provide environmental, hydrological, engineering and geotechnical services for the development of the reserve’s new reed beds. We prepared a hydrological and geotechnical design together with a dredging specification to make way for the new wetland area whilst allowing for the continuous supply of London’s potable water.
We reviewed specialist ecological survey data, prepared a draft design of reed bed planting plots (including reed stock, species mix and plant specifications) and prepared biosecurity notes to evaluate the presence of invasive species on or around the site during the reed bed creation works.
We also supported the client with the completion of the final design for tender, prepared ecological specifications for the contractor’s tendering for the project and provided a client’s representative for the construction process.
A cost and carbon-efficient solution
A significant driver for the scheme was Thames Water’s requirement to dredge the reservoirs. Two of the three reservoirs that were the focus of the reed bed creation are used by Thames Water as settlement lagoons for the copper mill water treatment works. To remain operational, the reservoirs have to be dredged every five to seven years. Prior to Ramboll’s involvement, the reservoir had not been dredged for over 10 years, meaning the water depths had been reduced, with an associated impact on settlement capacity and water flow.
A total of 619m of retention structure was constructed to create the reedbeds. The cost of the geotextile system, including all posts and tie back structures was just under £63,000. Making a high-level assessment about the design, if stone filled gabions had been used instead, the costs are likely to have more than doubled to over £140,000. A stone filled gabion solution would additionally require large amounts of material to be imported to site. For the high-level assessment design, an estimated 140 20-tonne lorry deliveries would be required to import the required stone. This has an estimated carbon cost of 9 tonnes. The timber post and geotextile scheme by comparison required less than 10 site deliveries.
The reedbed scheme delivered 1.8 hectares of new reedbed habitat and over 30,000 m3 of silt was dredged and placed behind the retaining structures.
Without the reedbed creation, Thames Water would have needed to dredge some of the sediment in the reservoir. Ramboll estimated that this would equate to approximately 9000m3. Because there were low levels of contaminants in the sediment, the material would likely not have been suitable for agricultural use, and we expect would have been disposed of as non-hazardous waste. Even taking relatively low costs for off-site disposal, pre-landfill tax disposing of this volume of material would cost in in the order of £450,000, and landfill tax could cost over £1 million. For the volume of material, over 600 tipper trucks would have been required to remove the sediment from site.