Anthony Guay

Director, Environment & Health

T: +44 7918 768 716

Project Data

CLIENT

London Borough of Waltham Forest

COMPLETION

2016

A wildlife experience on the edge of London

As well as a nature reserve with the Wetlands offers footpaths and cycle tracks twisting between the fully operational reservoirs. Excellent view of the Shard, the Gherkin and other City towers act as a reminder that this is the edge of London. What was once out of bounds to all but a few reservoir employees and fishermen has been transformed into a community recreational venue. The reservoirs continue to supply drinking water to 3.5million homes in north London. 

Ramboll was appointed by the London Borough of Waltham Forest to assist them in enhancing the existing wetlands and to provide a forward looking and sustainable approach to the disposal of dredged sediments from the reservoirs. We prepared a hydrological and geotechnical design together with a dredging specification to make way for the new wetland area and the development of the reserve’s new reed beds, whilst allowing for the continuous supply of London’s drinking water.

A cost and carbon-efficient solution

These reservoirs have to be dredged every five to seven years. However, prior to Ramboll’s involvement, they had not been dredged for over 10 years, which meant the water depths had been reduced, with an associated impact on settlement capacity and water flow. Dredging would require the disposal of millions of tons of silt was a problem, requiring over 600 vehicle movements to remove the sediment, with significant impact on the road network, air quality and carbon emissions. 

Ramboll experts undertook a root and branch reassessment of the site characteristics, developing the solution from concept design through to implementation. An initial assessment of the hydrological regime and the sediment quality established its suitability for re-use as a retaining structure for the reservoir. 

The solution was the creation a new reedbeds scheme delivering 1.8 hectares of new wildlife-friendly reedbed habitat through the re-use of 30,000 m3 of dredged silt placed behind 619m of retaining structures. This provided the Wetlands with an additional 1.8 hectares of new reedbed habitat. Over 30,000 m3 of silt was dredged and placed behind the retaining structures behind the structure, creating channels and ponds to create habitat for wading birds and fish fry. As well as removing the need to dispose of displaced sediment off-site, the solution saved an estimated 140 20-tonne lorry deliveries required to import the required stone, with only10 site deliveries required for the geotextile scheme. Future silt raised by dredging can be similarly re-purposed.

Design for a sustainable future

The project has shown how, through detailed environmental and geotechnical characterisation of the sediment, coupled with hydrological and ecological conceptual site models, sediment within the reservoir structures can be successfully re-used on-site. The re-use has provided various ecosystem services, through habitat generation and enhanced leisure opportunities for visitors to the wetlands. Going forward, as the reservoirs require re-dredging in another five to seven years’ time, the dredged sediment could again be used to create additional reedbeds in a similar manner or to supplement the existing reed bed structures.

The Walthamstow Wetlands opened to the public in 2017 to universal public and industry acclaim, attracting over 400,000 visitors in its first year, while still supplying drinking water to millions of north London residents. The project has received considerable industry recognition:

  • Ground Engineering Awards - Sustainability Project of the year
  • Brownfield Briefing Awards – Best Biodiversity Enhancement
  • Civic Trust Awards – Regional award – London

Sustainability facts

  • 30,000 m3 of dredged silt was used behind retaining structures, saving offsite disposal
  • 600 vehicle movements to remove the sediment obviated
  • >400k visitors to this fully functioning reservoir complex