HS2's Colne Valley Landscape & Habitat Creation Project: Creating a Lasting Legacy

• The plans represent the biggest single landscape and habitat creation project within HS2’s Green Corridor program
• The project will contribute towards a localized net gain in biodiversity within the Colne Valley
• 127 hectares of new chalk grassland, woodland, wood pasture and wetland habitats to be created
• 65,000 trees and shrubs of 32 species, and 3.5 kilometers of new hedgerows to be planted

An ambitious scheme, designed by HS2’s landscape architects, ecologists, engineers and soil specialists, including a Jacobs team, aims to reuse construction materials to create 127 hectares of new chalk grassland, woodland, wood pasture and wetland habitats that will substantially enhance the local natural environment.

The Colne Valley Western Slopes project will be a major contributor in meeting HS2’s carbon reduction targets by delivering significant ecological gain in the Colne Valley. It will also reduce the carbon footprint of the project by reducing road haulage and waste treatment activities and will enable habitats to sequester carbon by replacing arable land with natural habitats.

Plant species and habitats are being carefully selected to be resilient to a changing climate. Almost 65,000 trees and shrubs of 32 species will be planted together with nearly 3.5km of new hedgerows. Around 90 hectares of calcareous grasslands which once thrived on the valley slopes will be established and areas of historic woodland reinstated.

The project, being designed and delivered by HS2’s civils contractor Align, will provide wider health and recreation benefits for neighboring communities including the provision of new connected green spaces and around 4.5km of new footpath, cycling and horse-riding routes. Jacobs is design partner to Align, the high-speed rail consortium of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick.

Arable land that is initially needed for a major works compound to construct the Colne Valley Viaduct and Chilterns Tunnel will be transformed through the re-use of existing soils and recycling of three million m³ of chalk taken from the tunnel excavation, as well as concrete and limestone aggregate materials used in the construction process.

Reusing materials rather than transporting them away by road will significantly reduce carbon output. The excavated material will be used to re-establish the locally distinctive chalk valley landscape, replicate natural drainage flows and establish the right growing conditions for calcareous grassland across the site.

Calcareous grasslands, which develop on shallow soils overlying chalk or limestone, are a valuable, scarce and rapidly declining habitat in the U.K., with this decline reflected in both the Colne Valley and adjacent Chiltern Hills. The neighboring Chilterns Area of Outstanding Natural Beauty is currently thought to support around 700 hectares of chalk grassland. This collaborative project will provide a unique opportunity to create one of the largest extents of such habitat in this area.

New planting and seeding will create an extensive mosaic of habitat that will potentially be colonised by hundreds of species of flora and fauna, including invertebrates, birds, mammals, reptiles and amphibians.

HS2’s Environment Director Peter Miller said: “This project forms a major part of HS2’s Green Corridor program to establish better connected, sustainable and biodiverse landscapes along the route of the new railway that will contribute substantially to HS2’s carbon reduction target. It demonstrates HS2’s approach to addressing many of the complex issues surrounding climate change and which are central to protecting our environment, and is a great example of how good design and planning can mitigate the effects of climate change.”

Align Project Director Daniel Altier said: “This innovative, multi-discipline project is designed to create a huge expanse of new grassland, connected woodland and wood pasture in the Colne Valley, helping to preserve declining or rare flora and fauna in the area, and providing a lasting legacy for biodiversity and local people. We are pleased to be collaborating on the design with Natural England, Hertfordshire and Middlesex Wildlife Trust, the Colne Valley Regional Park Panel, local authorities, and specialist soils experts.”

Jacobs Ecology lead Matt Hobbs (on behalf of Align) said: “The Colne Valley Western Slopes is a unique opportunity to give something back for wildlife by creating a large new reserve dedicated to nature in the Colne Valley. The design will deliver habitats that are the biggest and best possible, and those that will succeed and thrive over the long term with minimal human input.

“We initially focused on soils and substrates, the essential building blocks for the habitats, and ensuring they are provided using site-won soils, chalk and temporary works materials. The key challenge is creating the necessary physical and chemical characteristics of the soil profiles to support resilient, biodiverse grasslands and the specialist species that thrive there. This is the subject of an innovative research study with Cranfield University and Tim O’Hare Associates that will involve both laboratory and field trials during construction to fine-tune the design.

“A key challenge with habitat creation is how to establish and then maintain the habitats over the long term. The team’s approach uses local land managers who will let natural processes operate and introduce free-roaming livestock to graze the habitats year-round. This will reduce management inputs, increase soil health, soil carbon, and overall biodiversity, while creating a dynamic landscape of wood pasture and wetland over time. The designs include all the necessary infrastructure to support grazing animals year-round including fencing, cattle grids, management areas and wetlands for watering.

“Numerous small-scale features have been included to diversify the habitat further. These include s-shaped sparsely vegetated banks for reptiles and invertebrates, and hibernacula to provide refuges for reptiles and amphibians, as well as localized variations in topography to provide niches for many species to colonize.”