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Integration of the HS2 rail line within the Colne Valley Western Slopes.
Integration of the HS2 rail line with the Western Valley slopes

Colne Valley Western Slopes

The ‘Colne Valley Western Slopes’ project – which was approved under Schedule 17 of the HS2 Act by Three Rivers District Council and Buckinghamshire Council – will see the transformation of what is now an HS2 construction site into one of the largest areas of new chalk grassland in the Chiltern hills.

Chalk grassland – a type of calcareous grassland – is a habitat of international conservation importance mainly found on limestone and chalk valleys of south-east England and the Isle of Wight.

Lime-rich, but low in nutrients, the thin soil holds little water and heats up quickly. These conditions encourage a huge variety of smaller herbs and wildflowers and over 40 species can be found in one square metre of grassland, including some of the UK’s rarest orchids and invertebrates. Only 700 hectares of chalk grassland exist across the whole of the Chilterns AONB.

As part of the ambitious scheme, 90 hectares of chalk grassland will be seeded into re-profiled soil layers using the nutrient poor subsoils on the site and mixing these soils with chalk from the tunnelling and recycled concrete and aggregates from construction works. The design of these soil profiles will be completed through collaboration with Cranfield University and Tim O’Hare Associates in an innovative research study that will use the results from both laboratory and on-site trialling of the soil layers.

This will sit alongside new areas of woodland, wood pasture and wetlands, including almost 65,000 trees and shrubs of 32 species and nearly 3.5km of new hedgerows. Around 4.5km of new footpath, cycling and horse-riding routes will give the public areas to large parts of the site, which sits between the Colne Valley Regional Park and the Chilterns AONB.

Architect's impressions of HS2’s ambitious Chiltern tunnel chalk grassland project - the Colne Valley Western Slopes

Engaging on the design of the Colne Valley Western Slopes

Since Spring 2018 we have been talking with the local community through a series of information events, to share details on the progress of the design of the Colne Valley Western Slopes, to seek views and to respond to feedback.

The plans were approved by Three Rivers District Council (which covers more than 99% of the site) and Buckinghamshire Council under Schedule 17 of the HS2 Act 2017. This followed extensive pre-application engagement and collaboration with the planning leads at Hertfordshire County Council, Three Rivers District Council and Buckinghamshire Council, as well as other key stakeholders.

As well as the landscaping, the approval also includes the design of the south portal itself and associated service buildings. Going forward, HS2 and its contractors will continue to work closely with the planning authorities to develop the final restoration of the site.

You can view or download our Chiltern Tunnel South Portal and Colne Valley Western Slopes ‘You said, We did’ information boards below, a key part of the design for HS2 in the Colne Valley.