Our response to HS2’s news of its intention to start tunnelling in the Chilterns

Chiltern Society Statement in response to HS2 announcement of its intention to commence tunnel boring operations beneath the Chilterns AONB

The Chiltern Society and its partners, the River Chess Association and Buckinghamshire Berkshire and Oxfordshire Wildlife Trust (BBOWT) are today warning that the fragile chalk aquifer beneath the Misbourne Valley – which provides drinking water to thousands of people in the region – is under threat from HS2 Ltd.’s planned tunnelling operations.

HS2 Ltd has announced that tunnelling beneath the Chilterns AONB is imminent, although a licence to commence tunnelling is still to be granted by the Environment Agency.  Despite more than a decade of seeking assurances that tunnelling operations will not damage the fragile chalk aquifer, HS2 Ltd has failed to allay our fears.

When plans to tunnel beneath the Chiltern Hills were first announced in 2010, the Chiltern Society and others raised concerns relating to the potential for significant damage to both the chalk aquifer, which supplies drinking water to many thousands of people regionally and in London, and to the Rivers Misbourne and Chess, which are internationally rare and threatened chalk streams that rely on the aquifer to support this very special habitat and the diverse wildlife that it supports.

The two tunnel boring machines (TBM’s), each more than 10 metres in diameter, will both carve through 16km (10 miles) of chalk from the Chalfont Lane South Portal site, just inside the M25, before emerging at top of the Misbourne valley above Great Missenden.  On the way, these machines will cross the Misbourne Valley twice, passing approximately just 20 metres beneath the River Misbourne at Chalfont St Giles and at the head of the historic Shardeloes Lake.

Our Geological Advisor, Dr Haydon Bailey, confirms that Chalk is an extremely permeable rock in this area and is known to contain intense fracturing and jointing that allow water to pass through the aquifer. There are very significant questions that, even at this late stage, remain unanswered, about how large-scale tunnelling will impact on this fragile groundwater system.

We are therefore seeking answers to the following questions:

  • Will HS2 tunnelling cause irreversible changes to groundwater flow? Will the tunnelling process disrupt water flow patterns, including affecting water supply to –
    • the River Misbourne and other rare chalk streams such as the River Chess?
    • the drinking water abstraction sites along the Misbourne valley and the Colne?
  • Should we expect partial or total loss of flow in the River Misbourne? Between Amersham and Chalfont St. Giles the River Misbourne is “perched” – the river bed is above the water table – and is therefore particularly fragile.  Will shallow tunnelling (at c.20m depth) beneath the river at Chalfont St. Giles cause disturbance to the riverbed and drainage of water from the river leading to permanent damage to this rare chalk stream habitat and loss of wildlife?
  • Will there be pollution to the aquifer? The two TBM’s will grind through the chalk creating a liquid slurry that will be pumped back through the tunnel. Will any turbid water flow be created within the adjacent aquifer and what impact will this have on water supplies and the health of our chalk streams? How will any resulting loss of potable water be mitigated?
  • Should we expect subsidence at Chalfont St Giles? The ground between the bed of the Misbourne and the solid chalk aquifer at Chalfont St Giles comprises unstructured chalk rubble and gravel at least 16m deep. There is potential for the vibration of the TBMs to create subsidence across the path of the twin bore tunnel and for the water to be diverted way from the Misbourne. How will this be mitigated?
  • The reduction of abstraction to protect the Chilterns’ chalk streams has been a long-fought battle. Will recent gains from water companies reducing abstraction be lost with HS2 demanding more water to carry out their tunnelling? Of particular concern, is the huge volume of water required for tunnelling. Precise volumes have still not been fully determined, with latest estimates of the machines requiring up to 10 million litres of water a day. Where will the water come from? Although some water will be recycled, 10 million litres is the equivalent of four days water supply for the whole of the population of Amersham.
  • Once the tunnelling starts, it is unlikely to stop. What contingency plans does HS2 have in place if it becomes apparent that the tunnelling is causing serious damage to the aquifer and to the River Misbourne?
  • Monitoring the impacts of construction relies on data provided by HS2. This is akin to asking students to mark their own homework. What measures will be in place to ensure effective monitoring of the entire tunnelling operation? We are concerned that mitigation plans are inadequate – the mitigation ‘plan’ for the River Misbourne amounts to little more than monitoring for an impact and then exploring how to address the damage, after it has occurred. By this stage irreparable damage will have been caused.

This project, from its very start, has been carried out in totally the wrong order. We would have expected that i) detailed ground surveys would be conducted along alternative routes, ii) detailed discussion would have taken place based on these surveys and finally, iii) a final route would have been selected. Instead i) the route was decided first, ii) public discussion took place based on inadequate data and iii) detailed ground surveys took place identifying all the problematic issues that could have been avoided had the route not already been fixed.

Tom Beeston, Chief Officer of the Chiltern Society, said “We believe that serious questions remain unanswered about the impact of HS2’s tunnelling operations on our precious chalk streams and the availability of drinking water for local communities.

“We are particularly concerned that the HS2 Act has weakened the Environment Agency’s powers to monitor and regulate the impacts of tunnelling upon the aquifer and our chalk streams. We want assurances that the Environment Agency remains able to protect our environment and public health.”

Unfortunately, we are now in the position of hoping for the best but fearing the worst. Only time will tell if our aquifer will remain intact and our chalk streams survive.

The only solution that would ensure that minimal damage is inflicted on the aquifer would be to tunnel through the clay under the aquifer


Notes to editors

The Chiltern Society was established in 1965, we have 7000 members, manage 12 conservation sites, and have 500 volunteers who work tirelessly to maintain and improve the Chilterns for the benefit of both residents and visitors alike.

Every year our volunteers contributed around 250,000 hours, to manage and improved several nature reserves and heritage sites, created long distance footpaths and cycleways (including the Chiltern Way), reviewed hundreds of planning applications and maintained thousands of miles of footpaths and bridleways.

We have campaigned for many years against the damaging effects of HS2; increased our activity and voice concerning many serious threats to the Green Belt across the Chilterns; promoted the heritage of the Chilterns through our annual Heritage Festivals; and offered an extensive programme of walks, cycle rides and events throughout the year.

Interviews are available with:
Haydon Bailey PhD CGeol FGS – Geological Advisor
Cllr. John Gladwin – Lead Trustee on all matters related to HS

Further information

For further information and interviews please contact Tom Beeston, Chief Officer at the Chiltern Society on 01494 936 473 or at TomBeeston@ChilternSociety.org.uk

Chiltern Society Letter to Sir James Bevan 28 April 2021 (1)

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