United Utilities will be carrying out investigation work as it prepares to embark on a project to replace six tunnel sections of the Haweswater Aqueduct.
Parts of the Forest of Bowland will see ground surveys taking place to determine the scale of the project. Work will focus on the Marl Hill area in the Ribble Valley and a section of the Bowland Fells, just south of Wray. It will last for around a fortnight in each location.
Water supplies will be unaffected during the investigations, which will also take place in the Hyndburn and Rossendale districts.
“We’re going to be quite visible over the summer months, as we try to understand the best solution,” Jemma Parkinson, United Utilities’ area stakeholder manager, told a meeting of the area of outstanding natural beauty advisory committee.
“Some of the kit which will be required is quite impressive.”
The meeting heard that previous repair work in 2016 had rectified some issues with the tunnel lining. But members were told that the quality of the concrete used when the tunnels were first constructed meant that there remained a risk to water quality and disruption of supplies if further improvements were not made.
The ground assessments will take place at depths of up to 1,200ft (365m).
James Cullen, principal environmental planner at United Utilities, said that the eventual work to replace the tunnels will be carried out sensitively.
“We are aware of our responsibilities in an area of outstanding natural beauty and that will have a bearing on how we decide to go about things,” he explained.
"To carry out the ground investigations, we will need fairly large rigs."
Committee member David Kelly, from the Ramblers’ Association, wanted to know “just how big a blot on the landscape” the full-scale work would be.
“At the moment, we don’t know what the final solution is going to look like…and I don’t want to set unrealistic expectations,” Jemma Parkinson told him.
Following this summer’s investigation work, formal public engagement is likely to take place between 2020 and 2022, with the project itself beginning the following year and continuing until 2028.
The stretch of tunnel due to be replaced at Marl Hill runs for 11 miles and the section beneath the Bowland Fells extends for four miles.
The Haweswater Aqueduct took almost 20 years to construct and was completed in 1955.
It now supplies two million customers – mostly in Greater Manchester – and moves the equivalent of an Olympic-sized swimming pool of water every six seconds.