Generating hydro power in the Snowdonia National Park with GPS PE Black

The Challenge

The National Trust had a unique challenge to create the Hafod y Llan hydro-electric power station amongst the steep slopes of Mount Snowdon, and turned to GPS PE Black for a complete penstock solution that would enable them to return the famous National Park to an outstanding destination for serious climbers, casual walkers and sightseers alike.

The Solution

Constructing such a significant scheme within such a challenging landscape has been no simple undertaking, and doing so while delivering the National Trust’s remit to conserve and protect the environment has added to the project’s complexities.  However, that environmental responsibility was central to Hydropower Services’ design for the scheme and local contractor, GHJ Civil Engineering & Construction leveraged its local and hydro scheme knowledge to manage both the project’s logistical and environmental challenges. While the 640kW scheme will permanently add to the uses that Snowdon has been put to over the years, careful planning, design and project management means that, even before the installation is fully completed, it’s already effectively disguised within the rugged landscape.  But this well-camouflaged scheme hides an extraordinary feat of engineering: once fully operational, the hydro-electric plant will play a key role in delivering the National Trust’s ‘Fit for the Future’ initiative to invest in renewable energy, with a generating capacity equivalent to the power requirements of all the Trust’s properties in Wales.

The Hafod y Llan scheme will extract water from the Afon Cwm Llan river at the head of the valley which will be fed through a 1km GPS PE Black penstock. At the foot of the penstock, GHJ will construct a power house, which has been designed to look like a typical Snowdonia out house. Here the rapid flow of water will enter the turbine, generating electricity before being returned to the river downstream without any effect on the water quality. GPS PE Black pipe was chosen thanks to its flexibility which allowed the contractor to bend and shape the pipe to the contours of the landscape, routing the penstock as close as possible to the river. Working with the Aliaxis technical team and bespoke manufacturing capabilities, the use of GPS PE Black has also enabled the design team to match the wall thickness (SDR) of the pipe to the required water pressure at differing points along the route, providing the most cost effective solution.

Commenting on the project, Alan Jones from GHJ explained: “Using GPS PE Black made the installation less invasive on the landscape because the pipe’s flexibility enables the penstock route to stay true to the contours of the hillside.  It also made the installation more cost effective because it required less excavation and gave us some leeway for small diversions around clumps of heavy rock."

The logistics of transporting the pipe to site, storing it during construction, constructing the water intake in the river and excavating the trench were all arduous thanks to the terrain and the particularly arduous conditions that lasted well into spring. Aliaxis delivered the pipe in 12m lengths in two separate consignments and GHJ constructed a secure compound at the bottom of the site where the pipe could be held safely until required. The pipe was transported to numerous safe drop-off points along the route of the trench using a helicopter and specially adapted handling equipment to protect the pipe from damage during transit, with wind and down drafts proving a significant challenge on the exposed site.  The pipe was laid out in sections and GHJ then used an innovative method of loading the welding machine and tracking it forward to weld the pipe in the trench in a continuous downhill operation, constructing a custom-made steel-frame structure over the welding equipment to protect the joints from the adverse weather conditions.

While the pipe is flexible, the need to install it in both vertical and horizontal alignment with the trench due to the natural contours of the land and the incline of the mountainside made the installation more complicated. Alan Jones continues: “We were able to bend the pipe but it would try to spring back to its natural shape so in some places we had to position stones in the trench to help the pipe keep to the required contours. We then backfilled as we went along, using special rotation buckets fixed to the excavators that could rotate to the angle of the mountain.  This sped up the installation and helped to return the mountainside to its natural state as quickly as possible, using the temporary road we’d constructed as part of the project as the backfill material for the trench.”

The majority of the penstock is GPS PE Black pipe in SDR 26 with a relatively thin pipe wall for the 560mm diameter, but as the water pressure increases within the sections of penstock closer to the turbines, the wall thickness was increased to SDR 17 and then SDR 13.6, while the diameter remains the same to maximise flow rates. To ensure that the changes in pipe dimensions were accommodated seamlessly along the route of the Penstock, Aliaxis provided custom-made change pieces to make the connections between the different SDR pipes sections. The company has also provided a bespoke flange to connect the final section of PE pipe to the ductile iron section at entrance to the power house.

The penstock is completely obscured by the backfill operation and, where possible GHJ has removed all the temporary culverts required during the installation, with the exception of those where the pipe rests on the culvert. Alan Jones adds: “Returning the site to nature has been an important part of the brief throughout the project and there are already few signs that a major civil engineering scheme has taken place on the mountainside. The turf transplantation and seeding, along with natural growth and the infamous Snowdonia rainfall, will help accelerate this restoration of the landscape so that there will be no evidence of the penstock snaking down the hillside until you enter the power house.”