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Marchlyn Mawr Dam and Reservoir
Mynydd Perfedd, Snowdonia, Gwynedd, Wales, UK
Marchlyn Mawr Dam and Reservoir
associated engineer
Central Electricity Generating Board
James Williamson & Partners
Binnie & Partners
date  1975 - 1979, 2007
UK era  Modern  |  category  Dam/Reservoir  |  reference  SH617619
ICE reference number  HEW 1237
photo  © Eric Jones and licensed for reuse under this Creative Commons Licence
The natural lake Marchlyn Mawr was dammed to form the upper reservoir of the Dinorwig Pumped Storage Scheme, the largest such hydroelectric scheme in Europe at the time of its construction. The dam is rock-filled and sealed with dense asphaltic concrete. Its crest level has been raised to increase storage capacity.
Llyn Marchlyn Mawr formerly held about 2.5 million cu m of water behind a natural moraine dam some 40m high. In the 19th century, the dam was enlarged a little and the lake used for public water supply.
The modern dam was constructed on the rock sill of the cwm (valley), on the north side of the lake, incorporating the moraine. It raised the water level by 33m to create a reservoir with a maximum capacity of 9.2 million cu m. The live storage of 6.7 million cu m is released to drive the hydroelectric turbines of Dinorwig’s underground power station (SH593601).
The dam is 620m long and 57m high. Its 6.5m wide crest is topped by an unsurfaced track and stands at a height of 636m above sea level. High water level in the reservoir is at 634m and low water level 600m. The dam was constructed with 1.5 million cu m of rock fill, taken from an abandoned quarry 2km away. It has side slopes of 1 in 2.
The upstream (south) face of the dam is sealed with an 80mm thick membrane of dense asphaltic concrete over a layer of binder asphaltic concrete, 60mm minimum thickness. It was only the second time this technique had been used in Britain. The downstream face is covered by vegetation and scree.
Dams require measures to prevent water seepage below them, known as a cut-off. Here, a 120m deep grout curtain was used. Grout was injected under pressure into a vertical zone, or curtain, in the underlying foundations below the dam wall. Where the dam wall meets the ground on the upstream side (at the ‘toe’), there is a long internal passageway, or gallery.
A 10.5m diameter low pressure tunnel runs down from the reservoir at Marchlyn Mawr, 1.7km on a slight gradient, to a 10m diameter vertical shaft 450m deep. From the bottom of the shaft, a 10m diameter high pressure tunnel of 670m leads to the power station. There it divides into six smaller tunnels, one feeding each of the turbines, which can be used for both power generation and pumping.
The dam was completed between 1975 and 1979. On 9th May 1984, the Dinorwig scheme opened officially, though it had been commissioned the previous year. It was built for the Central Electricity Generating Board and is now owned by the First Hydro Company.
Owing to the building of the power station, Marchlyn Mawr Reservoir could no longer be used for public water supply as the water was needed to generate electricity at a moment’s notice. Ffynnon Lligwy (or Llugwy) Reservoir (SH692627), 7.6km to the east, took its place. A new treatment works was constructed at Mynydd Llandegai (SH595654), some 6.2km north of Dinorwig. The scheme also involved some 18km of associated pipeline.
In 2005, maintenance repairs were carried out on cracks in the sealant. In 2006, two-thirds (20,000 sq m) of the sealant was replaced with a coat of hot mastic placed from the dam’s crest using a purpose-built spreader.
From June to October 2007, Marchlyn Mawr Dam was enlarged and its crest height was raised a further 2.5m, enabling the top water level to increase by 3.3m to 637.3m above sea level. The extra capacity allows increased storage and electricity generation.
The crest was heightened using a precast concrete wall along its upstream edge, with the access track raised on rock fill sourced from local slate tips. The wall was constructed in sections 2.7m high, 1.2m wide and 1.7m long, prefabricated offsite to minimise the need for on-site casting. Works also included enlarging the existing overflow structure, extending the original grout curtain and landscaping.
Resident engineer: Ithon Jones, James Williamson & Partners of Glasgow
Contractor: Gleeson Civil Engineering Ltd
Contractor (2007): Edmund Nutall
Waterproofing (2005-6): Hesselberg Hydro
Grouting (2007): Keller Ground Engineering
Crest wall (2007): Ruthin Precast Concrete
Research: PD, ECPK
"Dinorwig Pumped Storage Scheme" by J.A. Baines et al
Proceedings of the ICE, Vol. 74, Part 1, November 1983
"Dinorwig Power Station: Marchlyn Mawr Reservoir: Archaeological Assessment" by John Roberts, Ymddiriedolaeth Archaeolegol Gwynedd, July 2006
"Marchlyn Mawr additional stored energy project" by Giles Hird, Dams & Reservoirs, Vol.20, pp.9-15, March 2010
"Water management at Dinorwig pumped-storage power station" by Mark I. Bailes and Owen P. Williams, in Improvements in reservoir construction, operation and maintenance, Thomas Telford Ltd, London, 2006
reference sources   CEH Wales

Marchlyn Mawr Dam and Reservoir