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Tan y Grisiau Dam
Tanygrisiau Reservoir, Snowdonia National Park, Gwynedd, Wales, UK
Tan y Grisiau Dam
associated engineer
Central Electricity Generating Board
Freeman Fox & Partners
date  1957 - March 1963
era  Modern  |  category  Dam/Reservoir  |  reference  SH682447
ICE reference number  HEW 1238
photo  Paul Dunkerley
Tan y Grisiau Dam in Snowdonia holds back the waters of the lower reservoir serving the Ffestiniog Pumped Storage Scheme, a hydroelectric power generating plant. The reservoir was created artificially by damming the Afon Ystradau south of Tan y Grisiau village. The concrete dam has been camouflaged with rock cladding.
The hydroelectric scheme has two reservoirs Llyn Stwlan and Llyn Ystradau (Tanygrisiau Reservoir) which collect rainfall from a catchment area of 951 hectares of land in the Snowdonia National Park. Electricity is generated on demand by water falling under pressure from the higher to the lower reservoir, each of which can store 2 million cubic metres. Water is returned to the upper reservoir at off-peak times.
Tan y Grisiau Dam, a mass concrete gravity structure, impounds the waters of the lower Tanygrisiau Reservoir and is located on its north east side. It is 565.4m long and 15m high with a spillway 79m wide at its south end. The dam is faced with stone removed during the excavation works for its construction, which disguises its impact on the scenery.
Several streams discharge into the reservoir. Weirs in the streams above and below the reservoir monitor their flows. Outflows can be released through regulating valves in the dam, enabling the reservoir level to be managed to ensure incoming water from Llyn Stwlan can be accommodated and enough is retained for pumping back up to it later.
The 20.1m high power station building (SH679444) lies on the west shore of the reservoir. It is of steel frame construction faced with local stone and occupies a footprint of 83.2m x 21.9m. Electricity is generated at 16kV with four alternators of 90MW each and stepped up to 400kV for transmission to the National Grid.
Designed in association with: James Williamson & Partners, Kennedy & Donkin
Main contractors: Cementation Co Ltd, Sir Alfred McAlpine & Son
RCAHMW_NPRN 401209, 306315
Research: PD, ECPK
"Ffestiniog Pumped-Storage Scheme" by J.C.A. Roseveare, ICE Proceedings, London, Vol.28, pp.1-30, May 1964
reference sources   CEH Wales

Tan y Grisiau Dam