Stwlan Dam

Intake tower and dam

Intake tower and dam

Date

19 October 2013
Location

Llyn Stwlan, Moelwynion

SH 66623 44421; 52.98047°N, 3.98803°W

Information

Stwlan Dam

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Stwlan Dam

Stwlan Dam, looking towards Moelwyn Bach

Date

24 November 2012
Location

Llyn Stwlan, Moelwynion

SH 66623 44421; 52.98047°N, 3.98803°W

Information

Stwlan Dam was built as part of the Ffestiniog pumped-storage hydroelectric scheme, which was completed in 1963. Llyn Stwlan is located in a cwm below the mountains Moelwyn Mawr and Moelwyn Bach above the village of Tanygrisiau and before construction of the dam was a very small lake. Now it has a capacity of 2 million cubic metres and supplies water at a rate of 27 cubic metres per second to drive the turbines at the power station below. The concrete buttress dam has a total length of 373m, including the spillway; a maximum width of 4m; and a maximum height of 34m.

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Moelwyn Quarry

Looking down the incline towards Llyn Stwlan, Ffestiniog Power Station’s upper reservoir

Date

24 November 2012
Location

Llyn Stwlan, Moelwynion

SH 66089 44279; 52.97906°N, 3.99593°W

Information

Moelwyn Quarry, located on the eastern slopes of the Moelwyn mountains near Blaenau Ffestiniog, met with only limited success and closed around 1900. Exploration had started in the 1820s with a number of trial levels and the site was further developed in the 1860s when larger scale underground operations began. A slate processing mill, powered by waterwheel, was built at that time and its remains are visible below Stwlan Dam. The exit tramway descended the mountainside via a series of inclines to join the Ffestiniog Railway, the site of the actual junction now being submerged below the waters of Llyn Ystradau, Ffestiniog Power Station’s lower reservoir. Llyn Stwlan, the upper reservoir of this pumped-storage hydroelectric system built in the 1960s, now also covers part of the quarry complex.

Moelwyn Slate Mine (Royal Commission on the Ancient and Historical Monuments of Wales)

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Moelwyn Mawr and Moelwyn Bach

Llyn Stwlan, the upper reservoir for Ffestiniog Power Station, from Craigysgafn (B). The concrete dam and intake towers can be seen on the right, with the lower reservoir at Tanygrisiau below.

Date

26 June 2011
Location

Moelwynion

(A) SH 65821 44862; 52.98423°N, 4.00016°W
(B) SH 65991 44332; 52.97951°N, 3.99740°W
(C) SH 66028 43754; 52.97433°N, 3.99661°W
(D) SH 63574 43762; 52.97378°N, 4.03314°W

Information

The Moelwynion group of mountains in Snowdonia are to be found roughly in an area with Porthmadog and Capel Curig at its south-west and north-east ends, respectively. The highest peak in the range is Moel Siabod (872 m) and the name of the group comes from Moelwyn Mawr (Big White Hill), 770 m, and Moelwyn Bach (Little White Hill), 710 m. A rocky ridge, Craigysgafn, separates these two mountains and overlooks Llyn Stwlan, the top reservoir of Ffestiniog Power Station at Tanygrisiau below. This 360 MW pumped-storage hyroelectric power station began service in 1963 and was the first major facility of its kind in the UK.

Ffestiniog Power Station (First Hydro)

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