Maentwrog Dam

Maentwrog New Dam, Llyn Trawsfynydd

Date	8 December 2012
Location	Afon Prysor / Llyn Trawsfynydd	SH 67455 37711; 52.92112°N, 3.97334°W
Information	The artificial lake Llyn Trawsfynydd, which later also provided a supply of cooling water for Trawsfynydd nuclear power station, was originally created as part of the Maentwrog hydroelectric power scheme, constructed from 1925 to 1928. Four dams were built to contain the reservoir, the principal one being the Maentwrog Dam on the Afon Prysor at the northwest corner of the lake. The original structure was at the time Britain’s largest arch dam. However, it suffered from long-term leakage problems, caused by, amongst other things, vertical cracks in the arch near the abutments and leaching of porous areas. Work began in 1988 on a replacement dam at the end of the Ceunant Llennyrch gorge 75m downstream from the original. The new S-shaped gravity dam was completed in 1992. It is 219m long, 39m high, and consists of 54,500 cubic metres of concrete. The heat produced by the chemical reaction when cement cures can prose serious problems in massive concrete structures with regards to thermal cracking. During the construction of the new dam, pre cooling was employed to mitigate this problem: in the hot summer of 1989, the temperature of a 600 cubic metre pour was reduced by 6°C by injecting 3000 litres of -200°C liquid nitrogen into the mixed concrete. Maentwrog Power Station and Dam are currently owned by the Nuclear Decommissioning Authority and are operated on its behalf by Magnox. Maentwrog Hydroelectric Power Station (Magnox)

Spillway with road deck above

The capacity of the reservoir at overflow level is 33 million cubic metres

S-curve of the dam

Maentwrog Dam and valve control building

Road deck

Maentwrog Dam at the head of the Ceunant Llennyrch gorge

54,500 cubic metres of concrete

Looking down on the exit of the Afon Prysor