Silent Valley Reservoir

The Department's description

Exterior description and setting

Earth gravity dam across headwaters of Kilkeel River between Slieve Bignian and Slievenaglogh impounding 11350 million litres (2500 million gallons) of water. Dimensions of dam are: 496 m (1627ft) long, basal width 210 m (700ft)/ top width 6.1m (20ft), height above former river bed 27m (88ft). Equal slopes on both sides: that on wet side pitched with granite blocks to minimise erosion, grassed on air side. Puddled earth core, with 1.8m (6ft) wide concrete water cut-off trench in underlying bedrock. Pitching continues for a short distance around sides of reservoir. A road runs across the top, 2.5m (8ft) above high water level. It has an irregularly-coursed roughly-dressed granite block wall with rusticated copings on its reservoir side.

On the slope above the west end of the dam is the quarry from which stone was obtained.

Valve tower (HB16/02/022) and bell mouth overflow (HB16/02/023) at south-east corner of reservoir, and scour valve house/ overflow channel (HB16/02/024) a short distance below embankment.

Historical information

Luke Macassey, consultant engineer to the Belfast Water Commissioners, conceived the Mourne Scheme in the early 1890s. The first phase of this project to be completed was the diversion of water from the Kilkeel and Annalong rivers to Belfast in 1901. The next stage was the construction of this dam at what was then called the Happy Valley. Although exploratory borings commenced in 1913, war delayed the project until 1923 when a contract was signed with S. Pearson & Son (London). F.W. McCullough was the BWC engineer responsible for its design and in charge of the project for most of its duration (until his premature death). Work was delayed several years due to problems in finding bedrock in which to excavate the cut-off trench. Sir Ernest Moir & Co took over the contract in 1930 and the dam was completed in 1932 at a cost of £1.38m. The reservoir was opened by the Duke of Abercorn, Governor of N. Ireland, on 24 May 1933.

Primary Sources:
1. Plaques on valve tower wall.
2. Original drawing of the tunnel and weir in DOE Water Service, Westland House, Olf Westland Road, Belfast (cat. No.1C1).

Secondary Sources:
1. G. McIldowie ‘The construction of the Silent Valley reservoir, Belfast water supply’, in Proc. Institution Civil Engineers, N. Ireland Branch (1934).
2. Belfast City & District Water Commissioners 'The Mourne Scheme' (Belfast 1959).
3. A.Cooper, 'The reservoir safety programme in Northern Ireland’, paper presented at 1987 conference of Institution of Water Engineers and Scientists.

Criteria for listing: architectural interest

Not listed

Criteria for listing: historic interest

Not listed

Evaluation

It is of national importance in being the Mourne Scheme’s first reservoir and it impounds the largest volume of water in N. Ireland. Its spectacular mountainous setting is second only to that of the Ben Crom reservoir (HB16/02/028). The innovatory use of compressed air shafts during the construction of its cut-off trench is also of note. PHB have asked that consideration be given for inclusion on Schedule of Historic Monuments by Protecting Historic Monuments.