Silent Valley Reservoir, Head Road, Kilkeel, Newry, Co Down, BT34 4PU

Silent Valley Reservoir

This earth gravity dam across the headwaters of the Kilkeel River, built between Slieve Bignian and Slievenaglogh, is the first reservoir constructed as part of the Mourne Scheme and impounds the largest volume of water in Northern Ireland. It holds 11,350 million litres (2,500 million gallons) of water in a spectacular mountainous setting second only to that of the Ben Crom reservoir.

The dam is 496 metres long, with a basal width of 210 metres and a top width of 6.1 metres, standing 27 metres above the former river bed. It has equal slopes on both sides: the wet side is pitched with granite blocks to minimise erosion, whilst the air side is grassed. The structure features a puddled earth core with a 1.8-metre-wide concrete water cut-off trench in the underlying bedrock. The pitching continues for a short distance around the sides of the reservoir.

A road runs across the top of the dam, positioned 2.5 metres above high water level. This road is bounded on its reservoir side by an irregularly-coursed roughly-dressed granite block wall with rusticated copings. Above the west end of the dam is the quarry from which stone was obtained for its construction.

Associated structures include a valve tower and bell mouth overflow at the south-east corner of the reservoir, and a scour valve house with overflow channel positioned a short distance below the embankment. The innovatory use of compressed air shafts during the construction of the cut-off trench is of particular note.

The Mourne Scheme was conceived by Luke Macassey, consultant engineer to the Belfast Water Commissioners, in the early 1890s. The first phase—diversion of water from the Kilkeel and Annalong rivers to Belfast—was completed in 1901. Exploratory borings for the reservoir commenced in 1913, but the project was delayed by the First World War. A contract was signed with S. Pearson & Son (London) in 1923. F.W. McCullough was the Belfast Water Commissioners engineer responsible for its design and oversaw most of the project until his premature death. Work was further delayed several years due to difficulties in locating bedrock for 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 £1.38 million. The reservoir was formally opened by the Duke of Abercorn, Governor of Northern Ireland, on 24 May 1933.

The structure is of national importance both as an early major reservoir and as an example of innovative construction engineering practices. It is of significant industrial archaeological interest.