Uploaded May 2026 | Updated September 2026, 3 weeks ago
We are out for a dander along the Annalong Valley up towards the entrance to the Binnian Tunnel at Dunnywater. We've reached a spot on the bridge where we can see the entrance to the remarkable Binnian Tunnel that runs for 2½ miles through Binnian mountain to bring the water from the annalong river valley to the silent Valley reservoir of the other side of the mountain.
The story of the Binnian Tunnel is that between 1947 and 1950/51 a workforce of over 150 men drove a tunnel nearly two and a half miles long underneath Slieve Binnian in the Mourne Mountains.
The purpose of the tunnel was to carry water from the Annalong valley to top up the Silent Valley Dam which had been completed just twenty years earlier.
This was in response to the ever increasing demand for water in the greater Belfast area. The seemingly impossible task of cutting such a tunnel was undertaken by two work squads, starting at either end and meeting in the middle nearly 800 metres under the roof of the mountain. It was done without modern hi-tech aids! In fact they used candles to determine the straightness of their bore, yet when they did meet in the middle they were only inches off each others centre-mark!
A Mammoth Undertaking
The size of the task was immense. The tunnel had to pass right through the base of Slieve Binnian to reach the Silent Valley Reservoir. Binnian stands 747 metres tall. The tunnel measures about 8 feet square throughout it's length of two and a quarter miles (3.6km). Its entry point is at Dunnywater and its exit into the Silent Valley Reservoir can be seen at the roadside a short walk from the Silent Valley Visitors' Centre. A plaque there tells some of the facts about the tunnel.
Why was a tunnel needed?
The ever increasing demand for water meant that the Mourne catchment area had to be exploited more fully. The Silent Valley Reservoir alone was not enough. The Belfast City & District Water Commissioners had originally planned to build a second dam in the Annalong Valley however serious problems had been encountered in the building of the Silent Valley Dam. In short, what had at first seemed to be solid bedrock turned out to be huge glacial boulders with the real bedrock far below.
An enormous trench 210ft deep in the middle had to be dug and drained before foundations for the dam wall could be poured. This was an engineering challenge that almost brought the project to a close. It was assumed that the Annalong Valley would be no different and so the decision was taken not to attempt a second dam there.
The solution they came up with was to redirect all of the water coming down the Annalong Valley catchment area to top-up the Silent Valley Dam. The challenge however was that there was a huge mountain in the way - Slieve Binnian which stands 747 metres high.
So, late in 1947, work commenced on driving a tunnel through the mountain. The tunnel would have to be carefully planned and created with exactly the right gradient to deliver water into the Silent Valley at a pre-calculated rate and volume. This brought about major challenges both for the engineering experts who had to mastermind the plans and also for the tunellers who would have to blast an eight foot wide corridor through the mountain, starting from either end, maintaining a dead straight line and then meeting each other in the middle... by no means a simple job for anyone involved!
The tunnel was created using drilling and blasting techniques. The shelters for the workers can be found on Slievenaglogh.
The tunnel was designed to carry 90 million imperial gallons (410,000 m3) of water per day, be 2.5 miles (4.0 km) long, 7 feet (2.1 m) high and 8 feet (2.4 m) wide. The tunnel was officially opened in August 1952.
We are out for a dander along the Annalong Valley up towards the entrance to the Binnian Tunnel at Dunnywater. We've reached a spot on the bridge where we can see the entrance to the remarkable Binnian Tunnel that runs for 2½ miles through Binnian mountain to bring the water from the annalong river valley to the silent Valley reservoir of the other side of the mountain.
The story of the Binnian Tunnel is that between 1947 and 1950/51 a workforce of over 150 men drove a tunnel nearly two and a half miles long underneath Slieve Binnian in the Mourne Mountains.
The purpose of the tunnel was to carry water from the Annalong valley to top up the Silent Valley Dam which had been completed just twenty years earlier.
This was in response to the ever increasing demand for water in the greater Belfast area. The seemingly impossible task of cutting such a tunnel was undertaken by two work squads, starting at either end and meeting in the middle nearly 800 metres under the roof of the mountain. It was done without modern hi-tech aids! In fact they used candles to determine the straightness of their bore, yet when they did meet in the middle they were only inches off each others centre-mark!
A Mammoth Undertaking
The size of the task was immense. The tunnel had to pass right through the base of Slieve Binnian to reach the Silent Valley Reservoir. Binnian stands 747 metres tall. The tunnel measures about 8 feet square throughout it's length of two and a quarter miles (3.6km). Its entry point is at Dunnywater and its exit into the Silent Valley Reservoir can be seen at the roadside a short walk from the Silent Valley Visitors' Centre. A plaque there tells some of the facts about the tunnel.
Why was a tunnel needed?
The ever increasing demand for water meant that the Mourne catchment area had to be exploited more fully. The Silent Valley Reservoir alone was not enough. The Belfast City & District Water Commissioners had originally planned to build a second dam in the Annalong Valley however serious problems had been encountered in the building of the Silent Valley Dam. In short, what had at first seemed to be solid bedrock turned out to be huge glacial boulders with the real bedrock far below.
An enormous trench 210ft deep in the middle had to be dug and drained before foundations for the dam wall could be poured. This was an engineering challenge that almost brought the project to a close. It was assumed that the Annalong Valley would be no different and so the decision was taken not to attempt a second dam there.
The solution they came up with was to redirect all of the water coming down the Annalong Valley catchment area to top-up the Silent Valley Dam. The challenge however was that there was a huge mountain in the way - Slieve Binnian which stands 747 metres high.
So, late in 1947, work commenced on driving a tunnel through the mountain. The tunnel would have to be carefully planned and created with exactly the right gradient to deliver water into the Silent Valley at a pre-calculated rate and volume. This brought about major challenges both for the engineering experts who had to mastermind the plans and also for the tunellers who would have to blast an eight foot wide corridor through the mountain, starting from either end, maintaining a dead straight line and then meeting each other in the middle... by no means a simple job for anyone involved!
The tunnel was created using drilling and blasting techniques. The shelters for the workers can be found on Slievenaglogh.
The tunnel was designed to carry 90 million imperial gallons (410,000 m3) of water per day, be 2.5 miles (4.0 km) long, 7 feet (2.1 m) high and 8 feet (2.4 m) wide. The tunnel was officially opened in August 1952.
