Nant-y-Gro Dam Remains
The Nant-y-Gro Dam, nestled in the Elan Valley, holds a unique place in both engineering and wartime history. Built in the late 19th century to supply water to the workers constructing the dams and reservoirs that supplied Birmingham, it was later destroyed during explosive experiments that led to the development of the bouncing bomb used in the Dambusters Raid during the Second World War. Today, visitors can explore the peaceful valley trails, enjoy scenic views of the surrounding reservoirs, and discover how this modest structure contributed to one of history’s most daring missions.
Key info
| Location | Elan Valley Estate |
| County | Powys |
| Completed | 1885 |
| Destroyed | 1942 |
| Engineer | James Mansergh |
| Maintained by | Cadw |
| Heritage category | Scheduled Monument |
Visiting guide
Open 24 / 7
Free entry
Paid car park
What can I expect when visiting Nant-y-Gro Dam?
The Nant-y-Gro Dam is only accessible on foot. You can approach from multiple directions; however, this visiting guide will start from the bridge crossing the Elan Valley in front of the Caban Coch Dam. You can reach this bridge from the visitor centre or the Caban Coch car park. Cross the bridge and follow the path around the hydroelectric power station on the southern riverbank.
At the back of the power station, walk up the stairs. A section of the pathway contains an original staircase from the construction of the Caban Coch Dam. Take care as it can have uneven surfaces. Head through the gate at the top and turn right.
Walk past the Caban Coch Dam, which you will see to the right. Keep following the path.
The path will now snake its way around the edge of the Caban Coch Reservoir, which was still in the process of filling up when we visited in early autumn after a dry summer. Here you will start to take in the scenic views of the Elan Valley.
Continue along the path until you reach a staircase leading to the left. Signs marking the Nant-y-Gro Trail also point the way.
The stairs take you to a higher path that offers even more stunning views. Follow the path and you will reach where the Nant-y-Gro stream empties into the Caban Coch Reservoir. The stream was the reason why the engineers of the reservoir chose this spot to erect a temporary dam in 1893. It created a small reservoir that was used as a water source for drinking and bathing in the Elan Valley Worker’s Village located downriver past the Caban Coch Dam.
Remains of the mains pipe from the dam are still visible along the north east bank of the Caban Coch Reservoir. Thankfully, the dam was not full, and so the pipes were visible.
Continue walking the path along the stream and follow it down to the remains of the concrete tank used to store water before it entered the mains pipe.
Walk back to the main path and follow it up for a view of the spillway. Water would have flowed over the dam down into this channel before continuing to the Elan River. With the dam destroyed, it still functions as a small waterfall.
Further up the path, you will see the remains of the dam wall. After the wooden buildings in the workers’ village were decommissioned following the completion of the main reservoirs and dams, the Nant-y-Gro dam and reservoir became redundant.
In 1942, during the Second World War, the Corporation of Birmingham, which owned the land, gave permission to the Air Ministry to use the dam for experiments that later created the bouncing bomb used in the Dambusters Raid. A suspended explosive charge of 279 lbs (126 kg) was detonated against the dam wall, destroying it and releasing a huge spout of water shooting into the air. Today, the ends of the dam wall are still standing, exposing the wide gap where it was breached.
Look behind you for one of the best views of the Caban Coch Reservoir with the Garreg-Ddu dam in the distance.
How long does it take to visit Nant-y-Gro Dam?
A return walk of this visiting guide will take roughly 1 hour if starting from Caban Coch car park or 1.5 hours if starting from the visitors’ centre.
Is Nant-y-Gro Dam suitable for a picnic?
No, the dam is not suitable for a picnic. It is surrounded by a wire fence, and the path that leads to the site follows a rocky landscape. We prefer other luscious spots, such as beside the Caban Coch Dam.
How do I get to Nant-y-Gro Dam?
The dam is only accessible on foot. We recommend starting at the Caban Coch Dam or the visitor’s centre, which have car parks with charges. These starting points will require a short taxi ride from Rhayader, which is served by public buses.
History of Nant-y-Gro Dam
1893 – Work on constructing the Elan Valley dams and the workers’ village starts. James Mansergh, the chief engineer of the Birmingham Corporation Water Works, plans to build a temporary dam across the Nant-y-Gro stream higher than the level of the Caban Coch Reservoir to supply the workers’ village for drinking, washing, and supplying the baths and fire hydrants. People started pouring in to start their jobs, and the corporation had to ensure suitable accommodation and public services were available.
1895 – Construction of the Nant-y-Grow dam and the workers’ village is completed, and water starts flowing.
1904 – The Elan Valley water supply opens and starts flowing to Birmingham.
1907 – The workers’ village is dismantled and sold off. The water supply from the Nant-y-Gro reservoir becomes redundant.
1938 – Barnes Wallis, the assistant chief designer at a large engineering company called Vickers Armstrongs, was searching for a way to deliver massive explosive power to strategic targets in Germany, particularly the Ruhr Valley dams like the Möhne, Eder, and Sorpe.
These dams were crucial for hydroelectric power, steel production, and water supply; However, they were heavily defended by anti-aircraft guns, had thick walls made of reinforced concrete, and conventional bombing from high altitude lacked precision to hit them effectively. Wallis began thinking about how to deliver a bomb close enough to a dam. His ideas catch the attention of the Air Ministry.
1941 – Birmingham Corporation Water Works permits the Air Ministry to use the Nant-y-Gro dam for demolition tests because it coincidentally was a fifth-scale version of the Mohne dam in Germany.
1942 – Barnes Wallis conducts a test by detonating an underwater charge against the Nant-y-Gro dam, which was unsuccessful. He then set a suspended charge in contact with the dam, which successfully breached the wall. The 279 lbs (126 kg) explosive charge released a huge spout of water, shooting into the air as the chunks of concrete were scattered.
1943 – The experiments at Nant-y-Gro led Barnes Wallis to create the bouncing bomb used in Operation Chastise, commonly known as the Dambusters Raid. The attack breached the Möhne and Edersee dams, causing catastrophic flooding of the Ruhr Valley. Hydroelectric power stations, factories and mines were damaged and destroyed. An estimated 1,600 civilians (roughly 600 Germans and 1,000 enslaved labourers, mainly Soviet) were killed by the flooding. No. 617 Squadron used by the Air Ministry for the attack lost 53 airmen, and 3 were captured. Wallis went on to develop other bombs, then moved to aerospace projects after the war.
2008 – The dam remains are designated as a Scheduled Monument because it is acknowledged as a site of national importance for its potential to enhance our knowledge of the secretive scientific endeavour undertaken during the Second Wold War and the strategic consequences that might follow.
Sources
Elan Valley Trust information boards.
Blaenavon Ironworks