Nant Y Gro Dam is located a short distance upstream of caban Coch Dam. Map Ref SN921 634
The small masonry dam was built across the Nant-y-Gro stream in the early stages of the construction of dams in the Elan Valley. This dam created a one million gallon reservoir which was used to provide a water supply fed by a pipeline to the navvies village below Caban Coch Dam. The dam was capable of supplying up to 45,000 gallons of water per day. A concrete storage tank placed in front of the dam can be seen today. Some cast iron pipes used to pipe the water to the navvies village can be seen when the water level drops on Caban Coch reservoir.
Nant y Gro dam was abandoned once the Elan Valley water scheme was completed, water now being supplied to the new Elan Village via Caban Coch reservoir. The government requested the use of the 35 feet high dam for secret experiments during World War II. Barnes Wallis who invented the Bouncing Bomb carried out top secret experimental trials on Nant y Gro Dam.
Two carefully calculated explosive charges were used on the abandoned dam. On the first test in May 1942 the charge was 62lb of TNT fired 7.5 feet from the dam wall at a depth of 15feet. This resulted in some cracking of the construction joints of the dam and part of the spillway to become detached.
At another test site a team was preparing to demolish a model dam and decided to use a contact charge to do this, the results were unexpectedly dramatic with some parts of the model dam ending up 20ft downstream of the dam. With the results of this the research team returned to Nant y Gro Dam, this time a 280lb explosive packed in an anti-submarine bomb, suspended against the dam wall and submerged at a depth of 10ft. This test in July 1942 was successful, breaching the dam by 60ft wide and 24ft deep.
Trials continued with the development of the bouncing bomb and on the night of 16th May 1943 the Dam Busters Raid took place and successfully breaching the Moehne and Eder Dams in the Ruhr Valley in Germany.
Nant y Gro can be visited today via a footpath from the Caban Coch Dam, opposite side of the river from the visitor centre.