Bomb Testing at Nant-y-Gro Dam

nant-y-gro1The Elan Valley dams and more especially the Nant-y-Gro dam A small tributary of the River Elan, which joined it about half a mile upstream from the Caban Coch dam site, was to earn a place in the story of one of the most famous episodes of the Second World War, the Dambusters Raid of 1943. A small masonry dam was built across the Nant-y-Gro stream in the early stages of the construction of dams and reservoirs in the Elan Valley. This dam created a million gallon reservoir on the rocky slopes above Caban Coch which was used to provide a water supply fed by a pipeline to the navvies village below. The supply also filled water storage tanks used by the locomotives, steam cranes, stone cutting machines and other steam driven plant in the two valleys.

After the completion of the waterworks scheme the Nant-y-Gro dam was no longer needed, because the permanent stone-built Elan Village which replaced the temporary navvies village obtained a water supply under the new scheme. The Nant-y-Gro dam, however, was still intact at the time of the war when the government requested the use of the 35 feet high dam for secret experiments. These were linked to the wartime military objective of breaching a series of large dams in the Ruhr Valley in Germany in order to disrupt armaments production in the industrial heartlands of the Ruhr below the dams. A great deal of highly secret experimental work was being carried under the direction of Barnes Wallis, an aeronautical engineer, and the Nant-y-Gro dam, though much smaller, provided a valuable test bed for devising a practical means of breaching the huge Möhne, Eder and other masonry dams. A further advantage of the use of the Elan Valley site was its remoteness, ideal for top secret trials without fear of being observed.


nant-y-gro2The Nant-y-Gro experiments were preceded by trials on scale models at government research stations near London, involving the detonation of scaled amounts of explosives at varying distances from the walls of the model dams. In May 1942 the first live explosive tests on the Nant-y-Gro dam itself were carried out in the Elan Valley, watched by Barnes Wallis. These first attempts were spectacular, but they did not seriously damage the Nant-y-Gro dam. The location of the Elan Valley dams and reservoirs is shown on the sketch map. After further development work, another attempt was made in July 1942. A mine was suspended at the optimum depth by scaffolding from the mid point of the 180 feet long dam, and detonated remotely. A huge central section of the dam wall was successfully blasted away in a massive explosion.

This successful trial confirmed that it would be necessary to deliver an explosive device under water and in direct contact with a dam wall in order to do the job. It was known that all major dams would be safeguarded by protective netting positioned to prevent mines or torpedoes from reaching them, so the Elan Valley tests were followed by an extensive programme of highly secret trials at other locations in Britain which were to result in the famous “bouncing bomb” devised by Barnes Wallis.


nant-y-gro3Nant-y-Gro dam today. (Google Maps Location)The trees have taken over but the remains of the Nant-y-Gro dam can still be seen today in much the same condition as it was left after the wartime experiments of 1942. A walk which passes the site is signposted from the Elan Valley Visitor Centre, which is just below the Caban Coch dam. It is interesting to compare the above photograph with the 1942 picture taken from a similar viewpoint shown on page1 of this sequence. The storage tank below the dam is easily recognisable but mature trees now make the surviving parts of the original dam hard to spot. Part of the surviving structure of the dam photographed in March 2000, the remaining portion of the right hand side of the dam wall is visible through the trees in the photograph above. The breaching of the German dams in 1943 by the RAF led to the rapid introduction of improved defensive measures to protect the Elan Valley dams and other large dams elsewhere in Britain.