Discussions about attacking dams in Germany in the event of conflict in Europe had started before WW2.
In late 1940 Wallis had taken his idea of attacking the Möhne and Eder Dams to the Building Research Establishment (BRE), or as it was known at that time the Building Research Station (BRS) near Watford where engineering teams had been studying the use of explosives on various structures. His idea obviously didn’t seem so farcical, and it was decided that the best way to determine the requirements for explosive and best location to detonate would be to build an accurate scale model.
Between November 1940 and January 1941, a 1/50th scale model of the Möhne dam was built across a stream in a remote corner of the BRS site. For added reality the majority of the dam was constructed of special miniature blocks to replicate as near as possible the original construction of the Möhne Dam, and an inner wall of clay replicated the Möhne’s earth embankment. After seven weeks construction, it took just less than a week for the miniature reservoir to fill and testing began on 22 January 1941.
The tests consisted of detonating charges at various distances from the dam and at various depths. A probe on the dry side of the dam monitored the impact. After the sixth test the model dam was already leaking, and part of the parapet had been blown away. After the tenth explosion the model was so badly damaged it was no longer usable.
The model still exists in its original location and though it appears to have been repaired at some point in the past it is a testament to the incredible work that went on in preparation for the famous raid. In 2002 Historic England declared the model as being a historic monument of “not only of national but also international importance”.
The success at BRS was followed by a series of further tests by the Road Research Laboratory – RRL (now Transport Research Laboratory -TRL) in Harmondsworth (close to what is now Heathrow airport). Simplified replica dams were constructed of cast concrete rather than miniature blocks and were used in a series of tests. The results were encouraging, enabling them to calculate that to breach the full-size Möhne dam it would require 7,000 lbs (3,400 kg) of explosive detonated 30 feet below the water level against the inside face of the dam.
Sadly, none of the Harmondsworth model dams exist today but a plaque commemorating the involvement of the RRL in the tests was unveiled in 2013.
The final destructive tests took place on a small dam near Rhayader in Mid Wales where the secluded Nant-y-Gro stream flows into the Caban Coch Reservoir in the Elan Valley. During the construction of the Caban Coch Reservoir a small dam 190 feet long and 35 feet high was constructed across the Nant-y-Gro stream to supply the water needs of a navvy’s village and steam driven construction equipment.
The Nant-y-Gro Reservoir contained about a million gallons of water but once the Caban Coch Reservoir was completed in 1904 the water from Nant-y-Gro Reservoir was no longer needed. The dam was still standing in 1942 and it was requisitioned for use by Barnes Wallis.
Tests were undertaken in May 1942 but were not successful in destroying the dam. Following further development, a test was carried out on 24 July 1942 when 280lbs (127Kg) of explosives was suspended from scaffolding at the centre of the dam against the inner face of the dam at a depth of 10 feet and detonated remotely. The explosion blew a hole 60 feet wide and 25 feet deep.
The remains of the dam remain today much as they were left 80 years ago though they are now largely overgrown with trees. The walk to see it is well worth the effort and there is an information board nearby giving details.
For training several UK dams were used. Probably the most famous training site where many people will associate The Dambusters is Derwent Reservoir in Derbyshire. It was used for intensive low-level night-time flying exercises for about six weeks prior to the raid. It was selected and used because the Derwent Dam resembled the Möhne Dam with its twin towers.
But Derwent wasn’t the only reservoir used for Dambuster flight training. There are two other reservoirs slightly less well known but equally as important.
Eyebrook Reservoir, (sometimes quoted as Uppingham Lake) between Leicester and Peterborough was also used by 617 Squadron for low flying practise. Although its dam was more like that at the Sorpe, scaffolding towers were erected to give it the appearance of the Möhne Dam. It proved excellent for honing their difficult bomb aiming skills.
The third training reservoir was Abberton just south of Colchester as it was supposed to look like the Eder Reservoir from the air. It was used for final tuning of low flying techniques and was in fact used during the night of 14 May 1943 for a ‘full’ dress rehearsal for the raid.
Of course, no practise aerial bombing was ever done at any of these reservoirs, despite a scene in the 1955 film showing attempts to hit floating target. Images of Mosquito aeroplanes testing bouncing bombs on land at Ashley Walk Bombing Range in the New Forest, and at Loch Striven on the Firth of Clyde are of the spherical Highball Bouncing Bomb designed for use against battleships and not for the dams.
For Operation Chastise, seven German reservoir dams had been identified as potential targets.
Five were in the industrial Ruhr Valley (Möhne, Sorpe, Lister, Ennepe, and Henne) and two further to the east in the Weser Valley (Eder and Diemel). The Möhne, Eder, and Sorpe Dams were the primary targets. The others were secondary targets, but the Henne was removed from the list prior to the operation.
The Möhne, Eder, Lister, Ennepe, and Diemel Dams were gravity type dams. These were of a masonry or stone construction with a vertical upstream face to hold back the water. The weight of the dam and its resistance against its foundation opposed the horizontal pressure of water pushing against it.
Gravity dams required an attack straight on with the Lancaster flying at a height of 60 feet perpendicular to the centre of the dam releasing the backward spinning Upkeep to skip over torpedo nets to hit the upstream face of the dam and sinking to 30 feet before exploding.
The Sorpe (and Henne) Dams were earth (or embankment) dams These typically have a reinforced concrete core with compacted layers of earth, with earth and other more permeable substances on the upstream and downstream faces.
This required an attack along the crest of the dam with the Lancaster flying parallel and slightly to the water side of the dam before releasing the Upkeep without rotating as low as possible to roll down upstream face of the dam and sink to 30 feet before exploding.
On the night of the raid Townsend in AJ-O was given instructions to attack the Ennepe Dam but struggled to find it. It is believed that he actually released his Upkeep at the nearby Bever Dam which was another of earth construction.
My other Dambuster / Lancaster related blogs:
Operation Chastise – The Dambusters a minute-by-minute account
THE HIGH PEAK DAMBUSTER
The day a Lincoln bombed a Lancaster, and a Meteor shot a Meteor shooting a Lincoln
Milk run over the Eder Dam
Lancaster bombers operated by the Soviet Air Force in WW2
What happened to the Lancasters used on the Dambuster Raid?
From Manchester to Lincoln via Lancaster – An aviation aristocracy family tree
Testing and training for the Dambuster Raids – what’s left to see?