Many people credit Barnes Wallis with the idea of attacking the dams. Although he did independently think of attacking the dams during the early part of the war, it was the Air Ministry and Bomber Command who first thought up the idea in 1937. However, Wallis must be credited with brining the idea back to life during the war when the RAF and Bomber Command had other problems to deal with.
Nant-y-Gro Dam, Wales (1942)
How did Wallis plan to attack the dams when there were so many problems in doing so?
Most of the dams in the Ruhr Valley including five of the six targeted dams were gravity dams. The five targeted gravity dams were the Mohne (the most important), the Eder, the Ennepe, the Lister and the Diemel. The other targeted dam was the Sorpe, this was an earth dam.
Attacking a gravity dam and an earth dam takes completely opposite approaches:
A gravity dam, which is a solid wall of masonry, is held in place by its own weight. Spillage over the top of the dam during times of high water levels is completely normal and the water simply falls over the top of the dam, down the dry side and into the gathering basin. Dropping bombs onto the top of a gravity dam would therefore not be the most effective way to attack it. Dropping a bomb onto the top of the dam would simply create a breach at the top. Water would flow out of the breach but it would only do so until the water level reached the bottom of the breach and then stop. In effect it would be like a very large spillage over the dam. (For example, if a 10 feet breach were made, the water would flow out until the water in the lake was level with the bottom of the breach. The lake would be 10 feet shallower but that is all). Crucially the hydroelectric power station would still be able to operate and there would still be plenty of water left in the lake to supply the industry. In order to attack a gravity dam effectively, it must be struck at the base to weaken the structure and cause a breach near the bottom. The sheer volume and pressure of water that the dam holds back would then sweep away the wall and release the entire contents of the lake into the drainage basin.
Attacking an earth dam takes the opposite approach. An earth dam is essentially a reinforced concrete wall in the centre of the dam with thick banks of stone and earth on each side. These dams can withstand and soak up shock waves, the earth banks not only work as dampeners but a small amount of water is allowed to seep through so the stone mixed with the earth on the lake face of the dam is carried into the central core to fill in cracks that occur. Attacking an earth dam in the same way as a gravity dam would be useless because the earth walls are so thick at that depth the shock waves of the blast would be absorbed. The best way to attack this type of dam is to attack the top. The bank on the dry (air) side of the dam is made of porous soil which water is not allowed to pass over unlike a gravity dam. Overflow in an earth dam would cause the dry (air) side to be eroded away and leave the inner core under immense pressure from the weight of the water from the other side. This weight would cause the dam to collapse and leave the water to flow through and over. A breach in the top of the dam would therefore allow enough water through to erode the dry side and lead to the eventual collapse of the dam. A large breach would have to be made in the top to do this though.
As most of the dams in the Ruhr Valley including 5 of the targeted ones and the primary target, the Mohne, were all gravity dams, Wallis concentrated his efforts on a weapon to attack this type.
During his work on the problem, Wallis discovered that bombs that exploded on impact did relatively little damage to large, solid structures compared to if the explosion was underground or in water where the shock waves created by the explosion intensified the effect of the bomb. He also knew that it would take a much larger bomb than the 500lb ones used by the RAF to breach the dams.
Wallis' initial plans on attacking the dams concentrated on a giant bomb dropped from a great height that would penetrate deep down into the subsoil below the dam and create shockwaves so large, they would collapse it. Although this idea was not put into action with the dams raid, the development of this idea lead to the 12,000lb Tallboy bomb and later in 1945 the 22-ton Grand Slam earthquake bomb.
Wallis calculated that a 10-ton bomb dropped from 40,000 feet could land within 150 feet (45.7 meters) from the dam and still manage to break it. However in 1940, the RAF had only just started using 1,000lb bombs and still did not posses aircraft capable of flying at 40,000 feet or lifting a bomb of such weight. The Wellington designed by Wallis, was the best plane available but still it could only carry 10,000lb (approx 4,500Kg (4.5 ton)) at 18,000 feet. The Lancaster bomber which was still under development could only fly at 24,500 feet.
No existing or development aircraft met Wallis' needs for his weapon. He was therefore prepared to design his own! He put forward his proposals for 'Victory' a new bomber capable of delivering his proposed weapon. Victory would be in essence a giant Wellington powered by six engines and capable of carrying a 20,000lb (9-ton) bomb load at 40,000 feet. He estimated his new Victory bomber would take 2 years to develop (a relatively short time for bomber development). The Air Ministry were reluctant to give Wallis the necessary resources and personnel to develop his new bomber though. They would have to pull them from the development of the four engine 'heavies' coming through such as the Stirling, Halifax and later the Lancaster. Plans for the Victory bomber were scrapped in September 1941.
The only way Wallis was going to be able to deliver a bomb to the dams was to drastically reduce the weight of it to within the capability of an aircraft available. In his research Wallis requested help from the Road Research Laboratory at Harmondsworth. They were studying the effects of explosives on various structures and materials often testing on models. With the help of the Road Research Laboratory, Wallis performed scaled down explosions using a few ounces of gelignite on 1:50 scale models of the Mohne dam constructed from 3,000,000 tiny mortar 'bricks'. The explosions were carried out at various distances from the face of the dams. Although it is obvious that the closer the explosion was to the dam the more damage was done, Wallis discovered quite importantly that much less explosive was required to achieve the desired result. He also discovered that different types of explosives produced different results.
The tests were scaled up using a disused dam in Wales called the Nant-y-Gro measuring 30 feet high and 180 feet long. On July 24th 1942, 280lbs of explosives were detonated against the wall at a depth of 10 feet. The blast blew a hole 60 feet wide and 25 feet deep in the dam wall. An overwhelming success!
Scaling up the calculations Wallis believed that a similar effect could be achieved against a structure such as the Mohne dam with just 7,500lbs of explosives. This weight was well within the 8,000lbs lifting capability of the new Lancaster bomber which was just coming into service with the RAF in early 1942.
Now Wallis knew he could get a 7,500lb bomb which could breach the dams to the Ruhr Valley, he had to devise a way of dropping the bomb right next to the dam wall. The bomb would have to be dropped precisely in order to work, any inaccuracy and it simply would not work. This was the greatest problem of all in attacking the dams.
'Upkeep' was the solution Wallis developedů
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