ir Barnes Neville Wallis was famous for creating the bouncing bomb. Without it, 617 Squadron would not have destroyed the Mohne and Edersee dams in Germany’s Ruhr Valley, during Operation Chastise in May 1943. However, blowing up a dam is not easy, and twelve months earlier, in May 1942, Wallis hadn’t solved the problem. But thanks to a small dam few have heard of, hidden in the Elan Valley in Wales, he cracked the problem. Literally.

The Elan Valley sits a few miles west of Rhayader, in the mid-Wales county of Powys. In the late 19th century, the Birmingham Water Board Corporation selected the valley as the ideal location to build a series of dams, designed to create several reservoirs vast enough to capture and store a fresh, clean water supply for the ever-growing population of the UK’s second-largest city.

Originally, there were plans for seven dams, built in two phases: four along the River Elan and three along the River Claerwen. Work started in 1893, and the four Elan Valley dams were officially opened by King Edward VII and Queen Alexandra on 21st July 1904.

Navvy water supply

During this 11-year period, some 50,000 navvies worked on the project, housed in wooden huts further down the valley. These navvies also needed their own supply of fresh, clean drinking water. Additional water supplies were required to power the steam cranes and engines constructing the dam.

So, in a small side valley, about a kilometre from where they planned to build the Caban Coch dam, a small dam was built across the Nant Y Gro stream. This concrete and masonry construction was 58 metres long, just over three metres thick at its base, and 11 metres tall. It created a reservoir capable of holding a million gallons of water, while also supplying a nearby water storage tank to power machinery.

Once the construction work of the main dams was complete, the Nant Y Gro dam became obsolete.

When the Government hatched secret plans to destroy a series of dams in Germany’s Rhur Valley, they knew it wouldn’t be easy. Dams were heavily defended by artillery from attacks coming upstream, and thick, underwater nets stretched across reservoirs, preventing torpedoes from being dropped from behind.

What they needed was an engineer to come up with a different way of blowing up a dam, and a dam to practise on.

Wallis was born in Ripley, Derbyshire on 26th September 1887 and originally trained as a marine engineer. However, he later joined Vickers, going on to lead the design team behind the R100 airship.

Stone skimming

It was while skimming stones one day that Wallis had the idea of creating a bouncing bomb that could skip over torpedo nets and then hit a dam from behind. But where exactly would the bomb need to hit a dam?

The Government sought permission from the Birmingham Water Board Corporation to use the Nant Y Gro dam for secret explosives testing. It was perfect on two counts: firstly, the Elan Valley was so remote the experiments could be undertaken in complete secrecy. Secondly, the Nant Y Gro dam was of the same construction, and a one-fifth scale model of the dams in Germany’s Rhur valley.

It was in May 1942 that Wallis stood in the tranquility of the Elan Valley and watched a series of live explosive tests, where bombs were placed on the water’s surface near the top of the dam. While the results were spectacular, they failed to breach the dam.

Wallis calculated that the detonation needed to occur under the water, as close to the dam wall as possible. He returned two months later, in July 1942, and constructed a scaffolding unit that enabled them to submerge a 280 lbs bomb under the water at the right depth, against the dam wall. Once correctly in place, it was detonated remotely.

It worked. Such was the force, it created a hole in the dam 19 metres wide and eight metres deep, sending debris up to 30 metres downstream.

This proved to Wallis he needed to create a bomb that, if dropped at the right height, distance and speed, would bounce over all the torpedo nets in the reservoir, and then sink just before it reached the dam, exploding against the masonry structure.

Successful breach

Less than ten months later, after the more famous practise runs made by 617 Squadron over Ladybower Reservoir in Derbyshire, Eyebrook Reservoir in Rutland, and Abberton Reservoir in Essex, Operation Chastise and Wallis’ bouncing bomb successfully breached two dams, the Möhne and Edersee.

Over 330 million tons of water were released into the Rhur Valley, causing severe damage and loss of life.

Today, if you wander along a short path alongside Elan Valley’s Caban Coch reservoir, you’ll find the remains of Wallis’s experiment still clinging to the forested hillsides.

In the foreground stands one of the water storage tanks, used to help power the construction machinery. Today, all you can hear are the birds singing in the trees and the slow trickle of the Nant Y Gro stream as it continues to make its way, now unhindered, down the hillside into the main Caban Coch reservoir.

Little did the Birmingham Water Board Corporation know when they built this small dam that, fifty years later, it would play such a vital role in the Second World War.