Après moi le (petit) deluge

Hidden behind the fences, trees and flowers of a quiet Hertfordshire housing estate stands a remnant of one of World War 2’s most daring raids. Whilst the exact contribution of that operation toward victory in Europe eighty years ago is disputed, there is no argument that the science and technology behind it represent a towering achievement. Today – Celebration Day

– I finally sought out this astonishing survivor, regretting that my late father didn’t see it himself despite serving in that conflict in the same arm and being fascinated with the attack. So this is for you, dad.

On the night of 16/17 May 1943, nineteen Lancaster bombers of Royal Air Force Bomber Command targeted six hydroelectric dams (three were actually attacked) in the heavily-defended Ruhr valley, aiming to disrupt Nazi Germany’s industrial capacity. With the D-Day landings still a year in the future, Operation Chastise was deemed a necessary risk and a special squadron was formed to carry it out. Guy Gibson, only 24 years old yet already a Wing Commander, was chosen as its commanding officer. He would select the crews and plan the flight, but the concept and the weapons had already been conceived by another years before.

Naval and aeronautical engineer Barnes Wallis had been thinking about the problem of the dams ever since war broke out. Their unique nature meant conventional bombs would have no effect, even if they could be dropped accurately enough. Much larger bombs that would rely on deep penetration and the generation of shockwaves to destroy such structures were his preferred solution, but manufacturing limits and the lack of suitable aircraft forced Wallis to come up with another option. This was a smaller but still sizeable depth charge, that could be dropped from a conventional bomber onto water and then skipped or bounced along the surface until it encountered an obstruction. Only then would it sink, remaining in contact with whatever it had struck until a barometric fuze detonated the explosive contents underwater. The resulting energy, reflected by the non-compressible water, would be focused onto the dam, shattering its farthest face and causing a collapse.

That was the theory. It was not universally accepted and months of argument with the authorities and experimentation followed, testing the construction, shape and size of the mine, the correct height and range to drop it from and aids to help the crew fly an approach just 60 feet above the dams’ lakes, at night and under fire.

A key task was to determine the amount of explosive needed, and where it should be placed. Initial tests using models of the dams were disappointing, but continued using different scales and different construction techniques. Of the various examples built at facilities in Hertfordshire and Harmondsworth in west London, only one survives – that which I visited early this morning.

The Building Research Station in Bricket Wood, Hertfordshire was created by the government after the Great War to improve housing design. Its field of study expanded over time to embrace insect damage to timber, the use of concrete floors and the most appropriate standards for the humble brick, before being turned to war work in 1939. It was here then that Wallis and the BRS’s own scientists and managers commissioned a 1:50 model of the Möhne dam to help plan the Chastise attack.

Built over the winter of 1940/41 across a small stream in the grounds, its base and outer sections were formed by poured-in-situ concrete but to replicate the granite blocks of the actual structure as many as two million individual blocks of mortar about the size of a sugar cube were laid against both faces of a concrete core to make the central section. The completed model dam was forty five feet long and more than three feet high; a small lake was created behind it, with a pipe at its base allowing a controlled run off to prevent overtopping.

For the crucial tests small explosive charges, again carefully scaled, were emplaced at various distances from the dam and detonated to assess the right combination of size and placement. Ten were set off in total, causing cracks sufficient to breach the full-size structure and eventually rendering the model useless. As noted, the tests moved on and more models refined the idea before the programme culminated in the deliberate destruction of a redundant dam in Wales, confirming Wallis’s theory and leading to manufacture of the final Upkeep mines, small enough to be carried but large enough to blow a hole through the actual Möhne dam when exploded in the right place. Loaded onto the new 617 Squadron’s aircraft on 16 May 1943, three were in fact required, but the force of water released into the valley – at the rate of around 60,000 cubic feet per second, for hours – swept away steel bridges and stone buildings, ripped up railway tracks, stripped bark off trees where it did not uproot them and flooded thousands of acres of land.

Hundreds of civilians, soldiers and prisoners of war were killed; so were 53 of the 133 aircrew who participated in the attack, with some taken prisoner. Gibson survived and was awarded a Victoria Cross, though died in a plane crash the following year. The skills developed by 617 Squadron saw them employed throughout the remainder of the war in precision raids against V-weapon sites, bunkers and the feared battelship Tirpitz, many of these using Wallis’s earthquake bombs.

Still little-known outside aviation enthusiasts and professional historians and once only accessible on tours mounted by the Building Research Establishment, the BRS’s successor, the model dam can today be seen at any reasonable time a short stroll from that private housing development.

It is a slightly surreal sight when a ridge of rough grey concrete emerging from a carpet of spring blossoms and undergrowth is recognised for what it is – a winter visit might be more useful – and elevation of the path further diminishes the impact, so I’ve included a few images sourced online to emphasise the size of the model. It is clear, too, that the dam was repaired at some point after the tests, albeit with a new hole knocked into its parapet; this might have been a kind of unofficial tribute. A sign gives some brief facts (with a QR code inking to the BRE's site) and a good period photograph of the model before the test, though you do need to walk up and down the path a little to appreciate the size, setting and shape of the model.

But it’s still impressive, and well worth seeing if you have any appreciation for the wide-ranging, richly innovative and world-leading technology developed by Britain’s boffins during the war. Barnes Wallis was just one of them, so it seems fitting to end with an image of his son, Barnes Wallis Jnr, at the model in 1997.