September 30, 2011

WINDFALL FILMS and JAMES BELLAVANCE

Hugh Hunt sought to replicate a WWII RAF attack on Germany’s Ruhr Valley in 1943, dramatized in the film The Dam Busters, in which the original attack used a “bouncing bomb”. Almost 440 concrete blocks were used in construction for the project.

FEATURE | Concrete/Masonry

How a concrete dam was constructed in British Columbia to replicate German infrastructure bombed during WW2

PETER KENTER

correspondent

Wanted: Use of construction site, anywhere in the world, located on open water. Must be willing to allow construction and demolition of a concrete dam using Second World War-style aerial bomb.

That’s the gist of an advertisement placed on the Internet by Windfall Films of London, England and Hugh Hunt of Cambridge University’s department of engineering. Hunt wanted to replicate an intricately planned WWII RAF attack on Germany’s Ruhr Valley in 1943, dramatized in the 1955 film The Dam Busters. The original attack employed a “bouncing bomb” developed by British aeronautical engineer Sir Barnes Wallis. However, a flood had destroyed most of the papers surrounding the technical aspects of the operation in 1960. If a site could be found, the filmmakers would document the entire exercise.

The ad was seen by James Bellavance, at the time an economic development officer with the District of Mackenzie, British Columbia. Williston Lake, a reservoir engineered by BC Hydro 13 kilometres north of the community, would provide the body of water necessary to complete the experiment.

“They’d been looking for two years to build this project, but hadn’t found any place willing to allow them to blow up a dam,” says Bellavance. “I thought it would be a great economic benefit to the district, so we mailed our application to Windfall, and a few weeks later they were on site.”

View Larger Map

Bellavance had a background in cement production and prior experience as a building contractor. His ideas on how a one-third scale concrete dam could be constructed impressed the film’s producers.

“Initially, they wanted to form and pour a concrete dam on the bottom of the lakebed, but I convinced them that concrete lock blocks would provide a better construction material,” says Bellavance. “Rather than build a cofferdam at lake bottom level, we excavated 175,000 cubic metres of soil to 40 feet below lake level near the dam. Later, we would open the dam and allow the lake to flow into the area behind our dam.”

Bellavance had contemplated mortaring the 438 concrete blocks used in construction, but considering the short project life of the structure — just a few days — he decided to place a geotextile liner on the back of the dam, imbedding the bottom of the liner in clay and allowing water pressure to hold it in place.

All told, construction took five weeks, with one week lost to heavy rains.

Site permits would not allow for the explosion of an aerial bomb, so the filmmakers opted to separate the event into two sequences.

In the absence of a WWII-era Lancaster bomber, they chartered a similar vintage DC-4 to drop a dummy bomb onto the target. Ideally, the spinning bomb would bounce across the water, and then hug the wall of the dam as it sunk to its base. Later, real explosives would replace the dummy bomb.

“I painted a big red target on the back side of the dam,” says Bellavance. “Like the original RAF pilots, our pilots were given five chances to succeed in the mission. On its first run, the bomb bounced across the water, and hit the target within two-and-half feet from 475 metres away. The next day, Oct.10, 2010, when the lighting was identical, they detonated 230 kgs of plastic explosive at the base of the dam, exactly where the dummy bomb had landed. Eight million pounds of concrete blew up spectacularly, as the water poured through.”

View Larger Map

Bellavance appears in the final documentary, Dambusters: Building the Bouncing Bomb, which was broadcast in England on the BBC and in Canada on the History Channel.

“I was wired with a hidden mic which I forgot about from time to time,” recalls Bellavance. “A few times, the producers said: ‘I don’t think you should have said that — let’s back up and shoot that again.’”

Print | Comment