The sea mines laid off the coast of Denmark were designed to blow a massive hole in the hulls of any passing vessels from the Nazi Kriegsmarine.
When, 80 years later, the mines finally did explode, thanks to a new bomb clearance technique they barely bothered any nearby dolphins.
A scientific study has shown that a process of deflagration — in which undersea bombs are combusted using a small shaped charge rather than exploded using a larger charge — produces one hundredth of the noise.
The findings, published in the journal Marine Pollution Bulletin, point to a way to continue the expansion of offshore wind farms without battering the hearing of cetaceans, which have continued to be victims of the munitions of two world wars long after fighting has ceased.
“The North Sea and the seas around Europe are littered with unexploded ordnance,” said Stephen Robinson, principal scientist at the National Physical Laboratory (NPL). Between bombs ditched by returning Lancasters, mines laid to blockade opposing navies and the shells left over from firing exercises, the seas have been treated as a convenient dumping ground for two centuries.
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With increased marine construction, though, this practice has been increasingly inconvenient. “Usually, you leave the bombs alone,” Robinson said. “But if you are building a wind farm, you have no choice — because the cable laying and the construction of the turbines will disturb them. Then the traditional way is to blow them up with another bomb.”
As well as cratering the sea bed this makes, he said, a “robust” noise. “It’s one of the loudest man-made sounds in the ocean.”
Around the UK about 50 such detonations occur each year and the effect on the dolphins and other creatures — who need hearing to echo-locate and communicate — are poorly understood. Marine biologists have estimated that any marine mammals such as porpoises within a mile or more of the blast could potentially be deafened by the shockwave. Joanna Lumley, the actress, has led a campaign, Stop Sea Blasts, to find a solution.
The government stated in 2020 that less damaging approaches are to be used in preference, such as deflagration, developed by Alford Technologies, a British company. Rather than explode the bomb, this uses a high-temperature jet from a small blast to incinerate it. The work, published on Wednesday, has accurately quantified the difference it makes, in a real world test.
Working with the Danish navy, the researchers directly compared traditional approaches and deflagration on mines originally laid in a British plan to block a section of Denmark’s coastal waters. Scientists from NPL measured the shock wave. The difference came to 20 decibels, or in energy terms the explosion was about a hundred times more energetic than the deflagration.
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This is good news for porpoises, said Robinson, but also for green energy. “The need for wind is so great. There has to be an awful lot of these wind farms built in the next ten years, and you have to do it quieter if you’re not going to affect the marine environment adversely.
“Underwater noise pollution is one of the main barriers to wind farm expansion.”
