In 1919, a tank holding 2.3m gallons of molasses burst, causing tragedy. Scientists now understand why the syrup tsunami was so deadly

It may sound like the fantastical plot of a children’s story but Boston’s Great Molasses Flood was one of the most destructive and sombre events in the city’s history.

On 15 January 1919, a muffled roar heard by residents was the only indication that an industrial-sized tank of syrup had burst open, unleashing a tsunami of sugary liquid through the North End district near the city’s docks.

As the 15-foot (5-metre) wave swept through at around 35mph (56km/h), buildings were wrecked, wagons toppled, 21 people were left dead and about 150 were injured.

Now scientists have revisited the incident, providing new insights into why the physical properties of molasses proved so deadly.

Presenting the findings last weekend at the American Association for the Advancement of Science annual meeting in Boston, they said a key factor was that the viscosity of molasses increases dramatically as it cools.

This meant that the roughly 2.3m US gallons of molasses (8.7m litres) became more difficult to escape from as the evening drew in.

Speaking at the conference, Nicole Sharp, an aerospace engineer and author of the blog Fuck Yeah Fluid Dynamics said: “The sun started going down and the rescue workers were still struggling to get to people and rescue them. At the same time the molasses is getting harder and harder to move through, it’s getting harder and harder for people who are in the wreckage to keep their heads clear so they can keep breathing.”

As the lake of syrup slowly dispersed, victims were left like gnats in amber, awaiting their cold, grisly death. One man, trapped in the rubble of a collapsed fire station, succumbed when he simply became too tired to sweep the molasses away from his face one last time.



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“It’s horrible in that the more tired they get it’s getting colder and literally more difficult for them to move the molasses,” said Sharp.

Leading up to the disaster, there had been a cold snap in Boston and temperatures were as low as -16C (3F). The steel tank in the harbour, which had been built half as thick as model specifications, had already been showing signs of strain.

Two days before the disaster the tank was about 70% full, when a fresh shipment of warm molasses arrived from the Caribbean and the tank was filled to the top.

“One of the things people described would happen whenever they had a new molasses shipment was that the tank would rumble and groan,” said Sharp. “People described being unnerved by the noises the tank would make after it got filled.”

Ominously, the tank had also been leaking, which the company responded to by painting the tank brown.

“There were a lot of bad signs in this,” said Sharp.

Sharp, and a team of scientists at Harvard University, performed experiments in a large refrigerator to model how corn syrup (standing in for molasses) behaves as temperature varies, confirming contemporary accounts of the disaster.

“Historical estimates said that the initial wave would have moved at 56km/h [35mph],” said Sharp. “When we take models ... and then we put in the parameters for molasses, we get numbers that are on a par with that. Horses weren’t able to run away from it. Horses and people and everything were all caught up in it.”

The giant molasses wave follows the physical laws of a phenomenon known as a gravity current, in which a dense fluid expands mostly horizontally into a less dense fluid. “It’s what lava flows are, it’s what avalanches are, it’s that awful draught that comes underneath your door in the wintertime,” said Sharp.

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The team used a geophysical model, developed by Professor Herbert Huppert of the University of Cambridge, whose work focuses on gravity currents in processes such as lava flows and shifting Antarctic ice sheets.

The model suggests that the molasses incident would have followed three main stages.

“The current first goes through a so-called slumping regime,” said Huppert, outlining how the molasses would have lurched out of the tank in a giant looming mass.

“Then there’s a regime where inertia plays a major role,” he said. In this stage, the volume of fluid released is the most important factor determining how rapidly the front of the wave sweeps forward.

“Then the viscous regime generally follows,” he concluded. This is what dictates how slowly the fluid spreads out – and explains the grim consequences of the Boston disaster.

“It made a difference in how difficult it would be to rescue people and how difficult it would be to survive until you were rescued,” said Sharp.