Incredible physics behind the deadly 1919 Boston Molasses Flood

Physics 24 November 2016
Front page of the Boston Dail Globe with a headline about the Boston molasses flood

Boston Globe/Getty Images

It was the unlikeliest way to drown. The first physics analysis of Boston’s Great Molasses Flood shows that cold temperatures and unusual currents conspired to turn slow sticky goop into a deadly speeding wave.

On 15 January 1919, a storage tank in Boston’s North End ruptured and a wave of molasses more than 7 metres tall swept through the streets. The flood was no joke: it flattened buildings and killed 21 people and several horses. For nearly a century, historians have wondered why such a famously slow-moving fluid wreaked such widespread devastation.


Now, scientists have teamed up to explore the underlying physics for the first time.

Devastation and flattened buildings in the wake of the molasses flood
A sticky predicament

AP/Press Association Images

Nicole Sharp, an aerospace engineer turned science communicator who runs a Tumblr blog on fluid dynamics, and Jordan Kennedy at Harvard University gathered as much data as they could from historical records, including newspaper articles, old city maps and weather reports. They ran experiments on how molasses flows under various conditions, then fed the data into computer models.

Presented 21 November at the annual meeting of the American Physical Society Division of Fluid Dynamics in Portland, Oregon, their initial results offer a new view of why the flood was so deadly.

Molasses is what’s known as a non-Newtonian fluid of the shear-thinning variety, meaning it flows more easily in response to applied stress.

Slow as molasses in January

But when it’s cold, the stuff behaves more like a classic fluid. In particular, the viscosity increases dramatically as the temperature drops, as Sharp and Kennedy found when they tested molasses in a cold room.

“It took several minutes just to pour 48 milliliters into a graduated cylinder,” Sharp says. There’s a reason for the adage “Slow as molasses in January”.

So, given the cold temperatures in Boston at the time, the extent of the flood was a puzzle. Newspaper headlines speculated that a bomb may have caused the molasses to spread at such a rapid rate.

Aerial shot showing the extent of the molasses flow
Sickly-sweet tsunami

Fluid dynamics offers a better explanation. The true culprit: gravity currents, which come into play when a dense fluid spreads horizontally into a less dense fluid (in this case, molasses into air). It’s similar to how dense cold air will flow through an open door into a warm room, even if there is no wind to drive it. The density of the molasses alone would account for the speed of its initial spread. “Basically, you got bowled over by a tidal wave of molasses,” says Sharp, likening the effect to a sticky-sweet tsunami made of a substance 1.5 times as dense and several thousand times more viscous than water.

Temperature also played a critical role in the aftermath of the flood. On the fateful day, the molasses in the tank was slightly warmer than the surrounding air. But once the tank ruptured and the molasses spread, it cooled quickly, making it even more viscous – and much more dangerous.

The initial onslaught left victims covered in suffocating molasses as rescuers struggled to save them, waist-deep in the goo. Much like quicksand, the more people thrashed about, the more deeply they found themselves trapped in the treacle.

Sharp hopes to expand her analysis to what was happening in the tank just before the eruption, when a warmer layer of molasses was pumped into the bottom. “I’d expect there would be some crazy convective mixing going on, complicated by the fact that the viscosity of molasses changes substantially with temperature,” she says.

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