The Great Molasses Flood

     On 19 January 1915, a large molasses storage tank in Boston’s North End ruptured and a wave of molasses more than 24 feet tall swept through the streets at an estimated 35 mph flattening buildings, injuring 150 people, killing 21 people plus several horses and a number of dogs. People and animals were crushed and drowned by the molasses. Many of the dead were so glazed over in molasses they were hard to recognize. Coughing fits became one of the most common ailments after the initial blast. 
     By comparison to the 35 mph speed of the molasses, when Usain Bolt ran the 100-meter dash in 2009, his average speed was a little over 27 miles per hour. A "fast" ordinary person can sprint, maybe, 15 mph which is actually pretty slow. A bear can run up to approximately 37 mph and on land a hippopotamus can hit 19 mph. 
     At about 12:30 in the afternoon a molasses tank 50 feet tall and 90 feet in diameter, and containing as much as 2.3 million gallons collapsed. Witnesses felt the ground shake and heard a roar, a long rumble, a tremendous crashing, a deep growling, or a thunderclap-like bang.  Rivets, sounding like a machine gun, shot out of the tank. 
     The wave of molasses damaged the girders of the Boston Elevated Railway and momentarily tipped a railroad car off the tracks. As it went, buildings were swept off their foundations and crushed and for several blocks the molasses flooded the streets to a depth of 2 to 3 feet. 
     A Boston Post report read: Molasses, waist deep, covered the street and swirled and bubbled about the wreckage ... Here and there struggled a form‍—‌whether it was animal or human being was impossible to tell. Only an upheaval, a thrashing about in the sticky mass, showed where any life was ... Horses died like so many flies on sticky fly-paper. The more they struggled, the deeper in the mess they were ensnared. Human beings‍—‌men and women‍—‌suffered likewise...People were picked up by a rush of air and hurled many feet while others had debris hurled. 
     One schoolboy walking homeward with his sisters was picked up and carried, tumbling on its crest, almost as though he were surfing. When he hit the ground the molasses rolled him like a pebble as the wave diminished. He heard his mother call his name and couldn't answer, his throat was so clogged and then he passed out. 
     First to the scene were 116 cadets from USS Nantucket, a training ship of the Massachusetts Nautical School that was docked nearby. Soon, the Boston Police, Red Cross, Army, and other Navy personnel arrived. Some nurses from the Red Cross waded into the molasses, while others tended to the injured, keeping them warm and keeping the exhausted workers fed. Many worked through the night. 
     The injured were so numerous that doctors set up a makeshift hospital in a nearby building. Rescuers found it difficult to make their way through the syrup to help the victims and it was four days before they stopped searching for victims. 
     In the aftermath local residents brought a class-action lawsuit against the company that owned the tank, the United States Industrial Alcohol Company. In spite of the company's attempts to claim that the tank had been blown up by anarchists (some of the alcohol produced was to be used in making munitions), a court-appointed auditor found the company responsible after three years of hearings and they ultimately paid $600,000 (over 14 million today) in out-of-court settlements. Survivors of the fatal victims reportedly received around $7,000 ($160,000 today). 
     Cleanup crews used salt water from a fire boat to wash the molasses away and used sand to try to absorb it. The harbor was brown with molasses until summer. All of Boston was affected because people tracked molasses through the streets and onto subway platforms, seats inside trains and streetcars, pay telephone and homes. 
     We've all heard the expression, “slow as molasses in January” so how could such a slow-moving fluid cause so much damage? 
     Molasses is what’s known as a non-Newtonian fluid. Normally when we think of liquid we think of something that takes on the shape of the container it's poured into. Those are Newtonian fluids. But some fluids don’t follow this rule; they are non-Newtonian fluids.
     Non-Newtonian fluids change their viscosity or flow behavior under stress. If you apply a force, the sudden application of stress can cause them to get thicker and act like a solid, or in some cases it results in the opposite behavior and they may get runnier than they were before. Remove the stress and they will return to their earlier state. 
     You have probably seen this without knowing what it is. You want to get some ketchup out of the bottle, but when you turn the bottle upside down it won't come out. So what do you do? You smack the bottom of the bottle which causes the ketchup to become more liquid and it comes out. In this case, the ketchup's viscosity decreases and it gets runnier with stress. But, not all non-Newtonian fluids behave the same way when stress is applied. Some become more solid, others more fluid. Some react as a result of the amount of stress applied and others react as a result of the length of time that stress is applied. That's probably all you want to know about non-Newtonian fluids. 
     Fluid dynamics also comes into play. When a dense fluid (like molasses) spreads horizontally into a less dense fluid (in this case, into air) it's similar to how dense cold air flows through an open door into a warm room, even if there is no wind to drive it. The density of the molasses accounts for the speed of its initial spread and people, animals and things simply got bowled over by a tidal wave of molasses which is 1.5 times as dense and several thousand times more viscous than water. 
     Temperature also played a critical role. The molasses was slightly warmer than the surrounding air, but when the tank ruptured and the molasses spread, it cooled quickly, making it even more viscous and much more dangerous. As people were caught in the tidal wave, the molasses acted much like quicksand and the more people struggled, the more deeply sank into it. 
     Recent research has determined that the disaster was more fatal in the winter than it would have been during a warmer season. The syrup moved quickly enough to cover several blocks within seconds and thickened into a harder goo as it cooled, slowing down the wave but also hindering rescue efforts. If the tank had burst in warmer weather, it would have flowed farther but it would also have been thinner and less deadly. 
     When the molasses arrived in the tank, it was warmer by just a few degrees which made it less viscous and easier to transport to the storage tank. When the tank burst two days later, the molasses was still probably about 7-10 degrees warmer than the surrounding air which raised the viscosity of the molasses, trapping people who got caught in it. 
     In the recent study, a structural engineer calculated that the tank’s walls were at least 50 percent too thin and were made of a type of steel that was too brittle which probably contributed to the disaster.