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No. 1325:
The Day the Dam Broke

Today, an almost-forgotten catastrophe. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Kermit Pattison tells about the day the St. Francis Dam burst. In 1928, this dam on the San Fransisquito Creek north of Los Angeles burst, and it killed over 500 people. The St. Francis dam was the last of William Mullholland's efforts to bring water into Los Angeles.

Mulholland, Los Angeles's tough water superintendent, had built the great aqueduct from the Owens Valley agricultural region down into Los Angeles. With water the city grew, and as it grew it demanded more water. Owens Valley ranchers finally fought back.

They damaged Mulholland's aqueduct, took over pumping stations, and kidnapped city officials. Los Angeles would need a reservoir to sustain its water supply, so Mulholland chose the San Fransisquito Creek. He'd built many dams in his work. Now this dam would create a reservoir of thirty-thousand acre feet and a year's supply of water.

By now Mulholland's reputation as an engineer was too strong. As the dam went up, certain features weren't checked aggressively enough. This was a gravity-arch dam: heavy enough that the water wouldn't push it aside, but it also arched back into the reservoir where its curvature lent it extra strength.

As work began in 1924, a drought sent people clamoring for even more reservoir capacity. Mulholland raised the dam from 175 to 195 feet without demanding reinforcement. The reservoir began filling in 1926. Water rose to 165 feet in the first three months.

A year later, with water three feet from the top, cracks appeared. Mulholland deemed them the normal result of concrete curing. When leaks appeared, Mulholland pointed out that all dams leak. Then, on March 12, 1928, a strange thing happened. The water level had been standing just three inches from the top for some time. In the early evening, the water-level gage in the center suddenly dropped 3.6 inches. For that to happen, a million cubic feet would have to've been released. It made no sense.

Just before midnight, a chunk broke out of the east side of the dam. The center section moved just enough to break away from the west side, which then swept downstream followed by twelve billion gallons of water. No one who watched the dam burst open lived to tell of it. It was one the great engineering disasters of all time.

Dawn found Mulholland walking on the mud in the shadow of the still-standing center section. It seems he'd built part of the dam on silt without realizing it. The water level hadn't dropped 3.6 inches before the dam broke. Rather, mud had worked in below the center and buoyed that section upward, breaking the sides loose.

The tough Mulholland broke down and wept at the hearing. He took full blame. He lived only seven more years with his nightmare. History has debated his guilt ever since. One thing is clear: The failure owed much to his own expertise. The dam broke because people had trouble questioning his ability. And that's a position that no good engineer should ever want.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)

Pattison, K., Why Did the Dam Burst? American Heritage of Invention & Technology, Summer 1998, Vol. 14, No. 1, pp. 22-31.

See also the Wikipedia article about the St. Francis Dam Failure.