St. Francis Dam disaster is a warning for South Valley

St. Francis Dam disaster is a warning for South Valley

It’s a sad irony that California’s two worst disasters in terms
of human life lost have names derived from a gentle Catholic saint.
An estimated 3,425 individuals were victims of the San Francisco
earthquake and fire of 1906. And 22 years later, more than 600
people died after the collapse of the St. Francis Dam in southern
California.
It’s a sad irony that California’s two worst disasters in terms of human life lost have names derived from a gentle Catholic saint. An estimated 3,425 individuals were victims of the San Francisco earthquake and fire of 1906. And 22 years later, more than 600 people died after the collapse of the St. Francis Dam in southern California.

Most likely, you’ve never heard of the St. Francis Dam disaster. But with news this month that Morgan Hill’s Anderson Dam might collapse in a strong earthquake, it’s important to know.

The Santa Clara Valley Water District announced the results of a seismic study that determined the 60-year-old Anderson Dam might collapse if a “maximum credible earthquake” of 7.2 magnitude on the Calaveras fault struck within 1.25 miles of the dam. That quake has a less than 1 percent chance of happening. But if by bad luck it did, the sand and gravel in the foundation of the dam would liquefy, causing the structure to fail.

According to the study, if Anderson Dam collapsed, much of Morgan Hill would be hit with a 25-foot wall of rushing water. The flood would hit Gilroy two and a half hours later. Potentially, depending on what time of day the quake-induced dam failure happened, the thousands of deaths could make it the worst civil engineering disaster in U.S. history. That’s why the California Division of Safety of Dams banned Anderson Reservoir from being more than 68 percent full. The dam will undergo an estimated $110 million retrofit to start in 2015. 

The importance of making sure that Anderson Dam is safe connects to the St. Francis Dam catastrophe, which occurred at three minutes to midnight on March 12, 1928 in the San Francisquito Canyon, about 40 miles northwest of Los Angeles. I first learned about the disaster at the Ventura County History Museum. I was amazed when I read the exhibit detailing the tragic story that I’d never heard of what’s now considered America’s worst engineering disaster in the 20th century.

Built between 1924 and 1926, St. Francis Dam was a concrete gravity-arch dam designed by William Mulholland, the self-educated chief engineer of the Los Angeles Bureau of Water Works and the most famous civic engineer in California history. The Irishman from Belfast played a key role in developing the aqueduct systems and 19 dams that supplied Los Angeles County and the San Fernando Valley with enough water to grow the region into a major population center.

St. Francis Dam was a crown jewel of Mulholland’s complex water system. It measured 608 feet long, and rose to a height of 195 feet – nearly 20 stories – to impound the San Francisquito Creek. The stored water in the resulting three-mile reservoir weighed nearly 52 million tons.

Around noon on March 12, 1928, Mulholland paid an inspection visit to the dam because dam-keeper Tony Harnischfeger had discovered a leak of mud-colored water, an indication that the reservoir might possibly be eroding the foundation. Mulholland believed the leak was no danger and pronounced the dam safe. What Mulholland didn’t know was that the type of rock in the canyon couldn’t support the dam and reservoir. It was also built on an ancient landslide.

Twelve hours after the inspection, the dam failed, breaking into several sections. Nearly 13 billion gallons of water shot out of the reservoir in half an hour, creating a 125-foot high wave as it tore in the dark of midnight through the San Francisquito Canyon. The first to die was the Harnischfeger family, who lived in a cabin a quarter mile from the dam. The bodies of Tony Harnischfeger and his 6-year-old son Cody were never found.

The wall of water hit unsuspecting farm families as it raced through parts of the Santa Clarita Valley, following the Santa Clara River bed. It impacted the towns of Castaic, Fillmore, Bardsdale, Santa Paula and Montalvo. People drowned in the wave of water, were mangled by the wreckage or impaled by trees and lumber. Those who survived said the rushing water sounded like a hundred steam engines exploding all at the same time. Five and a half hours later, the water had grown to two miles wide and slowed down to 5 miles per hour when it emptied into the Pacific Ocean near Ventura. It had left a wash of devastation 70-miles long. Bodies were found in the ocean as far away as San Diego.

As a response to the disaster, California legislators the next year created the Division of Safety of Dams under the state’s Department of Water Resources. That organization oversees all dams in the state, including Anderson Dam. The lessons we learned from the St. Francis Dam catastrophe might help prevent a similar tragedy in our own South Valley region.

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