The Collapse of the St. Francis Dam

When one thinks of historical disasters in California, usually one of two events comes to mind…both involving earthquakes.  The first, and most obvious, is the earthquake and fire that struck San Francisco in 1906.  The second is the quake that again rocked the city in the fall of 1989.

But our topic of discussion this evening has nothing to do with earthquakes, though a whole lot of earth got moved.  Just before midnight on March 12, 1928, the St. Francis Dam collapsed, sending a tremendous wall of water (initially 125 feet high) screaming down the San Francisquito Canyon (roughly following the San Francisquito Canyon Road).

The causes of the failure were numerous, and go back almost to the start of construction in 1924.  Originally engineered as a 175-foot high dam, 10′ were added to the height in order to increase the water capacity.  Halfway through construction, yet another 10′ were added.  There were issues with material quality, proper design, and proper accounting for the bed on which the dam was built, but the height changes were probably the main problem.

When construction was completed, the dam began to fill without issue.  But as the dam reached capacity, the structural shortcomings came to light.  The additional height was not accompanied by enough width at the bottom to support the additional forces at the dam’s top.  So at 11:57pm, the pressure at the top of the dam sort of lifted the dam off its foundation and pushed it over.

More than 12 billion gallons ripped down the valley, at breakneck speed and with earth-gouging power.  Recall that when we talked about the terrible Johnstown Flood that occurred 40 years earlier, the collapsing South Fork Dam released less than half the volume of water.

Five and a half hours later, when the floodwaters reached the Pacific Ocean (more than 50 miles away), they carried homes, giant chunks of concrete and rock, parts of a hydroelectric plant, and the bodies of an estimated 600 people.  Upstream at the dam site, only the center section of the dam (shown above, aptly named the “Tombstone”) remained.

The collapse of the St. Francis Dam is a relative unknown in the list of California disasters, but it was one of the worst engineering disasters of the previous century.  In fact, in California’s history, only the dreadful 1906 earthquake resulted in a greater loss of life.

Recommended Reading: Directions to the St. Francis Dam – A nice pictoral of how to find the site of California’s second-worst disaster.