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Geo-Joint: The St. Francis Dam Disaster

Posted on March 07 2017

The story of how Los Angeles grew from a sleepy little village to a world-class megalopolis is well known. Though it began next to a river, that watercourse was unreliable—sometimes a raging torrent, but mostly a small flow, or in bad times, dry. If Los Angeles was to grow, it needed more of that liquid. William Mulholland and his fellow engineers developed the plans to get the needed water from the Owens Valley, over 200 miles away on the east side of the Sierra Nevada. Through fair means and foul, they got the water rights and built a massive waterworks of canals and tunnels to channel the precious stuff by gravity alone, down the valleys, through the mountains, and into the LA basin by 1913. It was, and is, a marvel of engineering even if it did have terrible effects on the people and environment of the Owens Valley. Owens Lake, far less than a shadow of its former self, is still the scene of massive dust storms as exposed lakebed sediment is lifted in clouds in the high winds there. But even before things got that bad, the lake was lowering fast, and in dry times might not have been reliable. Another threat to the continuous flow was the San Andreas Fault, which the canal crossed. In addition, angry farmers had detonated explosives along the canal, interrupting the flow. Mulholland realized he needed to create storage for water on the LA side of the mountains in order to maintain a steady supply.

This led to the construction of five dams around the Los Angeles area, and the filling of their associated reservoirs. Following these, Mulholland built the biggest of his storage works, the St. Francis Dam in San Francisquito Canyon. The canyon is situated in rough, steep hills about 50 miles north of Los Angeles, above what is now the city of Santa Clarita, a small town at the time. Built in a single year, the dam was almost 200 feet high and 700 feet along its curved rim, and its reservoir had an area of 600 acres. During construction, the height of the dam, originally set to be 175 feet, was increased to 195 feet. Like the other reservoirs, it was built to hold about a year’s worth of water for the growing city. The added capacity allowed for more water and therefore brought greater water pressure against the dam, but no work was done to strengthen the base. While not huge by today’s standards, it was the world’s largest arch-supported dam at the time, and cost the city 1.3 million dollars. In 1926, that was a hefty sum.

Los Angeles’ future was looking bright, with its water pipe full, and Mulholland, though despised in the Owens Valley, a hero to the city fathers. Things were soon to take a drastic turn. A couple of years after the dam’s completion, it began to show signs of trouble. Various cracks and leaks appeared, and while engineers made some repairs or assessed them as inconsequential, local farmers and townfolk living below the dam had their doubts and were known to make gallows humor regarding the possible danger. On March 12, 1928, a dam worker noticed a muddy flow coming out of the dam. Called to inspect the worrisome stream, William Mulholland himself assured the workers that it was nothing to be concerned about. In reality, it was the last hint the dam would give.

Just minutes before midnight that night, the St. Francis Dam gave way, and unleashed more than 12 billion gallons of water, which headed downstream to the sea. If anyone saw the failure of the dam, they didn’t live to describe it. Those living below heard the roar of the coming water and some were able to make it to higher ground or grab onto various pieces of flotsam, and survive. At first a 10-story-high wall of water, the contents of the reservoir spread out as it rolled down its 54-mile journey along the Santa Clara River bed to the Pacific. Frantic calls were made to towns farther down the line: Castaic, Saugus, Piru, Fillmore, Santa Paula, and Saticoy. A couple of CHP officers rode their motorcycles street to street with their sirens blaring and calling out warnings to the sleeping people in the flood’s path. Many were saved, but somewhere around 500 people died. Many in harm’s way were Mexican farm laborers who were little known, and possibly never found. The reservoir took 70 minutes to empty its watery contents. Moving at about 18 miles per hour, the flood spent itself into the sea at Ventura around 5:30 in the morning. The destruction was enormous. More than 1,200 houses had been destroyed, thousands of livestock drowned, farms and orchards plowed away by the raging waters or buried under sediment. It was perhaps the single worst American civil engineering disaster of the 20th century.

Clean-up took a lot of time, work, and money. It was estimated that damages ran to $20 million. For comparison, remember that the dam itself cost $1.3 million. As that gargantuan task was being addressed, questions about the cause of the failure were uppermost in people’s minds. Mulholland at first suspected sabotage by the same agents who were wrecking parts of the Owens Valley aqueduct. This theory was quickly disproven, and geologists and hydrologists determined the land upon which the dam was constructed was actually a huge former landslide, and as the waters of the reservoir seeped into old sliding surfaces, the weight caused the ground to give way. Other contributing causes may have involved weak concrete, inadequate foundation design, and uplift caused by the pressure of the reservoir’s water. The engineering of the dam had been done by Mulholland, who despite his vast self-taught knowledge and prodigious ability to inspire the building of huge projects, was not an expert in engineering geology. No professional geologists reviewed the design or damsite—it was all Mulholland’s work. He was personally devastated by the dam failure, and though the Department of Water and Power would not initially accept his resignation as Chief Engineer, his reputation no longer had the respect and admiration it once commanded amongst the citizens of Los Angeles. He died seven years later, a sad and diminished man.

One can scarcely calculate the damage that would result from the failure today of the troubled Oroville Dam, north of Sacramento. The problem with the spillways there are not based in old landslides, but while the operators there seem to have that situation in hand, physics, geology, and flowing water can sometimes surprise. Despite the difference in circumstances, all the caution being undertaken at Oroville is in some part a recognition of the terrible lessons taught by the events at the St. Francis Dam nearly ninety years ago.

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