St Francis Dam Disaster (1928): The Collapse in California

The St Francis Dam disaster was the collapse of a concrete dam in San Francisquito Canyon, north of Los Angeles, shortly before midnight on 12 March 1928. The flood that followed swept about 87 km (54 miles) to the Pacific Ocean and killed at least 431 people, making it one of the deadliest civil engineering failures in American history. It ended the career of William Mulholland and changed how dams are planned and supervised.

Background

In the early twentieth century Los Angeles grew from a modest town into a major city, and water was its central problem. William Mulholland, the self-taught chief engineer of the city’s Bureau of Water Works and Supply, built the Los Angeles Aqueduct, which brought water from the Owens Valley more than 350 km away and opened in 1913.

To store aqueduct water close to the city and protect against drought or sabotage of the line, Mulholland planned a reservoir in San Francisquito Canyon. Construction began in 1924 and the dam was completed in 1926. It was a curved concrete gravity dam, a type that holds water back mainly by its own weight, standing roughly 56 m (185 ft) high. During construction its planned height was raised twice without a matching widening of its base.

Warning signs

The reservoir first reached full capacity in early March 1928, holding about 38,000 acre-feet (roughly 47 million cubic metres) of water. Cracks and small leaks had appeared before, and engineers regarded them as normal for a new concrete dam.

On the morning of 12 March the damkeeper, Tony Harnischfeger, reported a new leak near the western abutment carrying muddy water. Muddy water can mean the foundation itself is being washed away. Mulholland and his assistant Harvey Van Norman drove up to inspect it, judged the leak to be clean enough and not dangerous, and left. Less than twelve hours later the dam failed.

What happened

The dam gave way at about 11:57 pm. Investigators later concluded that the eastern abutment failed first and the western side quickly followed, leaving only the central section standing. A wall of water estimated at more than 30 m (100 ft) high rushed down the canyon.

Harnischfeger and his family, who lived just below the dam, were among the first to die. The flood then destroyed the city’s Powerhouse No. 2 and struck a Southern California Edison construction camp near Kemp, where some 84 of about 150 workers were killed.

Through the early hours of 13 March the water spread across the Santa Clara River valley, hitting ranches, orchards and the towns of Fillmore, Bardsdale and Santa Paula. Telephone operators and motorcycle police raised the alarm in some places, which saved many lives. The flood reached the ocean between Oxnard and Ventura at about dawn, carrying bodies and debris out to sea.

Causes

Later geological studies have made the main causes clear. The eastern abutment was built against Pelona schist, a flaky rock, and on the remains of a large ancient landslide. The western abutment rested on a red conglomerate of the Sespe Formation that loses strength and can disintegrate when soaked.

Water seeping beneath and around the dam created uplift pressure and washed out weak material. The dam had little provision to relieve that pressure, and the raised height increased the load. No independent geologist had examined the site in detail before construction, which was common practice at the time but is not today.

Key people

• William Mulholland (1855–1935), chief engineer, who accepted responsibility at the coroner’s inquest and retired from the city’s water department soon afterwards.
• Harvey Van Norman, his assistant, who inspected the leak with him on 12 March.
• Tony Harnischfeger, the damkeeper, who reported the leak and died with his family.

Consequences

A coroner’s jury in Los Angeles found that the disaster was caused by the weak rock beneath the dam and by errors of engineering judgement. It held that no one was criminally responsible but said that dams of this kind should never again be left to a single engineer’s judgement. Los Angeles accepted liability and paid claims to victims and property owners, amounting to many millions of dollars.

In 1929 California passed legislation giving the state engineer authority over the design, construction and maintenance of non-federal dams. That system developed into today’s Division of Safety of Dams. The disaster also helped make engineering geology a standard step before any large dam is approved.

The standing central block, nicknamed the “Tombstone”, attracted sightseers and was demolished with explosives in 1929.

Timeline

• 1924: Construction of the St Francis Dam begins.
• May 1926: The dam is completed and the reservoir begins to fill.
• 5-7 March 1928: The reservoir reaches or nears full capacity.
• 12 March 1928, morning: The damkeeper reports a muddy leak; Mulholland inspects and declares the dam safe.
• 12 March 1928, about 11:57 pm: The dam collapses.
• 13 March 1928, about dawn: The flood reaches the Pacific near Ventura.
• 1929: California passes its dam safety law; the standing “Tombstone” section is demolished.
• 2019: The site becomes the St. Francis Dam Disaster National Memorial.

Why it matters

The St Francis Dam disaster showed how a single unchecked engineering decision could destroy hundreds of lives. Like the Johnstown Flood before it and the Banqiao Dam failure after it, it became a lesson in the risks of large reservoirs. The independent review and geological study it prompted are now a basic part of dam engineering worldwide.