 In 1926, engineers and construction workers congratulated themselves on the completion of the St Francis Dam. Little did they know, but two years later the fruit of their labour would be rubble and over 400 people would lose their lives. Dam disasters are intriguing as the vast structures seem indestructible, as they hold back mind-blowing quantities of water. However, as we have seen many times before on this channel, engineers can and have got it wrong and the results can be catastrophic. Today we're looking at the St Francis Dam and I'm going to rate it here 8 on the Painted Plainly Difficult Disaster Scale. St Francis Dam was located in LA County, California, USA, which is around here on a map. Our story starts with eventual superstar engineer William Moll Holland. Starting off from more humble routes, in 1878 he was hired by the Los Angeles City Water Company as a ditch tender. Proving to be a good employee, Moll Holland raised through the ranks eventually reaching superintendent. During his rise to power, Moll Holland had become a self-taught engineer studying in his free time. Being a bit of a future thinker, Moll Holland envisioned an aqueduct to supply Los Angeles with water from 233 miles away in the Owens Valley. Plenty to allow the city to grow. Taking on as chief engineer, he oversaw the project during his construction between 1908 and 1913. The successful completion of the project, which at the time was the largest of its type, brought fame to Moll Holland's door. Although clearly brilliant, Moll Holland had the ability to improvise on the spot, which is why the aqueduct was built in a relatively short period of time, but sometimes this would come at the cost of shortcuts. He also had an eye for saving money, which is good for budgets, but not good for redundancy and design. To save money, his own staff performed all their own engineering calculations, without any external review. Although frugal and ultimately okay on the aqueduct, when it comes to dam design and construction, cut corners can cause, well, we'll find out later. By the 1920s, the demand for water in Los Angeles had outstripped Moll Holland's aqueduct, reaching a population close to a million. Unfortunately, the rising population ran in parallel with a drought in the region. You see, the area was subject to below-normal rainfall, two out of every three years, as quoted by Moll Holland himself, and as such, rainfall was very boom or bust, leading to the idea of creating a large reservoir to store up rain from the good years to help the city through the bad. Moll Holland proposed an ambitious plan to increase the reservoir capacity around the city, near his aqueduct's terminus. The project would involve modifying existing waterworks and constructing seven entirely new dams, the largest of which would be the Saint Francis. The new dam would store 30,000 acre feet of water, roughly the amount consumed by city residents in 1922. Surveys for the location for the new dam began in 1922, eventually the site was settled upon and initial clearing works began. Strangely but understandably, the initial building works went on with little fanfare, but this was because Moll Holland had been burnt before. During the aqueduct construction, sabotage cost the project money as disgruntled locals and landowners sought to cause trouble. By July 1st, 1924, preliminary clearing works had been completed and main construction works would begin around six weeks later. But first we need to look at the design of this shiny new to be built dam. The basic design was actually copied from the Moll Holland dam built a year before, by, you guessed it, Moll Holland, designed by Edgar A. Bailey. This would be the second dam constructed of concrete by the Los Angeles Department of Water and Power. Chief Engineer, W. W. Holbert, supervised the transfer of the design from the Moll Holland location to the St. Francis location. The main changes were adjustments to fit the new setting of the St. Francis area. As such, Bill Moll Holland didn't manage the project on the micro level like he had on other designs before. This was exasperated by him being distracted by the concept of a Colorado River aqueduct. Before the designs of both dams, the engineering team studied a number of recent concrete dams such as the Arrow Rock Dam in Idaho and the Elephant Butte Dam in New Mexico. A storage capacity of 32,000 acre feet and an elevation of 1825 feet above sea level was envisioned in 1924 after initial survey work. The height of the dam was increased by 10 feet to allow a water capacity of 38,000 acre feet in 1925. The increased size necessitated the construction of a 588 foot long dike along the top of the ridge next to the western abutment. This was not in the original design. As such, the foundations were not widened for the extra height. The design incorporated a curved plan laid out on a radius of 500 feet with a distinctive series of 5 foot high vertical steps on a downstream face of the main dam. Each step was unique in its width and was a simple solution to building a complex curve. The dam didn't have a provision for groutable contraction joints however. A concrete batch plant was built in the canyon of the dam and local aggregate was incorporated with around one barrel of Portland cement per cubic yard of concrete. When the concrete was poured, little thought was given to contractions due to heat given off whilst curing. This combined with the oversight for contraction joints left the dam inflexible and potentially unstable. The reservoir that the dam would hold back began being filled in March 1926. With water diverted from the Owens River Aqueduct and the completed dam was opened in May 1926. During reservoir filling, a number of small cracks and leaks were detected but thought to be within expected tolerances. Between March and June, the reservoir filled at a rate of about 1.8 feet per day and on May 10th 1927, the reservoir pool reached an elevation of 1832 just 3 feet below the spillway crest. Summer demand kept the water level at the elevation. During the winter months between 1927 and 1928, the reservoir was allowed to fill up to just three inches below the spillway. By around February, all of LA's reservoirs were beginning to fill up and on the 7th of March, Aqueduct water was no longer diverted to the reservoir. Damkeeper Tony Harnish-Vega alerted the company on the morning of the 12th to dirty water cascading down the slope below the dam's right abutment. The presence of dirty water can give an indication of foundation erosion. Because the reservoir was at maximum fill and new cracks in the concrete were discovered over the previous week, Mulholland and Harvey Van Norman arrived to investigate the dirty water and the dam as a whole. Arriving at 11am, the two men began to investigate the instability of the structure. Because the dirty water was seen to be clear water mixing with loose dirt from a newly constructed road, although alarming, it was seen as a problem that could be rectified later on. The rest of the structure was inspected and given the seal of approval, well how wrong they would be just 12 hours later. Around 8pm, employees from one of the nearby powerhouses witnessed a 12 inch wide crack on a nearby road upstream from the dam, hinting towards the land around the dam beginning to shift. Ace Hopewell, powerhouse number one's carpenter, was riding his motorcycle along the road nearby, passing the crest of the dam around 11.50pm. He pulled up his bike and sidecar around a mile up from the dam to have a cigarette and to investigate a loud crashing sound. The Southern California Edison company experienced a loss of power at 11.57pm on their 70 kilovolt borough power line that ran near the dam's left abutment, extending eastwards to Palmdale. The shorting out the power line hints at large-scale landslide that enveloped the dam's entire left abutment, starting off the disaster. In the closing minutes of the 12th and the opening of the 13th of March 1928, the dam experienced a catastrophic failure. Within minutes, the majority of the dam had disappeared, leaving only a central section standing like an island. The flow of water was monumentous, forming a 140ft high flood wave, washing away large chunks of the wreck dam, as 12.4 billion gallons of water surged down. The wave, traveling at around 18mph, destroyed powerhouse number two, drowning nearly all the workers and their families who lived nearby. The flood water overflowed the Santa Clarita River's banks, flooding present-day Valencia and Newhall. By 1am, the entire Santa Clarita Valley was plunged into darkness. A temporary construction camp set up by Edison Company was washed away, killing at least 84. The flood damaged the towns of Fillmore, Bardstale and Santa Paula, eventually emptying bodies and debris into the Pacific Ocean, 54 miles downstream. The flood water was thought to have killed at least 431 people. Although this number doesn't count migrant workers, casual labourers and illegal immigrants, all of whom would have had no record of being in the region. Many dead were never found or identified, including dam keeper Tony Harnishfeger. Although his wife's body was found near the dam, suggesting that a late night inspection was being undertaken, potentially due to the landslide sounds heard by Ace Hopewell. So how did such an impressive structure fail so catastrophically? There was, and there still is, differing theories on the failure. It is thought that the failure started with the left east abutment suffering a landslide. The left fellside was then swept in front of the dam, leaving debris as evidence of the likely side of first failure. A committee appointed by the Los Angeles City Council set about working on an official report into the failure. The key failure was placed at defective foundations, and the report essentially said the dam was well designed but poorly executed. With an error in judgement about the suitability of the area's geology as a stable foundation for the dam, the investigation, though, claimed it was the right western abutment that had failed first, owing to cracks discovered leading up to disaster. But more modern analyses have speculated differently. Two alternate reports by Bailey Willis and Carl E Grunsky support the left eastern side failure theory. They also focused into a phenomenon called hydrostatic uplift. This is where water seeps under a dam and causes an upward force on the dam, reducing its effective weight. Although known at the time, the phenomenon was not properly understood and a St Francis dam didn't take this into account. Meaning that hydrostatic uplift mixed with an unstable land from historic landslips caused the failure at St Francis. The dam was not rebuilt and for a short while became a morbid attraction for locals to come and collect souvenirs, as the single standing part remaining known as the tombstone. The site would be cleared and the tombstone demolished in mid-1929 to detract a strange tourism to the area. Because the Mulholland Dam was essentially identical, 330,000 cubic yards was used to reinforce it from the risk of hydrostatic uplift. The Hollywood reservoir that Mulholland holds up was not allowed to exceed 4,000 acre feet of water. Mulholland's career also ended on the night of the 12th and the 13th of March 1928. During the inquest, he claimed full responsibility and retired in November the same year. No criminal charges were brought against the designers, with the inquest finding no evidence of criminal act or intent. Mulholland would die in 1935 at the age of 79. And odds and sods. As well as hints on future videos. I've got a Patreon and YouTube memberships as well so if you fancy check them out and all that's left to say is thank you for watching.