When Science Goes Wrong: Twelve Tales From the Dark Side of Discovery - Simon LeVay (2008)

ENGINEERING GEOLOGY: The Night the Dam Broke

THE URBAN TENTACLES of Los Angeles have not yet reached San Francisquito Canyon, an arid corridor that slices southward through the Transverse Ranges 40 miles northwest of downtown. A hiker can walk for miles there and see no life beyond the circling hawks and an occasional rattlesnake. But he couldn’t fail to notice the immense, eroded blocks of concrete, some weighing 10,000 tons or more, that lie half-hidden in the chaparral like the decaying ruins of a long lost civilisation. These monoliths serve as memorials to the victims of America’s worst civil engineering disaster of the 20th century - the failure of the St. Francis Dam on March 12, 1928.

The dam was the brainchild of William Mulholland, the famed water engineer whose projects made possible the explosive growth of Los Angeles during the early part of the 20th century. Born in Belfast in 1855, Mulholland arrived in Los Angeles at the age of 22 with little education and a few dollars in his pocket. He took employment as a ditch tender for the city’s water system, which was then dependent on the erratic local supply provided by the Los Angeles River. He quickly demonstrated an extraordinary aptitude for water engineering, and he rose through the ranks of the water company to become its superintendent. In 1902, when the city of Los Angeles turned the company into a public utility, Mulholland was appointed its chief engineer and general manager.

Mulholland’s most well-known achievement was the Los Angeles Aqueduct, begun in 1908, which brought water to the infant metropolis from the Owens River in the eastern Sierras. The aqueduct was an engineering marvel for its age, not only on account of its great length (233 miles) but also because, in spite of some very rugged terrain along the way, water moved from one end to the other entirely by the force of gravity. Where the aqueduct crossed a canyon, the water was carried down the slope, across the canyon floor, and up the other side within a V-shaped length of steel pipe called an inverted siphon; these siphons had to be tremendously strong to withstand the internal pressure generated by the drop. The aqueduct also had many tunnels: most notable among these was the five-mile long Elizabeth Lake tunnel, near the head of the Antelope Valley. Within this tunnel the aqueduct passed directly through the San Andreas Fault, and the torturous geology at the Fault posed special engineering problems. After emerging from the tunnel the aqueduct headed southward along the eastern flank of San Francisquito Canyon toward Saugus, terminating in a set of small reservoirs in the San Fernando Valley and nearby areas.

Thirty thousand Angelenos turned out in November 1913 to celebrate the opening of the aqueduct. The benefits to the city quickly became apparent, as irrigated orchards sprang up in the San Fernando Valley and plentiful water became available for homes and gardens in new subdivisions. Even today, the Los Angeles Aqueduct - extended in length and supplemented by an additional parallel pipe - supplies the bulk of the city’s water. The system brings almost half a billion gallons of water from the eastern Sierras to the thirsty metropolis every day.

Bill Mulholland became an almost godlike figure to the citizens of Los Angeles. He was urged to run for mayor, but he turned down the suggestion with a memorable comment. ‘Gentlemen,’ he told a crowd, ‘I would rather give birth to a porcupine backwards than be mayor of Los Angeles.’ Instead, he devoted himself to improving on what he had achieved. Over the following decade, he added four hydroelectric powerhouses to the aqueduct. Two of these were situated in San Francisquito Canyon: one was located at the head of the canyon, where the aqueduct dropped 1,000ft after exiting the Elizabeth Lake tunnel, and the other was sited six miles down the canyon. Over time, the powerhouses generated enough electricity to defray the entire costs of the aqueduct’s construction.

Godlike though Mulholland may have appeared to Angelenos, to the residents of Owens Valley he seemed more like the devil. The aqueduct sucked the lifeblood out of their soil; for every orchard that was planted in the San Fernando Valley, one had to be abandoned in Owens Valley. Agriculture came to a halt. Owens Lake, the natural terminus of the Owens River, dried up completely in 1924, and the lakebed gave off a cloud of arsenic-laced dust that choked the valley every time the wind blew. Meanwhile, the city’s agents began purchasing water rights farther to the north, enriching some settlers but threatening the rest with penury.

Infuriated by these developments, some of the settlers began a campaign of sabotage. In 1924 they seized the aqueduct’s headgates and diverted water back into the Owens River. This episode was followed by dynamite attacks on the inverted siphons and other parts of the aqueduct. The attacks led to interruptions in water delivery and necessitated expensive repairs.

The settlers’ campaign failed. Although the city’s acquisition of the local water rights involved some deception, it was done more or less in accordance with the legal requirements current at the time. The settlers gained little traction with the courts or with public opinion and eventually gave up. (The environmental problems in Owens Valley have remained unresolved to the present day: the city of Los Angeles is making some attempts at remediation, such as covering parts of the bed of Owens Lake with gravel and diverting a small portion of the Aqueduct water back into the Owens River.)

During the early 1920s, Mulholland came to realise that the city needed more water-storage capacity. One reason was the fear of drought. In fact, one three-year drought reduced flows to the point that the city had insufficient water to supply the needs of the farmers in the San Fernando Valley. In addition, there was the threat of sabotage. Finally, there was the always-looming danger of a rupture of the San Andreas Fault, an event that would block the aqueduct within the Elizabeth Lake tunnel. For all these reasons, Mulholland wanted to construct a set of reservoirs south of the fault that collectively could store a year’s supply of water for the city. The largest of these reservoirs, designed to hold half of the entire supply, was to be sited in San Francisquito Canyon, and construction of the necessary dam - its name anglicised to ‘St. Francis’ - began in 1924.

On the face of it, San Francisquito Canyon looked like an ideal location. In the northern part of the canyon lay a broad valley that could easily hold 32,000 acre feet (or about 10 billion gallons) of water. In fact, it had been the site of a large lake in prehistoric times. About halfway down the canyon, about a mile and a half north of the lower powerhouse, a rocky spur jutted out from the canyon’s western flank, constricting the canyon to a gorge barely 200ft wide at the canyon floor. Because the sides of the canyon were sloped, however, a dam would have to be considerably wider at its top - about 550ft if the dam were built straight across, and more if it were curved. Thus the greater portion of the dam would consist of its abutments, the sections that rested on the canyon’s sloping sides.

Mulholland had several geologists look at the proposed dam site before he made the decision to go ahead. These experts included John Branner, chairman of the geology department at Stanford University and the university’s second president. The geologists gave Mulholland their opinion that the site was suitable, but their inspections may not have been very detailed. After the disaster, Mulholland cited these geologists’ positive opinions but did not produce any written reports to back them up. Branner’s approval probably took the form of a verbal ‘looks OK to me’ after a single visit to the site, and he died before Mulholland made his final decision on the dam’s location.

One potentially troublesome feature of the site was already well known. A geological fault, the San Francisquito Fault, ran southward along the canyon, traversing the prospective site of the dam near the bottom of the sloping western wall of the gorge. Thus, if the dam were built, the fault would lie at the base of the dam’s west abutment (or ‘right’ abutment, according to the convention that the viewer is facing downstream). The fault was believed to be inactive, however, meaning that it was no longer subject to the slow accumulation of stress that might trigger an earthquake.

The reddish rock to the west of the fault, where the dam’s right abutment was to be located, is a conglomerate - that is, it consists of pebbles and cobbles in a matrix of hardened sand or silt. This particular example is known as the Sespe Formation, or Sespe Red Beds. The rock in this formation, especially in a zone near the fault, is rather crumbly and easily weathered, but its most startling behavior is evident when it becomes wet. When I visited the dam site in 2006, I pulled a loaf-size chunk of rock out of the slope where the right abutment once stood and placed it in the creek. The rock underwent a kind of slow-motion explosion: over a period of about 10 minutes, it gave off bubbles, started cracking, and then fell apart into a heap of stones and mud. In spite of this behaviour, Mulholland conducted tests that convinced him that the foundations of the right abutment would not fail or allow water to percolate through.

The rock to the east of the fault, which would carry the central section of the dam and its left abutment, is a mica schist. This is a sedimentary rock that has been altered by heat, pressure, and shearing forces so that its constituent grains are flattened, which gives the rock a laminated structure rather like slate. This particular formation is named the Pelona Schist. It is much harder than the rock of the Sespe Formation, but its layered structure gives it the tendency to split and slide along the plane of the layers, like a deck of cards. What was worse, the rock layers were tilted such that they roughly paralleled the slope of the east side of the gorge. At the fault itself, where the Sespe Formation and the Pelona Schist met, there was a layer of claylike ‘fault gouge’, about eight inches wide, that had been generated during innumerable ancient ruptures of the fault.

Undaunted by the problematic geological features of the site, Mulholland decided to go ahead. As the ‘chief’, his word was as good as law, and any review of his decision within or outside of the water department was perfunctory if it happened at all. As for environmental reviews or state safety inspections, such things did not happen in the 1920s.

Mulholland constructed the dam from concrete. Most of his prior dam-building experience involved earthen dams, but one year before starting the St. Francis Dam he had begun work on his first concrete dam. This was the dam now known as the Mulholland Dam, which confines the relatively small Hollywood Reservoir on the western fringe of Griffith Park. Apparently Mulholland chose concrete because of a lack of clay - needed to form the water-resistant core of an earthen dam - at the Hollywood and St. Francis sites. The design for the St. Francis Dam was very similar to that for the Mulholland Dam, and both were based on the then-current textbooks of dam design.

The two dams were gravity-arch dams. This means that they depended primarily on their weight to hold back the water in the reservoir. This weight - a quarter of a million metric tonnes in the case of the St. Francis Dam - thrusts directly downward, whereas the hydrostatic pressure of the reservoir water thrusts more or less horizontally downstream. The resulting combined thrust is aimed obliquely downward. For the dam to be stable against tilting, the combined thrust must be directed into the bedrock within the middle third of the dam’s footprint, not near the downstream edge or ‘toe’ of the dam or, even worse, downstream of the toe. The plans developed by Mulholland’s design engineers met this criterion, of course. An additional safety factor was added by the dam’s arched shape (curved convexly into the reservoir). This had the effect that some portion of the reservoir’s horizontal thrust was carried sideways into the dam’s abutments and thus into the canyon walls.

Conservative though this design was, Mulholland made it much less so by changes that he ordered after construction got under way. For one thing, he twice raised the height of the dam, from the original 175ft to 185ft and then to 195ft - an 11 per cent increase in height. His aim, of course, was to increase the holding capacity of the reservoir. But Mulholland did not order any compensatory thickening of the dam at its base. In fact, he actually omitted the lowermost portion of the dam’s toe, leaving the dam 20 feet (or 11 per cent) thinner at its base than called for in the design. This latter fact was only discovered decades later by Charles Outland, a local historian who made an extensive study of the disaster and wrote a detailed book about it. Outland spotted the shortened toe by closely examining photographs that had been taken during the dam’s construction. Because the dam was taller, and thinner at its base, than the original design had specified, it was significantly less stable against tilting.

The dam was completed in May 1926. Filling of the reservoir began during the construction phase but was not complete until March 7, 1928. At that point, the water lapped just a few inches below the spillway. Photographs taken at that time convey an image of graceful strength. The dam’s downstream face, rather than being smooth like most high dams that we’re familiar with today, was stepped. This gave the dam the look of a Roman amphitheatre and emphasised its curvature. Behind the dam, the reservoir spread out into the broad upper San Francisquito Canyon and its side-bays: it resembled a natural and serene lake.

Day-to-day inspections of the dam were left in the hands of the damkeeper, Tony Harnischfeger, who lived with his girlfriend, Leona Johnson, in a cottage located in the canyon a few hundred yards downstream from the dam. On the morning of March 12, 1928, five days after filling of the dam was complete, Harnischfeger telephoned Mulholland to tell him that muddy water was leaking from the base of the dam’s right abutment. High dams usually leak a certain amount of water, and the St. Francis Dam had already sprung several small leaks during the filling process, but the fact that this leak was muddy was novel and ominous. It suggested that water was not merely passing through the dam but was removing material as it did so.

Mulholland and his chief assistant, Harvey Van Norman, immediately drove from his downtown office to the dam site, which they reached around 10.30am. They confirmed that muddy water was indeed flowing down the foundations of the right abutment, at a rate of about 15 gallons per second. When they clambered up to the site where the water was emerging from the Sespe Formation, however, they saw that the water was clear. The mud was mixing in as the water ran down the slope and across the embankment of a dirt road. Some water was also leaking from the base of the other, left, abutment, but this water was also clear. Relieved to find that the dam was not being undermined, Mulholland made a mental note to have the leaks repaired at some later date, and he and Van Norman returned to the city.

The dam broke at two-and-a-half minutes before midnight that evening. There were no surviving eyewitnesses to the collapse, but the exact time could be pinned down because it caused an interruption in the transmission of electric power in lines that ran down the canyon. The interruption was experienced in Los Angeles only as a two-second dimming of lights, but areas closer to the dam were plunged into darkness.

It is likely that Harnischfeger and Johnson did witness the dam’s collapse before they became its first two victims. Leona Johnson’s fully clothed body was later found trapped amongst the slabs of broken concrete at the base of the dam. Most probably Harnischfeger and Johnson saw or heard some premonitory sign of the collapse, dressed and walked up to the dam to see what was wrong, only to be caught by a cascading mass of water and concrete. Harnischfeger’s body was never found, nor was that of his young son.

A few survivors did hear or feel the collapse. A motorcyclist named Ace Hopewell was driving up the valley shortly before midnight. The road ran along the canyon’s east wall: at the point that it passed the dam, it was cut into the Pelona Schist only 13 feet above the dam’s left abutment. Hopewell noticed nothing amiss as he passed the dam, but a mile farther up the road he heard a sound as of rocks falling. He stopped for fear of running into a landslide, but the sound seemed to be coming from behind him so he continued on his way.

EH Thomas was one of the staff of the lower powerhouse, which was situated in the canyon a mile and a half below the dam. His particular job, however, was to attend to the surge tank, high on the east rim of the canyon. The surge tank’s function was to damp out violent changes in hydrostatic pressure caused by the opening and closing of valves; from the surge tank, pipes (or ‘penstocks’) carried the aqueduct water down to the twin generators in the main powerhouse building. Thomas lived with his mother in a cottage not far from the surge tank. At about midnight the two were woken by a strong shock, followed by a continuous vibration. They first assumed that it was an earthquake. Thomas dressed, took a flashlight, and made his way toward the tops of the penstocks. Looking down into the canyon, he saw nothing but rushing water. The powerhouse, if it still existed, was completely overtopped by the flood, which scoured the canyon walls to a height of 120 feet above the creek. Thomas realised that the 28 other powerhouse workers and their families, whose homes were in the floor of the canyon, had most likely been swept away to their deaths.

Actually, three people did survive. One was Ray Rising. ‘I heard a roaring like a cyclone,’ he said later. ‘The water was so high, we couldn’t get out the front door… in the darkness I became tangled in an oak tree, fought clear, and swam to the surface… I grabbed the roof of another house, jumping off when it floated to the hillside… There was no moon and it was overcast with an eerie fog - very cold.’ Rising met up with a worker’s wife, Lillian Curtis, and her young son, who had had similar narrow escapes, and the three of them waited for the dawn together, but his own wife and three daughters died, along with 64 other members of the powerhouse community. The powerhouse itself, a 65ft-high concrete building, was swept away - only the generators remained in place, half-buried in mud.

The floodwaters rushed onward. About 15 minutes after the dam broke, the flood reached a cattle ranch near the southern end of the canyon. All the buildings were swept away, along with several of the residents, but a few had been woken by the noise and were able to scramble to higher ground.

At the southern end of San Francisquito Canyon, the creek joins with those from several other canyons to form the Santa Clara River, which runs 40 miles westward to the Pacific Ocean south of Ventura. Near where the creeks meet lay the small community of Castaic Junction - basically an auto park, with tourist cabins, a gas station, a cafe, and a few other buildings, which served travellers on the highway that connected Los Angeles with California’s Central Valley.

It took 50 minutes for the floodwaters to reach Castaic Junction, but they had lost little of their ferocity along the way. George McIntyre, the 19-year-old son of the auto park’s owner, was alerted by the rushing, crashing noises coming from the east. He, his father, and their cook went outside to see what was happening. They saw bright flashes from the direction of the Edison station at Saugus, and they concluded that there was something amiss there. Then they watched with amazement as one of the tourist cabins began moving off its foundations. Within moments the father and son were knocked off their feet by the floodwaters. While holding on to each other, the two men were swept away, but not before they caught a final glimpse of George’s younger brother struggling to get through the window of one of the cabins. After a while George and his father, were able to grasp onto a utility pole, but the father was injured, probably by floating debris, and after muttering a brief good-bye to his son he let go his grip. That was the last George saw of him. Eventually, George also let go of the pole and was sucked deep into muddy water. At long last, he found himself back at the surface, half-choked with water and mud. After drifting for some time he collided with the branches of a cottonwood tree, where he was able to hold on until the flood had receded. Besides George McIntyre, only one other person survived from the Castaic Junction community - the cook, who escaped the floodwaters by running to higher ground. All the structures were completely destroyed, and four miles of the north-south highway were inundated.

By this time, the wider world was beginning to find out about the catastrophe. The initial break on the Edison line had been followed by a cascade of wider electrical failures, as more lines were brought down, switching stations were shorted out and emergency connections were overloaded. Soon the city of Los Angeles, the Antelope Valley and all the coastal cities north to Santa Barbara had lost power. Charles Heath, Edison’s superintendent of transmission, later testified that he had guessed as early as 12.05am that the St. Francis Dam had failed, and that, after telling his subordinates to warn the towns in the Santa Clara Valley, he set out for Saugus in his official car, with siren blaring and red light flashing. He said that he reached Saugus at about 12.45am - just about the same time that the floodwaters were entering the head of the valley.

Heath knew that 150 Edison workers were sleeping in tents at a construction camp on the bank of the Santa Clara River, eight miles down the valley. He couldn’t reach them, because the road and the bridges were out, so he attempted to telephone the camp from the Saugus substation, which itself was being inundated by the rising water. No one answered the phone and then, after several repeated attempts, the line went dead. The flood, forming a wall of water 40 feet high or so, had struck the camp without any warning. Of the 150 workers sleeping there, 85 died. Few of them had even been able to get out of their tents.

Further down the valley were the towns of Fillmore and Santa Paula. The destruction of roads, bridges, power lines, and telephone lines had cut them off from any communication to the east, but at 1.30am the night telephone operator at Santa Paula received a call from the coast: the St. Francis Dam, she was told, had broken and floodwaters were descending the Santa Clara Valley. Working by candlelight, the operator began calling local police officers, town officials and the residents of the town whose homes were closest to the riverbed. Two police officers drove through the streets on their motorcycles with sirens blaring. They stopped at every third house or so, warning the sleepy occupants of the oncoming flood and telling them to alert their neighbours and move to higher ground. Soon the entire town was on the move. Meanwhile, squad cars raced up the valley to Fillmore and alerted the population there. As the cars attempted to drive farther east, however, they were stopped by the arriving flood, which filled the entire two-mile width of the valley and was advancing westward at about 12 mph.

Much of the Santa Clara Valley was used for citrus farming and other tree crops. The groves were obliterated by the oncoming water and mud. In addition, the low-lying portions of the valley towns, especially Santa Paula, were inundated. Houses were carried off, to be smashed up by the roiling water or to be left reasonably intact but several blocks away from their foundations. The bridges across the river were destroyed: the bridge at Santa Paula was demolished just minutes after a police officer ordered a crowd of would-be spectators to get off it.

As the floodwaters moved westward, they also slowed and spread out. By the time they reached the coast, five hours after the dam broke, they were moving at no more than a jogging pace. The coastal city of Oxnard was evacuated, but that turned out to be unnecessary.

The total death toll was estimated to be between 400 and 450 persons. This figure may be an underestimate, given that then, as now, many undocumented migrants from south of the border lived in the low-lying areas of the canyons and riverbeds. Though impromptu morgues overflowed with bodies, many of the dead were never found. Some were undoubtedly swept out to sea: bodies washed up on beaches as far south as San Diego. Others were buried under many feet of mud. In addition to the human toll, there was enormous destruction of livestock, buildings, infrastructure, orchards, and farmland.

Eventually, the city of Los Angeles settled with most of the victims of the disaster without litigation. Persons who had lost family members typically received $10,000 to $20,000 - or $100,000 to $200,000 in today’s dollars. In one case that did go to court, Ray Rising, the sole worker to survive at the lower powerhouse, was awarded $30,000 for the loss of his wife and three daughters.

What caused the dam to fail? William Mulholland himself, accompanied as always by Harvey Van Norman, rushed to the dam site in the small hours of the morning after the disaster. As dawn broke, an extraordinary scene revealed itself. A slim central section of the dam still stood in its original place, ranging 200 feet above the empty reservoir like a lone incisor in an otherwise toothless jaw. To its left, another huge section of the dam had slumped downward and across the surviving upright section, shearing off much of its stepped downstream face. This section had broken into three or four giant blocks. But the remainder of the dam, including its left and right abutments, were simply gone - carried as much as a mile down the canyon by the floodwaters. The rocky foundations of the abutments had also been scoured away to a depth of 20ft or more. The left wall of the canyon had fallen away in a giant landslide, carrying a stretch of the roadway with it. Evidently the material in this slide had mixed in with the floodwaters, turning them into a slurry that was dense enough to freight the huge blocks of concrete far down the canyon.

Surveying the scene, Mulholland immediately suspected sabotage. After all, the aqueduct itself had been the object of dynamite attacks just a few months earlier, so why not the dam, too? Mulholland mentioned or hinted at sabotage as the cause on several occasions, such as at the coroner’s inquest on the victims of the disaster. Some experts presented what purported to be evidence of sabotage, such as the presence of dead fish that were supposedly stunned by an explosion.

Still, the sabotage theory never really took hold, and with good reason. It would have been an enormous undertaking to bring down the dam, far beyond what could have been accomplished unnoticed by a small team of saboteurs. Furthermore, the seismological station at Caltech would have recorded the tremors induced by the blast, but inspection of the records showed nothing unusual at the time of the failure. To his credit, Mulholland did not use the sabotage theory to weasel out of responsibility for what had happened. He told the investigators, ‘If there is an error of human judgment, I am the human,’ and he was so laden by guilt that he professed an envy for those who had died.

There were, of course, investigations into the cause of the dam’s failure. The most high-level of these was a commission of inquiry set up by the governor of California. The commissioners consisted of six engineers and geologists, including professors from Caltech and the University of California, Berkeley. Perhaps responding to the pressure of a public outcry, the commissioners carried out their task with extraordinary speed. They met for the first time on March 19, made a single visit to the dam site, and issued their final report on March 24, just 12 days after the disaster.

During their visit to the dam site, the commissioners noted the unusual characteristics of the Sespe conglomerate near the San Francisquito Fault. ‘When entirely dry it is hard and rock-like in appearance,’ they wrote, but in reality it was ‘held together merely by films of clay,’ and when placed in water it ‘quickly softened and turned into a mushy or granular mass.’ The commissioners also had compression tests done, which revealed that, even when dry, the rock of the Sespe conglomerate in the dam’s right foundation was far weaker than the concrete of the dam itself.

It so happened that the remaining intact segment of the dam carried a gauge whose function had been to measure the depth of water in the reservoir. The depth was continuously recorded on a drum that was rotated by a clockwork mechanism. When the commissioners inspected the drum, they found that reservoir level appeared to have begun falling at about 11.30pm on the evening of the disaster and had fallen by about four inches by the time of the collapse. This suggested that there was an initial small breach in the dam or its foundations that had gradually widened itself until the overlying section of the dam lost its support and suddenly collapsed into the breach.

According to the commissioners’ final report, the disaster was not caused by any shortcomings in the design of the dam itself, but by defects in its foundations - most particularly, by the weakness of the Sespe conglomerate, especially when wet. Because some of the segments from the right (western) side of the dam had been swept a long way downstream, the commissioners concluded that the initial break had occurred on that side, probably at or near the location where the San Francisquito Fault passed under the dam. Reservoir water had eaten away at the fault gouge, or the weak conglomerate near it, until the dam’s right abutment had collapsed. The torrent of water, curving around the centre of the dam in a giant eddy, had eroded the foundations of the dam’s left abutment until it, too, collapsed, which in turn provoked the landslide on the canyon’s western slope.

Given the nature of the rock of the Sespe Formation, the commissioners concluded that failure of the dam was inevitable, unless water could have been prevented from reaching the dam’s foundation. They mentioned several steps that could have been taken to slow down the entry of water. These included the construction of a deep cut-off wall (a concrete wall built deep into the foundations at the upstream face of the dam), pressure grouting of the foundations (to fill spaces that would have permitted the entry of water), the drilling of drainage wells to remove water that did enter the foundations, and the construction of inspection galleries within the dam that would have allowed the state of its foundations to be monitored. But these steps, they believed, would only have postponed the dam’s ultimate collapse.

In spite of Mulholland’s suspicions about sabotage, he evidently did accept the notion that inherent problems with the dam or its foundations were the more likely cause of the disaster. That raised an urgent problem, because the near-twin of the St. Francis Dam was holding back the now-full Hollywood Reservoir. And the reservoir lay not in some undeveloped canyon 40 miles from Los Angeles but immediately above residential neighbourhoods within the city itself. To ward off a repeat of the disaster, Mulholland ordered a lowering of the reservoir.

The theory put forward by the commission - that the weakness of the Sespe conglomerate and its poor response to wetting were the prime reasons for the dam’s failure - was widely accepted. It was used, for example, at the coroner’s inquest on the victims, as part of an (unsuccessful) attempt to bring criminal charges against Mulholland. Even today, there are experts who agree pretty much word for word with the commission’s report. Geologist Jack Green of California State University at Long Beach, for example, has a web page devoted to the disaster in which he states that the dam broke because water eroded through the dam’s foundations in the neighbourhood of the fault, just as the commission concluded.

Even in 1928, however, there were experts who voiced disagreement with the commission’s theory of the disaster. Carl Grunsky was a well-respected consulting engineer based in San Francisco who had been retained by local ranchers during the construction of the dam, and who therefore had good opportunity to study it. Two months after the disaster Grunsky, along with his son and a Stanford geologist, presented an account of what had happened that differed markedly from that of the official commission. They argued that the dam broke because it was subjected to a kind of pincer action. When the dam was filled, the Sespe conglomerate began to swell as water invaded it, so the right abutment attempted to push the dam leftward. Meanwhile, the foundations of the left abutment responded to the pressure of the dam by starting to slide. This incipient landslide was slight enough to be unnoticeable, but it pushed the dam rightward. These forces, building gradually over the months prior to the failure, caused cracks to develop that partially separated the centre of the dam from its two abutments. These cracks had been seen, but they were attributed to irregular contraction of the concrete during drying, and they did not cause any concern.

The drop in the level of the reservoir during the 30 to 40 minutes prior to the dam’s collapse, as recorded by the gauge, was an illusion, according to Grunsky. He calculated that for the drop to be real, such an enormous amount of water would have been flowing past the lower powerhouse that it could not have escaped notice, and the workers and their families would have had plenty of time to save themselves. In fact, the operator of the upper powerhouse, a man named Henry Silvey, had spoken by telephone with the duty officer at the lower powerhouse just 10 minutes before the dam collapsed, and he did not report anything amiss.

What, then, had caused the appearance of a drop in water level? Grunsky found what he considered a decisive clue: while inspecting the remaining standing block of the dam, he noticed a ladder that had become trapped and crushed within a horizontal crack in its up-stream face. The crack must have opened by several inches to allow the ladder to enter; in other words, the dam must have tilted upward and forward. According to Grunsky, it was this upward tilting of the dam, caused by the pincer action described above, that had raised the gauge slightly out of the water, producing an illusory record of a loss of water prior to the collapse.

A further clue relating to the possibility of an incipient landslide was reported by Charles Outland, the historian mentioned earlier. Thirty-four years after the dam collapsed, a member of the staff of the upper powerhouse (probably Henry Silvey, but he requested anonymity) told Outland that he had driven up the canyon on the evening of the disaster. As he passed the left abutment of the dam, he was disturbed to notice that the roadway had sagged downward by up to 12 inches, producing a scarp of that height that he had to negotiate with great care. Given that the road was cut into bedrock just above the dam’s left abutment, the presence of the scarp suggested that a large block of the canyon slope under the dam’s left abutment had begun a downward slide well before there was any other sign of the dam’s failure.

Around 1990, a new analysis of the disaster was undertaken by J David Rogers, then an engineering geologist in private practice in the Bay Area and Los Angeles. (Rogers is now an associate professor at the University of Missouri.) First, Rogers pointed out that the east wall of the canyon, at the point where the dam was built, had been the site of an enormous natural landslide in prehistoric times, in which a giant sector of the mica schist had slid and tilted down, ending up against the Sespe conglomerate and completely blocking the canyon. It was this episode that created the ancient lake that once filled the upper canyon. Then, at some later time, the rising waters breached this natural dam: the lake emptied, leaving the broad, sediment-filled valley that struck Mulholland as so suitable for a reservoir. Thus, the dam’s left abutment was built on rock that was inherently unstable and liable to experience another slide, especially when subjected to the pressure of the dam and the lubricating effect of water that entered the schist. Rogers agreed with Grunsky that an incipient slide in the rocky foundations of the left abutment was a key causative factor in the dam’s collapse.

Another factor on which Rogers placed a great deal of emphasis was what is called ‘hydrostatic uplift’. This has to do with that staple of junior school physics, Archimedes’ principle. The principle can be stated thus: a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid displaced. This is why bodies that are less dense than water float at the water’s surface. But even bodies that are denser than water, such as those made of concrete, will experience a reduction in their apparent weight when they are immersed in water.

As long as a gravity dam rests on dry foundations, the full weight of the dam thrusts downward. If reservoir water enters the dam or its foundations, however, the dam is effectively immersed in the water. The dam then experiences a buoyant force equal to the hydrostatic pressure of the reservoir’s water column, and this upward force nullifies a significant portion of the dam’s weight. According to Rogers’s calculations, if the St. Francis Dam experienced full hydrostatic uplift, its apparent weight would have decreased by about 45 per cent. As a result, the thrust from the combination of the dam’s weight and the reservoir’s horizontal pressure would no longer have been directed into the bedrock within the middle third of the dam’s front-to-back extent, as called for in the dam’s design. Rather, it would be shifted 240ft downstream - well beyond the toe of the dam.

The theory of hydrostatic uplift, as applied to dam construction, was poorly understood in the 1920s - the textbooks on which Mulholland’s design engineers relied barely mentioned the topic. That is probably why Mulholland did not take effective precautions to prevent reservoir water from entering the dam’s foundations: he was concerned about such percolation only insofar as it might erode the foundations, not as a factor tending to lift the dam.

Mulholland did install a few relief wells in the dam to drain off any water that seeped under it. These wells were placed only under the relatively narrow central section of the dam, however - the portion that rested on the canyon floor. There were no relief wells in the dam’s abutments, which together formed the bulk of the dam’s width. Yet abutments are readily affected by uplift, because as they climb the sloping sides of the canyon, their height and weight decrease so that the stabilising downward thrust due to gravity becomes less.

Rogers found a photograph of the dam, taken some months before the filling of the reservoir was complete, in which the line of contact between the left abutment and the Pelona Schist, on the downstream face of the dam, was clearly wet. This observation, combined with the leakage of water in that area noted by Mulholland on the morning before the failure, is evidence that reservoir water had not merely entered the mica schist but had percolated all the way through to the downstream face of the left abutment. This water was not only facilitating slippage of the ancient landslide but also was exerting a gradually increasing uplift on the abutment. This tended to separate the abutment from the more stable central section of the dam.

As the mica schist crept progressively downward during the days and hours before the collapse, the incipient landslide pressed against the dam. Because of the dam’s curvature, the abutments were oriented quite obliquely to the canyon walls. The pressure of the slide was therefore exerted primarily on the upstream face of the left abutment, putting the entire upstream face of the dam into tension. A short while before the final collapse, in Rogers’s interpretation, a fairly small block of concrete near the base of the left abutment fell out of the dam. (This block was later found farther downstream than any other block, suggesting that it was the first to yield.) Water poured through this orifice: it undercut the mica schist, and it also entered transverse cracks in the dam, causing the central portion of the dam to experience full hydrostatic uplift and therefore to tilt upward by a few inches. This was what caused the water gauge to record an apparent fall in the reservoir level.

Then, at 11.57:30pm - the time defined by the power outage - the schist was undercut to the point that a giant landslide occurred, involving about 900,000 tons of rock. The landslide destroyed the remaining part of the left abutment, but the material in the slide plugged the gap in the dam for a short time. Then water tore through the slide and the catastrophic emptying of the reservoir began.

Rogers pointed to several observations that supported the idea of a landslide-induced breach in the left abutment as being the first event in the dam’s collapse, rather than a failure of the Sespe conglomerate under the rightabutment as the governor’s commission had concluded. For one thing, after the disaster a line of debris was found on the western shore of the reservoir near the dam, and this debris extended for several feet above what had been the reservoir’s high-water level. Rogers concluded that this line of debris was thrown up by a large wave - a tsunami, essentially - generated by the landslide. For the debris to have been left above the high-water level, the reservoir must have been full, or very nearly so, at the time the landslide occurred. Furthermore, although both abutments eventually failed, the rocky foundations under the right abutment were scoured away much less than under the left foundations, even though the Sespe conglomerate was relatively weak. This indicated that the reservoir level was already relatively low when the right abutment breached. As a further sign that the right abutment failed late in the proceedings, the scour level along the west side of the canyon just downstream of the dam was much lower than it should have been if water from a full reservoir had been pouring through the right abutment. A final clue was offered by a 30ft-long pipe, which was attached to the underside of the water gauge and which ran down the upstream face of the dam. After the collapse, the pipe was visibly bent in the direction of the left abutment. This happened, according to Rogers, because while the reservoir level was nearly full, water was rushing toward the breach in the left abutment caused by the landslide.

The complete order of events, as visualised by Rogers, was roughly as follows. Reservoir water entered the foundations of the left abutment, causing hydrostatic uplift of the abutment and promoting an incipient landslide. Failure of a segment of the dam near the base of the abutment opened an orifice that allowed high-pressure water to extend the hydrostatic uplift to the central section of the dam and to trigger a massive landslide that collapsed the entire abutment. As the landslide material was washed away, a scouring of the dam’s foundations caused the leftmost part of the central sector of the dam to collapse. The remaining portion of the centre of the dam did not fall, but it tilted and (as was determined by triangulation after the disaster) moved slightly to the east. This caused a separation from the right abutment, which therefore lost its stability and collapsed, allowing water to pour through on that side too. Rogers calculated that the maximum rate of flow past the collapsed dam and down the canyon was about 1.7 million cubic feet per second, which is nearly three times the average flow of the Mississippi River at New Orleans. The reservoir emptied in less than an hour.

The Los Angeles Department of Water and Power soon replaced the broken dam with a new, earthen dam in a nearby canyon, and with time the St. Francis Dam and its tragic demise faded from memory. Still, the disaster had wide-ranging consequences for dam-building elsewhere. For a start, several committees looked into the question of what to do with the Mulholland Dam. Eventually, it was decided that the dam could be operated at a lower reservoir level, but as a precaution against failure 300,000 cubic yards of earth fill were placed against the dam’s downstream face, completely burying its elegantly stepped facade. The construction of a much higher dam in San Gabriel Canyon, east of Los Angeles, was halted when a Berkeley geologist discovered that the western wall of the canyon, much like San Francisquito Canyon, was the site of an ancient landslide.

The disaster also caused great concern for the designers of the Hoover (or Boulder) Dam on the Colorado River, then in the planning stage. Politicians opposed to the dam, such as the governor of Arizona, used the disaster in their campaign to prevent the dam’s construction. Although the dam, when finally built, incorporated some design changes that took account of the failure of the St. Francis Dam, it nevertheless experienced some percolation of reservoir water into its foundations, and gradually increasing hydrostatic uplift pressures were measured. Finally, in the 1950s, a programme of pressure grouting of the dam’s foundations reduced seepage to an acceptable level.

A tragic repeat of the St. Francis disaster occurred in 1959. The Malpasset Dam, near Fréjus in the south of France, collapsed when the reservoir was filled for the first time, killing between 400 and 500 people. As with the St. Francis Dam, it appears that the collapse was caused by hydrostatic uplift of the dam’s left abutment, according to an analysis by Electricité de France.

Today, the problem of hydrostatic uplift is well understood, and extensive steps are taken during a dam’s design and construction to prevent seepage of water under a dam, to drain whatever water does penetrate, and to monitor uplift pressures. Still, other modes of failure are possible. If water enters a dam’s reservoir faster than the sluicegates or spillway can discharge it, for example, the reservoir will overflow the dam and likely destroy it. This occurred in China’s Henan Province in August 1985. Storms that had been spun off by a typhoon dropped 40 inches of rain on the area within the span of three days. A total of 62 different dams on two rivers overflowed and collapsed in a chain-reaction that cost the lives of an estimated 85,000 people.

After the failure of the St. Francis Dam and the subsequent inquiries, William Mulholland resigned his position as chief engineer and general manager of the Department of Water and Power. Already in his 70s, and beset by a neurological condition that may have been Parkinson’s disease, Mulholland lived the remaining seven years of his life out of the public eye. He is often described as a ‘broken man’ in his final years. Considering the torrent of verbal abuse that he experienced after the disaster, it would not be surprising if his spirit had been broken, yet it was not, according to a memoir penned by his granddaughter, Catherine Mulholland. Catherine describes her conversations with William Mulholland’s nephew, also named William, who worked with him and knew him intimately as a family member. ‘He was not broken by that mishap,’ the nephew told Catherine, ‘because he never accepted the responsibility of something that was beyond his power.’