On the morning of 5 June 1976, the Teton Dam — a brand-new 305-foot (93-metre) earthfill embankment built by the U.S. Bureau of Reclamation on the Teton River in eastern Idaho — eroded itself open from the inside and released roughly 250,000 acre-feet of water onto the towns of Sugar City, Rexburg and Wilford below. Eleven people died, some 25,000 were left homeless, around 16,000 head of livestock drowned, and property losses ran from hundreds of millions of dollars into the low billions. The reservoir had never been full before; the dam was failing the first time it was asked to hold its design load. The cause was not overtopping and not an earthquake. It was internal erosion — piping — driven by water that fractured the dam’s silt core and tunnelled through open joints in the rock the core was keyed into.
The Teton was a conventional zoned embankment with an impervious central core of wind-blown silt — loess-derived material that compacts well and seals beautifully, but that erodes readily once water moves through it and holds an open pipe without collapsing. That core was trenched down into the canyon’s foundation rock: a fractured, highly permeable volcanic rhyolite riddled with open joints. The job of sealing those joints fell to a grout curtain and to slush grouting along the key trench. That sealing was incomplete. The most heavily loaded interface on the site — the contact between an erodible silt core and a jointed rock that could carry concentrated seepage — was left as a path waiting for water.
The reservoir filled fast. Impounding effectively began in October 1975 behind the still-finishing dam, and through the spring of 1976 the level rose at roughly a foot a day, accelerating to about four feet per day by June as snowmelt poured in. Springs and small seeps appeared in the right abutment in the first days of June. On the morning of 5 June a clear leak turned muddy, then grew. By mid-morning a wet spot on the downstream face was discharging twenty to thirty cubic feet per second and a whirlpool was visible upstream. Crews drove bulldozers into the widening hole in a last attempt to plug it; the machines were swallowed. At about 11:57 the crest gave way and the reservoir emptied through the breach in a matter of hours.
Two federal inquiries — the Independent Panel impanelled by the Secretary of the Interior and the Governor of Idaho, and the Interior Department’s own Teton Dam Failure Review Group — reached the same family of conclusions. The dam failed by internal erosion of the silt core, most probably initiated by hydraulic fracturing of the key-trench fill and by seepage through unsealed joints in the rhyolite beneath the grout cap. The Teton became the United States’ canonical first-filling failure, the case that forced filters, controlled filling and independent review into the heart of American dam safety, and the disaster that ended Reclamation’s era of unquestioned authority over its own designs.
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In the early hours of 8 August 1975, the Banqiao Dam — a clay-core earthfill embankment about 24 metres high on the Ru River in Zhumadian Prefecture, Henan Province, China — was overtopped and washed away after Typhoon Nina stalled over the catchment and dropped more than a year’s rainfall in a single day. The official Chinese count of those killed directly by the flood wave was around 26,000; estimates that add the subsequent epidemics and famine across the inundated plain range up to roughly 145,000 more, for a total commonly cited between 170,000 and 230,000. The structure did not fail because the embankment was poorly built. It failed because its spillway and sluice gates could not pass the flood, and the water simply rose over the crest and cut the dam to pieces.
Banqiao had been the showpiece of a flood-control system thrown up across the Huai River basin in the early 1950s with Soviet engineering assistance. After cracking and repairs in 1955–56 it was reinforced and nicknamed the “Iron Dam” — a name that came to stand for misplaced confidence. The dam’s discharge works comprised five sluice gates and an undersized secondary spillway, together rated to pass a flood far smaller than the one that arrived. The engineer Chen Xing had argued during planning for twelve outlet gates; his recommendation was judged excessively conservative and cut to five. The single decision that governed the disaster was made on paper, years before the rain fell.
The rain, when it came, was without precedent. Typhoon Nina collided with a cold front and parked over southern Henan from 5 to 7 August 1975. More than 1,000 millimetres of rain fell in twenty-four hours near the storm centre — more than the region’s entire annual average — and three-day totals exceeded 1,600 millimetres in places. The dam had been designed for a “thousand-year” flood of roughly 300 millimetres per day. Nina delivered something closer to a two-thousand-year event, more than twice the design level. With the gates and spillway swamped and partly blocked by sediment, the reservoir crested above the dam at about 117.94 metres above sea level and overtopped. Around 01:00 the embankment breached, and some 600 million cubic metres of stored water emptied in roughly six hours.
What made Banqiao the deadliest dam disaster in recorded history was not the single breach but the cascade. The same storm overwhelmed dozens of other reservoirs in the same basin, including the Shimantan Dam on the Hong River, the second-largest in the system, which failed about half an hour before Banqiao. In total some sixty-two dams collapsed, releasing a combined flood across roughly 12,000 square kilometres of densely populated plain inhabited by more than ten million people. The investigation, conducted internally and kept secret for thirty years until the files were declassified in 2005, found what the engineering already showed: the dams were hydrologically under-designed, their discharge capacity grossly inadequate, and the basin had been packed with reservoirs whose failures fed one another.
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At about six o’clock on the morning of 9 February 1971, the magnitude-6.6 San Fernando earthquake shook the Lower San Fernando Dam — a 142-foot (43-metre) earth embankment built by the hydraulic-fill method between 1912 and 1916 at the head of the San Fernando Valley in Los Angeles — and the upstream face of the dam slid bodily into the reservoir. The loose sand at the core of the embankment liquefied, lost almost all of its strength, and flowed. The crest dropped, the upstream shell spread some 250 feet beyond its original toe, and when the movement stopped only about five feet of soil — roughly a metre and a half — separated a full reservoir from the breach. No one died at the dam. Below it lay a residential district of some 80,000 people, and the cause was the seismic liquefaction of the dam’s own fill.
The dam did not fail because the earthquake pushed it over. It failed because the shaking destroyed the strength of the soil holding it up. For about twelve seconds of strong motion, cyclic stress drove pore-water pressure in the saturated hydraulic-fill sand of the upstream shell until the effective stress between grains approached zero and the material behaved as a heavy liquid. Then — and this is the detail that made the case famous — the dam did not move during the shaking. The major slide occurred an estimated twenty to thirty seconds after the ground stopped moving, when the now-liquefied mass could no longer carry the dead weight of the embankment above it and the whole upstream slope ran out under static gravity alone.
Authorities did not know how close they had come until daylight. The Los Angeles Department of Water and Power began an emergency drawdown of the reservoir within hours and the police evacuated roughly 80,000 residents from the valley below while the lake was lowered over three days. The Lower dam held by a margin measured in feet. Had the reservoir stood a little higher, or the slide run a little farther, the case would read like Malpasset or Vaiont. Instead it became the most studied near-miss in geotechnical history.
The investigation, led by H. Bolton Seed and his colleagues at the University of California, Berkeley, and later re-examined by the US Army Corps of Engineers Waterways Experiment Station, established the modern understanding of how earthquakes destroy dams: not by inertia, but by liquefaction, and not necessarily during the shaking, but in the seconds after it. The finding ended the use of hydraulic fill for embankment dams in seismic regions and rebuilt the way every earth dam in earthquake country is analysed. The Lower San Fernando Dam is the canonical case of seismic soil liquefaction.
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