Banqiao Dam — the Spillway That Was Too Small, and a Cascade That Killed Thousands

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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