Banqiao Reservoir Dam Failure
The Dam was designed to survive a 1-in-1,000-year flood (300 mm of rainfall per day). In August of 1975, however, a 1-in-2,000 year flood occurred, and poured more than a year's rainfall in 24 hours (new records were set, at 189.5 mm rainfall per hour and 1060 mm per day, exceeding the average annual precipitation of about 800 mm), which weather forecasts failed to predict, produced by the collision of Super Typhoon Nina and a cold front. Communication to the dam was largely lost due to the collapse of buildings under heavy rain and wire failures. On August 6, a request to open the dam was rejected, because of the existing flood in downstream areas. On August 7, however, the request was accepted, but the telegrams failed to reach the dam.
The sluice gates were not able to handle the overflow of water, partially due to sedimentation blockage. On August 7 at 21:30, the People's Liberation Army Unit 34450, which was deployed on the Banqiao Dam, sent the first dam failure warning via telegraph. On August 8, 0:30, the smaller Shimantan Dam, which was designed to survive a 1-in-500-year flood, failed to handle more than twice its capacity and broke upstream, only 10 minutes after Unit 34450 sent a request that would open the Banqiao Dam by air strike. A half hour later, at 1:00, water crested at the Banqiao Dam and it too failed. This precipitated the failure of 62 dams in total. The runoff of Banqiao Dam was 13,000 m³ per second in vs. 78,800 m³ per second out, and 701 million tons of water was released in 6 hours, while 1.67 billion tons of water was released in 5.5 hours at upriver Shimantan Dam, and 15.738 billion tons of water was released in total.
The resulting flood waters caused a large wave, which was 10 kilometers (6.2 mi) wide and 3–7 meters (9.8–23 ft) high in Suiping (遂平), to rush downwards into the plains below at nearly 50 kilometers per hour (31 mph), almost wipe out an area 55 kilometers (34 mi) long and 15 kilometers (9.3 mi) wide, and create temporary lakes as large as 12,000 square kilometers (4,600 sq mi). Seven county seats, namely Suiping, Xiping (西平), Ru'nan (汝南), Pingyu (平舆), Xincai (新蔡), Luohe (漯河), Linquan (临泉), were inundated, as were thousands of square kilometers of countryside and countless communities. Evacuation orders had not been fully delivered because of weather conditions and poor communications. Telegraphs failed, signal flares fired by Unit 34450 were misunderstood, telephones were rare, and some messengers were caught by the flood. While only 827 out of 6,000 people died in the evacuated community of Shahedian just below Banqiao Dam, half of a total of 36,000 people died in the unevacuated Wencheng commune of Suipin County next to Shahedian, and the Daowencheng Commune was wiped from the map, killing all 9,600 citizens. Although a large number of people were reported lost at first, many of them returned home later. Tens of thousands of them were carried by the water to downriver provinces and many others fled from their homes. It has been reported that around 90,000 - 230,000 people were killed as a result of the dam breaking.
At the beginning of August in 1975, an unusual weather pattern led to a typhoon (Pacific hurricane) passing through Fujian Province on the coast of South China continuing north to Henan Province, (the name means "South of the (Yellow) River.") The rain storm that occurred when the warm, humid air of the typhoon met the cooler air of the north. This led to a set of storms which dropped a meter of water in three days. The first storm, on August 5 dropped 0.448 meters. This alone was 40 percent greater than the previous record. But this record-busting storm was followed by a second downpour on August 6 that lasted 16 hours. On August 7 the third downpour lasted 13 hours. The Banqiao and Shimantan Dams were only designed handle a maximum of about 0.5 meters over a three day period.
By August 8 the Banqiao and Shimantan Dam reservoirs had filled to capacity because the runoff so far exceeded the rate at which water could be expelled through their sluice gates. Shortly after midnight (12:30 AM) the water in the Shimantan Dam reservoir on the Hong River rose 40 centimeters above the crest of the dam and the dam collapsed. The reservoir emptied its 120 million cubic meters of water within five hours.
Around the same time, a think line of people stood strung out across the top if the Banquiao dam, toiling waist-deep in water trying to repair the rapidly disintegrating embankment. Suddenly there was a flash of lightning followed by deafening thunder. Silence followed, and, for a brief instant, the skies cleared and the stars appeared again overhead. Someone shouted, 'The water level is going down! The flood is retreating!' But just a few seconds later, the dam gave way, and 600 million cubic meters of water erupted with a demonic and terrifying force.