The Banqiao Reservoir Dam and Shimantan Reservoir Dam are among 62 dams in Zhumadian Prefecture of China's Henan Province that failed catastrophically or were intentionally destroyed in 1975 during Typhoon Nina. The resulting floods killed at least 171,000 people, making this the worst energy-related disaster in history in terms of loss of life.

The Banqiao dam was built in the early 1950s on the Ru River as part of a project to control flooding and generate electricity and as a response to severe flooding in the Huai River Basin in 1949 and 1950. The dam was 118 meters high (387 ft) and had a storage capacity of 492 million m³ (398,000 acre feet), with 375 million m³ (304,000 acre feet) reserved for flood storage. Cracks in the dam and sluice gates appeared after completion due to construction and engineering errors. They were repaired with advice from Soviet engineers and the new design, dubbed the iron dam, was considered unbreakable.

The Shimantan dam was a Soviet-style hydroelectric facility intended to be part of a flood control and electrification scheme intended to reduce the incidence of severe flooding in the Huai River Basin, and to provide local villages with energy services.

In early August 1975, Typhoon Nina dumped almost 8 inches of rain into the basin in 24 hours, exceeding the yearly precipitation rate, collapsing buildings and destroying thousands of villages.  The sluice gates of the Banqiao dam were not able to handle the overflow of water, partially due to sedimentation blockage. On August 7 at 7:30 p.m., 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, 12:30 a.m., 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 a.m., 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 cubic meters (11 acre·ft) per second in vs. 78,800 cubic meters (63.9 acre·ft) per second out, and 701 million tons of water was released in 6 hours, while 1,670 million 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, 3–7 meters (9.8–23 ft) high to rush downwards into the plains below at nearly 50 kilometers per hour (31 mph), almost wiped out an area 55 kilometers (34 mi) long, 15 kilometers (9.3 mi) wide, and created temporary lakes as large as 12,000 square kilometers (4,600 sq mi).

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. 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.

To protect other dams from failure, several flood diversion areas were evacuated and inundated, and several dams deliberately destroyed by air strikes to release water in desired directions.

The Jingguang Railway, a major artery from Beijing to Guangzhou, was cut off for 18 days, as were other crucial communications lines. Although tens of thousands of People's Liberation Army troops were deployed for disaster relief, all communication to and from the cities was cut off. Nine days later there were still over a million people trapped by the waters, relying on airdrops of food and unreachable to disaster relief.

According to the Hydrology Department of Henan Province, in the province, approximately 26,000 people died from flooding and another 145,000 died during subsequent epidemics and famine. In addition, about 5,960,000 buildings collapsed, and 11 million residents were affected.

The death toll of this disaster was declassified in 2005.

Sources

• Watkins, Thayer, The Catastrophic Dam Failures in China in August 1975, Department of Economics, San Jose State University, Accessed 26 Auhust 2008.
• Wikipedia contributors, Banqiao Dam, Accessed 26 August 2008.