The Banqiao Reservoir Dam () and Shimantan Reservoir Dam (Chinese: 石漫滩水库大坝; pinyin: Shímàntān Shuǐkù Dàbà) are among 62 dams in Zhumadian Prefecture of China's Henan Province that failed catastrophically in 1975 during a freak typhoon. Approximately 26,000 people died from flooding and another 145,000 died during subsequent epidemics. In addition, about 5,960,000 buildings collapsed.
History
The Banqiao dam was built in the early 1950s on the Ru River as part of a project to control flooding. The dam was 118 meters high and had a storage capacity of 492 million cubic meters, with 375 million cubic meters 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.
Chen Xing was one of China's foremost hydrologists and was involved in the design of the dam. He was also a vocal critic of the government dam building policy, which involved many dams in the basin. He had recommended 12 sluice gates for the Banqiao Dam, but this was scaled back to 5 and Chen Xing was criticized as being too conservative. Other dams in the project, including the Shimantan Dam, had similar reduction of safety features and Xing was removed from the project. In 1961, after problems with the water system surfaced, he was brought back to help. Xing continued to be an outspoken critic of the system and was again removed from the project.
Banqiao flood
The Dam was designed to survive a 1-in-1,000-year flood (306 mm rainfall per day). However, in August of 1975 a 1-in-2,000 year flood occurred, as a result of a freak typhoon and several days of wild-range record-high storms (new records were set, at 160.4 mm rainfall per hour and 1,631 mm rainfall per day, while the average annual precipitation was about 800 mm). The sluice gates were not able to handle the overflow of water, partially due to sedimentation blockage. On August 8, 12:30 AM, the smaller Shimantan Dam, which was designed to survive a 1-in-500-year flood, broke upstream. A half hour later, at 1 AM, 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/s (in) / 78,800 cubic meters/s (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, 10 km wide, 3-7 meters high, to rush downwards into the plains below at nearly 50 kilometers per hour, and created temporary lakes as large as 12,000 square kilometers. Seven county seats (Suiping , Xiping , Runan , Pingyu , Xincai , Luohe , Linquan ) were inundated, as were thousands of square kilometers of countryside and countless communities; evacuation orders had not been fully delivered because of the weather condition, inadequate communication and some messengers being caught by the water. For example, only 827 out of 6,000 people died in the evacuated community of Shahedian just below Banqiao Dam, but half of a total of 36,000 people died in the unevacuated Wencheng commune of Suipin County next to Shahedian. 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.
The Jingguang Railway , a major artery from Beijing to Guangzhou, was cut off for 18 days, as were other crucial communications lines. Nine days later there were still over a million people trapped by water, inaccessible to disaster relief, while epidemics and famine devastated the trapped survivors. Chen Xing was again brought back to the project and aided in clearing the river channels.
Many of the dams have been rebuilt, including Banqiao in 1993.
See also
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