Banqiao Dam

The Banqiao Reservoir Dam (simplified Chinese: 板桥水库大坝; traditional Chinese: 板橋水庫大壩; pinyin: Bǎnqiáo Shuǐkù Dàbà) is a dam on the River Ru in Zhumadian Prefecture, Henan province, China. It infamously failed in 1975, causing more casualties than any other dam failure in history, and was subsequently rebuilt.

The Banqiao dam and Shimantan Reservoir Dam (simplified Chinese: 石漫滩水库大坝; traditional Chinese: 石漫灘水庫大壩; pinyin: Shímàntān Shuǐkù Dàbà) 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 dam failure killed an estimated 171,000 people; 11 million people lost their homes. It also caused the sudden loss of 18 GW of power , the equivalent of roughly 9 very large modern coal fired power stations or about 20 nuclear reactors, equalling about 1/3 the peak demand on the UK National Grid.

History

The Banqiao dam was begun in April 1951 on the Ru River with the help of Soviet consultants as part of a project to control flooding and to generate electricity. It was a response to severe flooding in the Huai River Basin in 1949 and 1950. The dam was completed on June 1952. Because of the absence of hydrology data, the design standard was lower than the standard. After the 1954 Huai River great flood, the upstream reservoirs including Banqiao were extended, constructed and consolidated. Banqiao Dam was increased in height by three meters. The dam crest level was 116.34 meters above sea level and the crest level of the wave protection wall was 117.64 meter above sea level. The total capacity of reservoir was 492 million m³ (398,000 acre feet), with 375 million m³ (304,000 acre feet) reserved for flood storage. The dam was made of clay and was 24.5 metres high. The maximum discharge of the reservoir was 1742 m³/s.

Cracks in the dam and sluice gates appeared after completion due to construction and engineering errors. They were repaired with the 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 five. Chen Xing was criticized as being too conservative. Other dams in the project, including the Shimantan Dam, had a similar reduction of safety features and Chen was removed from the project. In 1961, after problems with the water system surfaced, he was brought back to help. Chen continued to be an outspoken critic of the system and was again removed from the project.

1975 Flood

The Dam was designed to survive a once-in-1000-years flood (300 mm of rainfall per day). In August 1975, however, a once-in-2000-years 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,China Central Television reports that the typhoon disappeared from radar . According to Xinhua, the forecast was a rainfall of 100 mm by the Beijing-based Central Meteorological Observatory. 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 (namely the 2nd Artillery Division in residence at Queshan county), 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 at the Banqiao crested at the 117.94 m level above sea level, which was 0.3 meter higher than the wave protection wall on the 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 were released in 6 hours, while 1.67 billion tons of water were released in 5.5 hours at upriver Shimantan Dam, and 15.738 billion tons of water were 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 wiping out an area 55 kilometers (34 mi) long and 15 kilometers (9.3 mi) wide, and creating 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.

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 Nihewa and Laowangpo flood diversion areas downstream of the dams soon ran out of their capacity and gave up part of their storage on August 8, forcing more flood diversion area to begin to evacuate. The dikes on the Quan River collapsed in the evening of August 9, and the entire Linquan county in Fuyang, Anhui was inundated. As the Boshan Dam, with a capacity of 400 million m³, crested, and the water released from the failures of Banqiao and Shimantan was rushing downstream, air strikes were made against several other dams to protect the Suya Lake dam, which was already holding 1.2 billion m³ of water. Suya Lake only won a temporary reprieve, and both it and Boshan became targets as well. Finally, the Bantai Dam, which was holding 5.7 billion m³ of water, was bombed.

The Jingguang Railway, a major artery from Beijing to Guangzhou, was cut off for 18 days, as were other crucial communications lines. Although 42,618 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. Epidemics and famine devastated the trapped survivors. The damage of the Zhumadian area was estimated to be about RMB 3.5 billion (US$513 million) . The Zhumadian government appealed to the whole nation for help, and received more than RMB 300 million (US$44,000,000) in donations.

After the flood, a summit of National Flood Prevention and Reservoir Security at Zhengzhou, Henan was held by the Department of Water Conservancy and Electricity, and a nationwide reservoir security examination was performed after this meeting. Chen Xing was again brought back to the project and aided in clearing the river channels.

Casualties

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. Unofficial estimates of the number of people killed by the disaster have run as high as 230,000 people. The death toll of this disaster was declassified in 2005.

Reconstruction

Within eleven years after the dam failure, the lower reach of the River Ru, esp. Zhumadian City, experienced several more disastrous floods. After many feasibility studies, the new Banqiao Reservoir reconstruction was listed as a key national project of The Seventh Five-Year Plan of China. The project owner was Huai River Water Resources Commission. The construction contractor was Changjiang Gezhouba Engineering Bureau. By the end of 1986, the rebuilding project commenced. On June 5, 1993, the project was certified by the Chinese government.

The reconstructed Banqiao Reservoir controls a catchment area of 768 km². The maximum reserve capacity is 675 million m³, a capacity increase of 34% above the capacity of the failed dam. The effective storage is 256 million m³ and the corresponding normal high water level is 111.5 m above sea level. The flood control storage is 457 million m³. The dam is made of clay and is 3,720 m long, and 50.5 metres high. The dam crest level is 120 m above sea level. The maximum discharge of the reservoir is 15000 m³/s.

Legacy

After the disaster of the Banqiao dam failure, the Chinese government became very focused on surveillance, repair, and consolidation of reservoir dams. China has 87,000 reservoirs across the country; most of them were built in the 1950s-1970s using low construction standards. Most of these reservoirs are in serious disrepair, posing challenges to the prevention and control of flood-triggered geological disasters in areas with a population of 130 million or more. China's medium and small rivers are considered to be the Achilles' heel in the country's river control systems. According to statistics from the Ministry of Water Resources, China has invested RMB 64.9 billion (US$9.72 billion) since the 1998 Yangtze River floods in repairing and consolidating the country's 9197 degraded reservoirs, in which 2397 are large or medium sized, and 6800 are key small reservoirs. All of the above projects should be finished before the end of 2010. However, there are 5400 small (1) reservoirs and a great many even smaller (2) reservoirs in need of repair. The capacity of a small (1) reservoirs is defined to be greater than 1 million m³ but less than 10 million m³. The capacity of small (2) reservoirs is defined to be greater than 100,000 m³ and less than 1 million m³. Projects to repair and consolidate 5400 small (1) reservoirs will be completed before the end of 2012. Projects for the remaining smaller (2) reservoirs will be completed within 3–5 years.