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1975 – Super Typhoon Niña (2)

By: Floro Mercene

China has been plagued with severe floods from time immemorial. In 1950, shortly after an episode of severe flooding in Henan Province, the government built two major dams, the Banqiao Dam and Shimantan Dam. There are numerous smaller dams built as well.

Soon after Banqiao Dam was completed in 1952, cracks appeared. They were then repaired, reinforced and expanded with the help of Soviet engineers. After renovations, Banqiao Dam was dubbed the “Iron Dam”, a dam that could not be broken.

On August 5, 1975, a storm arrived in the east-central province of Henan. Although typhoon Niña was downgraded from a typhoon to a tropical storm, the remnants of the storm produced widespread torrential rainfall as it stalled over this area for 3 days.

Approximately 1,060 millimeters of rain fell during those three days. The storm produced more rainfall in 24 hours than an average year’s worth of rain in the Henan Province, 800 millimeters. By August 8, the Banqiao and Shimantan Dams, which had been built to protect against heavy rainfall, were both filled over their capacity. The dam could not release the water as fast as its reservoir was filling.

Shortly after midnight of August 8, the Shimantan Dam collapsed. The reservoir emptied 120 million cubic meters of water within five hours pouring out onto the Henan Province. Shortly after 1 a.m. of August 9, the main part of the Banqiao Dam collapsed, sending a wall of water 6 meters high and 12 kilometers wide rushing down the river channel, followed by 600 million cubic meters of water, destroying 62 smaller dams in its path. The flood spread over more than a million hectares of farm land throughout the province.

The wall of water was traveling at about 50 km per hour or about 14 meters per second.