Black Hills flood

The Black Hills Flood of 1972, also known as the Rapid City Flood, was one of the most detrimental floods in the history of South Dakota. It took place on June 9–10, 1972 in the Black Hills of Western South Dakota. 15 inches (380 mm) of rainfall over six hours sent Rapid Creek and other waterways overflowing, flooding many residential and commercial properties in Rapid City. It also caused flooding of Battle, Spring, Bear Butte, and Boxelder Creeks.

During the night of June 9, Canyon Lake Dam became clogged with debris and failed, resulting in 238 deaths and 3,057 injuries. Several bodies were never found. Over 1,335 homes and 5,000 automobiles were destroyed. The value of the damaged property was over US $160 million in 1972 dollars.

A few days before the Rapid City Flood “earlier rains had left the soil saturated, increasing the amount of runoff of the flood to come.” On the afternoon of the 9th, substantial rains fell on the area, caused by “an almost stationary group of thunderstorms.”

Cloud seeding experiments being conducted by the Institute of Atmospheric Sciences on clouds west of Rapid City were speculated to have contributed to the unusual amount of rain. However, there is no evidence that the two phenomena were related.

Additionally, "a strong low-level easterly flow which forced the moist unstable air up-slope on the hills. This sustained orographic effect helped the air to rise, cool, and release its moisture in repeating thunderstorms. Another contributing factor was the unusually light winds at a higher atmospheric levels which did not disperse the moist air nor move the thunderstorms along to prevent an extreme concentration of rainfall."

According to Herbert Thompson the air pattern causing this storm was noticed in big measure over the Great Plains, with a minor scale to the east of the Rockies. There were only what appeared to be light winds, so nothing indicated that there was a huge storm underway. Thompson further indicates that a cold high pressure region was pushed from Canada into the Great Lakes region leading into the western part of South Dakota. A “mesoscale cloud mass” from Colorado also moved towards Rapid City. One of the cloud masses amplified the “mid-level moisture over Rapid City,” while the other mass caused the pressure to stay, as the high pressure pushed downward, thus creating the right conditions to produce rainfall. The rainfall from the upper “cloud mass” above the Black Hills formed into new smaller masses downwind which reprocessed the rain allowing for the constant rainfall. The storm was described by Nair, Hjelmfelt, and Pielke as “convective cells of high precipitation efficiency a characteristic of tropical precipitation systems. The immense precipitation was based over the “Rapid, Boxelder, Spring, and Battle Creeks,” creating run-off, along these creeks which led to flooding throughout the surrounding areas.

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Wikipedia