One of the basic rules of the Boy Scouts, advises campers to never pitch tents in areas that could be flooded. That’s why I have been puzzled as to why people buy houses built on flood plains on or near greenways below dams or other flood-prone areas. Worse, I’ve wondered why developers are sometimes given permission to construct subdivisions in such places.
A tragic flood swept over a large portion of Rapid City, South Dakota, in 1972. The disaster turned out to be one of the most devastating floods in US history. Flooding left a couple hundreds of fatalities and a few thousand injured people. When I read the news accounts, the Boy Scout lesson came to mind.
The soils in the Black Hills of South Dakota were already saturated because of steady rains that had fallen during the previous days. Then on the afternoon of June 9th, two large areas of thunderstorms mushroomed to life, setting the stage for still more heavy rains.
Heavy showers from one storm fell across the northeastern area of the Black Hills, then slowly spread southward and intensified as the storms approached the central Black Hills. The other storm system, centered over the southern Black Hills moved northward and also grew as it approached the central Black Hills. The two systems joined into a continuous line of thunderstorms that poured rain over the eastern Black Hills near Rapid City. East winds kept the line of activity trapped in place.
The large volume of water soon washed over the saturated ground and channeled through the steep canyons of the Black Hills. Small streams and drainages soon overflowed past their banks with rushing flash flood waters. All of the streams eventually emptied into the Rapid Creek, just above Rapid City. The creek had swollen to more than 300 times the normal volume. By late night time, the drastic rise in water levels, caused officials to issue the first flood warnings.
The highway 40 bridge over Rapid Creek was torn from its footings from a 20 feet high wall of water. Trailer houses were swept from their parking spaces and washed into Canyon Lake, the reservoir of the Pactola Dam. The trailer wreckage, trees, and other debris smashed into the dam embankments. By late evening the dam’s spillway had become clogged with flood debris preventing the use of its flood gates. The 15 inches of rain that had fallen within six hours had nowhere else to go.
Electrical power was lost shortly after the first warnings, most of the broadcasting stations went silent cutting off further public warnings. At nearly midnight, the flood waters topped the Pactola Dam, leading to its total collapse. A five-feet tall wall of flood water, carrying large chunks of concrete and pavement, pushed through the valley towards Rapid City. The rush quickly washed out more bridges, smashed homes and businesses, destroyed natural gas mains, and divided the city in half.
Shortly afterwards 1,800 South Dakota National Guardsmen who had coincidentally been undergoing their annual two-week training duty near the city, began rescue operations. They were soon helped by airmen from nearby Ellsworth Air Force Base. Rescue work was hampered because major bridges had been washed out and more than 80 blocks of street pavement was torn up.
The first daylight of June 10th revealed that fully one third of Rapid City was under water. Wreckage was all along the path of the flash flooding. Eventually, the statistics were gathered. 230 People were killed in Rapid City, 8 died in nearby Keystone. More than 3,000 people were seriously injured. 1,336 homes were lost. More than 5,000 motor vehicles were destroyed. 15 out of 23 bridges over Rapid Creek were wrecked. Monetary damage was tallied at $160,000,000 in 1972 dollars ($914,000,000 in 2015 dollars).
Following the clean up, it was decided the floodplain should not be used for residences. Instead, a six-mile floodway area was converted into nature areas, a golf course, ball fields, and a 4,000-feet long levee to protect one neighborhood area. The conversion was completed in 1979.