The Rapid City Flood of 1972
Weather
In a 6-hour time frame on June 9, 1972, a rush of water poured through Rapid City and canyons in the surrounding area, destroying homes, vehicles, businesses, bridges, and claiming 238 lives.
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| Surface map, June 9, 1972. |
At the surface, strong easterly winds pushed warm, moist air over the Black Hills, and temperatures climbed quickly into the 70's and 80's. Dew point temperatures (a measure of how much moisture is in the air) were in the middle to upper 60's, about 10 degrees higher than usual for early June.
The National Weather Service (NWS) forecast called for thunderstorm development over the Black Hills, with some storms becoming severe.
An early morning surface map shows a cold front and an area of high pressure (the blue "H") moved southward across North Dakota. The front separated the moisture laden Canadian air mass from a drier airmass over the Black Hills. The green arrows represent the strong easterly winds pushing the moist air against the Black Hills.
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| Idealized depiction of orographic lifting. |
By late morning, the atmosphere was unstable enough for growing storm clouds, what meteorologists call “towering cumulus” clouds, to develop over the hills near Rapid City. Towering cumulus clouds are precursors to thunderstorm development and formed through a process called "orographic lifting". Since air cannot move through mountains, it is lifted over them.
As the air rises, it cools and the water in the airmass condenses into clouds. The particles of water in the cloud eventually get so heavy, the column of rising air, known as an updraft, can no longer hold them aloft. The drag created by the sinking water droplets is called a downdraft. The updraft and downdraft are collectively referred to as a “cell”. When a cell develops, a thunderstorm is in the mature phase. This is when rain, thunder, lightning and hail are present. Eventually, when many downdrafts form, the storm is cut off from the warm, moist air and no new cloud droplets are formed. This is known as the dissipating stage of a thunderstorm.
During the early afternoon hours, convective cells built in northeastern Wyoming, about 60 miles northwest of Rapid City and by 3:00 p.m., a line of thunderstorms develops to the southeast of Rapid City, moving west northwest. The strong easterly winds and orographic lift kept the storms "anchored" to the Black Hills. The terrain of the Black Hills channeled the precipitation, concentrating the highest volumes of water in the canyons.
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| Precipitation totals (in inches) from sites 5 miles west of Rapid City, at Pactola Dam and 5 miles south-southwest of Sturgis. |
Rain began at Pactola Reservoir at 5:00 p.m., the same time another radiosonde was released at the National Weather Service office in Rapid City. Data from the radiosonde indicated heavy rain and thunderstorms would continue. By 6:00 p.m., an area of low pressure developed along the frontal boundary and a Canadian high-pressure system continued to slide into the eastern Dakotas. Winds increased to more than 40 miles an hour in some locations and nearly continuous line of thunderstorms covered the eastern slopes of the Black Hills.
Meteorologists at the National Weather Service office called State Radio for reports and were told of a foot of water over roads near Sturgis. Law enforcement reported high water in Boulder Canyon between Sturgis and Deadwood and State Radio requested commercial radio and television stations announce motorists should avoid Boulder Canyon. By 6:30 p.m., the Pennington County Civil Defense Director reported 3 inches of rain at Pactola Dam. The South Dakota School of Mines and Technology was operating a weather radar during a portion of the event. The radar operator contacted the NWS about strong thunderstorms in the Hermosa area and estimated 2 inches of rain per hour have fallen in Boulder Canyon. Reports of 4 inches of rain in 2 hours in Galena in Lawrence County.
Shortly after 7:00 p.m., the National Weather Service relayed precipitation reports and requested guidance from the River District Office in Sioux City, Iowa (the duties of River District Offices have since been transferred to National Weather Service Forecast Offices and River Forecast Centers). By 7:15 p.m., a flash flood warning was issued by the NWS for the northern Black Hills. At 7:45 p.m., the River District Office advised the NWS office in Rapid City to expand the warning southward to include Rapid City and a citizen in the Nemo area contacted the NWS about Box Elder Creek being in flood, and evacuations in low-lying areas of Nemo.
The expanded warning was issued by the NWS at 8:00 p.m. and the Pennington County Emergency Operations Center opened. About an hour later, commercial phone service, the National Weather Service's only link to law enforcement, media and other NWS offices, became intermittent and unreliable. Despite the breaks in communication, at 9:30 p.m., the NWS told radio and television stations the heavy rain will continue until midnight.
Rapid Creek overtopped its banks at 10:15 p.m.. A short time later, Mayor Don Barnett urged the evacuation of all low-lying residential areas. Canyon Lake Dam failed at 10:45 p.m., adding to the flood waters. However, Barnett said the failure of Canyon Lake Dam was like spilling a "cup of water in a bathtub". (Data from the gauge on Rapid Creek above Canyon Lake later reveals that from 9:00 p.m. until the dam failed the water level rose approximately 12 feet). Data evaluation after the storm indicated the heaviest precipitation fell between Pactola Reservoir and Rapid City. Recording precipitation gages indicated the highest rainfall rates occurred between 6:00 p.m. and midnight.
Shortly after midnight on June 10th, a flood crest estimated at 50,000 cubic feet per second reached downtown Rapid City. Within five hours, Rapid Creek was once again within its banks.
The picture above shows the precipitation that fell over the Black Hills during the flood. On the above image, the higher the precipitation amount, the darker the shade of blue. The storms dropped an average of over 6 inches of rain, with up to 15 inches of rain reported in some locations. The rain fell at rates of 2 to 6 inches an hour.
