Flash flood hits Rapid City           

A flash flood in Rapid City, South Dakota, kills more than 200 people on this day in 1972. This flood demonstrated the danger of building homes and businesses in a floodplain region.. The settlers built the town of Rapid City well south of the floodplain and for 75 years there were few flooding problems for the residents.

In 1952, the Pactola Dam was built 10 miles from the city. The new dam controlled the floods, setting off a boom in development of the floodplain area. Eventually, the Rapid City area became home to 50,000 people.

In the spring of 1972, torrential rains battered the Black Hills. Warm moist air from the Gulf of Mexico collided with a Canadian cold front, causing 15 inches of rain to come down in only six hours. The spillway for the Pactola Dam got clogged with debris during the storm, leading to the total collapse of the dam and a devastating wave of water crushed most of the nearby buildings and swept away 238 people.

Residents, most of who were not insured for flood damage, suffered $160 million in damages. In the wake of this tragedy, it was decided that the floodplain should no longer be used as a residential area. It is now a golf course and a park with several ponds (FEMA, 1987).

Four types of floods can occur in South Dakota. The first type is commonly called a flash flood. A flash flood is the result of several inches or more of rain falling in a very short period of time, often tens of minutes (Ward, 1978).

The second type of flooding is sometimes termed the long-rain flood, and is the most common cause of major flooding (Ward, 1978). This type of flood results after several days or even weeks of fairly low-intensity rainfall over a widespread area, often hundreds of square miles. As a result, the ground becomes "water logged," and the water can no longer infiltrate into the ground;

The third type of flood in South Dakota is the result of melting snow in the spring. This type has characteristics that are almost a combination of the flash flood and long-rain flood. The area covered by this type of flood is generally not as large as that covered by the long-rain flood, but is typically larger than that covered by the flash flood. Some of the largest floods that have occurred in South Dakota were the result of melting snow and ice. These large floods have occurred along the entire length of the Missouri River.

The fourth type of flood results from the failure of dams. The four largest dams in South Dakota -- Oahe at Pierre, Big Bend at Fort Thompson, Fort Randall at Pickstown, and Gavins Point at Yankton -- are all located on the Missouri River. Large dams in the Black Hills are the Deerfield, Pactola, Sheridan and Angostura dams. If any of these large dams were to fail, flood damage could be very great. –Shekita Maxwell, 1^st Year Graduate Student MPH

References

 

Federal Emergency Management Agency (FEMA), 1987, Proceedings: Flash Flood Mitigation Symposium, Rapid City, South Dakota, June 9-10, 1987, Denver.

Ward, Roy, 1978, Floods: A Geographical Perspective: Macmillan Press LTD, London