Types of Floods

Nearly every day of the year, it floods somewhere in the United States. Flooding causes more damage in the United States than any other weather related event, with an average of eight billion dollars per year and an average of 95 fatalities per year in the past 30 years. Flooding can occur in any of the 50 states anytime of the year, sometimes very quickly. Flood effects can be local, impacting a neighborhood or community, or very large, affecting entire river basins and multiple communities. The steep and varied terrain of the Black Hills, coupled with diverse soil conditions, allows for different types of high water scenarios. The following sections explain the different types of high water scenarios and when they are most likely to occur.

1972 Flood Image

White River near Highway 83

Flooding in Rapid City, June 11, 1909 (photo courtesy of the Rapid City Journal).

Flood damage in Rapid City on June 10, 1972
(photo courtesy of the Rapid City Journal).

White River south of Murdo near Highway 83
 on March 8, 2010

Flash Flood on August 17, 2007 in Hermosa SD

RAINFALL-INDUCED FLASH FLOODING

Most thunderstorms that affect the northeastern Wyoming and western South Dakota are of the high-based, “pulse” variety. These storms commonly produce brief, extremely heavy rains and small hail during their fifteen to thirty minute lifetimes. Such storms may cause an “inconvenience” to those under them, especially if they occur over an urbanized area.

The majority of our flash flood events are associated with persistent, stationary storms over the Black Hills. These storms typically occur during the spring and summer. Some flash floods occur well downstream from where the rain fell, and take a considerable amount of time (1-3 hours) to reach a location. Therefore, even if the weather is dry where you are located, you could still be susceptible to a flash flood. This is especially true if you are located near steep terrain. Flash floods can be accompanied by a dangerous wall of water that carries rocks, mud, and other debris sweeping away most things in its path.

The June 9-10, 1972 flash flood in Rapid City, the most devastating flood event in South Dakota history, dumped up to 15 inches of rain in less than six hours on the eastern slope of the Black Hills. Other significant flood-prone areas include Battle Creek, Bear Butte Creek, Beaver Creek, Box Elder Creek, Fall River, and Spring Creek.

LOW ELEVATION SNOWMELT RUNOFF FLOODING

A rapid warm-up within a snow covered basin can cause rivers to suddenly rise out of their banks. The presence of large amounts of low elevation snow presents the greatest threat for rapid snowmelt problems. In our area, the greatest snowfall with high moisture content occurs during the warmer months, which can often lead to this type of flooding

ICE JAM FLOODING

The formation of ice jams can lead to river flooding. Cold temperatures cause the development of ice layers as much as one to four feet thick on the rivers and streams. While this ice has great strength, it is also very brittle. If the water flowing beneath the ice can exert enough pressure to break this cover, huge chunks of ice (floes) move downstream until they are stopped by some constriction in the channel. These constrictions can be natural (a sand bar or a sharp bend in the channel), or man-made (a bridge). As the ice builds behind the barrier, it forms a dam which holds the water back until the pressure exerted on the ice dam either removes it, or it fails. Even though this ice jam is gone, a new jam can form downstream. The Belle Fourche, Cheyenne, White, Little Missouri, and Grand Rivers are prone to ice jam flooding. 

DAM BREAK FLOODING

Although this type of flooding is probably the least likely to occur, the loss of property and lives can easily be the highest of all flooding situations. Northeastern Wyoming and western South Dakota have hundreds of dams ranging in size from small stock ponds to large reservoirs. All of these have a potential for failure, and it is the responsibility of the National Weather Service to warn all persons in the affected area if this were to occur. 


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