Dangers of flash floods

We haven’t written much about flooding with this drought situation, but flash floods are among the most dangerous types of flooding because they can develop rapidly, sometimes within minutes or hours. They occur when water accumulates faster than the ground, streams, rivers, and drainage systems can absorb or carry it away. The most common cause is intense rainfall, particularly from thunderstorms that produce large amounts of rain over a short period of time.

Flash floods usually cause more fatalities in the United States in an average year than lightning, tornadoes, or hurricanes. They can also deposit large quantities of sediments on floodplains and destroy vegetation cover not adapted to frequent flood conditions, especially in drier areas.

Many may have heard about the flash flood disaster in central Nepal near the China border. Last week, Aug. 26, a catastrophic flood and debris flow struck central and northern Nepal. The disaster wasn’t due to heavy rainfall but began when part of a glacier and surrounding rock collapsed. The collapsing ice, water, and debris surged into the river system, creating a fast-moving flood that traveled about 60 miles downstream. The event generated seismic energy equivalent to approximately a magnitude 5.2 earthquake according to the U.S. Geologic Survey resulted in heavy damage and thousands of people missing.

Slow-moving or repeated thunderstorms can be especially dangerous. When storms repeatedly pass over the same area, called “training storms,” rainfall can quickly overwhelm small streams and drainage systems. Mountainous and hilly areas are also vulnerable because water flows rapidly downhill and collects in valleys and low-lying locations.

One of the deadliest flood disasters in United States history occurred in 1972 when intense rainfall from the remnants of Hurricane Agnes caused devastating flooding in Rapid City, S.D. During the night of June 9, thunderstorms produced exceptionally heavy rain over the Black Hills. Rapidly rising water overwhelmed Rapid Creek that destroyed homes, bridges, and businesses.

 The Johnstown Flood of 1889 was one of the most famous flooding events. After days of heavy rain in Pennsylvania, the South Fork Dam failed, releasing an enormous volume of water toward Johnstown. The resulting flood destroyed much of the city and killed more than 2,200 people.

Other causes include dam or levee failures, sudden releases from ice or debris jams, and heavy runoff from recently burned landscapes. This type of flash flood occurred in Idaho. The Teton Dam disaster of June 5, 1976, was one of the worst and most destructive sudden flooding events in the state’s history. Although it was caused by the catastrophic failure of a dam, the resulting flood behaved much like an enormous flash flood, racing through southeastern Idaho with little time for communities downstream to prepare.

Despite the enormous destruction, warnings and evacuations helped save many lives. The Teton Dam disaster became an important lesson in dam engineering, emergency warnings, and public safety. According to the Bureau of Reclamation, it also led to major changes in dam safety practices in the United States.

With an expected very strong El Niño this winter season, Southern California and other regions in the southern U.S. may go from drought to floods as the sub-tropical jet stream gains strength.

An example of this potential was the Great Flood of 1938. This event is widely regarded as one of the worst flood disasters in Southern California history. From Feb. 27 through March 4, a series of powerful El Niño-driven storms produced extraordinary rainfall across the region. Downtown Los Angeles received more than 11 inches of rain, while parts of the San Gabriel Mountains received more than 30 inches.

During this event, saturated ground in the foothills and mountains could no longer absorb the intense rainfall, causing water to rush down narrow canyons and streams. In some small mountain watersheds, floodwaters reached their peak in less than an hour. The 1938 disaster was a major reason Southern California expanded its system of flood-control dams, debris basins, channels, and other protective infrastructure.

In terms of our local weather, August was a month with no measurable rainfall at Cliff Harris’s station and the Spokane International Airport. Other regional locations also didn’t receive any measurable rainfall. The first eight months of the year are now the driest in history. Occasional showers are expected over the next week to 10 days, but the overall weather pattern through the middle of September is still looking drier than normal.

In this pattern of wide weather “extremes,” this region has gone from very dry to very wet. A good example was in 2012. From July 16 through Oct. 12, only 0.67 inches of rain fell in Coeur d’Alene. However, the skies opened up and that year turned out to be the wettest in history with a total of 43.27 inches of moisture.

Eventually, moisture from the Pacific is expected to move into the region and bring some relief to the current drought situation, but not likely until at least late September or October. As I’ve mentioned earlier, drought patterns are difficult to break.

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Contact Randy Mann at [email protected].