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My document Flash Flooding and Debris Flows
My document National Flood Safety Awareness Week
March 17-21, 2014
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National Flood Safety Awareness Week continues today with the theme of Flood Hazards. Here we will discuss flash flooding and debris flows.

What are debris flows and what causes them?

Debris flows, sometimes referred to as mudslides, mudflows, lahars, or debris avalanches, are common types of fast-moving landslides. These flows generally occur during periods of intense rainfall or rapid snow melt. They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that are typically about 10 miles per hour, but can exceed 35 miles per hour. The consistency of debris flows ranges from watery mud to thick, rocky mud that can carry large items such as boulders, trees, and cars. Debris flows from many different sources can combine in channels, and their destructive power may be greatly increased. They continue flowing down hills and through channels, growing in volume with the addition of water, sand, mud, boulders, trees, and other materials. When the flows reach flatter ground, the debris spreads over a broad area, sometimes accumulating in thick deposits that can wreak havoc in developed areas.

Wildfires can also lead to destructive debris-flow activity. In areas burned by forest and brush fires, a lower threshold of precipitation may initiate debris flows. In the summer of 2000, a moderate wildfire, the Cave Gulch Fire, swept the area near Canyon Ferry Lake, northeast of Helena, denuding the slopes of vegetation. During the summer of 2001, a near stationary thunderstorm dropped heavy rain on the area resulting in a debris flow through Kim's Marina.

More information may be obtained from your National Weather Service offices in Montana...

Cave Gulch Debris Flow
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The Cave Gulch Fire in July 2000 burned 45,000 acres on Hedges Mountain near Canyon Ferry Lake in Montana. The fire was mainly of moderate intensity, but denuded enough land to allow debris and sediment to move during precipitation events. Following the fire, five flash floods occurred in Cave Gulch between May 2001 and August 2002. The severity and force of the flash floods caused major damage to businesses and homes in the area. Citizens and government agencies worked together to determine the best methods to convey and reduce floodwater across private, federal, and state properties in the Cave Gulch drainage area into Canyon Ferry Lake.

Two businesses and five homes near Canyon Ferry Lake were impacted by debris flows caused by flash floods. The floods also caused such erosion that the drainage, which usually only ran water during periods of snow melt or high rainfall events, was eroded down to the water table creating a perennial stream in certain areas. Conservation measures were applied to protect buildings and roads from future floods and to provide for a permanent streambed.

Laird Creek Debris Flow - July 2001

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