- Mudslides, mudflows, lahars, and debris avalanches are common types of fast-moving landslides, also known as debris flows.
- These debris flows generally occur during periods of intense rainfall or rapid snowmelt.
- On steep hillsides, debris flows begin as shallow landslides that liquefy and accelerate. A typical landslide travels at 10 miler per hour, but can exceed 35 miles per hour.
- The consistency of debris flows range from thin or thick mud to rocky mud that can forcefully carry larger items of destruction such as boulders, trees, and cars.
- When flows reach flatter ground, the debris spreads over a broad area, and can accumulate in thick deposits that wreak havoc in developed areas.
- Every year, landslides in the U.S. cause roughly $3.5 billion in damage and kill between 25 and 50 people.
- Casualties in the U.S. are primarily caused by rock falls, rock slides, and quick-moving debris.
- Landslides often accompany earthquakes, floods, storm surges, hurricanes, wildfires, or volcanic activity. They are usually more damaging and deadly than the triggering event.
- The May 1980 eruption of Mount St. Helens caused the largest landslide in history. A rockslide debris avalanche large enough to fill 250 million dump trucks to the brim traveled about 14 miles, destroying nine highway bridges, numerous private and public buildings, and many miles of highways, roads, and railroads. The debris avalanche also formed several new lakes by damming the North Fork Toutle River and its tributaries.
- In July 1994, a severe wildfire swept Storm King Mountain, Colorado, denuding the slopes of vegetation.
- Heavy rains on the Storm King Mtn. in September of ‘94 resulted in numerous debris flows, one of which blocked Interstate 70 and threatened to dam the Colorado River.
Learn the warning signs and disaster tips for landslides. GO
Sources: US Geological Survey, FEMA, American Red Cross