A massive mudslide that has authorities searching for three missing men in remote western Colorado highlights the dangers of heavy rain saturating slopes.
Half a mile of a rain-soaked ridge near the small mountain town of Collbran, Colorado, collapsed Sunday, sending a mudslide careening down Grand Mesa, one of the world's largest flat-topped mountains. The slide was three to four miles (five to six kilometers) long and a two miles across, according to news reports, making it considerably larger than the mudslide that struck Oso, Washington, in March.
"It's an understatement to say that it is massive," Sheriff Stan Hilkey told CNN. "The power behind it was remarkable."
Rescue workers are searching for Clancy Nichols, 51; his son Danny, 24; and Wes Hawkins, 46. The men had gone to the area to check on reports of an initial slide on Sunday, according to the Mesa County sheriff.
Rescue efforts have been hampered by the instability of the mudslide area. The two-mile-wide mudslide is up to 30 feet (nine meters) deep at the edges and is believed to be several hundred feet deep in the middle.
The mudslide occurred on land with restricted access, so officials say they doubt anyone else was caught up in it. (See "After Washington Mudslide, Questions About Building in Nature's Danger Zones.")
Science of a Mudslide
A mudslide, also called a debris flow, is a type of fast-moving landslide that follows a channel, such as a river. A landslide is simply when rock, earth, or debris moves down a slope. (See photos of a mudslide and a video on landslides.)
Mudslides occur after water rapidly saturates the ground on a slope, such as during a heavy rainfall.
According to Jim O'Connor, a research hydrologist with the U.S. Geological Survey in Portland, Oregon, it doesn't take high relief in the topography to create a slide. Rather, it just takes a pull of gravity strong enough to bring down material that is made fluid by water.
Mudslides tend to happen during wet seasons, says O'Connor. (Related: "Rescue Dogs Tested by Washington Mudslide Recovery.")
Mudslides are also often triggered by earthquakes or by disturbances in hillsides caused by fires or human activity.
In the United States, mudslides and landslides result in an average of 25 to 50 deaths a year, according to the Centers for Disease Control and Prevention (CDC).
Strategies to decrease the risk of mudslides include draining water off hillsides, reinforcing the bases of hills so they are not undercut by rivers, and "loading the toe," says O'Connor.
In the case of loading the toe, engineers put heavy objects, such as large rocks, at the base of a hill to try to anchor the slope and prevent it from coming loose.
O'Connor says the piles of rock that are often seen at the base of roadcuts are the most visible example of that strategy. (See "Mudslide Buries More than 350 in Afghan Village.")
The CDC recommends that people exercise caution around steep slopes during rainfall. Immediate signs of a pending slide include tilting trees and sudden increases or decreases in rivers.