How Landslides Get Slippery
On 5 December, the heaviest rains in Colombia's history triggered a landslide that wiped out a poor hillside community and left more than 145 people dead or missing. Now, researchers have an inkling of how such sediment flows can grow to monster proportions.
Although many landslides are meager affairs that quickly sputter to a stop, others can eventually involve more than 1 billion cubic meters of soil and rock—enough to fill the Louisiana Superdome more than 275 times—flowing hundreds of meters at speeds of more than 10 meters per second. Field observations and small-scale lab studies have hinted that sediments saturated by intense rainfall or melting snow contribute to the rapid growth and acceleration of landslides, but hard data on full-sized debris flows have been hard to come by. Now, tests conducted at a sediment flume in the mountains of Oregon have helped fill the hole in the data.
The flume is a 2-meter-wide, football-field-length facility that hugs the mountainside at an angle of 31 degrees. In each of eight experiments, Richard Iverson, a research hydrologist at the U.S. Geological Survey in Vancouver, Washington, and colleagues dumped 6 cubic meters of wet sand, mud, and gravel—about a small dump truck load—at the top of an open channel, which had been packed with a partially saturated mix of the same sediments. The mixture is meant to replicate typical hillside sediments, but to protect the scientists and the facility, the material didn't include large boulders.
The tests were designed to mimic a sudden slump of wet material slipping down a hillside, says Iverson. A sprinkler system allowed the researchers to control the amount of water in the flume sediments during each test, and sensors anchored throughout the material, as well as high-speed video of the tests, provided information about what was happening in and under the schussing debris.
Getting the tests started was simple, says Iverson. "We just piled the material behind the gate and then let it loose."
For the first 3 seconds of all eight tests, the faux landslide behaved the same. The material dumped at the head of the flume rapidly accelerated downhill and spread out across the sediments below. When material packed in the flume was relatively dry, the landslide soon fizzled.
But when the flume sediments were almost or fully saturated, the landslide grew explosively, in some cases ending up with four times the momentum of the original sediment dump (see video). In these cases, sensors revealed, the pressure of the overriding material boosted water pressure in the underlying sediments in less than 1 second, nearly liquefying them and dramatically decreasing any friction that might slow down the flow. As a result, flume sediments eroded and joined the accelerating, ever-swelling flow.
Many scientists have long suspected that water plays a big role in landslide behavior, says Anne Mangeney, a geophysicist at the Institute of Geophysics of Paris, who wasn't involved in the research. But until now, she notes, no one had come up with hard data to quantify the effect.
The team's results, reports online today in Nature Geoscience, "are a huge find," says hydrologist Jason Kean of the U.S. Geological Survey in Denver. "This is the first paper that clearly identifies why friction is reduced inside a landslide ... and how little debris flows become big ones," he notes.
"I'm very pleased with this work," adds Oldrich Hungr, a geological engineer at the University of British Columbia in Canada. "While many of us have been postulating such things from geological observations, [Iverson] has actually measured them."