Using artificial landslides created in a giant outdoor laboratory, scientists have uncovered key details on how these disasters happen — research that could help them improve forecasts of landslides' size and impact.

Many landslides are triggered when intense rainfall or snowmelt loosens steep, debris-laden slopes. As the torrentssurge downward, they can then grow dramatically in size and speed by sweeping up extra material from their beds and banks, but the precise way in which they enlarge is unclear. In fact, it might seem as if the energy needed to drag in additional matter should actually slow these flows down.

Although scientists have studied miniature landslides on a benchtop scale before, the properties of real landslides are likely to depend strongly on their size. As such, researchers built a unique system where they could recreate full-scale landslides.

Only slide of its kind

Scientists built a 6-foot-wide, 310-foot-long (2-meter-wide, 95-meter-long) slide or "flume" and have sent tons of gravel, sand, mud and water crashing down its roughly 30-degree slope over the years. They monitor how these artificial landslides grew or slowed with electronic pressure and erosion sensors inside the slide, and tinkered with the material lining of the flume.

Each artificial landslide requires a lot of manual labor.

"Running a single experiment takes about 10 to 15 person-days of effort to get it set up and run, involving loading the sediment, wetting it and letting it go," said Richard Iverson, a research hydrologist at the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash. "Others who have thought of building something similar have come to visit us and then decided not to do it because they thought it quite daunting. It's the only facility of this kind in the world."

Wet vs. dry

Their experiments revealed the key to landslides gaining mass and speed was the presence of a bed of wet sediment. Debris flowing over dry dirt slowed down and grew less in size.

"The thing that was unanticipated, and also the most important, was how much the momentum of flows could grow," Iverson said. "In the course of less than 100 meters [330 feet] of interacting with beds of wet sediment, these flows quadrupled in momentum when on beds of wet sediment compared to flows that moved across bare beds."

The researchers suggested landslides grow over wet ground because the weight of the flow increases the water pressure between sediment grains. This lubricates the base of the torrent, causing it to gain mass and speed.

"These findings help us refine our mathematical models for forecasting hazards," Iverson said. "We can't prevent natural events from happening in most cases, but we can try to issue increasingly accurate warnings based on information from experiments like this."

Future research can analyze how rocky particles within landslides can rapidly separate based on size. "The larger grains first move up and then are carried to the front, changing the character of frictional resistance in the flow's 'snout,' which can have a huge impact on the flow's dynamics," Iverson said.

And the research even applies beyond landslides.

"The motion of granular materials has many industrial applications, such as with cereal grains, crushed rock in mining or when making pharmaceuticals, so there's a lot of interest in understanding this," Iverson said.

The scientists detailed their findings online Dec. 19 in the journal Nature Geoscience

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