Landslides moving over wet ground gain momentum because the weight of the flow increases the water pressure between sediment grains. This lubricates the base of the flow, causing it to gain mass and speed, reports a paper published online this week in Nature Geoscience.
Richard Iverson and colleagues used a unique, 95-metre-long flume to assess the mechanisms by which landslides grow or slow. They found that the key to growth in mass and speed was the presence of a wet sediment bed; debris flows moving over dry dirt gained less material and speed than those moving over a wet base.
Many landslides are triggered by intense rainfall or snowmelt, and this mechanism can, in part, explain how they grow dramatically in scale.