Debris-flow initiation promoted by extension within a slow-moving landslide

The dynamics of slow landslide motion can predispose oversteepened and extended slide regions to debris-flow initiation. For more than 20 years, our real-time monitoring, combined with repeat high-precision GPS surveys, of the Cleveland Corral landslide complex, California, USA, reveals that debris flows initiate from slow-moving kinematic elements of this complex. Different slide elements move in different wet years, and all remain dormant in dry years. To explore controls on landslide-element kinematics, we use triaxial testing to define the critical state behavior of the landslide material, and use a large-diameter sampling ring to determine in situ material porosities in both extensional and compressional regions of the slide. Regions undergoing extension contain materials looser than their critical state, potentially aiding liquefaction and debris-flow mobilization from shallow, secondary slides. Although intense rainfall serves as a trigger for debris-flow initiation, slow deformation of the larger landslide promotes debris-flow formation by oversteeepening toe and lateral margins and by preferentially extending, and effectively loosening, material in these steep regions.