Debris-flow initiation experiments using diverse hydrologic triggers

Controlled debris-flow initiation experiments focused on three hydrologic conditions that can trigger slope failure: localized ground-water inflow; prolonged moderate-intensity rainfall; and high-intensity rainfall. Detailed monitoring of slope hydrology and deformation provided exceptionally complete data on conditions preceding and accompanying slope failure and debris-flow mobilization. Groundwater inflow and high-intensity sprinkling led to abrupt, complete failure whereas moderate-intensity sprinkling led to retrogressive, block-by-block failure. Failure during ground-water inflow and during moderate-intensity sprinkling occurred with a rising water table and positive pore pressures. Failure during high-intensity sprinkling occurred without widespread positive pore pressures. In all three cases, pore pressures in most locations increased dramatically (within 2-3 seconds) during failure. In some places, pressures in unsaturated materials rapidly "flashed" from zero to elevated positive values. Transiently elevated pore pressures and partially liquefied soil enhanced debris-flow mobilization.