Abrupt debris-flow mobilization in a hillslope experiment

Debris-flow mobilization from shallow landsliding is widespread, can be deadly, and is the focus of warning systems worldwide. One challenge of forecasting is that some shallow landslides transform into rapid debris flows, whereas others do not. Although early warning systems can depend on forecasting this poorly understood transition, high-resolution hydrologic and deformation data from debris-flow mobilization events are rare. As part of the APERIF project (Ochiai et al., Landslides, 2004), we performed a field-scale artificial rainfall (sprinkling) experiment on a planar 33º natural hillslope near Mt. Kaba-san, Japan. Using 100-Hz sampling, we recorded synchronous subsurface pore-pressure response and ground-surface motion throughout the transition from slide (~1 m thick) to flow, including slow precursory deformation, abrupt rapid failure, and subsequent debris-flow motion down a small channel. The experiment began with ~6 hours of high-intensity sprinkling (at 78 mm/hr), which triggered motion of the slide in response to rising positive pore-water pressures. Continued sprinkling led to persistent slow motion of the slide for ~1 hour, until acceleration and abrupt failure. Our data revealed that during the several seconds of rapid failure, pore pressures dropped (indicative of soil dilation) then oscillated greatly in response to deformation, thereby enhancing liquefaction and flow.

 

An abrupt transition from slow to fast motion in dilative soils can present a mechanical conundrum. Although loose, contractive soils may collapse and liquefy, many hillslope soils are dense and dilate in shear, thereby impeding motion. We explored slide behavior using a 1D model (Iverson, 2005) that fully couples slide motion and pore pressure with evolving shear-zone dilatancy, and utilized measured and estimated parameters from our hillslope experiment. Simulations demonstrated that dilative soils impede motion, as observed initially in our experiment. However, dilatant systems can evolve dynamically through persistent landslide motion driven by prolonged rainfall. When motion-inhibiting dilatant effects are exhausted, our analysis showed rapid acceleration during a swift drop and subsequent increase in pore pressures (within a few sec), as was also observed in the field experiment. This behavior provides a mechanism to mobilize debris flows from shallow landsides in dense hillslope soils. Our results suggest that although high-resolution monitoring might detect precursory motion, forecasting liquefaction and debris-flow genesis is still dependent on soil properties and transient hydrologic conditions.