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[1] The hydrologic behavior of shallow weathered soils commonly determines the propensity for slope failure. Here we use laboratory data and field data collected by an automated monitoring system to assess the character of pore water pressure responses in a natural clay slope subject to intermittent rainfall. Although we did not measure pore pressure distributions that triggered slope failure, we obtained three years of field data that provided reliable and largely reproducible documentation of transient pore pressure responses. At depths of tens of centimeters to a few meters below the ground surface, moisture and pressure sensors recorded relatively fast, transient responses to precipitation. The speeds of pore pressure pulses advancing downward in the saturated zone were much larger than those of advective fronts driven by gravity, and the amplitudes of the pulses attenuated with depth. Statistical assessment of 129 pressure head responses demonstrates that this behavior is consistent with predictions of a linear, one-dimensional pore pressure diffusion model. However, the model best simulates measurements if diffusivity is treated as a calibration parameter and if initial moisture conditions match model assumptions. For regional assessment of slope stability, the predictive accuracy of the linear-diffusion model is limited by inherent uncertainties in defining the initial conditions and in assigning the values of hydraulic parameters.

Aggregation of data collected in 28 controlled experiments reveals reproducible debris-flow behavior that provides a clear target for model tests. In each experiment ∼10 m3 of unsorted, water-saturated sediment composed mostly of sand and gravel discharged from behind a gate, descended a steep, 95-m flume, and formed a deposit on a nearly horizontal runout surface. Experiment subsets were distinguished by differing basal boundary conditions (1 versus 16 mm roughness heights) and sediment mud contents (1 versus 7 percent dry weight). Sensor measurements of evolving flow thicknesses, basal normal stresses, and basal pore fluid pressures demonstrate that debris flows in all subsets developed dilated, coarse-grained, high-friction snouts, followed by bodies of nearly liquefied, finer-grained debris. Mud enhanced flow mobility by maintaining high pore pressures in flow bodies, and bed roughness reduced flow speeds but not distances of flow runout. Roughness had these effects because it promoted debris agitation and grain-size segregation, and thereby aided growth of lateral levees that channelized flow. Grain-size segregation also contributed to development of ubiquitous roll waves, which had diverse amplitudes exhibiting fractal number-size distributions. Despite the influence of these waves and other sources of dispersion, the aggregated data have well-defined patterns that help constrain individual terms in a depth-averaged debris-flow model. The patterns imply that local flow resistance evolved together with global flow dynamics, contradicting the hypothesis that any consistent rheology applied. We infer that new evolution equations, not new rheologies, are needed to explain how characteristic debris-flow behavior emerges from the interactions of debris constituents.