Hydrological response of hillslope soils above a debris-slide headscarp

The Jamieson Creek debris slide initiated in thin soils over a competent be drock surface, on a planar section of hillslope, during a heavy rainstorm i n November 1990. An array of automated piezometers and tensiometers was pla ced along a 22 m wide section of the headscarp in 1997 to monitor the tempo ral variation of pore-water pressures. Interpretation of the data addresses the hydrologic response to the storms in October and November 1997. The pi ezometers, which were designed for installation by driving, reveal very loc alized responses in what otherwise appears to be a uniform soil matrix. Pea k positive pressures occur at the time of maximum rainfall intensity. The t ensiometers indicate the hydrological response at the ground surface appear s uncoupled from that at the bedrock interface. Implications of the extreme spatial variability in pore-water pressure are evaluated for conceptual mo dels of hillslope hydrology. The assumption of parallel seepage flow is wid ely adopted in translational slope stability analyses, imposing a linear di stribution of pore-water pressure with depth. None of the reported field da ta are consistent with such a linear distribution with depth or a uniform r esponse across the slope.