Instrumental record of debris flow initiation during natural rainfall: Implications for modeling slope stability

Abstract

[1] The middle of a hillslope hollow in the Oregon Coast Range failed and mobilized as a debris flow during heavy rainfall in November 1996. Automated pressure transducers recorded high spatial variability of pore water pressure within the area that mobilized as a debris flow, which initiated where local upward flow from bedrock developed into overlying colluvium. Postfailure observations of the bedrock surface exposed in the debris flow scar reveal a strong spatial correspondence between elevated piezometric response and water discharging from bedrock fractures. Measurements of apparent root cohesion on the basal (C b ) and lateral (C l ) scarp demonstrate substantial local variability, with areally weighted values of C b = 0.1 and C l = 4.6 kPa. Using measured soil properties and basal root strength, the widely used infinite slope model, employed assuming slope parallel groundwater flow, provides a poor prediction of hydrologic conditions at failure. In contrast, a model including lateral root strength (but neglecting lateral frictional strength) gave a predicted critical value of relative soil saturation that fell within the range defined by the arithmetic and geometric mean values at the time of failure. The 3-D slope stability model CLARA-W, used with locally observed pore water pressure, predicted small areas with lower factors of safety within the overall slide mass at sites consistent with field observations of where the failure initiated. This highly variable and localized nature of small areas of high pore pressure that can trigger slope failure means, however, that substantial uncertainty appears inevitable for estimating hydrologic conditions within incipient debris flows under natural conditions.

Sediment connectivity: a framework for understanding sediment transfer at multiple scales

“…For example, surface runoff or snowmelt may increase pore-water pressure/slope weight to initiate Coulomb slope failure. Intensive monitoring at a site in Oregon has revealed that hydrologically connected subsurface stormflow through a shallow bedrock zone can increase pore pressure, triggering landslides, in some instances, even in non-convergent topography (Montgomery et al, 2009). In this example, hydrological processes, via vertical and horizontal hydrological connectivity, are an important driver of sediment detachment (Duvert et al, 2011).…”

Epistemic uncertainties and natural hazard risk assessment. 1. A review of different natural hazard areas

“…require greater computational power and may still not represent the water flow processes adequately, such as the role of preferential flows and bedrock fracture systems in inducing conditions for failure (e.g Montgomery et al, 2009;Hencher, 2010;Beven, 2010;Beven and Germann, 2013). Even at sites where the costs of extensive field investigations can be justified, there is much that remains unknown about the subsurface including the detail of water flow pathways and knowledge of the hydro-mechanical behaviour of soils.…”

The temporally varying roles of rainfall, snowmelt and soil moisture for debris flow initiation in a snow dominated system: the compound trigger concept

“…Similarly, there is no consensus on the 15 level of soil moisture, i.e. the water volume stored in near-surface layers of the unsaturated substrate, required to trigger debris flows under different rainfall conditions (Johnson and Sitar, 1990;Montgomery et al, 2009). Essentially omitting the temporally variable yet cumulative influences of evaporation, transpiration and drainage on the soil wetness state, these concepts of antecedent wetness should be treated with caution and may hold only limited information.…”

“…Johnson and Sitar, 1990;Coe et al, 2008;Montgomery et al, 2009) and other potentially relevant variables, such as canopy interception (e.g. Sidle and Ziegler, 2017), are typically not available at sufficient spatial and temporal resolutions.…”