Factors controlling debris avalanche initiation

Abstract

Heavy rainfall during a January 1983 storm triggered numerous debris avalanches and associated debris torrents in the Smith Creek basin, western Whatcom County, Washington, United States. Four classes of debris avalanches are recognized: wedges, drainage depressions, logging roads, and discontinuity surfaces. Nine different debris avalanche headscarps that are representative of these four classes are studied in detail. The geometric configuration of each headscarp, the properties of soils comprising the headscarps, and water tables based on numerical models of hillslope hydrology are used in limit equilibrium slope stability analyses to back-calculate root cohesion (Cr) values at failure. The calculated values are consistent with those reported by others. Four groups of Cr values are proposed and correlated with forest vegetation: group I, understory, Cr range from 1.6 to 2.1 kPa; group II, scrub forest, Cr range from 2.1 to 2.5 kPa; group III, mixed forest, Cr range from 2.5 to 3.0 kPa; and group IV, old growth forest, Cr greater than 3.0 kPa. Circular and noncircular failure surfaces are tested to determine the most likely failure configuration. Most computer-generated failure surfaces are consistent with those observed in situ. One debris avalanche is attributed to hydraulic erosion rather than Coulomb shear. Soil matrix suction contributes significantly to slope stability under drained, initial conditions. Loss of soil matrix suction during rainstorms and loss of root cohesion at failure cause a sharp reduction of soil shear strength. The stability of each headscarp is modeled during a moderately severe comparison storm in December 1979. Failure was probable at only two of the nine sites, and field evidence of prior failure is found at one of these. Sites with group I vegetation were also susceptible to failure during this storm. Pore-pressure increases triggered the debris avalanches; however, soil depth, soil density, the presence of smooth bedrock discontinuities, and root cohesion are shown to be important factors controlling initiation locations. The conclusions of this study are based on modeling results, which are only verified by post-failure observations at the avalanche headscarps. Direct field measurement of the hydrologic and shear strength parameters is required before the modeling results and conclusions drawn can be confirmed. Key words: slope stability, debris avalanche, hydrology, root cohesion, forested hillslope, factor of safety, Whatcom County, Washington.

Forest fire, bank strength and channel instability: the ‘unusual’ response of Fishtrap Creek, British Columbia

“…After the fi re, Equation 6 predicts a rapid decline in C r to a minimum value close to 0·5 kPa, followed by a more gradual recovery towards pre-disturbance values ( Figure 16). Our cohesion value of 1·9 kPa is similar to previous estimates of reach-average root cohesion made using this model for similar types of streams (Eaton, 2006;Eaton and Church, 2007), and similar to hillslope-average root cohesions for similar vegetation densities estimated using pore-pressure slope failure models (Buchanan and Savigny, 1990). As a point of reference, Eaton (2006) showed that the channels studied by Hey and Thorne (1986) were associated with reachaverage C r values ranging from 1·0 kPa for weakly vegetated channels to 3·0 kPa for densely forested channels, while Eaton and Church (2007) showed that the downstream hydraulic geometries of several gravel bed streams in the continental United States were consistent with C r values ranging from 1·7 kPa to 3·0 kPa.…”

Modelling effects of forest canopies on slope stability

“…This paradigm originated from investigations that revealed root density and strength declines following logging (Burroughs and Thomas, 1977;Ziemer, 1981). Confirmatory evidence includes field observations that total root biomass and strength are less in failed slopes than in unfailed slopes (Wu et al, 1979;Buchanan and Savigny, 1990;Schmidt et al, 2001), and controlled experiments that have found strength of soil in direct shear increases with rooting density Dakessian, 1981, 1982;Wu et al, 1988).…”

Identification of mechanisms for landslide type initiation of debris flows

“…There are numerous other proposals to identify typical topographic features of debris flow initiation locations, e.g., Buchanan and Savigny (1990), Fannin and Rollenson (1993), Johnson and Sitar (1990), Costa (1984), Johnson and Rahn (1970), Takahashi (1980) or Statham (1976). All of these studies observe different features in the topography of debris flow initiation sites.…”