Root strength, colluvial soil depth, and colluvial transport on landslide-prone hillslopes

I evaluate factors governing shallow-soil, slope stability in forested landscapes by measuring root strength, colluvial soil depth, colluvial soil transport, and topography within the Oregon Coast Range. Cohesive root reinforcement within different vegetation communities was quantified by determining the tensile strength, species, depth, orientation, relative health, and root-area-ratio for all root threads $\ge$1 mm in diameter. Examination of 41 sites, including soil pits and landslides triggered during 1996, reveals that cohesion values within clear cuts are uniformly <10 kPa, while cohesion values range from <2 kPa in industrial, anthropogenically altered forests with significant understory and deciduous vegetation to over 150 kPa in old growth forests dominated by coniferous vegetation.; Colluvium depth measurements (n = 721) over a 12,400 m2 headwall area range from 0 to >3 m. Near the ridgetop, at small upslope drainage areas, saprolite is preserved (up to 1 m thick) and the bedrock surface is characterized by closed depressions (>2 m deep and 6 m in diameter) filled with colluvium. At large upslope drainage areas colluvium is thin, saprolite is absent, and the bedrock surface is devoid of depressions. I observed no correspondence between local slope and colluvium depth. Regional geographic-information-system-based, slope stability assessments using measured colluvium depths predict localized areas of potentially unstable ground that are concordant with observed landslide locations.; Differential tilting of 212 piezometer standpipes emplaced to varying depths within a 43° catchment reveals a general inverse relationship between sediment transport rate and below-ground length of standpipe. Measurements made over 860 m2 provide a four to nine year record of surface sediment velocities ranging from 0 to greater than 10 cm yr-1 with mean annual volumetric fluxes varying from 20 to 44 cm3cm -1yr-1. Bedrock-to-soil production rates inferred through measurements of (i) the gradient of the volumetric colluvial transport vector delineate a median production rate of 0.14 mm yr-1 and (ii) the magnitude of topographic divergence surrounding bedrock outcrops and the coefficient of diffusivity delineate a median production rate of 0.07 mm yr-1. No inverse dependence between production rate and depth of colluvial soil cover was discernible.