Creep and earthflow erosion from undisturbed and management impacted slopes in the Coast and Cascade Ranges of the Pacific Northwest, U.S.A.

Abstract. Soil creep, slumping, and earthflows are major processes ofnatural slope erosion and sediment transport on slopes underlain bydeeply weathered parent material in the Cascade and Coast Ranges ofOregon, Washington, and northern California, USA. Creep movement isquasi-viscous, occurring under shear stresses too small to producediscrete failure. The soil mass is primarily mobilised by breakdownof included clay structures. Remolding transforms the clay fractioninto a slurry which lubricates the remaining soil mass. Where shearstresses are great enough, slumps and earthflows develop and enlargeby progressive failure of the mantle materials. Substantial sectionsof drainage basins in the region are affected by these processes andare highly sensitive to any manipulations or impacts that change thestress distribution in the soil mass.
Creep and earthflow rates in various geologic terrains have beenmonitored since 1972 by bore-hole inclinometer methods. Depth ofmovement varies from near-surface to more than 17 meters. Undisturbedrates of movement are variable within all parent material types butare within the range of 0.5 to 104 mm/year. The highest rates occurabove zones of incipient shear or extension flow marking changes inparent material type, degree of alteration, or structural weakness. Slope exerts little control over variations in movement rate. Ratesare most sensitive to water content of the parent materials. Monitoredsites show clear relationships between seasonal rainfall and totalyearly movement, and movement predominantly occurs during the falland winter rainy season or during the spring snowmelt. Movement ona storm by storm basis was not detectable. Preliminary analysis ofone harvested slope indicates a marked increase in movement beginningthe second year after disturbance with post-impact rates more than
twice undisturbed rates.