| Movement patterns of large, complex, persistently active, earthflow-like landslides commonly vary in time and space. In many cases these movement patterns appear to be catalyzed by temporally or spatially variable hydrologic or loading conditions or by variable sediment-flux conditions at the landslide headscarp or toe. Unsteady, nonuniform landslide responses to these variable conditions are investigated in this study through physically based mathematical analysis and detailed landslide monitoring.;Perturbation analysis shows that small, unsteady, nonuniform components of landslide sediment flux exhibit convective-diffusive behavior that is controlled largely by a single dimensionless parameter, here called the landslide Peclet number. Physical interpretation of the Peclet number demonstrates that migration of landslide sediment-flux perturbations is dominated by convection if the landslide material behavior is primarily viscous and by diffusion if the behavior is more rigidly plastic. Landslides that deform plasticly along thin boundary shear zones are thus inferred to respond relatively rapidly and globally to localized, transient perturbations. Landslides that deform in thick, viscous creep zones, in contrast, are inferred to respond over periods as long as several decades, with zones of perturbed sediment flux that translate downslope as kinematic waves.;Data that reflect a decade of movement and deformation of Minor Creek landslide in northwestern California corroborate important elements of the mathematical theory. Minor Creek landslide moves in response to seasonal piezometric fluctuations each year. Additionally, lower portions of the landslide have moved and deformed significantly in response to transient stream erosion of the landslide toe. The rates and styles of the movement and deformation patterns correspond well with deductions based on the generalized viscoplastic constitutive model and perturbation theory.;The mathematical analysis employs a postulated, generalized viscoplastic constitutive model for landslide material. The constitutive model can represent widely ranging landslide deformation styles, including nonlinear viscous flow and rigid-plastic frictional slip. Using the constitutive model, physical conservation laws and an idealized landslide geometry, analytical solutions for steady landslide shear deformation and sediment flux are developed. These solutions provide the basis for a perturbation analysis of the kinematics of unsteady, nonuniform downslope landslide sediment flux. |
