UC Santa Cruz

When an initial landslide does not fully evacuate all the failed material, remaining deposits may be susceptible to recurring landsliding in the future. These types of consecutive landsliding systems present significant natural hazards and play an important role in landscape evolution, however they are difficult to incorporate into hazard assessments and geomorphic analyses because they violate a commonly held assumption of stationarity. Illuminating the dominant processes that control the behavior and triggering conditions of recurring landslides is critical for improving these assessments.

Landslide movement in the simplest form occurs when the balance of shear stress driving material downslope exceeds the shear strength within a hillslope. In the regime of Coulomb friction, this can be articulated as: τ=(σ-p)tanφ+c Where τ is shear stress, σ is normal stress, p is pore-water pressure, φ is friction angle, and c is effective cohesion. For rainfall driven landslides, infiltration of rainwater into the subsurface increases pore-water pressure within the landslide body, thereby reducing effective normal stresses (σ-p) and shifting the balance towards landslide movement. Field observations suggest that pore-water pressure rise (and consequently landslide deformation) is controlled by the details of how rainfall infiltrates through the vadose zone, i.e., the unsaturated region below the ground surface and above the water table.

In this dissertation, I present three studies combining field observations and numerical data to explore the influence of vadose zone processes on recurring landslides. In the first chapter, I synthesize field observations of a large, slow-moving landslide near San Jose, California, USA to explore how lithologically controlled weathering patterns influence earthflow hydrology and deformation. I find that the landslide has a thin weathered zone, comparable to other landslides in the same geologic formation, which results in a small amount of dynamic storage. I propose that this storage capacity constrains the possible range of stress within the landslide body and consequently limits landslide movement as excess water is shed via springs and saturation overland flow once the water table reaches the surface. In the second chapter, I use dynamically downscaled climate model projections to model variably saturated groundwater conditions over 150 years at this same landslide. Using field data from chapter 1, I link the groundwater water data to an empirical model of landslide movement to explore how multiyear precipitation variability will affect future landslide movement. I find that future increases in precipitation variability may lead to a decrease in slow-landslide movement in California, particularly in areas where the amount of possible recharge is precipitation limited. Declining landslide movement is due in part to greater precipitation intensity which diverts rainfall to runoff rather than recharge. Additionally, the impact of wet years on landslide movement is limited by storage constraints and the impact of dry years may propagate forward for multiple years due to the importance of antecedent saturation state. Finally, in the third chapter, I use numerical experiments to investigate the dominant controls on the reactivation of coastal bluff landslides along Puget Sound, Washington, USA. I find that the changes in hydraulic conductivity and strength from root reinforcement have the greatest impact on the likelihood of landslide reactivation, followed by changes in the properties of the soil water retention curve. Collectively, these studies advance our understanding of vadose zone processes within landslides and the resulting effects on landslide movement.