Process-based characterizations of subsurface fluid pressures for a devil's slide-like system [electronic resource]
Keywords: Geological and Environmental Sciences.

Coastal margins host slope stability hazards that are influenced by hydrologic, geologic, and / or anthropogenic perturbations. This dissertation is motivated by the hydrologically-driven, creeping and episodic deep-seated bedrock slides that intersect a former section of the Pacific Coast Highway in the active landslide zone at Devil's Slide near Pacifica, California. Serendipitously, the timing of this study correlated with an outpouring of subsurface data associated with geotechnical exploration along the Tom Lantos Tunnels at Devil's Slide. The focus of this effort weighs heavily on process-based hydrology in that the main objective was to test the hypothesis that simulated hydrologic response within the saturated subsurface, for a Devil's Slide-like system, can produce the elevated fluid pressures needed to generate slope instability. In the first phase of this study, an extensive photo record, historically-based digital elevation model, and a loosely coupled water-balance / limit-equilibrium approach are leveraged to better understand the spatial and temporal characteristics of the slope instability problem at Devil's Slide. In the second phase of this study, numerical simulation of three-dimensional (3D) subsurface flow is utilized to investigate fluid pressure response for the active landslide zone. The foundation of these simulations rests upon a newly conceptualized geology for a Devil's Slide-like system. In the third phase of this study, concept-development simulation is employed to quantify the influence of historically-based climate forcings and end-member parameterizations of the subsurface on fluid pressures throughout the study area. This effort combines previously unavailable information, careful field measurements, and sophisticated numerical simulations. Contributions from this study demonstrate that, for a Devil's Slide-like system, (i) the careful dissection of photo records is critical to inventory the distribution / components of a landslide complex, (ii) observed episodic slip rates can easily differ from average rates of retreat by an order of magnitude, (iii) a water-balance alone cannot provide process-based insights, (iv) specific climatic conditions facilitate variable lag times associated with water table dynamics, (v) recharge is the most sensitive parameter to establish risk-averse estimates of fluid pressure, (vi) nuances in the 3D flow field related to fault zone characteristics markedly influence fluid pressures, and (vii) it is unlikely that seasonal fluctuations in the regional water table account for severe failure modes. The 3D saturated-zone simulations conducted for this study estimate slope instability on the average, but they do not resolve event-based failure. Perched water, identified here as a plausible hydrologic response for Devil's Slide, likely promotes slope failure at the site. The perched-water hypothesis, born out of the concept-development simulations conducted for this study, encourages new, interdisciplinary data discovery to characterize the unsaturated near surface and parameterize / evaluate transient simulations of 3D variably-saturated flow.