Landslide Recurrence: Geomorphic and Hydrologic Feedback Cycles

The triggering mechanism for shallow rainfall-induced landslides typically involves a storm sequence or even a single, heavy rainfall event, with extremely consequential impacts on humans and the built environment. In contrast, the topographic and hydroclimatic conditions that predispose slopes to instability co-evolve over geologic timescales, but have equally significant consequences for society. While geomorphic processes may set the landscape conditions that favor landsliding, recent field evidence suggests that in settings where failed materials are not completely evacuated from steep slopes, landslide-related disturbances to hydrologic processes may actually promote recurring slope failures. This may explain, in part, why landslides tend to occur where they have already occurred in the past, but also introduces challenges for quantitative characterization of both landslide hazards and landscape evolution. This is important on human timescales because landslide recurrence has been responsible for several of the deadliest landslide events in recent U.S. history, but it is also potentially important for quantifying sediment fluxes and denudation rates. Here we demonstrate the feedbacks between hydrology and landsliding using continuous monitoring data and numerical modeling for the landslide-prone coastal bluffs near Seattle, WA. These coastal bluffs have experienced repeated cycles of landsliding and subsequent recovery since deglaciation, but the landslide recurrence and recovery rates remain poorly characterized. Continuous monitoring with subsurface hydrologic instrumentation, time-lapse cameras, extensometers, and repeated site visits, confirm that landslide disturbances to vegetation, hydraulic properties, and topography promote a positive feedback cycle for landslide recurrence. Coupled hydro-mechanical modeling of variably saturated subsurface flow and slope stability for landslide-triggering storm sequences provides insights into which disturbance impacts exert the greatest influence on hydrologic response and landslide recurrence for these slopes. Understanding these feedbacks has implications for how landslides and pedogenesis interact to shape spatial variations in soil thickness and properties.