GROUNDWATER, UNSATURATED FLOW, AND LANDSLIDE INITIATION: RICK HEALY’S INADVERTENT CONTRIBUTIONS TO SLOPE STABILITY ANALYSIS

Landslides triggered during major storms or after periods of elevated groundwater recharge are controlled by subsurface hydrological and mechanical processes related to the loading, weakening, and eventual failure of earth materials. Due to the complexity of these coupled processes, many hydro-mechanical models for slope-stability analyses include a vastly over-simplified representation of the infiltration and drainage processes associated with hillslope hydrological response to rainfall or snowmelt. In particular, assumptions about 1D vertical infiltration and uniform initial conditions have limited investigations into important nuances associated with how unsaturated soil-hydraulic properties and subsurface geometries may affect the initiation and magnitude of potential landslides. The quantitative understanding of groundwater recharge and variably saturated subsurface flow pioneered by Rick Healy and coauthors, and the open-source nature of their variably saturated two-dimensional flow (VS2D) model and other U.S. Geological Survey codes they developed, have allowed landslide scientists to more correctly simulate the unsaturated flow processes related to both shallow and deep-seated landslides. Here we present two examples from previous work using VS2D simulations to inform improved slope stability models that (1) yielded more accurate rainfall thresholds for landslide early warning systems across areas in southern Italy with layered volcanoclastic deposits, and (2) elucidated the role of complex glacial stratigraphy in controlling the magnitude and potential mobility of large deep-seated landslides in western Washington State. These two examples show that Rick Healy’s legacy in quantitative unsaturated zone hydrology and method development go well beyond contaminant transport and water resources; his groundbreaking work also facilitated improved landslide hazard assessments for risk reduction in very different settings around the world.