Hydrologic Impacts of Landslide Disturbances: Implications for Remobilization and Hazard Persistence

Landslides typically alter hillslope topography, but may also change the hydrologic connectivity and subsurface water-storage dynamics. In settings where mobile materials are not completely evacuated from steep slopes, influences of landslide disturbances on hillslope hydrology and susceptibility to subsequent failures remain poorly characterized. Since landslides often recur at the site of previous failures, we examine differences between a stable vegetated hillslope (VH) and a recent landslide (LS). These neighboring hillslopes exhibit similar topography and are situated on steep landslide-prone coastal bluffs of glacial deposits along the northeastern shore of Puget Sound, Washington. Our control hillslope, VH, is mantled by a heterogeneous colluvium, supporting a dense forest. In early 2013, our test hillslope, LS, also supported a forest before a landslide substantially altered the topography and disturbed the hillslope. In 2015, we observed a clay-rich landslide deposit at LS with sparse vegetation and limited root reinforcement, soil structures, and macropores. Our characterization of the sites also found matrix porosity and hydraulic conductivity are both lower at LS. Continuous monitoring during 2015-2016 revealed reduced effective precipitation at VH (due to canopy interception), an earlier seasonal transition to near-saturated conditions at LS, and longer persistence of positive pore pressures and slower drainage at LS (both seasonally and between major storm events). These differences, along with episodic, complex slope failures at LS support the hypothesis that, despite a reduced average slope, other disturbances introduced by landsliding may promote the hydrologic conditions leading to slope instability, thus contributing to the persistence of landslide hazards. Plain Language Summary The most deadly landslides in recent U.S. history resulted from reactivation of existing landslides. Motivated by these tragedies and other landslide-related losses, we investigated why recent landslides may be less stable than similar hillslopes nearby. We collected observations and monitoring data from two neighboring hillslopes sharing similar geology and steep topography, near Seattle, Washington; one hillslope where a landslide occurred recently and a second that was stable and forested. We found that this second hillslope remained stable in part because roots reinforce the soils, but also because the vegetated soils drain efficiently following major rainstorms. In contrast, the existing landslide on the other hillslope disrupted vegetation and altered the soil properties, reducing drainage efficiency. As a result, the landslide materials became wetter earlier in the winter rainy season than the soils of the neighboring vegetated hillslope and also stayed wetter longer after the rains ended. The combination of reduced root reinforcement and the persistence of wetter soil promoted repeated slope failures at the landslide hillslope. These new insights into the self-driving factors contributing to landslide recurrence can be used to inform residents in landslide-prone areas and improve landslide hazard assessments to help reduce landslide-related losses.