Field and Remote‐Sensing Evidence for Hydro‐mechanical Isolation of a Long‐Lived Earthflow in Central California

Abstract

Persistent motion of slow-moving landslides has been linked to entrapment of water in slide masses by weak, low-permeability shear zones at their basal and lateral margins. This so-called "bathtub effect" should have remotely sensible effects on soil moisture and vegetation health. Here we assess this effect at a seasonally active earthflow in northern California with analysis of soil properties and nine years (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) of multispectral satellite imagery. The shear zone has low hydraulic conductivity, and the earthflow shows elevated water content relative to its surroundings. Spectral indices suggest that the earthflow is perennially wetter, and its vegetation thrives for longer into the dry season as a result. The magnitude of the bathtub effect is correlated with cumulative multiyear rainfall, and its surface expression disappears during the driest year of the 2012-2015 drought. These findings demonstrate a new method for identifying and monitoring mechanical and hydrological interactions that enable persistent landslide motion. Plain Language SummaryThe ability of some landslides to move slowly and persistently for many years without accelerating catastrophically may be linked to entrapment of rain water in slide masses by slow-draining boundaries at their sides and bottoms. The existence of this so-called "bathtub effect" can be tested for directly with soil analysis but should also have visible effects on soil moisture and vegetation health that can be measured remotely using satellite imagery. We examined a seasonally active landslide in northern California with slow-draining boundaries and elevated water content relative to its surroundings. Nine years (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) of satellite imagery suggest that the earthflow is perennially wetter and its vegetation thrives for longer into the dry season as a result. The magnitude of this effect is correlated with cumulative multiyear rainfall. These findings demonstrate a new method for identifying and monitoring the hydrology and movement of persistent landslides.

Inferring the Subsurface Geometry and Strength of Slow‐Moving Landslides Using 3‐D Velocity Measurements From the NASA/JPL UAVSAR

“…Direct comparison with friction angle values measured in the laboratory and back‐calculated for Franciscan mélange rocks and landslide materials provides some insight into our findings. For saturated conditions, we find that the inferred friction angles for medium to large earthflows and landslide complexes overlap the measured friction values from the Two Towers earthflow (Schulz et al., 2018), Minor Creek earthflow (Iverson & Major, 1987), and Oakridge landslide complex (Nereson et al., 2018). The majority of the smaller slumps have saturated friction angles that are significantly higher than these three landslides.…”

“…Our study is the first (to our knowledge) to apply the conservation of volume approach to invert for the thickness of multiple landslides in a given region. Previous work (Booth et al, 2020; (Nereson et al, 2018), Two Towers (Schulz et al, 2018), and Minor Creek landslides (Iverson, 2000;Iverson & Major, 1987) and the Calaveras Dam, which is founded on Franciscan mélange (Roadifer et al, 2009). The Calaveras Dam samples are plotted for two different block-in-matrix proportions, which are reported as percentages.…”

Geomorphic and Sedimentary Effects of Modern Climate Change: Current and Anticipated Future Conditions in the Western United States

“…During the severe 2012-2015 drought, earthflow velocities decreased by an order of magnitude to the slowest in eight decades for which measurements exist (slowing from~1 to <0.1 m/year). Using unmanned aerial vehicle synthetic aperture radar (UAV-SAR) to measure deformation in a 4,700-km 2 study area within the (Nereson et al, 2018;Nereson & Finnegan, 2019); 6, Olympic Mountains landslides (Smith & Wegmann, 2018); 7, Glacier Bay rockslides studied by Coe et al (2018); 8, set of 38 landslides studied by Geertsema et al (2006); 9, Denali National Park (Robert et al, 2019); 10, debris flows from 2006 storm at Mount Rainier (S. T. Lancaster et al, 2012;Legg et al, 2014); and 11, debris flows from 2013 Front Range storm (Patton et al, 2018).…”

Widespread Initiation, Reactivation, and Acceleration of Landslides in the Northern California Coast Ranges due to Extreme Rainfall

“…Similar types of slow‐moving landslides occur in mountainous areas around the world (Cerovski‐Darriau & Roering, ; Malet et al, ; Miao et al, ; Rutter & Green, ; Simoni et al, ). The slow‐moving landslides occur in a mechanically weak (friction angle ∼ 15°), clayey granular soil (chlorite, illite/mica, and smectite) with low hydraulic diffusivity (∼10 −6 m 2 /s; Iverson & Major, ; Keefer & Johnson, ; Nereson et al, ; Schulz, Smith, Wang, Jiang, & Roering, ; Schulz, Smith, Wang, Jiang, Deuell, et al, ).…”