Unsaturated flow processes and the onset of seasonal deformation in slow-moving landslides

Journal of Geophysical Research: Earth Surface
By: , and 

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Abstract

Predicting rainfall-induced landslide motion is challenging because shallow groundwater flow is extremely sensitive to the preexisting moisture content in the ground. Here, we use groundwater hydrology theory and numerical modeling combined with five years of field monitoring to illustrate how unsaturated groundwater flow processes modulate the seasonal pore water pressure rise and therefore the onset of motion for slow-moving landslides. The onset of landslide motion at Oak Ridge earthflow in California’s Diablo Range occurs after an abrupt water table rise to near the landslide surface 52–129 days after seasonal rainfall commences. Model results and theory suggest that this abrupt rise occurs from the advection of a nearly saturated wetting front, which marks the leading edge of the integrated downward flux of seasonal rainfall, to the water table. Prior to this abrupt rise, we observe little measured pore water pressure response within the landslide due to rainfall. However, once the wetting front reaches the water table, we observe nearly instantaneous pore water pressure transmission within the landslide body that is accompanied by landslide acceleration. We cast the timescale to reach a critical pore water pressure threshold using a simple mass balance model that considers variable moisture storage with depth and explains the onset of seasonal landslide motion with a rainfall intensity-duration threshold. Our model shows that the seasonal response time of slow-moving landslides is controlled by the dry season vadose zone depth rather than the total landslide thickness.

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Publication type Article
Publication Subtype Journal Article
Title Unsaturated flow processes and the onset of seasonal deformation in slow-moving landslides
Series title Journal of Geophysical Research: Earth Surface
DOI 10.1029/2020JF005758
Volume 126
Issue 5
Year Published 2021
Language English
Publisher American Geophysical Union
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center, Pacific Coastal and Marine Science Center
Description e2020JF005758, 24 p.
Country United States
State California
Other Geospatial San Francisco Bay area
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