Unsaturated Flow Processes and the Onset of Seasonal Deformation in Slow-Moving Landslides

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. Plain Language Summary Landslides are often triggered by rainfall events that increase water pressure within rock and soil. A key impediment to predicting landslide motion is that movement of water in the ground is extremely sensitive to preexisting moisture content. Hence, rainfall history exerts a strong control on water movement into the ground. For large landslides, it is commonly assumed that the ground is saturated to the surface, which simplifies the modeling of pressure changes. Here we show, however, that the dynamics of infiltration through the unsaturated ground at the start of the wet season fundamentally control both the style and timing of landslide response to rainfall, which we verify through field monitoring of a large, slow-moving landslide in the California Coast Range. At the start of the wet season, we observe no pressure response at depth for weeks to months. However, eventually, a sudden pore pressure rise in the landslide body marks the shift to a regime where pressure transmission and landslide acceleration from rainfall is nearly instantaneous. This bimodal behavior, which we can predict by comparing the seasonal rainfall rate to the unsaturated groundwater velocity, is an expected consequence of infiltration into initially unsaturated ground with the material properties observed.