Monroe landslide, CA
Monroe landslide, CA
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The Shasta-Trinity and Six Rivers National Forest lands in northern California are vulnerable to landsliding due to steep slopes, disturbances such as wildfire, clearcut logging, road construction , and a seasonally wet climate . Our focus of this study is a deep-seated, rotational debris slide -- Monroe landslide. It is located upslope from Trinity County Road 311, approximately 5 km northwest of Hyampom, California. The 2004 Sims Fire and 2015 Saddle Fire struck the same place to the north of the active landslide. The toe had failed and damaged the road twice in 2002 and 2004.
Figure 1. Landscape of the Monroe landslide complex. The lateral margins of the active slide zone inferred from the geomorphic features are highlighted by thick black lines.
The Monroe landslide has been moving very slowly at a rate of ~2 cm/yr for at least 70 years and has been continuously delivering sediments to the South Fork Trinity River. The knowledge about the Monroe landslide is still fairly incomplete. The complex landslide mobility is tied to heterogeneities of the soil materials, drainage from the incised streams, and disturbances from wildfires and road constructions. Here we use spaceborne L-band ALOS-1 PALSAR-1 and ALOS-2 PALSAR-2 data to retrieve the spatio-temporal displacements of the Monroe landslide, and we further explore its geometry, triggering mechanisms, and hydraulic properties (e.g., diffusivity) [Hu et al., 2019, JGR].
Figure 2. Landslide velocity of the Monroe landslide.
The largest LOS velocities (>350 mm/yr) occur near the longitudinal center (element 3) where there is a slightly oversteepened (>15°) facet. Hillslopes adjusted to the oversteepening by mass wasting processes, that is, quickly transporting the sediments and reducing the slope angles downslope (element 4). The head and toe have larger slope angles, yet the velocities are smaller than in the middle transport zone. This might be due to the wider landslide body at the head and toe.
Landslide toe is partially overlapped with the road-damaging failures from 2002 and 2004, and exhibits complicated in displacement patterns. The derived velocity fields further suggest that the south part at the toe moves upward and eastward while the north part remains nearly stationary during the observation period. This contrast suggests subsurface disconformities and/or temporal variability.
Figure 3. Ground surface and basal surface elevation along the kinematic elements of the landslide transport zone (dotted-line profiles in Figure 2).
We inverted for the landslide thickness based on mass conservation. Results show a pronounced thickening near the transition zone from subsidence to uplift between elements 3 and 4. Upslope of the key transition, elements 1 and 2 remain stable or experience subtle uplift, and the derived basal surface depth does not vary much. Landslide thickness declines slightly where the landslide enters the narrow part of element 3 where the slope steepens and the surface subsides, and then the thickness increases in order to compensate for the inflow of mass until the key transition. Downslope of the key transition the landslide at element 4 becomes thinner to maintain mass conservation.
Based on the one-dimensional pore pressure diffusion model, we performed the temporal cross correlation between the normalized landslide velocity and transient pore pressure at depth, and constrained the effective hydraulic diffusivity to be 6.8(-2.0/+2.9)×10-5 m2/s.
Figure 4. The normalized velocity against the normalized pore pressure over one calendar year, superimposed on the average daily rainfall.
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