The influence of stormwater pathways on pore-pressure timescales near shallow landslide sites
Abstract
Intense storms striking the west coast of the US from mid-December through March have a higher likelihood of triggering positive pore pressures and shallow landslides because the steep slopes they rain on often already store significant soil moisture. USGS monitors soil moisture at four landslide-prone sites around San Francisco Bay Area to understand conditions that precede widespread landsliding. The record of historic events from here and elsewhere in California indicates that shallow landslide abundance in a landscape, normalized by susceptible area, increases as a power law with ~6-hour storm rainfall intensity. We hypothesize that this timescale represents a combination of the movement of rainfall towards the failure plane by pore-pressure diffusion and by film flow along macropores. We explored this hypothesis using rainfall and pressure head records from winter storms in 2012, laboratory estimates of soil water retention characteristics, field experiments and one-dimensional modeling. Storms with hourly rainfall rates in excess of 20 mm/hour generated pressure heads commonly up to several decimeters at soil bases. Peaks in the rainfall rate were mirrored at later times in the pressure head at depth. Modeling of pressure heads observed at several sites indicates hydraulic diffusivities of 10-5 m/s, with response times to rainfall input of 5-8 hours. This instrumental response was more rapid, and the variation in head more detailed than that predicted by one-dimensional forward modeling with Richards' equation with laboratory parameters. To refine this effort and to explore the role that film flow might have, we used tracer experiments to explore moisture pathways through this soil. At historic landslide scars near the monitoring sites, we used falling head tests to explore the rate at which the saprolite can transmit water as a lower boundary condition on pressure head. These field measurements and post-storm observations of exfiltration from saprolite fractures present the possibility that some landslide generation during storms is a consequence of rapid film flow. Understanding how to detect landscapes that exhibit such behavior might play a crucial role in forecasting the effects of storms on landslide-prone hillslopes in California and elsewhere.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.H11I1259S
- Keywords:
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- 1810 HYDROLOGY Debris flow and landslides;
- 1826 HYDROLOGY Geomorphology: hillslope;
- 1848 HYDROLOGY Monitoring networks