First Advisor

Adam Booth

Date of Award

2016

Document Type

Thesis

Degree Name

Bachelor of Science (B.S.) in Geology and University Honors

Department

Geology

Subjects

Landslides -- Oregon, Isotopes -- Analysis, Landslide hazard analysis, Groundwater recharge

DOI

10.15760/honors.270

Abstract

Landslides pose a significant threat to property and personal injury, estimated at 3.5 billion dollars and 25-50 deaths per year respectively (Schuster, 1996). The triggers that cause landslides are well understood and include intense precipitation events, seismic shaking, destabilizing activities, and the over-steepening of slopes. However, these triggers were absent when the Silt Creek Landslide initiated, and determining this landslide’s subsurface hydrologic conditions is the focus of this study. Located on the western slopes of the Cascade mountain range in Oregon and within an active logging area, the landslide reactivated during the summer of 2014 at a site that had previous landslides dating back to at least 1994. The area is underlain by poorly welded tuffs capped with basaltic flows. Considering the near-drought to drought conditions that existed 3 years’ prior, it has a relatively high rate of slip of approximately 15 m/yr. On March 4th, 2016 I obtained twelve water samples from surface flow and springs on the landslide for isotope analysis of oxygen (18O) and deuterium (D). Comparing the resultant mean, -10.09‰ δ18O, against well-defined linear elevation/ δ18O relationships for western Oregon and the Global Meteoric Water Line for δ18O / δD the source of surface flow and groundwater likely originates from a pond above the head scarp. Analysis of precipitation data over a 42-year period prior to reactivation confirms the drought-like conditions existing at time of reactivation. These conditions align with the ‘bathtub’ model wherein an older landslide creates the conditions for repeated failures by aligning platy clay minerals such that the hydraulic conductivity is reduced to the point that a perched aquifer forms. This provides lubrication for subsequent landslides even during periods of low precipitation. Understanding the mechanisms operating on this landslide can inform future assessments of hazard and risk as well as appropriate mitigation strategies.

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Persistent Identifier

http://archives.pdx.edu/ds/psu/17409

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