Advisor

Scott Burns

Date of Award

Winter 3-13-2013

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Physical Description

1 online resource (vii, 81 pages)

Subjects

Urban hydrology -- Oregon -- Portland, Urban runoff -- Oregon -- Portland, Drainage -- Oregon -- Portland, Slopes (Soil mechanics) -- Oregon -- Portland -- Stability, Stream restoration -- Oregon -- Portland

DOI

10.15760/etd.625

Abstract

The Stephens Creek Watershed in southwest Portland, Oregon was chosen by the city as a pilot project for urban stream restoration efforts, and the infiltration of stormwater was identified as a potential restoration strategy. The Stephens Creek Watershed has historically been known to be unstable during high precipitation events (Burns, 1996), and the need to address the response of slope stability to anthropogenically-driven changing groundwater conditions is the focus of this study. Airborne light detection and ranging (LiDAR) and geotechnical data from the City of Portland were employed to create a high resolution (0.84 m2) physics-based probabilistic slope stability model for this watershed, using the map-based probabilistic infinite slope analysis program PISA-m (Haneberg, 2007). Best and worst case models were run using fully dry and fully saturated soil conditions, respectively. Model results indicate that 96.3% of the watershed area had a probability [less than or equal to] 0.25 that the slope factor of safety (FOS) was [less than or equal to] 1 for fully dry conditions, compared to 76.4% for fully saturated conditions. Areas that had a probability [greater than or equal to] 0.25 that the slope factor of safety (FOS) was [less than or equal to] 1 were found to occur mainly along cut/fill slopes as well as within the deeply incised canyons of Stephens Creek and its tributaries. An infiltration avoidance map was derived to define areas that appear to be unsuitable for infiltration. Based on these results, it is recommended that stormwater continues to be directed to existing sewer infrastructure and that the "storm water disconnect" restoration approach not be used by the city.

Persistent Identifier

http://archives.pdx.edu/ds/psu/9352

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