Sponsor
Portland State University. Department of Geology
First Advisor
Adam Booth
Term of Graduation
Fall 2024
Date of Publication
12-12-2024
Document Type
Thesis
Degree Name
Master of Science (M.S.) in Geology
Department
Geology
Language
English
Subjects
3D modeling, hazards, landslide, NDVI, slope stability, White Bluffs
Physical Description
1 online resource (viii, 90 pages)
Abstract
Slope failures shape the landscape by moving unstable material downslope and, in doing so, become a great hazard to all those in their path. Understanding the mechanisms that control landslide location, timing, size, type, and runout - and therefore hazards - requires a deep understanding of the local geology, stratigraphy, and groundwater conditions. Along White Bluffs, in south-central Washington, landslides of several failure styles have likely been triggered by an increased water table in recent decades, threatening both farmland behind the bluffs and habitat below. This study focuses on characterizing potential groundwater sources and determining controls on spatial and temporal patterns of landsliding from 1996-2019. We digitally mapped and characterized the White Bluffs landslides using aerial imagery and lidar, determined the affected land area and volume, and compared patterns to potential controlling mechanisms.𝑥
We found that 91 individual landslides occurred affecting a land area of 5x106 m2 with a total estimated volume of 10.9x106 to 11.2x106 m3. The dominant landslide failure modes observed were rockslide-rotational slumps and earthflows. Normalized difference vegetation index (NDVI) was used as a proxy for the relative water content of groundwater seeps within and adjacent to landslides. Geochemical data indicated those seeps are consistent with surface irrigation waters and ponds above the bluffs. We found a weak positive correlation between NDVI of seeps and the mobility index of their associated landslides (r^2 = 0.355), suggesting that water content partly controls, but is likely not the only important variable controlling landslide runout.
Future landslide hazard potential along the bluffs was also estimated with 3D slope stability modeling for current and alternate hydrogeologic conditions. The models showed that some portions of the bluffs which are oversteepened along the Columbia River are prone to slope instability even under dry conditions. Under the current groundwater conditions our factor of safety (FoS) models show that the areas of greatest future slope instability are the regions which have suffered past slope failures. The percent of model area shown to be at or below a FoS of 1 is 1.6% for dry conditions, 3.3% for current groundwater conditions, and 7.9% for fully saturated conditions. These results show that the higher the water table is along the bluffs, the lower the FoS will be, and therefore, the more prone the bluffs will be to landslides. Our results imply that water table height and groundwater flow are the most important factors besides slope controlling variations in stability along the bluffs. Our landslide volume models have a volume range of 104 m3 to 107 m3. Results show landslides on the lower end of the range are likely all along the bluffs whereas landslides on the higher end of the volume range are most likely where the FoS is the lowest, i.e where the water table is the highest. Ultimately, our models show that the areas most prone to future landslides are the areas which have already experienced landslides and many of these areas are likely to remain unstable even with a groundwater table 50% lower than the current position. These results may prove useful for future site planning along the bluffs to reduce the risk from landslide damage.
Rights
©2024 Anna Tsitsivas
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Persistent Identifier
https://archives.pdx.edu/ds/psu/43005
Recommended Citation
Tsitsivas, Anna, "Landslide Mapping, Characterization, and Hazard Assessment with 3D Slope Stability Modeling in White Bluffs, Washington State" (2024). Dissertations and Theses. Paper 6754.