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UBC Theses and Dissertations

The landslide response of alpine basins to post-Little Ice Age glacial thinning and retreat Holm, Kristopher William


The retreat of glaciers following the end of the Little Ice Age (since approximately 1850 AD) has been cited as a cause for increased landslide activity in alpine basins. The primary reasons for this are the combined effects of removal of ice buttressing from bedrock slopes and deposition of glacial drift in steep areas prone to instability. Nineteen alpine basins along the upper Lillooet River valley, northwest of Pemberton, B.C., were studied to investigate the controls of post-Little Ice Age Neoglacial retreat on landslide activity . Terrain containing landslides was characterized by identifying how Neoglacial scouring and retreat have modified terrain, thus affecting slope stability. A decision-making flowchart was constructed to assist future identification of landslide hazards associated with ongoing glacial retreat. This work was based on field observation, GIS analysis, statistical associations between landslides and terrain attributes, and comparison of Neoglaciated and non-Neoglaciated terrain within each basin. Examples of landslides influenced by Neoglacial retreat include debris slides and debris flows, rockfall, rockslides, rock avalanches, and, slow deep-seated slope movements. In bedrock, landslide response to post-Little Ice Age Neoglacial retreat ranges from severe in the Meager Creek volcanic area to low in many granitic rock basins. In general, the magnitude of landslide response depends on the intensity of glacial scour below the Neoglacial trimline. Basins underlain by weak volcanic rocks experienced significant oversteepening by Neoglacial scour, and active rockfall, deep-seated slope movements and large failures occur near glacial trimlines. Basins underlain by granitic rock rarely show increased bedrock instability resulting from Neoglacial retreat, except for shallow rockfall along some glacial trimlines and failures in previously unstable slopes. In surficial materials, landslides associated with Neoglacial retreat occur in till and colluvium and are concentrated within the Neoglacial Limit along trimlines. Landslide processes in surficial materials are classified as primary and secondary Neoglacial effects. Primary effects involved evidence of direct Neoglacial influence on landslide activity, and included glacial undercutting of colluvial or drift embankments and deposition of glacial drift in areas prone to instability. Secondary effects involved deposition of drift material in locations entrainable by failures initiating upslope of the trimline. The Neoglacial effects investigated in this thesis concern only one of many factors contributing to landslide activity in alpine basins. Nevertheless, the study results suggest that Neoglacial scour, retreat, and deposition are significant factors for increased landslide density in areas already prone to instability.

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