Quantifying hillslope response to glacier retreat : landslide mechanics, processes and impacts : a dissertation presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physical Geography at Massey University, Palmerston North, New Zealand

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Massey University
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Landslides are a natural hazard experienced in all countries around the world. Their formation is heavily influenced by internal and external processes including geology, earthworks, rainfall and in some instances glacier ice. This thesis investigates the response of alpine hillslopes to glacier retreat through the utilisation of movement monitoring techniques, subsurface investigations and geotechnical slope stability analysis. Three objectives are proposed and guide the research outcomes of this thesis. The first objective is to investigate the underlying preconditions, preparatory factors and triggers which control and enhance paraglacial rock slope failures through the study of the Mueller Rockslide in New Zealand’s South Island. Preconditioning and progressive weakening of the rockslide began over 8000 years prior to current glacier retreat, with the rockslide forming ~7500 years ago. Movement monitoring shows the rockslide to accelerate gradually over its history, culminating in maximum movement rates of ~6 m per year. Movement is constrained along structurally conditioned discontinuities, identified through a novel ground investigation combining geophysical and geotechnical techniques. Glacier debuttressing is considered a primary trigger of controlled slope failure, with increased displacement in spring attributed to snow melt and rainfall. Step-path failure of the rockslide is ongoing with tensile stress accumulating in the landslide body leading to segmentation of the rockslide into four zones. Ongoing progressive failure will likely lead to block sliding of the rockslide. The second objective of this thesis was to identify drivers of movement in paraglacial sediment slopes. A novel suite of monitoring techniques was used at Fox Glacier which showed clear correlation between hillslope failure and glacier retreat and downwasting. Unlike previous studies, this thesis found debuttressing to be the primary trigger of slope failure with rainfall acting as an accelerant. Debris flows, once thought to be a dominant process in sediment slope adjustment did not begin until debuttressing was completed. Finally, this thesis investigated the potential impacts of paraglacial landsliding on the broader environment. Both the rock and sediment slope failures in this study have deformed their supporting glaciers and a combination of monitoring techniques have uncovered to how sediment is delivered to the proglacial zone. Due to both sites being located within a high seismic hazard zone, catastrophic failure of both sites is possible. Continued failure of the Mueller Rockslide will likely result in a landslide dam and will also lead to continued retrogression of the headscarp and weakening of the surrounding hillslope. Continued failure of the sediment slopes at Fox Glacier will contribute large volumes of sediment to the proglacial zone, allowing for remobilisation of sediment in flood conditions. Both landslides have apparently created morphological change in their supporting glaciers. A unique combination of monitoring methodologies was used to provide insight into landslide movement and evolution, but their usage should not be limited only to studies of landslides. The methodologies used in this thesis provided context across a range of spatial and temporal scales.
The published versions of Chapter 3 and Chapter 5 may respectively be found at: 10.1007/s10346-019-01316-2 and https://doi.org/10.1016/j.geomorph.2020.107411.
Fig 2-4 (=Wohl et al., 2019 Fig 4) was removed for copyright reasons.
Glaciology, Slopes (Physical geography), Landslides, New Zealand, South Island