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UBC Theses and Dissertations

Forest and hydrogeomorphic processes in shallow landslide initiation zones Sakals, Matthew Egan


Following a shallow landslide, hydrogeomorphic processes in the initiation zone respond to the discontinuities of soil depth, topographic expression, and hydrologic and forest conditions. The shallow landslides occurring at the study sites are episodic erosion processes punctuating periods marked by the deposition of hillslope material. Recovery of soil depth and topographic expression occurs through small-scale processes that infill the failure area. Rates of small-scale redistribution of hillslope material decline with time and are attributable to both vegetation re-establishment and diminished surface topographic variability. Understory forest vegetation in failure areas resemble the vegetation on adjacent undisturbed slopes within approximately 100 years of failure; re-establishment of the forest canopy may require several centuries longer. Stochastic elements of the surrounding forests strongly affect soil accumulation through influences on material transport and deposition. The central tendency of soil accumulation approximates a sigmoid curve with the majority of accumulation occurring within 100 years of failure. Soil depth on adjacent hillslopes positively influences soil accumulation in failure areas, but repeated shallow landslides can deplete hillslope materials from the contributing area. The sediment balance of weathering, storage, and evacuation strongly influences future cycles of failure and recharge. The frequency of preferential flow pathways in shallow landslide initiation zones was found to be spatially variable with fewer preferential flow pathways in the infilled soils of failure areas; thus landslide occurrence and subsequent infilling may negatively influence future slope stability. As a result of this study, recommendations for future research are made regarding the effects of forests on the regime of small-scale material transport processes occurring after shallow landslides, on the spatial and temporal development of preferential flow pathways in shallow landslide initiation zones, and on the long-term stability of sites of shallow landslides.

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