UBC Theses and Dissertations
Environmental variability and geomorphic responses on alluvial fans : an experimental study Leenman, Anya S.
The dynamic behaviour of channels on alluvial fans can present hazards to infrastructure and human lives. In particular, avulsion, lateral channel migration, and erosion and deposition of sediment can present problems. Much of our understanding of these dynamics comes from small-scale physical experiments with constant flow and sediment supply. Recent research suggests that constant inputs to steep streams on fans do a poor job of representing the temporally varying inputs to natural streams. This dissertation presents a suite of physical alluvial fan experiments which further explore the impacts of temporally varying inputs. First, an experiment with constant inputs shows that complex, unpredictable avulsion dynamics can arise in simple conditions due to in-channel sedimentation, particularly around large grains. Second, a set of experiments with temporally varying flow demonstrates that flood hydrograph shape governs the geomorphic impact of floods on fans; non-linear relations between the instantaneous flow and the magnitude of geomorphic change mean that more material was moved by short, high-peaked floods than by long, low-peaked floods. Third, experiments with temporally varying sediment feed show that rapid erosion and channel adjustment can be triggered by both increases and decreases in the feed rate. Moreover, longer-duration feed variations generate lower-gradient, elongate fans with a more incised fan-head trench. Together, these experiments highlight the distortions introduced when temporally varying inputs are replaced with constant flow or sediment feed. In addition, the experiments are significant for understanding hazards on fans, as they indicate how fan morphology and channel dynamics might vary as climate change alters the frequency and severity of high flows and sediment inputs in mountain streams.
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