Effects of changes in valley geomorphology on the behavior of volcanic mass-flows
Stinton, Adam James
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Geophysical mass-flows such as pyroclastic flows, lahars and avalanches, are commonly associated with explosive volcanism. These flows are primarily gravity-driven and are strongly influenced by the nature of the surface over which they move. Any spatial or temporal changes in the surface can affect the behavior of these flows. A series of investigations were conducted to determine the effects of changes in channel morphology on pyroclastic flows and lahars, the effects of long-term (century scale) changes in valley morphology on the behaviour of pyroclastic flows, and the effects of changes in the geology of a surface encountered by a debris avalanche during emplacement. A combination of numerical modeling (using the TITAN2D geophysical flow model) and fieldwork were used to gather data on the behavior of the mass-flows in question. Spatial changes in the morphology of a channel can cause pyroclastic flows to slow and thicken when encountering a break in slope, when entering a channel and when passing through a channel constriction. A change in channel width affects the flow thickness by a factor equal to that of the width change. Confined pyroclastic flows reach longer runout distances than unconfined flows with the same volume and bed friction. Long-term changes in the geomorphology of a valley can result in the rapid formation of thick pyroclastic flow deposits that choke existing channels leaving little or no space for deposition from subsequent flows to occur in. Temporal changes in the morphology of a channel caused by deposition from lahars resulted in an increase in the deceleration of flows in the deposition area, shorter runout distances, and increases in both the inundation and deposition areas. Assigning different values for basal friction to the various geological or geomorphological features encountered by a debris avalanche during emplacement significantly improved the accuracy of simulations when compared to documented examples of debris avalanches.