Influence of flow composition and flow regime on fan morphodynamics

Abstract:

Alluvial fans and debris flow fans -- large, often conical-shaped, features -- form when particles carried with or by a flow transition from a relatively steep confined channel into a broader, mildly sloping open area. Morphology can vary dramatically from one fan to the next including channelization, avulsion dynamics, and apparent shape of the feature (from semi-conical to much more irregular) for otherwise similar boundary conditions. At the same time, small to moderate scale features, from local and global particle size segregation and fluid-particle separation, also vary from one fan to the next. There is little understanding of how or whether these different features relate to one another. Loose categorizations, like “sediment transport” vs. “debris floods” and “debris floods” vs. “debris flows” have not been used to distinguish potentially related landscape features in the different fans they create. We investigate the co-variation of these dynamics with a focus on the effect of changing flow composition in mixtures of sand, clay and water on the flow dynamics and fan morphology. Towards this, we initiate both short-term and long-term discharge events down a channel onto open beds of both porous and non-porous boundaries. We use flow observations and slump tests to guide us in distinguishing among the transport and segregating behavior of clear-water bed-load transport, debris-floods, and debris-flow events. Then we use high resolution laser measurements and photogrammetry calculations to quantify differences in topography, channelization, and avulsion features from the distinct flow types. We find all these experimental fans develop channels and avulsion dynamics though the details change dramatically from one type of mixture to the next. We also sort the particles at different regions of the channels and their fans and find notable differences in well-known segregation effects reported in the literature, from down-stream coarsening to apparent coarse levies. We discuss these distinctions in the context of detailed fans observed in the field that developed under different climates and are comprised of sediment of different rock lithologies.