The Effective Viscosity of Ash-Laden Flows

Author

Kirstin Anne Burns, The University of MontanaFollow

Year of Award

2007

Document Type

Thesis

Degree Type

Master of Science (MS)

Other Degree Name/Area of Focus

Geology

Department or School/College

Department of Geosciences

Committee Chair

Rebecca Bendick

Commitee Members

Emmanuel Gabet, Scott Woods

Keywords

debris flow, effective viscosity, fine-grained sediment, hyperconcentrated flow, rheology, vegatative ash, wildland fire

Abstract

Debris flows can drastically alter ecosystems and damage infrastructure, and there is an increased risk of debris flows following a wildfire. In general, runoff and erosion drastically increase following a wildfire as a result of many processes, one of them being debris flows. Large debris flows are common in the months and years after wildfire in mountainous areas throughout the western U.S.A. The progressive bulking of surface runoff may be the dominant triggering mechanism of these debris flows. Vegetative ash on the hillslope becomes entrained in the flow, along with other fine-grained sediment and increases the effective viscosity of the flow. The increase in effective viscosity decreases the settling velocity of the sediment within the flow, which in turn increases the bulk density of the flow. The increase in bulk density increases the erosivity of the flow. While previous research has shown that the addition of fine-grained particles can increase the effective viscosity of a flow, little research has been done to determine how the addition of ash changes the effective viscosity of the flow. A viscometer was used to test the effective viscosity of a variety of sediment and water slurries. The tests varied in shear rate, sediment composition (ash, silt, sand) and sediment concentration. These parameters mimicked natural conditions where debris flows occur. Comparisons were made between slurries containing different sediment types and sediment concentrations at various shear rates to form equations that related these parameters. All of the slurries tested could be classified as a power-law fluid; more specifically, all slurries exhibited pseudoplastic (shear thinning) behavior. Slurries containing only ash behaved differently than slurries containing only silt or sand. Also, in the slurries containing ash, silt and sand, it was found that slurries containing a high percentage of ash behave differently than those containing mainly silt and sand. Using the data collected, two equations were generated that relate the effective viscosity to shear rate and sediment concentration. One equation is for use with ash-rich slurries, the other ash-poor slurries.

Recommended Citation

Burns, Kirstin Anne, "The Effective Viscosity of Ash-Laden Flows" (2007). Graduate Student Theses, Dissertations, & Professional Papers. 1215.
https://scholarworks.umt.edu/etd/1215