Elementary theory of bed-sediment entrainment by debris flows and avalanches

• Published in: Journal of Geophysical Research: Earth Surface Vol. 117; no. F3
• Main Author: Iverson, Richard M.
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01.09.2012; American Geophysical Union
• Subjects: Avalanches; Bed load; bed sediment; Contaminated sediments; Debris flow; Detritus; Earth sciences; Earth, ocean, space; Entrainment; Exact sciences and technology; Flow profiles; Flow velocity; Friction; Geomorphology; Hydrology; Landslides & mudslides; Liquefaction; momentum conservation; pore pressure; Porosity; Sediment transport; Sediments; Soil erosion; Entrainment (atmosphere, ocean); thickness; pore fluid; debris flows; Velocity distribution; slopes; Deformable body; Continuum; mechanical controls; fluid pressure; Friction coefficient; flow; efficiency; shear; friction; depth; avalanches; interpretation; materials; Rate equation; discontinuities; prediction; Flow velocity; slope angle; theory
• ISSN: 0148-0227; 2169-9003; 2156-2202; 2169-9011
• DOI: 10.1029/2011JF002189

Analyses of mass and momentum exchange between a debris flow or avalanche and an underlying sediment layer aid interpretations and predictions of bed‐sediment entrainment rates. A preliminary analysis assesses the behavior of a Coulomb slide block that entrains bed material as it descends a uniform slope. The analysis demonstrates that the block's momentum can grow unstably, even in the presence of limited entrainment efficiency. A more‐detailed, depth‐integrated continuum analysis of interacting, deformable bodies identifies mechanical controls on entrainment efficiency, and shows that entrainment rates satisfy a jump condition that involves shear‐traction and velocity discontinuities at the flow‐bed boundary. Explicit predictions of the entrainment rateEresult from making reasonable assumptions about flow velocity profiles and boundary shear tractions. For Coulomb‐friction tractions, predicted entrainment rates are sensitive to pore fluid pressures that develop in bed sediment as it is overridden. In the simplest scenario the bed sediment liquefies completely, and the entrainment‐rate equation reduces toE = 2μ1gh1 cos θ(1 − λ1)/ v¯1 , where θ is the slope angle, μ1 is the flow's Coulomb friction coefficient, h1 is its thickness, λ1 is its degree of liquefaction, and v¯1 is its depth‐averaged velocity. For values ofλ1ranging from 0.5 to 0.8, this equation predicts entrainment rates consistent with rates of 0.05 to 0.1 m/s measured in large‐scale debris‐flow experiments in which wet sediment beds liquefied almost completely. The propensity for bed liquefaction depends on several factors, including sediment porosity, permeability, and thickness, and rates of compression and shear deformation that occur when beds are overridden. Key Points Momentum conservation constrains bed‐sediment entrainment rates by mass flows Pore pressures generated in overridden bed sediment influence entrainment rates Flow momentum can grow unstably in the presence of entrainment