The effective viscosity of slurries laden with vegetative ash

• Vegetative ash increases the viscosity of runoff.
• Runoff with vegetative ash is intensely shear-thinning.
• Settling velocities of fine particles are slowed in ash-laden flows.
• Ash-laden runoff may transform into bulking debris flows.

After a forest fire, ash can blanket the soil surface to depths of tens of centimeters. It has been proposed that the incorporation of vegetative ash into hillslope runoff can, by increasing the flow's effective viscosity, reduce the settling velocity of entrained particles and lead to the development of runoff-generated debris flows. To investigate how the addition of this material might affect the rheology of runoff in burned areas, we measured and compared the effective viscosity of slurries composed of varying concentrations of ash and similarly sized mineral particles (i.e., silt). All the slurries were pseudoplastic (i.e., shear-thinning) and, at low weight concentrations (≤ 0.3 g g− 1), the rheologies of ash and silt slurries were nearly identical. At higher concentrations, however, their rheologies diverged significantly. For example, whereas the viscosity of the silt slurries increased monotonically with concentration, the viscosity of the ash slurries decreased. Ash incorporated into silt slurries induced this same complex behavior, indicating that ash alters the rheological properties of runoff in ways that would not be predicted simply based on its size. In addition, we found that, at high concentrations, the intensity of shear-thinning in the ash slurries was greater than in the silt slurries. Finally, we compared the settling velocities of small particles in clear water and ash slurries and conclude that the rheological characteristics of ashy runoff could promote runoff-generated debris flows in burned landscapes.