On the development of an unsaturated front of debris flow

Abstract

A visibly granular debris flow front, where large boulders accumulate, is often observed in nature. Although there is abundant evidence of particle sorting and solid-fluid segregation processes, little is known about the specific segregation mechanisms and what factors control the relatively dry coarse snout. To investigate the conditions associated with the development of an unsaturated front, experiments have been conducted with grain-fluid mixtures of different compositions. To create long-lived, accessible, stationary flows we used two vertically rotating drum setups with a diameter of 2.4 m and 4 m respectively. The presence of an unsaturated front was detected by pore pressure transducers installed at the base of the flows. Additionally normal stress and flow depth are measured. We carried out a series of runs with varying fluid viscosity, sediment concentration, channel bed roughness, and mean velocity. The unsaturated fronts developed in faster ows with higher sediment concentration. The dry fronts formed even in well-sorted coarse particles (hence differential particle inertia is not necessary). High fluid viscosity may favor dry front formation due to the tendency for the fluid to stick to the boundary but also reduce the front formation by retarding segregation processes at the front.