The effect of grain size on erosion and entrainment in dry granular flows

TL;DR

This study investigates how grain size ratios influence erosion and flow dynamics in dry granular flows, revealing that measurement methods significantly affect erosion rate estimations and interpretations.

Contribution

The paper introduces a novel particle-size-based method for estimating erosion depth and compares it with the traditional critical velocity approach, highlighting discrepancies and insights.

Findings

Maximum flow velocity depends on grain size ratio.

Different methods yield contrasting erosion rate trends.

Measurement technique choice critically affects erosion rate assessment.

Abstract

The entrainment of underlying erodible material by geophysical flows can significantly boost the flowing mass and increase the final deposition extent. The particle size of both the flowing material and the erodible substrate influence the entrainment mechanism and determine the overall flow dynamics. This paper examines these mechanisms experimentally by considering the flow of particles over an erodible bed using different particle size combinations for the incoming flow and the base layer in a laboratory-scale inclined flume. Dynamic X-ray radiography was used to capture the dynamics of the flow-erodible bed interface. The experiments found that the maximum downslope velocity depends on the ratio between the size of the flowing particles and the size of the bed particles, with higher ratios leading to faster velocities. Two techniques were then applied to estimate the evolving erosion depth: an established critical velocity method, and a novel particle-size-based method. Erosion rates were estimated from both of these methods. Interestingly, these two rates express different and contradictory conclusions. In the critical-velocity-based rate estimation, the normalized erosion rate increases with the flow to bed grain size ratio, whereas the erosion rates estimated from the particle-size-based approach find the opposite trend. We rationalise this discrepancy by considering the physical interpretation of both measurement methods, and provide insight into how future modelling can be performed to accommodate both of these complementary measures. This paper highlights how the erosion rate is entirely dependent on the method of estimating the erosion depth and the choice of measurement technique.