  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.
