  In this paper, transient granular flows are examined both numerically and
experimentally. Simulations are performed using the continuous 3D granular
model introduced in Daviet & Bertails-Descoubes (2016), which represents the
granular medium as an inelastic and dilatable continuum subject to the
Drucker-Prager yield criterion in the dense regime. One notable feature of this
numerical model is to resolve such a non-smooth rheology without any
regularisation. We show that this non-smooth model, which relies on a constant
friction coefficient, is able to reproduce with high fidelity various
experimental granular collapses over inclined erodible beds, provided the
friction coefficient is set to the avalanche angle - and not to the stop angle,
as generally done. In order to better characterise the range of validity of the
fully plastic rheology in the context of transient frictional flows, we further
revisit scaling laws relating the shape of the final collapse deposit to the
initial column aspect ratio, and accurately recover established power-law
dependences up to aspect ratios in the order of 10. The influence of sidewall
friction is then examined through experimental and simulated collapses with
varying channel widths. The effective flow thickness is estimated in relation
to the channel width, thereby challenging previously held assumptions on its
estimation. Finally, we discuss the potential extension of the constant
coefficient model with a hysteretic model to refine predictions of early-stage
collapse dynamics, illustrating the impact of such phenomenology on transient
flows and paving the way to more elaborate analysis.
