Title: Comments on avalanche flow models based on the concept of random kinetic energy,
Abstract: In a series of papers, Bartelt and co-workers developed novel snow-avalanche
models in which \emph{random kinetic energy} $R_K$ (a.k.a.\ granular
temperature) is a key concept. The earliest models were for a single, constant
density layer, using a Voellmy model but with $R_K$-dependent friction
parameters. This was then extended to variable density, and finally a
suspension layer (powder-snow cloud) was added. The physical basis and
mathematical formulation of these models is critically reviewed here, with the
following main findings: (i) Key assumptions in the original RKE model differ
substantially from established results on dense granular flows; in particular,
the effective friction coefficient decreases to zero with velocity in the RKE
model. (ii) In the variable-density model, non-canonical interpretation of the
energy balance leads to a third-order evolution equation for the flow depth or
density, whereas the stated assumptions imply a first-order equation. (iii) The
model for the suspension layer neglects gravity and disregards well established
theoretical and experimental results on particulate gravity currents. Some
options for improving these aspects are discussed.