Mechanisms governing incipient motion of ellipsoidal rock fragments in concentrated overland flow

Abstract

A modified Shields criterion for incipient motion of ellipsoidal rock fragments in concentrated overland flow was previously defined based on laboratory data. In order to apply this new criterion, an estimate of the angle of repose for the considered rock fragment is needed. Therefore, a set of laboratory experiments was conducted in order to develop an algorithm for this estimation. The main result of these experiments shows that each rock fragment lying on a rill bed of a given roughness is characterized by a normally distributed set of angles of repose. This distribution reflects the different local slopes that a rock fragment can experience. The new criterion has been successfully validated for single rock fragments lying on an erodible rill bed. Application of this criterion to field conditions, however, did not explain all variation in the observed data. In fact, rock fragments having sizes larger than the predicted flow competence were removed during a moderate rainfall event. A consideration of several mechanisms reveals that collisions between rock fragments seems likely to be a significant mechanism explaining the observed discrepancy.