ABSTRACT
Whenever a confined granular material flows into an unbounded region, there is a well defined boundary to the flowing grains, as the material moves into initially grain-free domain. For example, such granular vacua are found in the lee of obstacles placed in free-surface chute flows; particles are deflected around the stationary obstacle and do not infill the region immediately downstream of it, forming a grain-free region in its wake. In fact such deflection patterns often underlie the design of many avalanche defense barriers (Gray et al. 2003). Also large-scale natural rockfalls and avalanches are often initiated in gulleys from which the flows may spread laterally when they are no longer confined. In this paper we examine the generic processes by which free-surface granular flows expand into grain-free regions by studying in detail the lateral spreading of a channelized flow when the boundaries that confine it are removed.
