Computer simulations of cobble structure on a gravel stream bed

In Harris Creek, a gravel bed river in southcentral British Columbia, the large stones protruding from the surface armour of the bed are arranged as curvilinear ridges oriented at varying angles to the flow, defining a net-like surface structure. A computer simulation, using various rules governing the entrainment and deposition of stones, was written to reproduce these structures. The working hypothesis for the simulation is that the patterns are the result of the mutual interaction of stones as they are mobilized into the flow and collide with downstream obstacles. An opposing hypothesis would be that the pattern being imposed on the stones by the flow. A statistical algorithm was developed to identify cobble ridges and their orientations and to allow a comparison between the simulation results and the real example. Simulations using different sized stones in a bed with 10% of its area covered by stones produce flow-parallel, linear clusters resembling cluster bedforms. Simulations using a more densely populated bed, up to 25% covered area, produce a variety of gravel bedforms including cluster bedforms, transverse ribs, some trending at oblique orientations, and arcuate ridges. These results indicate that the magnitude of the stone population influences the suite of bedforms that emerge. Cluster bedforms may be the result of obstacles to the downstream movement of stones in a sparsely populated bed. Furthermore, cluster bedforms may be nuclei from which oblique ridges develop out of excess sediment. Finally, simulations in which large stones are rarely entrained and travel shorter distances than small stones, produce structures very similar to those found in Harris Creek.