Bedrock Channel Formation by Dry Granular Flows

Channel formation in bedrock is generally explained by fluvial or debris flow processes. Channelization occurs because topographic steering of water-sediment mixtures focuses flows on routes of steepest descent, which causes preferential erosion and the formation of channels. In contrast, dry granular flows tend to spread laterally over loose granular beds due to dispersive pressures and hence tend to smooth out incipient channels, as visible in the shape of sediment cones. However, it remains unknown whether dry granular flows can carve channels in bedrock, and their role in forming gullies and debris chutes in steep rocky terrain is not constraint. In addition, given the abundance of gullies and chutes on rocky crater walls on the Moon and Mars, the potential contribution of dry granular flows to gully formation has important implications for the role of surface water on these planetary bodies.

In this study we report on first results of laboratory experiments on the formation and evolution of bedrock gullies driven by dry grain flows. The experiments use a synthetic polyurethane foam bed, which has been shown to quantitatively scale with natural bedrock undergoing abrasion by grain saltation, and semi-rounded grains of 2 cm diameter. The physical experiments include measurements of particle impact energies, particle travel paths and the spatial patterns of bedrock erosion, and are used to test a discrete cellular-based model of gully formation. Preliminary results show that dry granular flows are effective at eroding the analog bedrock and, with repeated grain flows, the initially planar and smooth bedrock slope develops a dendritic network of chutes, similar to those seen in steep mountain terrain and on crater walls. Thus, even in the absence of a fluid, dry grain flows appear to be capable of self-channelization in bedrock. In addition, we find that dry grain flows can develop chutes at bed slopes below 23 degrees, which is significantly lower than the angle of repose of the used gravel (35 degrees), because particles more easily move over smooth surfaces. Our results imply that granular flows also might be responsible for relatively low angle chutes on Mars and the Moon, even in the absence of water, where transported boulders are large compared to underlying bedrock roughness.