A Mechanistic Model and Experiments on Bedrock Incision and Channelization by Rockfall

Rockfall and rock avalanches are common in steep terrain on Earth and potentially on other planetary bodies such as the Moon and Mars. Since impacting rocks can damage exposed bedrock as they roll and bounce downhill, rockfall might be an important erosive agent in steep landscapes, even in the absence of water. We developed a new theory for rockfall-driven bedrock abrasion using the ballistic trajectories of rocks transported under gravity. We calibrated this theory using laboratory experiments of rockfall over an inclined bedrock simulant. Both the experiments and the model demonstrate that bedrock hillslopes can be abraded by dry rockfall, even at gradients below the angle of repose, depending on the bedrock roughness. Feedback between abrasion and topographic steering of rockfall can produce channel-like forms, such as bedrock chutes, in the absence of water. Particle size has a dominant influence on abrasion rates and runout distances, while the hillslope angle has a comparatively minor influence. Rockfall transport is sensitive to bedrock roughness; terrain with high friction angles can trap rocks creating patches of rock cover that affect subsequent rockfall pathways. Our results suggest that dry rockfall can play an important role in eroding and channelizing steep, rocky terrain on Earth and other planets, such as crater degradation on the Moon and Mars. Rockfall is common on Earth and other planets. Falling rocks bounce down rocky slopes and likely also erode them. However, it has not been explored how erosive this process is, nor what landforms it might generate. We developed a numerical model for this erosion process and calibrated it with experiments of dry grains hopping down an inclined erodible surface. Both experiments and modeling showed that bedrock erosion from rockfall can occur even on relatively low-grade hills. Small hollows were carved by rockfall, which over time coalesced into larger troughs that captured the path of subsequent rocks. This process led to a self-enhancing feedback that produced a bumpy surface with rocky chutes. Rock size had a larger effect on erosion amounts than the steepness of the hill. Our work suggests that dry rockfall can play an important role in the evolution of mountain slopes on Earth and craters on the Moon and other planets. Rockfall can erode rocky hillslopes even below the angle of repose Grain size has a dominant effect on impact abrasion; slope is of minor importance Topographic steering of grain results in self-formed bedrock channels