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Published September 1, 2015 | public
Journal Article

New insights into the mechanics of fluvial bedrock erosion through flume experiments and theory

Abstract

River incision into bedrock drives the topographic evolution of mountainous terrain and may link climate, tectonics, and topography over geologic time scales. Despite its importance, the mechanics of bedrock erosion are not well understood because channel form, river hydraulics, sediment transport, and erosion mechanics coevolve over relatively long time scales that prevent direct observations, and because erosive events occur intermittently and are difficult and dangerous to measure. Herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory. We also build on this work by providing new theory for rock resistance to abrasion, thresholds for plucking by vertical entrainment, sliding and toppling, and by assessing bedrock-analog materials. Bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength, sediment supply has a dominant control over bed roughness and abrasion rates, suspended sediment is an efficient agent of erosion, and feedbacks with channel form and roughness strongly influence erosion rates. Erodibility comparisons across rock, concrete, ice, and foam indicate that, for a given tensile strength, abrasion rates are insensitive to elasticity. The few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed, and the dominance of block sliding and toppling at knickpoints. These observations are consistent with new theory for the threshold Shields stress to initiate plucking, which also suggests that erosion rates in sliding- and toppling-dominated rivers are likely transport limited. Major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river-bed parallel; waterfall erosion by toppling and plunge-pool erosion; feedbacks between weathering and physical erosion; erosional bedforms; and morphodynamic feedbacks between channel form and erosion rates. Despite scaling challenges, flume experiments continue to provide much needed tests of existing bedrock-erosion theory, force development of new theory, and yield insight into the mechanics of landscapes.

Additional Information

© 2015 Elsevier B.V. Received 24 May 2014; Received in revised form 26 February 2015; Accepted 2 March 2015; Available online 10 March 2015. With pleasure we thank Paul Carling, Sean Bennett, and Cheryl McKenna Neuman for formal reviews that strengthened the paper, Richard Marston for his detailed edits, and the conveners and scientific committee of the 46th Annual Binghamton Symposium for bringing this special issue to fruition. MPL and JSS thank Daniel Lo for helping in foam erosion rate calibration studies and Brian Fuller for bringing river experimentation back to life at Caltech. Ian Dubinski kindly shared photos and insight from his study. Paul Carling made available data from his block sliding experiments. This work was supported by NSF grant EAR-1147381 and NASA grant 12PGG120107 to MPL, and NSF grant EAR-0345344 to LSS. JSS was partially supported by an NSF Graduate Student Research Fellowship. Author contributions: All authors contributed equally to this work.

Additional details

Created:
August 22, 2023
Modified:
October 23, 2023