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

Abstract

Please cite this article as: Lamb, Michael P., Finnegan, Noah J., Scheingross, Joel S., Sklar, Leonard S., New insights into the mechanics of fluvial bedrock erosion through flume experiments and theory, Geomorphology (2015), AbstractRiver 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 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 A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 2 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 topplingdominated rivers are likely transported 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.

Canyon formation constraints on the discharge of catastrophic outburst floods of Earth and Mars

“…This latter result strongly argues against the hyperconcentration hypothesis. Consistent with the more reasonable volumetric water‐to‐rock ratios suggested by Craddock and Howard [], a water‐to‐rock ratio of 5 × 10 4 yields total flood duration that is very similar to those we estimate from the maximum‐discharge model and more typical of fluvial bedrock incision on Earth [e.g., Lamb et al ., ].…”

“…Undercutting occurs as a result of scouring of rocks where the water jet impinges the plunge pool, by the combined mechanical action of water and transported sediments [ Mason and Arumugam , ; Stein et al ., ; Bollaert , ; Flores‐Cervantes et al ., ; Pagliara et al ., ], while plucking and toppling occur through the action of bed shear stress applied by water flow upstream of the waterfall brink [e.g., Coleman et al ., ; Chatanantavet and Parker , ; Lamb and Dietrich , ; Lamb et al ., ]. In the case of vertically fractured lithologies, Lamb and Dietrich [] proposed that toppling of rock columns by overland flow could explain the morphology of amphitheater‐headed canyons.…”

“…Lamb et al . [] showed that toppling is the dominant erosion mechanism in fractured bedrock as long as block height is at least half of block width, which is typical of canyons carved in basalt with subvertical cooling joints, a common lithology in megaflood terrain on Earth and Mars. For example, toppling during large‐scale floods is thought to have been the main mechanism for erosion at Box Canyon and Malad Gorge, Idaho [ Lamb et al ., , ], and at Ásbyrgi in Iceland [ Baynes et al ., , ].…”

Seeking the Shore: Evidence for Active Submarine Canyon Head Incision Due to Coarse Sediment Supply and Focusing of Wave Energy

“…Shepard and Emery (1941) noted that the largest submarine canyons offshore California are clustered around three mountainous coastal promontories (cf. The effectiveness of this mechanism is likely strongly modulated by the grain size and supply of clastic detritus in the system Lamb et al, 2015;Sunamura, 2018), which is ultimately related to tectonic uplift and the presence of durable bedrock lithologies (Smith et al, 2017). Several classifications have been proposed for submarine canyon littoral connectivity (Romans et al, 2016;Sweet & Blum, 2016); however, no single mechanism has been identified to explain why a select group have avoided being disconnected during post-LGM sea level rise.…”

Bedform genesis in bedrock substrates: Insights into formative processes from a new experimental approach and the importance of suspension-dominated abrasion

“…(2) 227 13 where s is sediment density, Ui is particle impact velocity, D is grain diameter, and µ is 228 dynamic fluid viscosity (e.g., Lamb et al, 2015). For saltating grains the particle impact 229 velocity is calculated using the equation proposed by Wiberg and Smith (1985); however, 230 expressions are not available for suspended load particle impact velocity.…”