New experiments to probe the role of fractures in bedrock on river erosion rate and processes

Abstract. River erosion is a fundamental process that impacts, among others, mountain landscape evolution. Mountain rock lithologies often exhibit bedding, joints, and fractures that are thought to alter the incision efficiency of rivers compared to intact, massive rocks. The presence of close enough planar mechanical discontinuities allows the creation and entrainment of large blocks through plucking, a process that adds to abrasion, and potentially macroabrasion, by the transported sediment. Despite preliminary attempts to include shallow fracturing in theoretical models of bedrock incision and a couple of studies that quantified the relative importance of abrasion and plucking processes in situ, we are still lacking ways to systematically probe the role of fractures on bedrock erosion rates and processes. Due to the complex interactions at play, here we investigate this question via an experimental approach, using a new erosion mill designed to erode a fractured concrete disk with a diameter of 17 cm. We simulate vertical or dipping fractures by embedding a 3D-printed plastic mesh in the concrete, using BVOH – a plastic that softens in cold water – creating mechanical weaknesses with a controlled pattern. We explore 10 different geometries and run 4 additional experiments without fractures for control. We record the topographic evolution every 2 min by photogrammetry and derive erosion maps by measuring elevation changes between successive scans. Our results show that fractures influence the morphodynamical evolution of the disks and the relative contributions of abrasion and plucking. However, abrasion systematically remains the dominant erosion mechanism, with plucking contributing at most to 29 % of the total erosion for vertical fractures spaced by 20×20 mm2, 40 % for one specific dip angle (67°), and less than 10 % for most experiments. Average erosion rates show a modest (20 %) increase with the fraction of plucking, but do not show a clear relationship with fracture density and the presence of fractures. We suggest that the rate of erosion by plucking is limited by the depth and slow rate of horizontal fracture propagation between pre-existing vertical fractures, such that in our experimental setup, abrasion is systematically a dominant component. These findings emphasize the critical role of block preparation and loosening for plucking to be an effective process compared to abrasion. This new setup allows abrasion, macroabrasion, and plucking driven by bedload impacts to be studied in controlled situations, albeit with the well-known limits of abrasion mills and without the variety of natural processes that can drive fracture propagation. Further experiments should expand the parameter space of the erosion efficiency problem (i.e., sediment mass, grain size, flow velocity, intact rock mass strength, 3D fracture patterns) to help in developing mechanistic models applicable in natural environments.