Flow in bedrock canyons

Abstract

Bedrock erosion in rivers sets the pace of landscape evolution, influences the evolution of orogens and determines the size, shape and relief of mountains. A variety of models link fluid flow and sediment transport processes to bedrock incision in canyons. The model components that represent sediment transport processes are increasingly well developed. In contrast, the model components being used to represent fluid flow are largely untested because there are no observations of the flow structure in bedrock canyons. Here we present a 524-kilometre, continuous centreline, acoustic Doppler current profiler survey of the Fraser Canyon in western Canada, which includes 42 individual bedrock canyons. Our observations of three-dimensional flow structure reveal that, as water enters the canyons, a high-velocity core follows the bed surface, causing a velocity inversion (high velocities near the bed and low velocities at the surface). The plunging water then upwells along the canyon walls, resulting in counter-rotating, along-stream coherent flow structures that diverge near the bed. The resulting flow structure promotes deep scour in the bedrock channel floor and undercutting of the canyon walls. This provides a mechanism for channel widening and ensures that the base of the walls is swept clear of the debris that is often deposited there, keeping the walls nearly vertical. These observations reveal that the flow structure in bedrock canyons is more complex than assumed in the models presently used. Fluid flow models that capture the essence of the three-dimensional flow field, using simple phenomenological rules that are computationally tractable, are required to capture the dynamic coupling between flow, bedrock erosion and solid-Earth dynamics.

Graffiti for science – erosion painting reveals spatially variable erosivity of sediment-laden flows

“…This passage may channelize the sediment flow even when these blocks are submerged by the flushing water. Further, secondary currents due to turbulence induced by the boulders are also likely to have influenced the sediment distribution (Venditti et al, 2014). We do not have direct measurements of the spatial sediment transport distribution during the flushings, but the erosion painting technique was able to document the crucial influence of sediment routing in setting local erosion rates.…”

Flow structure in large bedrock‐channels: The example of macroturbulent rapids, lower Mekong River, Southeast Asia

“…Widespread occurrences of erosional features such as potholes and flutes are evidence of abrasion by both bedload and suspended load (Meshkova and Carling, 2012). Consequently, the aspect ratios (B/h) of bedrock sections, including high velocity reaches, such as rapids, may be small and likely deviate strongly from those typical of equilibrium alluvial channels (Carter and Anderson, 2006;Venditti et al, 2014). Consequently, the aspect ratios (B/h) of bedrock sections, including high velocity reaches, such as rapids, may be small and likely deviate strongly from those typical of equilibrium alluvial channels (Carter and Anderson, 2006;Venditti et al, 2014).…”

Morphological dynamics of an englacial channel

“…the plunge pool and cave, as water rapidly attains equilibrium temperature , and secondary circulation currents (Venditti et al, 2014) rapidly reduce the excess heat energy.…”

“…Vallé and Pasternack (2006) observed dichotomous hydraulic responses to a change in discharge for both sloping and ballistic jets: the projection length of the ballistic jet increased by 60 % in response to a doubling of discharge, but decreased by 20 % for the sloping jet. The effect of this is that the impact point of a ballistic jet changes more markedly with discharge (Mason and Arumugam, 1985;Stein et al, 1993), and will affect plunge pool geometry through the resulting flow circulation patterns (Venditti et al, 2014). Independent from the types of jet and hydraulic jump, knickzones provide exceptionally high hydraulic resistance and heat dissipation, and it has been argued that step-pool morphology is mutually adjusted with flow and energy ex-penditure (Abrahams et al, 1995;Chin, 2003;Curran and Wohl, 2003).…”

“…1), defined as a critical slope segment preceded and followed by a shallow slope segment (Milzow et al, 2006). Commonly, the step riser is proceeded by a pool segment that is characterised by gradual changes in flow, although the high-velocity plunging flow or jet can cause erosional scour, thereby influencing pool geometry (Stein et al, 1993) and secondary circulation patterns (Venditti et al, 2014).…”