Flow in bedrock canyons
| Autor: | Michael Church, Colin D. Rennie, Ryan W. Bradley, Malcolm Little, James Bomhof, Jeremy G. Venditti |
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| Rok vydání: | 2014 |
| Předmět: | |
| Zdroj: | Nature. 513:534-537 |
| ISSN: | 1476-4687 0028-0836 |
| DOI: | 10.1038/nature13779 |
| Popis: | A survey along the Fraser Canyon in Canada reveals complex flow dynamics involving velocity inversions and upwelling, which suggests ways to improve flow and bedrock incision modelling. Canyons carved into the bedrock in areas of actively uplifting terrain are important for our understanding of the link between tectonics, climate and topography. Fluid flow and sediment transport are processes associated with canyon bedrock incision in models, but to date there are no field observations of the flow structure in bedrock canyons that can be used to test the model components that represent fluid flow. This paper describes a survey along the Fraser Canyon in Canada that provides detailed measurements of flow in a narrow bedrock canyon. The data reveal a more complex flow structure than currently assumed in flow models, implying that a revision in our understanding of bedrock canyon incision is required. Bedrock erosion in rivers sets the pace of landscape evolution, influences the evolution of orogens and determines the size, shape and relief of mountains1,2. 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 developed3,4,5. 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. |
| Databáze: | OpenAIRE |
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