Geometrical mechanism of inverse grading in grain-flow deposits: An experimental revelation |
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| Authors: | Prabir Dasgupta Priyanka Manna |
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| Institution: | 1. Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113–0033, Japan;2. Institute of Geological Sciences, Jagiellonian University, Oleandry 2a, 30–063 Kraków, Poland;1. Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 23062, USA;2. Skidaway Institute of Oceanography, University of Georgia, Savannah, GA 31411, USA;3. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208, USA;4. Geological Sciences, California State University Northridge, Northridge, CA 91330, USA;5. School of Oceanography, University of Washington, Seattle, WA 98195, USA;6. National Institute of Water and Atmospheric Research (NIWA), Private Bag 14-901, Kilbirnie, Wellington, New Zealand;7. Department of Geological Sciences, University of Oregon, Eugene, OR 97403, USA;8. Anchor QEA, LLC, San Francisco, CA 94102, USA;9. Geologic Resources Division, National Park Service, Denver, CO 80225, USA;10. Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington, New Zealand;11. Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA;12. East Carolina University, UNC Coastal Studies Institute, Greenville, NC 27858, USA;13. Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235, USA;14. Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695, USA;15. GNS Science, PO Box 30-368, Lower Hutt 5040, New Zealand |
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| Abstract: | The grain-flow has so far been defined with reference to the distinctive sediment-support mechanism, the dispersive pressure. The role of sediment-support mechanism, however, is required in a multiphase flow to prevent the gravitational settling of the particles through the driving medium during the flow. In a single-phase flow of non-cohesive grains no such secondary mechanism is required to counteract the gravitational pull, the driving force of the flow. So the definition of grain-flow needs a critical revision. This, in turn, involves proper understanding of the grain-flow mechanism, so that the relation between the process and the product can be properly established. The most distinctive feature often demonstrated by a grain-flow deposit is the particle size segregation, which leads to the development of inverse grading. The available explanations for this phenomenon find theoretical constraints. In the present study an attempt was made to understand the mechanism of single-phase non-cohesive granular flow of different flow regime and the particle segregation pattern in the resultant deposit through laboratory simulation. The experimental observations revealed that no sustained granular flow sets in on a slope deviating much from the limiting value of the angle of repose of the granular material. A persistent simple shear flow develops on slopes of this critical value. Each of the grains rolls in response to simple shearing. If the shear stress attains a critical value, theoretically the larger grains can even climb up the adjacent smaller ones towards the down-slope direction. In reality, however, high angle climb is not very common. The larger grains preferably roll over the smaller grains when the common tangent becomes almost horizontal or makes a very low angle with the direction of flow, and by this process gradually reaches the upper surface of the flow causing the development of inverse grading. The upper surface of the resultant deposit remains parallel to the sloping substratum. These properties readily distinguish this variety of granular flow from the other natural flows, and the flow may thus be assigned the distinct status of grain-flow. |
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