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Published byCurtis Caldwell Modified over 9 years ago
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Sediment Transport over Ripples and Dunes Stephen R McLean, UC Santa Barbara Jonathan Nelson, USGS, Denver, CO Thanks to: Sandro Orlandi, University of Bologna Lindsay Gary, UCSB
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Ultimate Goal: predict bedform evolution System: Response of flow to topography Response of sediment to flow Response of topography to sediment flux divergence What is the sediment flux over a dune?
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Early, fixed bed experiment- effects of acceleration
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S Shift relative to upstream ramp crest
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Shift relative to reattachment
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Research Goal: predict bedform evolution Predict transport rate (Nelson, et al. tomorrow) Measure transport rate Measure near-bed velocity statistics Determine effects of ripples
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Experiments Goal: determine relationship between flow and transport 1) Characterize flow 2) Measure transport rate
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Instrumentation: Acoustic Doppler Profiler Velocity along four beams (~1 mm bin spacing) ~20 Hz sampling freq; not simultaneous
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Quad set-up
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Streamwise velocity profile- no upstream ramp
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Streamwise velocity profile- with upstream ramp
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Reynolds stress- with upstream ramp
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Instrumentation: Acoustic Doppler Profiler Velocity along four beams (~1 mm bin spacing) ~20 Hz sampling freq; not simultaneous Multiple transducer array (MTA) 32 acoustic transducers (20mm spacing)
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Estimating Transport rate For two-dimensional flow erosion equation yields:
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Flat bed downstream of ramp
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Time stack- no upstream ramp
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Change in bed elevation- no upstream ramp
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Time stack – with upstream ramp
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Change in bed elevation- with upstream ramp
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Stress and transport- no upstream ramp
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Nondimensional transport- no upstream ramp
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Stress and transport- with upstream ramp
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Non-dimensional transport- with upstream ramp
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Stress and transport with and without upstream ramp
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Comparison with and without upstream ramp
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Stress and transport with and without upstream ramp
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Comparison between beginning and end of run
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General Observations Flow/transport relation not strongly affected by acceleration –Transport decreases as acceleration increases –Near-bed flow decreases as acceleration increases also Concept of critical shear stress plays no role in lee of a separation zone Ripples form almost immediately for the flat bed case Ripples form on steep slopes with no upstream flow separation Ripples do not form on steep slopes downstream of flow separation Ripples seem to enhance transport
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Erosion equation versus ripple migration
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