Sediment Transport over Ripples and Dunes Stephen R McLean, UC Santa Barbara Jonathan Nelson, USGS, Denver, CO Thanks to: Sandro Orlandi, University of.

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Presentation transcript:

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

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?

Early, fixed bed experiment- effects of acceleration

S Shift relative to upstream ramp crest

Shift relative to reattachment

Research Goal: predict bedform evolution Predict transport rate (Nelson, et al. tomorrow) Measure transport rate Measure near-bed velocity statistics Determine effects of ripples

Experiments Goal: determine relationship between flow and transport 1) Characterize flow 2) Measure transport rate

Instrumentation: Acoustic Doppler Profiler Velocity along four beams (~1 mm bin spacing) ~20 Hz sampling freq; not simultaneous

Quad set-up

Streamwise velocity profile- no upstream ramp

Streamwise velocity profile- with upstream ramp

Reynolds stress- with upstream ramp

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)

Estimating Transport rate For two-dimensional flow erosion equation yields:

Flat bed downstream of ramp

Time stack- no upstream ramp

Change in bed elevation- no upstream ramp

Time stack – with upstream ramp

Change in bed elevation- with upstream ramp

Stress and transport- no upstream ramp

Nondimensional transport- no upstream ramp

Stress and transport- with upstream ramp

Non-dimensional transport- with upstream ramp

Stress and transport with and without upstream ramp

Comparison with and without upstream ramp

Stress and transport with and without upstream ramp

Comparison between beginning and end of run

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

Erosion equation versus ripple migration