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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #75638

Title: THE FLUID AND SEDIMENT DYNAMICS OF UPPER-STAGE PLANE BEDS

Author
item Bennett, Sean
item BRIDGE, JOHN
item BEST, JAMES

Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/28/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: To understand more fully the fluid and sediment dynamics of upper-stage plane beds, laboratory experiments were conducted where turbulent motions of both fluid and sediment were measured using Phase Doppler Anemometry (PDA). Experiments were also conducted on fixed, slightly wavy and flat beds with no sediment in motion in order to isolate the effects of sediment and low-relief bed waves. The bedforms on mobile upper-stage plane beds were a few millimeters high and close to a meter long. Vertical profiles of time-averaged flow velocity, eddy length-scale and eddy viscosity (strength) are represented well by the law of the wall for the entire flow depth. For the mobile bed, von Karman's coefficient is about 0.33 and equivalent sand roughness-to-mean bed-grain size ratio varies from 9 to 17, due to the presence of bed load and low-relief bed waves. For the fixed beds with no sediment transport, von Karman's coefficient is about 0.41 and equivalent sand roughness is approximately equal to the mean bed-grain size, as expected. The turbulent movement of suspended sediment is shown to be very similar to the movement of the fluid. The Rouse equation for equilibrium suspended-sediment concentration fits the data well near the bed, but under-predicts concentration higher in the flow because the sediment diffusion term is underestimated. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds due to enhanced wake formation and eddy shedding from the downstream-side of near bed grains and the bedforms.

Technical Abstract: To understand more fully the fluid and sediment dynamics of upper-stage plane beds, laboratory experiments were conducted where turbulent motions of both fluid and sediment were measured using Phase Doppler Anemometry (PDA). Experiments were also conducted on fixed, slightly wavy and flat beds with no sediment in motion in order to isolate the effects of sediment and low-relief bed waves. Bed-elevation fluctuations on mobile upper-stage plane beds ranged from a fraction of a millimeter to several millimeters, and periods ranged from a few seconds to several hundred seconds. Vertical profiles of time-averaged flow velocity, mixing length and eddy viscosity are represented well by the law of the wall for the entire thickness of the boundary layer. For the mobile bed, von Karman's coefficient is about 0.33 and equivalent sand roughness-to-mean bed-grain size ratio varies from 9 to 17, due to the presence of bed load and low-relief bed waves. For the fixed beds with no sediment transport, von Karman's coefficient is about 0.41 and equivalent sand roughness is approximately equal to the mean bed-grain size, as expected. Sediment diffusivities are shown to be very similar to fluid diffusivities. The Rouse equation for equilibrium suspended-sediment concentration fits the data well near the bed, but under- predicts concentration higher in the flow. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds due to enhanced wake formation and eddy shedding from the lee-side of near bed grains and low-relief bed waves. Analysis of upward and downward velocity fluctuations reveals a net upward momentum flux throughout the boundary layer, as predicted by Bagnold [1966].