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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #103378

Title: SPECTRAL ANALYSIS OF TURBULENT FLOW AND SUSPENDED SEDIMENT TRANSPORT OVER FIXED DUNES

Author
item VENDITTI, JEREMY
item Bennett, Sean

Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Sediments eroded from agricultural areas due to farming practices are introduced to streams and rivers. The transport and deposition of these solids and their associated agrichemicals can have detrimental effects on water quality, ecology and river flow processes such as conveyance and the severity of floods. Transport of suspended solids has traditionally be related to the characteristics of the flow, yet little quantitative information exists which defines this link. A laboratory study was conducted to determine how the characteristics of the flow can been related to the transport of suspended solids. This study used a laboratory channel with triangular-shaped roughness elements on the bed. Flows velocity was measured with an acoustic probe, and the concentration of the suspended solids within the water was measured with a turbidity meter. Analysis of the data showed that the time variation in the downstream velocity component and the sediment concentration were very similar. This demonstrates the very close link between the transport of suspended solids and flow velocity in rivers. The goal of this research is to provide faster and more accurate technologies for predicting total sediment load in streams and rivers. This information is important to federal agencies in their assessment of sedimentation problems such as loss of ecological habitat and increased flood potential.

Technical Abstract: Laboratory measurements of turbulent fluctuations in velocity and suspended sediment concentration were obtained over fixed, two-dimensional dunes synchronously in a sediment-starved flow. The flow separation cell and a perturbed shear layer are the main sources of turbulence production, and that the distribution of suspended sediment is controlled by macroturbulent flow structures. Spectral analysis reveals that peak spectral energies generally occur at 1 to 2 Hz for the downstream velocity component and 2 to 4 Hz for the cross-stream and vertical velocity components. Peak spectral energies for suspended sediment concentration occur near 1 Hz throughout the flow. Co-spectral analysis shows that ejection-events and sweep-events are well correlated with suspended sediment concentration, but over a broad range of frequencies. Integral time-scale for velocity ranges from 0.2 s for the downstream component to 0.06 s for the cross-stream and vertical components. Integral length scale for velocity ranges from 0.065 to 0.135 m for the downstream component, which is comparable to flow depth, and from 0.02 to 0.03 m for the cross-stream and vertical components, which is comparable to dune height. For suspended sediment concentration, integral time and length scales were similar to the downstream velocity component.