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United States Department of Agriculture

Agricultural Research Service

Research Project: INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS Title: Effects of silt loading on turbulence and sand transport

Authors
item Wren, Daniel
item Kuhnle, Roger

Submitted to: International Journal of Sediment Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 10, 2012
Publication Date: December 1, 2012
Citation: Wren, D.G., Kuhnle, R.A. 2012. Effects of silt loading on turbulence and sand transport. International Journal of Sediment Research. 27(4):451-459.

Interpretive Summary: The transport of bed material and fluid turbulence are affected by many factors, including the amount of silt being carried in a channel. Current research has focused on sand-sized particles introduced to gravel beds, while the effect of silt load on sand transport has received less attention. Experiments were performed over a range of silt concentrations, flow conditions, and bed configurations. This data can used to assess the effect of silt concentrations, such as soils eroded from plowed fields, on sediment transport in receiving channels. Measurements of flow and sand/silt transport were made using an instrument that used the density of the fluid/sediment mixture to determine the concentration. Three acoustic instruments were used to measure water flow and turbulence. Sand transport in the presence of increasing silt load did not change in a consistent manner. Sand transport increased with silt loading up to 4 g/l for dune-covered beds and up to 2 g/l for dune-covered beds with lower flow strength and then declined to near the clear water case with increasing concentrations of silt. There was low range of transport rates and inconsistent trend with silt addition for the ripple case. The length and height of dunes were generally lower with increasing silt concentration. A clear trend of decreasing turbulence with increasing silt concentration was observed in the ripple case, with a 33% reduction in one key turbulence parameter caused by an 8.9 g/l concentration of silt.

Technical Abstract: The transport of bed material and fluid turbulence are affected by many factors, including the fine sediment load being carried in a channel. Current research has focused on sand-sized particles introduced to gravel beds, while the effect of silt load on sand transport has received less attention. Experiments on the effects of silt load on sand transport were performed with a recirculating laboratory flume using three different sand bed configurations: ripples, dunes (Fr-0.34), and dunes (Fr=0.48). For each configuration, the effects of a range of silt loadings were evaluated; the range of silt concentrations in the study was 0-26.9 g/l. Sand and silt load were measured using physical samples of the water/sediment mixture and a density cell. Three Acoustic Doppler Velocimeters were arranged to measure simultaneously in one vertical and used to measure flow and turbulence quantities. Sand transport did not change in a consistent manner with increasing silt load, increasing up to 4 g/l for dunes (Fr=0.48) and up to 2 g/l for dunes (Fr=0.34) and then declining to near the clear water case with increasing concentrations of silt. Silt addition for the ripple case caused a relatively small change in the rate of sand transport, decreasing with added silt up to approximately two g/l and then increasing as the silt concentration went up to approximately 10 g/l. The length and height of dunes were generally lower with increasing silt concentration. A clear trend of decreasing Reynolds stress with increasing silt concentration was observed in the ripple case, with a 33% reduction in near-bed Reynolds stress caused by an 8.9 g/l concentration of silt.

Last Modified: 12/21/2014
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