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United States Department of Agriculture

Agricultural Research Service

Title: Lateral variations in suspended sediment concentration over dunes

Authors
item KUHNLE, ROGER
item Wren, Daniel - UNIV. OF MISSISSIPPI

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 21, 2006
Publication Date: December 1, 2006
Citation: Kuhnle, R.A., Wren, D.G. 2006. Lateral variations in suspended sediment concentration over dunes. Journal of Hydraulic Engineering. 132(12), 1341–1346.

Interpretive Summary: Accurate determinations of the rate of sediment being moved in the water column of stream channels are necessary because the sediment may fill reservoirs and reduce their capacity, may fill channels and cause flooding, may degrade water quality, and may cause instability of the channel banks which can cause the destruction of valuable agricultural and other lands. Knowledge about the variability of sand transport across a channel is poorly known. During water flow the sand bed of many streams becomes molded into a series of high and low features (dunes) which vary dramatically both across and down the channel. These dunes greatly affect the amount of sediment carried in the water column. A series of experiments were conducted in a model stream channel in the laboratory to measure and characterize the changes in the amount of sediment in the water column across the channel. Two samples collected at the same time across the channel were found to vary by as much as 42%. This study has led to a more complete understanding of the variation of sand movement across a sand-bottom stream or river channel. The information from this study is critical for improving sediment prediction and sampling techniques and will lead to advances which will allow agricultural and other watersheds to be managed in a more informed and environmentally sensitive manner.

Technical Abstract: The magnitude of the lateral variations in the concentration of suspended sediment over dunes in an alluvial sand-bed channel are poorly known. Characterizing the lateral distributions of suspended sediment is important for understanding its causes and for accurate measurement of the rate of sediment transport. A series of laboratory experiments were conducted in a laboratory flume to characterize the lateral variations of suspended sediment over dunes. Forty experimental runs were made using flow depths of 0.33 and 0.13 m in a 1.2 m wide flume channel. The Froude number was 0.5 and the median diameter of the bed material was 0.52 mm. Samples of suspended sediment and acoustic backscatter data were collected simultaneously at two lateral positions at spacings of 0.40, 0.20, 0.10 and 0.05 m (one third to one twenty fourth of the channel width). Suspended sediment grain size was found to decrease from 0.17 to 0.13 mm over 0.2 to 0.8 of the flow depth. Mean absolute differences between the paired samples were shown to increase and then level off to values between 25 and 40 percent of the mean concentration as the lateral spacing between samples was increased to between 1 to 2 times the flow depth. An analysis of the sediment concentration distributions indicated that from 8-16 sample positions across the channel would be required to define the mean suspended sediment concentration with a confidence interval of ± 4 to 7.5% of the mean concentration with a probability of 0.95. This study provides new information for understanding lateral variations in the concentration of suspended sand over dunes and new information for devising effective strategies for sediment sampling.

Last Modified: 9/29/2014
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