Submitted to: Proceedings of Sediment Monitoring Instrument and Analysis Research Workshop
Publication Type: Proceedings
Publication Acceptance Date: 2/11/2004
Publication Date: 5/1/2005
Citation: Kuhnle, R.A., Wren, D.G. 2005. Cross-stream variations in suspended sediment transport over dunes, implications for sampling. In: Gray, J.R. (Ed.),Proceedings of the Federal Interagency Sediment Monitoring Instrument and Analysis Research Workshop, September 9-11, 2003, Flagstaff, AZ, U.S. Geological Survey Circular 1276, Appendix 4. Interpretive Summary: The rate of sediment being moved in the water column of stream channels is necessary information 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 the 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. Twenty experimental runs were made using a flow depth of 0.13 m in a 1.2 m wide flume channel (Fr = 0.5, median diameter = 0.52 mm). Suspended sediment concentrations were simultaneously sampled at two points located at the same depth but spaced laterally at intervals of 0.40, 0.20, 0.10 and 0.05 m. Also, acoustic instruments wee used to obtain high-resolution bed profiles and depth-integrated suspended sediment concentration for the lower region of the flow. Mean absolute differences between paired suspended sediment concentrations 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. An analysis of the sediment concentration distributions indicated that 16 sample positions across the channel would be required to define the mean suspended sediment concentration with a confidence interval of less than 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.