Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/9/2003
Publication Date: 6/15/2003
Citation: KUHNLE, R.A., WREN, D.G. SPATIAL VARIATIONS IN SUSPENDED SEDIMENT TRANSPORT OVER DUNES. WORLD WATER & ENVIRONMENTAL RESOURCES CONGRESS, ASCE, PHILADELPHIA, PA. 2003. CD-ROM. Interpretive Summary: Excess sediment being moved by streams has the potential to deposit in reservoirs and reduce their capacity for water storage, to deposit in channels which reduces their capacity to carry flows and may cause flooding, to degrade water quality, and to cause instability of the channel boundary which can cause the destruction of valuable agricultural and other lands, as well as other property. Thus accurate information on the rate of sediment carried in the water column of streams is necessary for informed management of the lands of a watershed. During the flow of water, sand on the bottom of a stream generally becomes formed into a series of high and low places (dunes) which vary appreciably across and down a stream channel. The formation and movement of dunes along the bottom of a stream greatly affects the amount and distribution of sediment carried in the water. Experiments were conducted in a model laboratory stream channel to measure and characterize the variation in sediment movement across the channel. Samples collected at the same time at two locations across the channel varied by up to 50%. This information is needed to improve prediction and sampling techniques which will allow better management practices for agricultural and other watersheds.
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 was 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. 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 (one third to one twenty fourth of the channel width). Also, acoustic instruments were 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 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.