ARS | Publication request: Size of Suspended Sediment Over Dunes

Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 15, 2007
Publication Date: October 16, 2007
Citation: Kuhnle, R.A., Wren, D.G. 2007. Size of Suspended Sediment Over Dunes. In: Proceedings of Hydraulic Measurements and Experimental Methods Conference, Lake Placid, NY, p. 274-279.

Interpretive Summary: Reliable and accurate determinations of the rate of sediment being moved by the flowing water in stream channels are necessary because the sediment may fill reservoirs and reduce their capacity, may fill channels and cause increased incidences of flooding, may degrade water quality and adversely affect aquatic ecosystems, and may cause instability of the channel banks which can cause the destruction of valuable agricultural lands and key components of our infrastructure. Physical, chemical, and biological sediment damage in North America has been estimated to exceed 16 billion dollars annually. The rates of sand transport in streams and rivers is poorly known because accurate predictions or measurements are difficult and expensive to make largely because of complex features such as dunes which commonly occur on stream bottoms. Dunes are triangular shaped forms which have a strong influence on the rates and sizes of sediment the stream is able to move. This study focused on characterizing the size of the sediment grains in motion above the bottom of a model stream channel in which dunes had formed in the laboratory. Being able to predict the sizes of sediment in movement in streams is necessary to allow accurate calculations and to allow new technologies, such as acoustics, to measure sediment movement rates accurately. The results from this study will allow an improved understanding of the interactions between sediment and flowing water and lead to improved measurement and prediction technologies for sediment movement in streams.

Technical Abstract: Samples of suspended sediment were collected at four elevations simultaneously over two-dimensional mobile dunes in 0.5 mm sand in a laboratory flume channel. A constant sampling position relative to the dunes was maintained by adjusting the translation rate of the sampling carriage to be the same as the migration rate of the dunes. The change of the median grain size of the suspended sediment with distance from the bed was found to be related to the part of the dune over which the samples were collected. The median grain sizes of the suspended sediment over the lowest third of the flow depth was nearly constant with depth (slope = -5.1) over the trough of the bed forms, while over the brink point the grain size varied rapidly with depth (slope = -0.4), with intermediate rates of change over the rest of the dune. At heights of about 0.5 the flow depth, the grain size profiles tended to a value of about one half the bed material median size.