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Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/29/2009 Publication Date: 6/9/2009 Citation: Kuhnle, R.A., Wren, D.G. 2009. Size of Suspended Sediment over Dunes. Journal of Geophysical Research. 114, F02020, doi:10.1029/2008JF001200 Interpretive Summary: Accurate knowledge of the rate of sediment being moved by the flowing water in stream channels is important 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 organisms, and may cause instability of the channel banks which can lead to the destruction of valuable agricultural lands and critical 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 ridges which form as flowing water moves the sand along the bottoms of streams and rivers. They have a strong influence on the rates and sizes of sediment the stream is able to move. This study focused on characterizing and predicting the size of the sediment grains in motion above the bottom of a model stream channel with dunes in the laboratory. Being able to predict the sizes of sediment in movement in streams is necessary to produce reliable rate 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 the movement of sand-sized sediment in streams. Technical Abstract: Samples of suspended sediment were collected at four elevations simultaneously over two-dimensional mobile dunes in two mixtures of 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 rate of change of concentration of the suspended sediment with distance from the bed for individual grain sizes was found to be related to the part of the dune over which the samples were collected. The suspended sediment concentration over the lowest third of the flow depth was nearly constant with depth over the trough of the bed forms, while over the brink point concentration varied rapidly with depth, with intermediate rates of change, similar to measurements over a flat bed, over the rest of the dune. The concentration of coarser grains was reasonably explained by assuming they were in intermittent suspension from the brink point of one dune to that of the next. Differences between the vertical changes in grain size were reduced when the sizes were plotted relative to the mean crest elevation of the dunes rather than the local bed elevation. The Rouse equation was found to yield accurate predictions of median grain sizes above the crest level using a reference sample, while median grain size was found to be nearly constant below the level of the crest. |
