Submitted to: International Journal of Sediment Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/24/2007
Publication Date: 3/1/2007
Citation: Kuhnle, R.A., Wren, D.G., Chambers, J.P. 2007. Prediction of the grain size of suspended sediment: implications for calculating suspended sediment concentrations using single frequency acoustic backscatter. International Journal of Sediment Research, 22(1): 1-15. Interpretive Summary: Physical, chemical, and biological sediment damage in North America has been estimated to exceed 16 billion dollars annually. The Environmental Protection Agency has determined that sediment is one of the largest pollutants of the nation’s waterways. Yet accurate rates of sediment moved by streams are difficult to determine because of the problems with having equipment and personnel on site during often infrequent sediment movement events and because of the high cost (20,000 to $65,000 per year) associated with collecting daily sediment rates. One method which holds promise to decrease the cost of obtaining sediment movement information is the use of backscattered sound energy (acoustics) off sediment grains suspended in the flowing water of streams and rivers. To allow the use of acoustics to measure sediment movement, however, the size of the sediment grains in the water column must be known independently. Information collected on two rivers and in a model river channel allowed methods to be tested which predicted sediment size and ultimately may make the determination of sediment rates in rivers and streams using acoustics more reliable. This information is a key component needed to make measurements of sediment moving in the flows of streams and rivers using acoustics practical. The information from this study is an important step towards making sediment sampling affordable in many more areas and will potentially provide improved information to allow agricultural and other watersheds to be managed in a more informed and environmentally aware manner.
Technical Abstract: Collection of samples of suspended sediment transported by streams and rivers is difficult and expensive. Emerging technologies, such as acoustic backscatter, have promise to decrease costs and allow more thorough sampling of transported sediment in streams and rivers. Acoustic backscatter information may be used to calculate the concentration of suspended sand-sized sediment given the vertical distribution of sediment size. Therefore, procedures to accurately compute suspended sediment size distributions from easily obtained river data are badly needed. In this study, techniques to predict the size of suspended sand are examined and their application to measuring concentrations using acoustic backscatter data are explored. Three methods to predict the size of sediment in suspension using bed sediment, flow criteria, and a modified form of the Rouse equation yielded mean suspended sediment sizes that differed from means of measured data by 7 to 50 percent. When one sample near the bed was used as a reference, mean error was reduced to about 5 percent. These errors in size determination translate into errors of 7 to 156 percent in the prediction of sediment concentration using backscatter data from 1 MHz single frequency acoustics.