Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/1/2005
Publication Date: 4/2/2006
Citation: Kuhnle, R.A., Wren, D.G., Chambers, J.P. 2006. Prediction of the grain size of suspended sediment; implications for calculating suspended sediment concentrations using single frequency acoustic backscatter. In: Proceedings of the 8th Federal Interagency Sedimentation Conference, April 2-6,2006, Reno, Nevada. CDROM. Interpretive Summary: Accurate determinations of the rate of sediment being moved in the water column of stream channels are necessary because the sediment may fill reservoirs and reduce their capacity, may fill channels and cause 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. Knowledge of the rates of sand transport in streams and rivers is poorly known because accurate measurements are difficult and expensive to make. One method that shows promise to decrease the cost of obtaining sediment movement information is the use of backscatteed sound energy (acoustics) off sediment grains suspended in the water column 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 channel in a laboratory allowed methods to be tested which predicted sediment size and ultimately allowed sediment rates in rivers and streams to be determined using acoustics. This information is a key component needed to make measurements of sediment in the water column using acoustics practical. The information from this study is critical for 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: The collection of samples of suspended sediment transported by streams and rivers is difficult and expensive. Emerging technologies, such as acoustic backscatter, have demonstrated the promise to decrease the cost and allow more thorough sampling of sediment in transport in the water column. One critical piece of information required to use single frequency acoustic backscatter to calculate the concentration of sand-sized sediment in the water column, is the vertical distribution of sediment particle size. In this study, techniques to predict the size of suspended sand particles are examined and their use with acoustic backscatter data to predict sediment concentration is explored. Methods to predict the size of sediment in suspension using bed sediment and flow criteria had mean absolute differences of from 7 to 50 percent as compared to measured values. When the sample nearest to the bed of the stream was used as a reference, the mean absolute differences between calculated and measured sizes were reduced to 5 percent. These errors in size determination translate into errors of 12 to 84 percent in the prediction of sediment concentration using backscatter data from 1 MHz single frequency acoustics.