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Submitted to: Joint Federal Interagency Sedimentation and Hydrologic Modeling
Publication Type: Proceedings Publication Acceptance Date: 2/17/2015 Publication Date: 4/19/2015 Citation: Kuhnle, R.A., Wren, D.G. 2015. Effective particle sizes of cohesive sediment in north Mississippi streams. Joint Federal Interagency Sedimentation and Hydrologic Modeling. Reno, Nevada, April 19-23, 2015. pp. 259-268. Interpretive Summary: Knowledge of the sizes of sediments being carried by a stream or river is necessary for the study of sediment transport by flowing water. The grain size along with other physical properties of the sediment, including the composition, density, and shape are needed for determining how the sediment will be transported and how it will interact with materials in the water column such as chemicals and aquatic biota. For many years it was assumed by practitioners that fine sediments (less than 62 microns in diameter) in streams and rivers were transported as individual particles. Recent studies have shown that fine sediments in motion in stream channels are transported as a collection of particles often many times the diameter of the primary particles. To improve our knowledge of the actual or effective size that fine particles assume when in motion in channels, measurements of their size in place during flow events were made in stream channels at three locations in north Mississippi. Measurements from these three indicated that effective grain sizes of the fine particles ranged from six to twenty one times the diameter of the primary particles. This information will add to our knowledge of effective sizes of fine sediments and allow constraints to be placed on fine sediment size for these channels as well as for others. This data is required for the improvement of the predictive capabilities of channel and watershed models which are important tools for the management and control of sediment movement by streams and rivers that drain agricultural and other lands. Technical Abstract: Knowledge of the size of cohesive sediment particles transported in streams is important information for predicting how the sediment and contaminants the sediment may be carrying will be transported by the flow. Cohesive sediments (less than 0.062 mm in diameter) generally are not transported in their primary dispersed particle sizes, but commonly assume larger sizes as aggregates and/or as flocs. Due to a lack of models for predicting effective particle sizes of fine sediments in streams, it is common for researchers and practitioners to assume that the sediment transported in a stream assumes the primary size of the sediment. Information on the effective sizes of fine sediment in streams is available for only a small number of streams. More data on the effective size of cohesive particles in streams is needed to allow more accurate modeling of their transport. In this investigation, measurements of the effective particle size were made using a portable laser particle size analyzer which was deployed in three streams during runoff events. The primary particle sizes were determined from samples of the suspended sediment which were collected in close proximity to and at the same time as the effective particle sizes were measured. The effective particle sizes were found to be coarser than the primary sizes and did not vary in a predictable manner with the stage of the streams during the runoff events. Ratios of the effective mean sediment size to the primary mean sediment size ranged from 6 to 21 for the three streams considered. |
