Combined Geomorphic and Numerical-Modeling Analyses of Sediment Loads for Developing Water-Quality Targets for Sediment

Abstract

The principle objective of the study was to determine sediment loads for James Creek, Mississippi and for similar, but stable “reference” streams to develop water-quality targets for sediment. “Reference” sediment-transport loads were determined from stable streams with historical flow and sediment-transport data in the Southeastern Plains Ecoregion. Using the discharge that occurs, on average every 1.5 years (Q1.5) as the “effective discharge,” an initial “general reference” of 0.31 T/d/km was obtained. This value, however, is skewed towards streams with sand beds and does not accurately reflect conditions along James Creek. A refined “reference” condition was developed for stable silt/clay-bed streams in the Southeastern Plains resulting in a “reference” suspended-sediment yield of 3.23 T/d/km at the Q1.5. A weighted-reference condition based on the percentage of the drainage area encompassed by the various bed-material types results in a reference yield at the Q1.5 of 2.2 T/d/km. Similarly, a weightedreference concentration of 160 mg/l was obtained. “Actual” sediment-transport loads were obtained by: simulations of flow and sediment transport using the Simon is a Research Geologist at the USDAARS, National Sedimentation Laboratory, Oxford, MS 38655. E-mail: asimon@ars.usda.gov. Bingner is an Agricultural Engineer, Langendoen is a Research Hydraulic Engineer, Wells is a Postdoctoral Researcher , and Alonso is Research Leader, all at the USDAARS, National Sedimentation Laboratory, Oxford, MS 38655. . model AnnAGNPS and by simulations of channel flow and sediment transport by the channel-evolution model CONCEPTS. Average sediment loads at the mouth of James Creek over the 35-year period are about 250,000 T/y with 88% emanating from channels and 12% from upland sources. This loading value, however, is somewhat misleading in that severe channel erosion occurred between 1967-1968 following channel clearing and snagging over the lower 17 km. Since this time, sediment loads attenuated and the contribution from channels and uplands over the period 1970-2002 shifted to 70% and 30%, respectively. “Actual” simulated suspended-sediment loads at the Q1.5 show a 35-year average of 675 T/D/km; 155 T/D/km over the past 10 years. Following the installation of low-water crossings in 1999 loads decreased to about 39 T/D/km. This value is more than an order of magnitude greater than the “reference” yield.