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

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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. Without historical sediment-transport data for James Creek a combination of methods was used including analysis of historic data from other sites, data collection, and numerical modeling of uplands and channels. “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/km2 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/km2 at the Q1.5. A weighted-reference condition based on the percentage of the drainage area encompassed by the various bedmaterial types results in a reference yield at the Q1.5 of 2.2 T/d/km2. 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 model AnnAGNPS; direct comparisons of measured cross sections from 1967 and 2002; and by simulations of channel flow and sediment transport by the channel-evolution model CONCEPTS. AnnAGNPS was used for uplands and CONCEPTS for the main-stem channel. 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/km2; 155 T/D/km2 over the past 10 years. Following the installation of low-water crossings in 1999 loads decreased to about 39 T/D/km2. This value is more than an order of magnitude greater than the “reference” yield.
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Document Type: Research Article

Publication date: 01 January 2003

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