|Langendoen, Eddy - UNIV. OF MISSISSIPPI|
Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: December 23, 2002
Publication Date: December 23, 2002
Citation: SIMON, A., BINGNER, R.L., LANGENDOEN, E.J., ALONSO, C.V. ACTUAL AND REFERENCE SEDIMENT YIELDS FOR THE JAMES CREEK WATERSHED - MISSISSIPPI. USDA-ARS NATIONAL SEDIMENTATION LABORATORY RESEARCH REPORT. 2002. NO. 31. 185 PP. Interpretive Summary: The development of sediment Total Maximum Daily Loads (TMDLs) requires knowledge of the current or impaired condition and the reference or unimpaired condition of a watershed. A study was performed to link USDA technology that determines reference suspended transport rates from stable streams from similar watersheds to an upland watershed in Mississippi at the current conditions where impairment has been determined. Information was collected from field measurements or available data to assess the reference sediment transport rates and to evaluate the current conditions of the watershed by applying USDA watershed and channel evolution computer technology. Sediment transport rates from current impaired conditions were found to be over 17 times greater than unimpaired conditions, where 70 percent of the sediment from impaired conditions originated from the streams. Thus, remedial measures may need to be focused on instream erosion control practices, while also working with the agricultural producers, to prevent sediment from impairing the watershed. From this approach, action agencies can develop sediment TMDLs that are targeted for specific watersheds using readily available technology that is based on sound science and then implement plans that reduce sediment based on its source.
Technical Abstract: This study determined "actual" sediment-transport rates and rates for similar, but stable or unimpaired ("reference") streams that could be used to develop water-quality targets for sediment. "Reference" rates were obtained from historical stream flow and sediment load data in the Southeastern Plains Ecoregion. Using the 1.5-year return flow as the effective discharge, a weighted-reference condition based on the percentage of the drainage area encompassed by the various bed-material types resulted in a reference yield at the effective discharge of 2.22 T/d/km2. "Actual" transport rates were obtained from a combination of: simulations of sediment yield using the watershed model AnnAGNPS; direct comparisons of measured cross sections from 1967 and 2002; and simulations of channel flow and sediment transport by the channel-evolution model CONCEPTS. "Actual" suspended-sediment yields at the effective discharge show a 35-year average of 675 T/d/km2. However, the average over the past 10 years is 155 T/d/km2 and, following the installation of additional low-water crossings in 1999 further reduced yields to 39 T/d/km2. Results of this study indicate that a significant proportion of the sediment in the James Creek watershed emanates from stream channels. Decisions regarding reducing sediment loadings will need to pay particular attention to stream-channel processes and stabilizing eroding reaches and tributaries of the watershed.