Using low-grade weirs as a best management practice for phosphorus and sediment mitigation

Author(s): Baker, B.; Kroger, R.; Prevost, D.; Pierce, T.

Widespread concern for nutrient enrichment of freshwater and marine environments led to the formation of the Mississippi River/Gulf of Mexico Hypoxia Task Force, which aims to reduce riverine loads of total phosphorus from the Mississippi and Atchafalaya River Basins by 45% by 2015. Recent studies highlighted advantages of using low-grade weirs, situated in drainage ditches, to reduce effluent nutrient loads, as opposed to traditional control drainage practices such as variable height risers. The overall objective of this study was to quantify the effects of low-grade weir frequency and spatial arrangement on phosphorus and sediment reduction efficiencies of agriculture runoff using field-based experimental design in the Mississippi Delta. Low-grade weirs are an innovative, relatively low-cost and low-technology best management practice in comparison to large water reservoir systems or bioreactors, making them a suitable option for large and small-scale farmers alike. Study sites were located in the Yazoo Delta Region of Northwestern Mississippi. Results of the phosphorus and sediment load reduction efficiencies of low-grade weirs showed positive reductions in most ditches, with and without weirs. Low-flow and storm-flow outflow concentrations were found to be variable between sites, with no clear significant differences between sites with or without weirs. Mean percent differences between low flow and storm flow at each site resulted in low-flow samples having significantly lower phosphorus and sediment concentrations than during storm flows. A complimentary investigation of hydraulic retention highlighted that control ditches, while not engineered for such purposes, retained water. However, because this was an unintended consequence, these systems also exhibited flooding into producer fields; this phenomenon did not occur in ditches with weirs.
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