Effectiveness of low grade weirs to reduce sediment and nutrients loads in agricultural ditches of the Mississippi Delta

Author(s): Ramirez-Avila, J.; Poganski, B.; Kröger, R.

Drainage ditches are an essential component of the agricultural landscape. Ditches mediate the flow of pollutants from agroecosystems to downstream water bodies. Low-grade weirs established along drainage ditches, as an agricultural best management practice, have been evaluated as an effective measure to mitigate nutrient and sediment loads to downstream aquatic systems. A study was performed to estimate sediment and nutrient loading reductions and to determine the cost-effectiveness curve of implementing low grade weirs in agricultural drainage ditches systems in the Mississippi Delta. The study goals were addressed by combining field data collection and computational modeling techniques. Runoff volumes flowing downstream of low grade weirs along different drainage ditch systems during stormflow and irrigation events were estimated by using the Hydrologic Engineering Center - River Analysis System (HEC-RAS 4.1) model. Monitored sediment and nutrient concentrations and the generated runoff flows were used to develop representative rating curves for each low grade weir on each drainage ditch system. Runoff hydrograph flows on each event were routed through the generated rating curves to estimate instantaneous and total sediment and nutrient loads at each location. Loads were compared to determine low grade weirs efficiency inside each drainage ditch and between drainage ditch systems. A second part of the study used the System for Urban Stormwater Treatment and Analysis Integration (SUSTAIN 1.2) to determine the most cost-effective solutions for meeting proposed agricultural runoff water quality conditions. Results from this study will provide more insights to further enhance the nutrient reduction strategy within the Mississippi Delta region.

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