Sediment and Phosphorus Accumulation Dynamics behind Newly Installed Low-Grade Weirs in Agricultural Drainage Ditches

Low-grade weirs are controlled surface drainage management structures that are being advocated for improving water quality of downstream ecosystems within agricultural landscapes. Low-grade weirs are small controlled drainage structures that are placed in series in an agricultural drainage ditch to create biogeochemical conditions for enhanced nutrient retention but also to slow runoff velocities and encourage sedimentation. This study evaluated eight low-grade weirs for sediment and P accumulation in agricultural drainage ditches 1 yr after installation. On average, sediment (weir: 54 +/- 25cm; reference: 13 +/- 7 cm; P <= 0.001) and water (weir: 9 +/- 7cm; reference: 1.16 +/- 2 cm; P <= 0.05) depth was greater upstream of weirs when compared within ditch references sites. There were no significant differences in total P concentrations of sediments or between any P fractions between 1-yr-old weirs and their respective references. Bioavailability ratios of P (i.e., the ratio of potentially bioavailable to nonbioavailable P fractions) were also found to be similar between weir and reference sites. Based on these results, weirs increase the hydrological capacity of drainage ditches and significantly retain more sediment and P within the drainage ditch within 1 yr of construction. Future research will examine temporal changes in weir sediments and associated P concentrations to aid our understanding of how maintenance of weirs should occur to maximize physical and chemical characteristics for greatest sediment and P retention.