STAGE-DISCHARGE RELATIONSHIPS OF DRAWDOWN PLATES FOR DENITRIFYING WOODCHIP BIOREACTORS

Denitrifying woodchip bioreactors are a conservation drainage practice used to reduce nitrate loads in tile drainage water from field crops. Prolonged saturation in the woodchip media under no flow or low flow conditions can result in the production of hydrogen sulfide and methane. The US Department of Agriculture Natural Resources Conservation Service’s (NRCS) Conservation Practice Standard for denitrifying bioreactors recommends a low-level orifice at the bioreactor outlet to drain the woodchip media within 48 h to prevent prolonged stagnation. This study determined stage-discharge relationships for three drawdown configurations in an AgriDrain water level control structure (15.2 cm nominal tile size) to improve accuracy of estimating flow in field bioreactors. Tested drawdown configurations consisted of a custom-made inverted-v orifice plate currently used by the NRCS (3.6 cm² orifice area); and a 2 and an 8 mm gap between the base of the structure and the bottom stop log (2.6 and 12.3 cm² orifice area, respectively). The stage-discharge relationship for all three orifices was accurately described by the orifice flow equation (R² > 0.99). Stage-discharge relationship for the inverted-v drawdown plate in combination with a stainless steel-edged v-notch weir was the sum of their respective stage-discharge relationships for orifice flow and flow over the weir. Contraction coefficients for the inverted-v, 2 mm gap, and 8 mm gap configurations were 0.74, 0.74, and 0.76, respectively. The stage-discharge relationships for the inverted-v, 2 mm gap, and 8 mm gap configurations were Q = 3.26*A*h^0.5, Q = 3.29*A*h^0.5, and Q = 3.38*A*h^0.5, respectively, where Q is in m³ s^-1, A is the orifice area in m², and h is the water head differential in meters.