An ASAE Meeting Presentation DESIGN OF AN ORIFICE AND WEIR OUTLET FOR POORLY DRAINED FORESTED WATERSHEDS

Orifice-weir structures are being used to reduce peak drainage rates and to store water during the growing season in poorly drained pine plantations. Earlier studies have demonstrated their effectiveness in reducing drainage outflows while conserving water during the growing season. This study reports on criteria and preliminary guidelines for designing such structures. The guidelines were developed using both observed data and results of DRAINMOD simulations for multiple combinations of outlet sizes and poorly drained mineral and organic soils. The main objective functions were reducing flow rates higher than 5 mm/day, minimizing wet days e.g. water table depths within 30 cm depth from the surface, and maintaining water table within 45 to 120 cm depth. Results show that the size and the depths of the orifice and the weir primarily depend upon the main objective function chosen followed by the soil type and the watershed area. Wet days increased with smaller orifice sizes and shallower weir depths. For example, for a watershed of 25 ha, a 7.5cm or a 10 cm diameter orifice in a flat weir with the crest 25 cm below the surface would result in the same % time having peak drainage rates for both the organic and the mineral soils. However, the number of wet days was 3 times higher in the organic soil than in the mineral soil. Although an orifice placed above ditch bottom yielded higher frequency of water table depth between 45 cm and 120 cm depth as desired compared to the one near ditch bottom, this must be weighted against two other objectives. This indicates two things: 1) for the same drainage density and area, a larger orifice size may be required in the organic soil to achieve the same reduction in wet days as in the mineral soil; 2) the orifice size and its location depends upon the primary objective function chosen.