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Detention ponds are important units of drainage system for managing excess rainfall and runoff quality. To design detention ponds the outflow characteristic must be accurately known. Perforated riser pipes are a popular outlet control structure for stormwater detention basins. This study examines the discharge characteristics of a circular orifice cut into a vertical riser pipe as an outflow mechanism for the detention pond. In particular, the effects of the head above the orifice, the size of the orifice and the size of the riser pipe on the discharge coefficient are evaluated. A physical model is built and various size riser pipes are installed in the tank to simulate a detention basin. The discharge through the orifice is determined by measuring the rate of change of the water level in the tank against time. A water level against volume drained calibration is used to find the rate of change of volume over time, and hence the discharge coefficient. The study shows that the discharge coefficient increases as the head and the ratio of the orifice diameter to pipe diameter decreases. It is found that the hydrostatic pressure variation across the orifice at the small head has a minimum impact on the discharge. A generalised function is developed to estimate discharge under a varying head over the orifice and for different ratios of orifice diameter to riser pipe diameter, which can be used for the design of detention ponds.

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