Modelling of Stormwater Detention Pond Outlet for Dry and Wet Seasons

Abstract

Existing large stormwater detention pond outlets are designed to cater for high flow, for example, due to 100-year ARI design storms. As such, the outlets are usually large, which are unable to detain smaller storms. To improve the outlet design to have dual function to sustain both small and extreme storms, a new outlet design is proposed with two separate orifice openings within a single outlet. An early investigation was conducted by applying a case study, in which a real-life detention pond was selected and simulated in Storm Water Management Model embedded with scenarios of existing outlet with 0.45 m diameter orifice opening, and another with proposed 0.1 m diameter orifice opening before the existing orifice. The model was run through with 10-year ARI design storm according to the local equatorial climate. The modelled outflow indicated that the existing orifice produced a peak of 0.35 m3/s, while the new orifice was found to succeed the existing orifice to produce 0.03 m3/s, a ten times reduction. Further comparison between the two outlet designs, improvement to the detention capability was obtained, to mention a few parameters: delayed time to peak from 40 minutes by existing orifice to 60 minutes by new orifice and expanded hydrograph base time from 6 hours by existing orifice to 12 hours by the new orifice. The modelling efforts indicated that the smaller orifice to augment the existing orifice would achieve better stormwater control objectives in detaining the urban runoff on site.