Cap-orifice as a Flow Regulator for Rain Garden Design

Abstract Rain gardens have been recognized as an effective device for on-site runoff volume reduction and storm water quality enhancement. A rain garden is designed as an infiltration basin with a shallow, wide water storage volume. The sub-base underneath the basin bottom is structured as a two-layered filtering medium. The upper sand layer provides the required filtering process and then the lower gravel layer provides a reservoir for a gradual release of the stored water through a perforated underdrain pipe. A newly constructed rain garden can have a high infiltration capacity. After several years of service, the accumulation of solids intercepted in the filtering layer will develop a clogging effect or the infiltration capacity in the sub-base continues decreasing. When the drain time exceeds the safety criteria, the rain garden needs to be replaced. Coping with a decaying infiltration rate, a rain garden needs to reduce its flow release in the early years. In this study, it is suggested that a cap orifice be installed at the exit of the underdrain pipe. This cap orifice is sized to reduce the initial infiltrating rate in the early years and then to work with the clogged infiltration rate throughout the life cycle of a rain garden. The design methodology presented in this paper allows the engineer to size this cap orifice according to the decay of infiltration rate, required drain time, and allowable flow release.