Cap-Orifice as a Flow Regulator for Rain Garden Design

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 subbase 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 subbase 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.