Operation of Cap Orifice in a Rain Garden: Fid-Bau Portal

Operation of Cap Orifice in a Rain Garden

Guo, James C. Y. / Luu, Toan M.

The rain garden is one of the most commonly used, low-impact development (LID) devices. Rain gardens are designed to reduce runoff volume, delay peak flow, and improve water quality in stormwater. A rain garden consists of surface and subsurface components: The surface component is a storage basin that stores the water quality capture volume (WQCV), whereas the subsurface structure is formed with a two-layered filtering medium. The drain time is the most important design parameter for the rain garden’s operation. A short drain time will lead to an inadequate residence time for the sedimentation process, whereas a long drain time will result in potential hazards to the public. A new rain garden usually has a high infiltration capacity through its filtering layers. Over years of service, the clogging effect will slow down its infiltration rate. To improve the performance of a rain garden through its life cycle, it is recommended that a cap orifice be installed at the outlet of the perforated underdrain pipe. Such a cap orifice can be adjusted to achieve the targeted flow release, according to the stage of rain garden’s life cycle. This paper presents a numerical algorithm that predicts the infiltrating process through the unsaturated and saturated sand-mix zones underneath a rain garden. With field data, the calibration parameters for the numerical model are identified to be hydraulic conductivity and initial soil moisture content. The numerical procedure developed in this paper provides a quantifiable guidance on how to turn the cap orifice up and down to satisfy the design condition. This numerical procedure can be further incorporated into a web-based remote control system that can assist the engineer to operate a rain garden at a distance.