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Where underdrains are specified, the following information provides guidance for underdrain requirements. Underdrain Material Types Underdrain systems may be composed of a variety of materials, with PVC pipe material being the most commonly used. Alternative pipe material may include flexible ADS pipe. Other pipe materials may be substituted at the designer’s prerogative and with concurrence of the reviewing agency. Underdrain Connections Pipe joints and storm drain structure connections must be adequately sealed to avoid piping conditions (water seeping through pipe or structure joints). Pipe sections shall be coupled using suitable connection rings and flanges. Field connections to stormdrain structures and pipes shall be sealed with polymer grout material that is capable of adhering to surfaces. Underdrain pipe shall be capped (at structure) until completion of site construction. Underdrains connected directly to a storm drainage structure shall be non-perforated for an appropriate distance from the structure interface to avoid possible piping problems. Underdrain Perforations The perforation locations are not too critical for proper function, as long as the total opening area exceeds the expected flow capacity of the underdrain itself. Commonly marketed perforated PVC pipe has ¼ or ½" perforations, 6" center to center, along two or three longitudinal rows. Whether or not the perforations are placed at the invert of pipe or elsewhere, depend upon the design of the facility. Typically, the perforations are placed closest to the invert of the pipe to achieve maximum potential for draining the facility. The perforations can be placed near the top of the pipe if an anaerobic zone is intended. Water below the perforated portion of the underdrain will have a tendency to accumulate during periods of saturation. Otherwise, water will have a tendency to infiltrate into the surrounding insitu soils. Underdrain Location and Spacing Underdrains are typically located at the invert of the bioretention facility to intercept any filtered water that does not infiltrate into the surrounding soils. Soil and gravel cover over the underdrain shall be at least 2' in depth. Placement of 2-3" gravel bedding is recommended beneath the discharge points. Underdrains must "daylight" or connect to an existing drainage system to achieve positive flow. Various methods may be applied to establish spacing of the underdrains. Consistent distribution throughout the filter bed can affect the de-watering. Software is available that applies several methods to determine appropriate spacing. Underdrain Gravels Gravel bed materials should used to protect an underdrain pipe and to reduce clogging potential. Placement of the gravel over the underdrain must be done with care. Avoid dropping the gravel high levels from a backhoe or front-end loader bucket. Spill directly over underdrain and spread manually. Recommended construction specifications for gravel used to protect bioretention underdrains follows:
Observation/Cleanout Standpipe An observation/cleanout standpipe should be installed in every bioretention facility that has a depth greater than 2’ and/or an underdrain system. The standpipe will indicate how quickly the bioretention facility dewaters following a storm, provide maintenance cleanout port, and it will be connected to the underdrain system to facilitate cleanout. The observation well should consist of a rigid non-perforated PVC pipe, 4 to 6 inches in diameter. It should be located in the center of the structure and be capped flush with the ground elevation of the facility. The top of the well shall be capped with a screw, or flange type cover to discourage vandalism and tampering.
The above information was derived from the Prince George's County Bioretention Manual, |
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