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Gravel bed materials are sometimes used to protect an underdrain pipe 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. The construction specifications for gravel used to protect bioretention underdrains follows:

• Gravel stone size shall be no greater than ½”-1½” in diameter.

• The use of "pea gravel" in place of geotextile fabric is preferred

• Depth of the gravel shall not exceed 12"

• River-run, washed gravel is preferred.

Older specifications for bioretention utilized a geotextile fabric to filter water and soil before passing through to the underdrain gravel blanket. The use of a pea gravel diaphragm has gained acceptance because of the reduced likelihood of blockage. If a pea gravel diaphragm is used in this manner, it should have a minimum thickness of 3-4” and a maximum thickness of 8”. Where situations permit, a greater depth may be applied. A permeable filter fabric shall be placed over the underdrain gravel blanket and beneath the pea gravel diaphragm only where the underdrain is located and extending 2’ to either side.

Filter fabric is needed for controlling transport of silt, and controlling the direction of flow. In some older designs, the filter fabric placed on top of the gravel bed is used to control sediment transport into the gravel bed, which otherwise may become clogged. This filter fabric must meet a minimum permittivity rate of 75 gal/min/ft² and should not impede the infiltration rate of the soil medium. Filter fabric may be placed along the "walls" of the facility to help direct the water flow downward and to reduce lateral flows. Filter fabric must be placed along the sidewalls, (from the subgrade and over the stone) when installing a facility in a median strip or parking lot landscape island to prevent lateral flow under pavement.

Where bioretention is used for areas that require groundwater protection (karst, stormwater hot spots, or source water protection) or in close proximity to basements, an appropriate impervious liner should be specified. Clay liners should be of an appropriate impervious material as specified by a geotechnical engineer.  Synthetic liners, such as HDPE or PVC should be of appropriate thickness (at least 30 mil recommended). Any underdrain systems shall be placed above the liner with a provision to cap the underdrain discharge pipe to confine drainage if needed. Care during placement of the liner is necessary to avoid puncture. Soil medium placed over the liner should be placed by hand shovel rather than construction equipment.

The above information was derived from the Prince George's County Bioretention Manual,

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The information on this website was compiled by T.E. Scott & Associates, Inc. from various sources.

We make no guarantees or warranties to the applicability of this material to specific applications. 
Last modified: 04/27/08