Earlier designs for bioretention with an underdrain system included a filter fabric to separate the soil medium from the gravel blanket around the underdrain. This type of system is susceptible to failure from premature clogging. Therefore, a pea gravel diaphragm is preferred. The pea gravel diaphragm provides a greater porosity and is less likely to block. The thickness of the diaphragm should be between 3” and 9” in thickness to be affective. In some cases, pea gravel is used in-place of the underdrain gravel blanket itself. If the designer utilizes pea gravel around the underdrain pipe, the perforations should not exceed ¼” in diameter. 

Use of Geotextile is cautioned because the fabric may clog prematurely, resulting in limited infiltration. Fabric porosity should allow water to reach underdrain. The nonwoven variety is recommended.  Click here for more information on pea gravel and other filter materials.

CONTINUE TO Underdrain or Outlet

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.

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Last modified: 10/23/09