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The role of an underdrain in the bioretention facility is to ensure proper drainage for the plants and to ensure proper infiltration rates occur. The use of an underdrain system to provide a discharge point precludes the need for extensive geotechnical investigation.

Typically, a bioretention facility located on a private lot will require an underdrain system to avoid drainage problems when facilities are located in prominent locations. Underdrains or other discharge methods are required when siting facilities on residential lots. 

Underdrains are configured in many different ways and typically include a gravel/stone "blanket" encompassing a horizontal, perforated discharge pipe. A pea gravel diaphragm as described above can be used to protect the underdrain from blocking. No underdrains are necessary for in-situ soils with infiltration rates greater than 1" an hour and where  water table depths are known to be greater than 2' below the proposed invert of the bioretention facility. For added protection, the drainage area shall be less than 1 acre. A percolation test must be performed to ensure soil infiltration suitability.  The use of an underdrain increases the ability of the soil to drain quickly, thereby reducing the problem of mosquitoes.  Underdrains keep the soil at an adequate aerobic state, allowing plants to flourish. Vertical adjustments to the location of the underdrain may be made to help establish anaerobic zones for the processing of nitrates. Software is available that includes several different methods for underdrain spacing design.  Click here for more information on underdrains. 

In some cases, a liner may be necessary to avoid infiltration into surrounding soils and/or groundwater.  Examples include facilities located in karst topography, stormwater hot spots, source water protection, or in close proximity to basements.  Click here for more information about liners.

CONTINUE TO Surface Overflow

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

Photo credit:  T.E. Scott & Associates, Inc.

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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