The ponding area provides surface storage of the storm water runoff, and provides for the  evaporation of a portion of the runoff. Settling of the particulates occurs in the ponding area and provides an element of pretreatment. Ponding design depths have been kept to a minimum to reduce hydraulic overload of in-situ soils/soil medium and to maximize the surface area to facility depth ratio, where space allows.  The maximum ponding depth has been set to 6” in order to ensure that the hydrologic loading capacity is not compromised.  Pooling times are kept below 4-6 hours by utilizing soil mediums with a high percentage of sand.

Several sizing methods are used to size the ponding area.  Some are strictly volume-based while others are a function of the filter media area required to drain the water in a specified time period.  Software is available that to assist in computing the volume and/or area requirements for these methods.

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

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

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Last modified: 10/23/09