As in the case of any impoundment, overflow of storm events that exceed the storage capacity of the facility should be addressed appropriately.

In a residential setting, overflow may not present a problem if the drainage area and facility capacity are relatively small, and the system is located within grassy areas that provide a safe, non-erosive surface for any overflow conditions that may arise.  Even in these cases, downstream conveyance must be designed appropriately.  Regardless, an evaluation of overflow risks should always be performed for each facility.

In commercial or industrial settings, design for overflow is more critical. Often, facilities in commercial settings are incorporated into the parking lot landscape islands. The paved surfaces flowing to the facilities can generate large quantities of runoff.  Therefore, overland conveyance of overflow water and flow through bioretention facilities are discouraged. Designers must always provide a safe discharge point, using the appropriate design storm event.

Often, overflow from bioretention facilities is accomplished by including a yard stormdrain inlet within the facility. 

Methods of determining discharges and overflow capacities should apply typical storm drain hydrology and hydraulics methods.

CONTINUE TO DESIGN DETAILS

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

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Last modified: 10/23/09