Changes

This type of bioretention facility provides the highest level of stormwater volume control.  Although this design does not include an underdrain, an overflow pipe, a weir or other outlet structure must be included.

This type of bioretention facility provides the highest level of stormwater volume control.  Although this design does not include an underdrain, an overflow pipe, a weir or other outlet structure must be included.

==Planning considerations==

==Planning considerations==

<h3>Inlets</h3>

===Inlets===

All forms of bioretention benefit hugely from the inclusion of pretreatment in the overall treatment train. The best type of pretreatment will depend on how the water enters into the facility. Mostly commonly this would be:  

All forms of bioretention benefit hugely from the inclusion of pretreatment in the overall treatment train. The best type of pretreatment will depend on how the water enters into the facility. Mostly commonly this would be:  

*sheet flow

*sheet flow

*concentrated piped flow.  

*concentrated piped flow.  

Distributed sheet flow is preferable to reduce the erosive energy of the water, and to promote infiltration across the surface of the practice.  

Distributed sheet flow is preferable to reduce the erosive energy of the water, and to promote infiltration across the surface of the practice.  

<h3>Overflow and active storage</h3>

===Overflow and active storage===

A plan should be in place for the conveyance of excess flow in the event that the facility is inundated during an extreme event. This may be to permit controlled overland flow to another facility, or to direct the flow to a pipe. In this case the invert of this pipe must be below the surrounding landscape to prevent accidental spilling. The depth between the top of the [[filter media]]/[[mulch]] and the invert of the overflow pipe is active storage of limited duration ponding and is sometimes referred to as the bowl depth. To prevent [[mosquito]] activity (and fear of mosquito activity), surface ponding time should be limited to 24 hours and should be calculated as 1D infiltration.       

A plan should be in place for the conveyance of excess flow in the event that the facility is inundated during an extreme event. This may be to permit controlled overland flow to another facility, or to direct the flow to a pipe. In this case the invert of this pipe must be below the surrounding landscape to prevent accidental spilling. The depth between the top of the [[filter media]]/[[mulch]] and the invert of the overflow pipe is active storage of limited duration ponding and is sometimes referred to as the bowl depth. To prevent [[Mosquitoes|mosquito]] activity (and fear of mosquito activity), surface ponding time should be limited to 24 hours and should be calculated as 1D infiltration.       

==Design==

==Design==

<h3>Calculating Size</h3>

===Calculating Size===

Bioretention facilities can be sized as other forms of subterranean infiltration facility.  

Bioretention facilities can be sized as other forms of subterranean infiltration facility.  

'''[[Infiltration: Sizing and modeling]]'''

'''[[Infiltration: Sizing and modeling]]'''