BIORETENTION FACILITY DESIGN :

A. Provide a site specific design for all bioretention facilities.

Text Box: •	Do not use a generic specification from the Virginia SWM Handbook or other source.    •	Provide specific elevations and dimensions for all pertinent components (e.g., separate layers, tree and shrub locations, berms, storm sewer overflow, underdrains, etc.)    •	  Provide the specific composition of the engineered soil / filter media (i.e., % organics, sand, compost, etc.)    •	Provide the specific trees and shrubs to be planted and their locations.

B. Evaluate location of bioretention facilities early on in the design process.

Text Box: Do not locate bioretention measures in wetland areas, other areas with high seasonal water tables, fill areas, or in areas with no room for a pre-treatment filter strip or a mechanism to provide for sheet flow.

C. Characteristics of discharge entering bioretention facility:

Text Box: •	All stormwater discharge entering a bioretention facility must be in sheet flow.  Use measures such as level spreaders, cub cuts and stone diaphragm, etc.    •	Spread out discharge along the length of the facility to keep water from concentrating in one area of the facility and displacing the mulch layer.    •	Use 5 cfs and 1 acre of as the threshold for the maximum drainage to a single facility or separate cell.

D. Depth of engineered soil/filter media affects pollutant removal.

Text Box: Although the recommended depth of the soil media ranges from 2.5’ to 4’, the use a 3’ depth provides maximum pollutant removal potential; additional depth does not provide additional benefit.

E. Enhanced location of trees and shrubs:

Text Box: To prevent the disruption of sheet flow, provide easier maintenance access, and to provide adequate shading, locate trees at the perimeter of the bioretention facility and away from the direct flow path into the facility.  An expanded area for growth for shrub root systems within the measure typically creates more efficient pollutant uptake.

F. Underdrains are required when minimum insitu soil infiltration rate is > 0.52 in/hr.

Text Box: •	Use a main (longitudinal) underdrain combined with laterals at a maximum spacing of 10’ on center.  Both drains must be a minimum 4” (preferably 6”) diameter pipe to help delay clogging.     •	Locate a non-perforated PVC clean-out at each main-lateral intersection.  Provide a watertight cap flush with the ground surface.    •	Use a 45° connection between the clean-out and the underdrain in order to facilitate maintance.      •	Extend all underdrains to the periphery of the facility in order to promote exfitration between storms.    •	Terminate perforations in the underdrain when it is located within 5’ of a stormwater appurtenance (e.g., drop-inlet, manhole, etc.)

G. Provide an observation well to facilitate inspection and maintenance of a bioretention measure.

Text Box: §	Regardless of whether or not an underdrain system is required, provide all bioretention facilities with an observation well located at the center of the measure, and extending down to the bottom of the BMP.    §	In the typical situation when underdrains are required, the observation well can take the form of a centrally located clean-out.    §	Similar to clean-outs, use non-perforated PVC pipe.       

H. Incorporate geotechnical report findings into the design of a bioretention facility.

Text Box: Provide a minimum 2’ of clearance between the bottom of the measure and the seasonal high water table and bedrock.  Even with the use of underdrains, groundwater must not move into the facility as it can inhibit dewatering and create undesired anaerobic conditions.  

I. All infiltration-type BMP’s, including bioretention facilities, are not well-suited to be located in fill material.

Text Box: §	Regardless of whether or not an underdrain system, placing a bioretention facility directly over top of fill material is not recommended.    §	Underlying rock (e.g., bedrock) must not be blasted to allow installation of a bioretention facility – resulting fissures can convey stormwater directly to groundwater.        §	If site constraints dictate that fill be placed along the vertical sides of the measure, impervious filter cloth must encompass it except along the bottom where there is the interface with in situ soil.

J. Separation between internal layers – stone vs. filter cloth.

Text Box: §	To help prevent premature clogging, use 4” of choke stone, or simply “pea gravel” (VDOT #8 stone), as the separator between the engineered soil (filter media) and the underdrain (stone) layer and between the bottom of the underdrain layer and the in situ soil.     §	 This pea gravel separator replaces filter cloth in those locations.   

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