Porous / Pervious Pavement Design Considerations :

A. At this time, consult additional design standards for detailed information on pervious concrete as there is only limited information available in the Minimum Standard 3.10D of the Virginia SWM Handbook.

Text Box: Use Fairfax County and/or the Georgia Stormwater Manual (on the web) specification for designing pervious concrete, with the additional stipulations found below.                   

B. For meeting water quality treatment criteria for porous asphalt or pervious concrete, 1 of the following 2 options may be utilized.

Text Box: •	If no additional impervious area is to be treated by porous pavement or pervious concrete, the area defining the BMP can be taken out of the impervious area calculation.*    •	If the porous pavement or pervious concrete is to treat any additional area, apply a pollutant removal rate (efficiency) to the entire impervious area that drains through the system (to include the surface area of the pervious concrete).   If the underlying in situ soil has a minimum infiltration of 0.52 in/hr, set the removal efficiency to 65%; for lower infiltration rates, set the efficiency to 35% (equivalent to an extended detention facility).     

* Note: a storm drainage easement is still required as water quality of the runoff would be compromised if the pavement is not maintained or is removed.

C. The depth of the stone reservoir layer must be based upon site conditions and storage needs – not just an arbitrary design feature. Use the greater of the following 2 design depths:

Text Box: •	A minimum depth of 24” is required between the surface of the pavement to the bottom of the stone reservoir layer as a result of the potential for hard, wet freezes in Loudoun County (i.e., it must extend below the frost line).  This depth leads to a typical configuration of 6” of porous concrete + 18” of #57 stone;     or,    •	A minimum depth equal to 1” x the impervious area draining to the facility     

D. There are site limitations affecting the use of porous pavement or pervious concrete on a development site.