(Answer specific to the City of Austin)
A typical project site has a large amount of impervious cover that is generated by drives, parking, and sidewalks, but are needed to provide access to the site or are required by a regulatory department. All the additional impervious cover leads to bigger water quality controls, which will diminish the useable site area of a project site.
The City of Austin has a water quality credit system that will allow the reduction of the required water quality volume if certain design criteria is met. One way to help reduce the impervious cover and water quality volume is by using porous pavement for pedestrian use.
Porous pavement consists of a porous concrete surface with a minimum 3” gravel bed and subgrade beneath it, capable of supporting daily pedestrian use. In short, the gravel and subgrade temporary store water underneath it prior to infiltration into the ground. The temporary storage allows pollutants to be removed/reduced prior to entering streams or underground water. The porous concrete on top removes fine materials but keeps coarse materials, and combined with cement, allows more voids within the concrete, which in turn allows water to seep and penetrate through. The pavement and subgrade must have a minimum hydraulic conductivity greater than 0.20 inches per hour and the gravel layer must have an effective porosity greater than 0.30.
Certain restrictions do apply, as porous pavement cannot be used for water quality credit within the Barton Springs Recharge or Contributing Zone, and it cannot be used as a water quality control for other impervious cover. For further information, please see the City of Austin Environmental Criteria Manual, Section 1.6.7.E.