Impervious coverage refers any surface which has been compacted or is covered with a layer of material that makes it highly resistant to infiltration by water. Examples of impervious pavement surfaces include buildings and recreational facilities (patios, pool decks, tennis courts, etc.)
In certain jurisdictions, there are limits on the amount of impervious cover that can exist on a site in order to control either the amount of runoff that is generated and/or the water quality of the runoff. In North Carolina for example, there are Built Upon Area (BUA) restrictions on new developments, which is a percentage of impervious cover that is allowed on a given site and is based on a number of factors including allocated density, location in the watershed, etc.
How to Reduce Impervious Surfaces with Permeable Pavers
The use of Permeable Interlocking Concrete Pavement (PICP) typically allow for credits against the impervious cover requirements because of their ability to control and treat the rain falling on them, as well as any run-on from other areas accounted for in the design. The following is an example of BUA credit calculations per the North Carolina Department of Environment and Natural Resources.
Meeting Impervious Cover Requirements with PICP System
1. Conventional Pavement
By designing a PICP system that can handle all of the direct rainfall and run-on from the building, the required water quality and quantity requirements are met
2. Using Permeable Pavement as a BMP
As a low density development, there is a 24% BUA limit. Building and pavement consume all available impervious cover quantities.NOTE: This slide is taken directly from a presentation prepared by the NC Department of Environment & Natural Resources.
Simply put, using PICP allows for the site design to accommodate a wider range of possibilities that would otherwise not have been possible, including a larger building footprint or a separate garage, addition of outdoor recreational facilities, and/or more parking. This not only meets the intended environmental requirements, but also increases the property value due to the increased flexibility of the lot.