Determine the imperviousness of the existing site. The Rational Method (see Resources for more information) is most commonly used to determine the weighted runoff coefficient. Then multiply by 100 to get the percent imperviousness. The imperviousness of t

Determine the imperviousness of the existing site. The Rational Method (see Resources for more information) is most commonly used to determine the weighted runoff coefficient. Then multiply by 100 to get the percent imperviousness. The imperviousness of the site determines which compliance path the project must take.

• If the average imperviousness of the pre-development site is 50% or less (typically, most of the site is vegetated or permeable), follow requirements for Case 1.
• If the average imperviousness of the pre-development site is more than 50% (typically, most of the site is hardscape or impermeable), follow requirements for Case 2. Redevelopment projects often fall into this category.