The green infrastructure and low-impact development practices
provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas equal or less than 10,000 square feet, this Part
1 establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology and other factors.
A. The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1) Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year, twenty-four-hour duration
precipitation.
(2) For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
Twenty percent of existing impervious area, when present, shall
be considered meadow in good condition in the model for existing conditions.
B. The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities equal to or greater than 10,000 square feet. For new impervious
surfaces:
(1) Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2) At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of this commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
(3) Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
(4) This method is exempt from the requirements of §
140-16, Rate controls.