Green infrastructure and low impact development practices provided in the BMP Manual4 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. This Chapter establishes that only the Design Storm Method may be used for regulated activities that add greater than one-half acre of new impervious area or disturb greater than one acre of total area.
A. The Design Storm Method (CG-1 in the BMP Manual4) 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, 24-hour duration precipitation.
(2) For modeling purposes:
(a) Existing (predevelopment) non-forested pervious areas must be considered meadow in good condition.
(b) Twenty percent of existing impervious areas to be disturbed by the project, when present, shall be considered meadow in good condition in the model for existing conditions. Regulated activities not requiring an NDPES permit are exempt from this requirement.
B. The Simplified Method (CG-2 in the BMP Manual4 ) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable for regulated activities that add greater than one-half acre of new impervious area or for projects that require routing of stormwater storage facilities. 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 half-inch of the permanently removed runoff should be infiltrated.
(Ord. 2022-01, 7/25/2022)