The green infrastructure and low impact development practices provided in the BMP Manuals shall be utilized for ah 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. Water volume controls shall be implemented using the Design Storm Method in Subsection A. or the Simplified Method in Subsection B. below, or alternative design criteria as allowed by PA Code Title 25, Chapter 102.
A. The Design Storm Method (CG-1 in the BMP Manuals) is applicable as a method to any size of regulated activity. This method requires detailed modeling based on site conditions. The following shah be incorporated into the CG-1 method:
(1) Do not increase the post-development total runoff volume for all storms equal to or less than the 2-year 24-hour duration precipitation.
(2) At least the first one inch of runoff from the net increase in 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. If the developer provides justification that the listed removal options are not feasible, and the Designated Plan Reviewer agrees, runoff shall be detained in a facility designed for a 24 to 72 hour dewatering time in an area with a dedicated stormwater system (not contributory to a combined sewer system) and shall be detained in a facility designed for a 72 hour dewatering time in an area contributory to a combined sewer system before discharge to local stormwater systems or the environment.
(3) For modeling purposes:
(a) Existing (predevelopment) non-forested pervious areas must be considered meadow in good condition.
(b) Twenty percent (20%) 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 Manuals) 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 greater than one acre or for projects that require design of stormwater storage facilities. For new impervious surfaces:
(1) Stormwater facilities shall capture at least the first two inches of runoff from the net increase in impervious surfaces.
(2) At least the first one inch of runoff from the net increase in 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. If the developer provides justification that the hsted removal options are not feasible, and the Designated Plan Reviewer agrees, runoff shall be detained in a facility designed for a 24-hour dewatering time in an area with a dedicated stormwater system (not contributory to a combined sewer system) and shall be detained in a facility designed for a 72-hour dewatering time in an area contributory to a combined sewer system before discharge to local stormwater systems or the environment.
(3) Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in ah 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 § 26-124, Rate Controls.
(Ord. 946, 11/8/2018, §303)