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, or alternative design criteria as allowed by Pa.
Code Title 25, Chapter 102.
A. The Design Storm Method (CG-1 in the BMP Manual) is applicable as
a method to any size of regulated activity. This method requires detailed
modeling based on site conditions. The following shall be incorporated
into the CG-1 method:
(1) Does 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) 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 twenty-four- to seventy-two-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 seventy-two-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) nonforested pervious areas must be
considered "meadow" in good condition.
(b)
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 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 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 listed removal options
are not feasible, and the designated plan reviewer agrees, runoff
shall be detained in a facility designed for a twenty-four-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 seventy-two-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 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 §
473-17, Rate controls.