The green infrastructure and low impact development practices provided in the BMP Manual (NOTE: See §
19-901, Subsection
3.) shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection
1 or the Simplified Method in Subsection
2 below. Water volume controls shall be implemented using the Design Storm Method in Subsection
1 or the Simplified Method in Subsection
2 below, or alternative design criteria as allowed by 25 Pa. Code Chapter 102.
1. The Design Storm Method (CG-1 in the BMP Manual) (NOTE: See §
19-901, Subsection
3.) 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:
A. 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.
B. 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 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.
C. For modeling purposes:
(1) Existing (predevelopment)
nonforested pervious areas must be considered meadow in good condition.
(2) Twenty percent of existing
impervious area, when present, shall be considered meadow in good
condition in the model for existing conditions.
2. The Simplified Method (CG-2 in the BMP Manual) (NOTE: See §
19-901, Subsection
3.) 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:
A. Stormwater facilities shall
capture at least the first two inches of runoff from the net increase
in impervious surfaces.
B. 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.
C. 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.
D. This method is exempt from the requirements of §
19-304, Rate controls.