The green infrastructure and low-impact development practices
provided in the BMP Manual3 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 25 Pa. Code
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) 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) 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 Manual3) 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 §
190-13, Rate controls.
Post-development discharge rates shall not exceed the pre-development discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour storm events. This is the equivalent to a 100% release rate area when compared to those rates shown in the maps contained in
Appendix A (New and Existing Release Rate Management Districts). This pre-development to post-development control is not
to be misconstrued as the same as the conditional direct discharge
areas on the Release Rate maps. If it is shown that the peak rates
of discharge indicated by the post-development analysis are less than
or equal to the peak rates of discharge indicated by the pre-development
analysis for one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year,
twenty-four-hour storms, then the requirements of this section have
been met. Otherwise, the applicant shall provide additional controls
as necessary to satisfy the peak rate of discharge requirement. Peak
flows should be computed using the methods included in the chapter
titled "Stormwater Calculations and Methodology" of the DEP Stormwater
Management BMP Manual.