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)
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 forty-eight
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. Justification, as
prepared and sealed by a qualified professional, for assertion that
permanent removal is not feasible must be provided, at a minimum,
in the form of field measured infiltration rate testing and geotechnical
evaluation of the existing site soils with regard to the impact of
proposed infiltration. Applications omitting this justification, including
infiltration rate testing will be considered incomplete. BMPs proposing
use of extended detention in lieu of permanent reduction through the
listed methods shall include a valve to adjust or regulate dewatering
time to achieve the performance standards noted herein. The valve
system shall include an inspection/monitoring port for review of dewatering.
(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 §
156-304, Rate controls, if the project qualifies for exemption by the provisions of §
156-302A.