Approvals issued and actions taken under this Part 3 do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
The design of all regulated activities should include the following to minimize stormwater impacts: (See
Subappendix A-2 for a nonstructural project design checklist.)
A. The applicant should find practicable alternatives to the surface
discharge of stormwater, the creation of impervious surfaces, and
the degradation of waters of the commonwealth and must maintain as
much as possible the natural hydrologic regime of the site.
B. An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
C. All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
Note: Philadelphia County, Bucks County, and Montgomery County
will follow different groundwater recharge criteria.
A. Infiltration best management practices (BMPs) shall meet the following minimum requirements unless the site qualifies for an exemption from the infiltration requirements of this Part
3 as listed in §
154-76.
(1) Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum soil depth of 24 inches between the bottoms of the
infiltration BMPs and bedrock or other limiting zones.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's qualified person.
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(d)
All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
(e)
Pretreatment (see §
154-80) shall be provided prior to infiltration.
(2) The size of the infiltration facility shall be based upon the following
volume criteria:
(a)
Bucks County and Montgomery County portions of the Watershed:
[1]
Where practicable and appropriate the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
inch of runoff (I) over all proposed impervious surfaces.
[2]
The Rev required shall be computed as:
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Rev = (1/12) * (I)
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Where:
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|
|
Rev
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=
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Recharge volume (cubic feet)
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|
|
I
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=
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Impervious area within the limits of earth disturbance (square
feet)
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|
|
An asterisk (*) in equations denotes multiplication.
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(b)
Philadelphia County Portion of the Watershed:
[1]
The recharge volume shall be equal to one inch of runoff over
all DCIA within the limits of earth disturbance.
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Rev = (1/12) * (I)
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|
Where:
|
|
|
Rev
|
=
|
Recharge volume (cubic feet)
|
|
|
I
|
=
|
DCIA within the limits of earth disturbance (square feet)
|
|
|
An asterisk (*) in equations denotes multiplication.
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B. Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified person, and at a minimum address
soil permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
(1) Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
(2) Provide field tests such as double ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(3) Design the infiltration structure for the required recharge volume
(Rev) based on field determined capacity at
the level of the proposed infiltration surface.
(4) If on-lot infiltration structures are proposed by the applicant's
qualified person, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
(5) An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
(Note: Philadelphia County, Bucks County, and Montgomery County
will follow different water volume control requirements.)
A. Bucks County and Montgomery County portions of the watershed: The low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Water volume controls shall be implemented using the design storm method in Subsection
A(1) or the simplified method in Subsection
A(2) below. For regulated activity areas equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, this Part
3 establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors. All regulated activities greater than one acre must use the design storm method.
(1) The design storm method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(a)
The post-development total runoff volume for all storms equal
to or less than the two-year, twenty-four-hour storm event shall not
be increased.
(b)
For modeling purposes:
[1]
Existing (predevelopment) nonforested pervious areas must be
considered meadow.
[2]
Twenty percent of existing impervious area, when present, shall
be considered meadow in the model for existing conditions.
(2) 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:
(a)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces. (Note: An asterisk (*)
in equations denotes multiplication.)
Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
(b)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of the commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
Volume (cubic feet) = (1/12) * impervious surfaces (square feet)
(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 Section 408,
Peak Rate Controls.
B. Philadelphia county portion of the watershed: The following equation
is to be used to determine the water volume control storage requirement
in cubic feet for regulated activities within the Pennypack Creek
Watershed in Philadelphia County:
Water volume control (cubic feet) = (1/12) * (I)
Where: I = DCIA within the limits of earth disturbance (square
feet)
Note: Philadelphia County, Bucks County, and Montgomery County
will follow different stream bank erosion requirements. If the municipality
has adopted a riparian corridor ordinance, the more restrictive requirement
shall apply.
A. If a perennial or intermittent stream passes through the site, the
applicant shall create a stream buffer extending a minimum of 50 feet
to either side of the top-of-bank of the channel. The buffer area
shall be established and maintained with appropriate native vegetation
(refer to Appendix B of the BMP Manual for plant lists). If the applicable
rear or side yard setback is less than 50 feet, the buffer width may
be reduced to 25% of the setback to a minimum of 10 feet. If an existing
buffer is legally prescribed (i.e., deed, covenant, easement, etc.)
and it exceeds the requirements of this Part 3, the existing buffer
shall be maintained. [Note: The municipality may select a smaller
buffer width (above) if desired, but the selected buffer may not be
less than 10 feet]. This does not include lakes or wetlands.
B. Bucks County and Montgomery County portions of the watershed. Applicants
shall adhere to the following stream bank erosion/channel protection
requirements:
(1) In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream bank erosion),
the primary requirement is to design a BMP to detain the proposed
conditions two-year, twenty-four-hour storm event to the existing
conditions one-year flow using the SCS Type II distribution. Additionally,
provisions shall be made (such as adding a small orifice at the bottom
of the outlet structure or a sand filter) so that the proposed conditions
one-year, twenty-four-hour storm event takes a minimum of 24 hours
to drain from the facility from a point when the maximum volume of
water from the one-year twenty-four-hour storm event is captured (i.e.,
the maximum water surface elevation is achieved in the facility).
Release of water can begin at the start of the storm (i.e., the invert
of the water volume control orifice is at the invert of the facility).
(2) The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small drainage areas
contributing to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake. It is recommended that the design,
to accommodate maintenance, include a sand or porous media filter.
C. Philadelphia County portion of the watershed:
(1) Redevelopment sites with less than one acre of earth disturbance
or redevelopment sites that demonstrate a twenty-percent reduction
in DCIA from predevelopment conditions are exempt from this requirement.
(2) Applicants shall adhere to the following stream bank erosion/channel
protection requirements:
(a)
To meet the requirement, stormwater management practices shall
retain or detain the runoff from all DCIA within the limits of earth
disturbance from a one-year, twenty-four-hour Natural Resources Conservation
Service (NRCS) Type II storm event in the proposed site condition
such that the runoff takes a minimum of 24 hours and a maximum of
72 hours from the end of the storm event to drain the management facility.
(b)
The infiltration and water quality volumes may be incorporated
into the channel protection portion of the design provided the design
meets all requirements concurrently.
(c)
In "Conditional Direct Discharge Districts" (District C) only (see §
154-93), the objective is not to attenuate the storms greater than the two-year recurrence interval. This can be accomplished by configuring the outlet structure not to control the larger storms or by a bypass channel that diverts only the two-year stormwater runoff into the basin or conversely, diverts flows in excess of the two-year storm away from the basin.