Approvals issued and actions taken under this chapter 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.
Note: Philadelphia 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 chapter as listed in §
290-6B:
(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 bottom of the
infiltration BMPs.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's design professional.
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the start of the design storm.
(d)
All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water quality
volume within three days (72 hours) from the start of the design storm.
(e)
Pretreatment shall be provided prior to infiltration. See §
290-11 for definition.
(2) The size of the infiltration facility shall be based upon the following
volume criteria:
(a)
Montgomery County portion of the watershed.
[1]
Where practicable and appropriate, the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
(1.0) inch of runoff (I) over all proposed impervious surfaces.
[2]
The Rev required shall be computed as:
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Rev = (P/12) * (I)
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Where:
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|
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Rev
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=
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Recharge volume (cubic feet)
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|
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P
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=
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1.0 inch
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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. The recharge volume
shall be equal to one (1.0) inch of rainfall over all DCIA within
the limits of earth disturbance.
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Rev = (P/12) * (I)
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Where:
|
|
|
Rev
|
=
|
Recharge volume (cubic feet)
|
|
|
P
|
=
|
1.0 inch
|
|
|
I
|
=
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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 design professional 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 retention (Rev) volume 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
design professional, 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 and Montgomery County will follow
different stream bank erosion requirements. 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 maintained with and encouraged
to use appropriate native vegetation [Refer to Appendix B of the Pennsylvania
Stormwater Best Management Practices (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
chapter, 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.
A. Montgomery County portion 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 design storm 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) so that the proposed conditions one-year
storm takes a minimum of 24 hours to drain from the facility from
a point where the maximum volume of water from the one-year storm
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 quality 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 sand or a porous media filter.
B. 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 design storm 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 §
290-25), 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.
All wet basin designs shall incorporate biologic controls consistent
with the West Nile Guidance found in Appendix G.