Permits must comply with any and all applicable
local, county, state and federal regulations.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration stormwater management facilities shall give consideration to providing groundwater recharge to compensate for the reduction in the percolation that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs, which can be over-designed to compensate for the infiltration losses due to parking areas. It is recommended that roof runoff be directed to infiltration BMPs, which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with §
297-3 and take advantage of utilizing any existing recharge areas. Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, due to seasonal high water table, soil permeability rate, soil depth or setback distances from special geologic features, then the design professional shall be responsible to show that this cannot be physically accomplished. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from §
297-15A(3), depending on demonstrated site conditions and shall be the greater of the two volumes.
A. Infiltration
BMPs shall meet the following minimum requirements:
(1) Infiltration requirements. Regulated activities will be required to infiltrate, where site conditions permit, a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be infiltrated shall be determined from §
297-15A(3)(a) or
(b), depending upon demonstrated site conditions.
(2) 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
depth of 24 inches between the bottom of the BMP and the limiting
zone.
(b) An infiltration
and/or percolation rate sufficient to accept the additional stormwater
load and drain completely as determined by field tests conducted by
the applicant's design professional.
(c) The infiltration
facility shall be capable of completely infiltrating the required
retention (infiltration) volume within four days (96 hours).
(d) Pretreatment
shall be provided prior to infiltration.
(3) The size
of the infiltration facility shall be based upon the following volume
criteria:
(a) NRCS curve
number equation.
[1] The
NRCS runoff equation shall be utilized to calculate infiltration requirements
(I) in inches.
Equation 297-15.1
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I (Infiltration requirement, in inches) = (200/CN)
– 2
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Where:
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CN = SCS (NRCS) curve number of existing conditions
contributing to the infiltration facility.
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[2] This
equation is displayed graphically in, and the infiltration requirement
can be determined from Figure 297-15.1.
[3] It
has been determined that infiltrating 0.46 inch of runoff from the
impervious areas will aid in maintaining the hydrologic regime of
the watershed. However, the rounded number 0.5 inch will be used.
Figure 297-15.1. Infiltration Requirement
Based Upon NRCS Curve Number
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[4] The
retention (infiltration) volume (Rev) required
to meet the infiltration requirement would therefore be computed as:
Equation 297-15.2
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Rev =
|
(0.5 or I, whichever is greater) * impervious
area (square feet) / (12 inch/feet) = cubic feet
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Where:
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I = infiltration requirements (in inches).
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(b) Annual
recharge = water budget approach.
[1] If the goals of §
297-15A(3)(a) cannot be achieved, then 0.5 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation 297-15.1 or the curve in Figure 297-15.1 should be used to determine the infiltration requirement.
[2] The
retention (infiltration) volume (Rev) required
again would therefore be computed as:
Rev =
|
(0.5 or I, whichever is greater) * impervious
area (square feet) / (12 inch/feet) = cubic feet
|
B. Soils. A detailed
soils evaluation of the project site shall be required where practicable
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
is not permitted to 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 Township of Muhlenberg that the soils
are conducive to infiltrate on the lots identified.
C. Carbonate areas. The applicant is required to investigate the ability of all areas on the site which are not underlain by carbonate rock to meet the infiltration requirements of §
297-15A. If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure 124-B-1 in Appendix B. However, the applicant is not required to use infiltration in carbonate areas even if the site falls into the "Recommended" range on Figure 124-B-1 in Appendix B. If infiltration is not proposed, the calculated infiltration volume (§
297-15A) shall be treated by an acceptable BMP. Acceptable methods for treating this volume are storage and reuse, bioretention, wet ponds and soil composting. Other methods may be acceptable, and all methods should generally follow the design guidelines outlined in the Pennsylvania Stormwater Best Management Practice Manual, latest edition.
(1) Infiltration
BMP loading rate percentages in Figure 124-B-1 in Appendix B shall
be calculated as follows:
(2) The area
tributary to the infiltration BMP shall be weighted as follows:
Area Description
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Weighting
|
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All disturbed area to be made impervious
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100%
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All disturbed areas to be made pervious
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50%
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All undisturbed impervious areas
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100%
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All undisturbed pervious areas
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0%
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(3) Soil thickness
is to be measured from the bottom of any proposed infiltration BMP.
The effective soil thickness in Figure 124-B-1 in Appendix B is the
measured soil thickness multiplied by the thickness factor based on
soil permeability, as follows:
Permeability Range
(inches per hour)
|
Thickness Factor
|
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6.0 to 12.0
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0.8
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2.0 to 6.0
|
1.0
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1.0 to 2.0
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1.4
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0.75 to 1.0
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1.2
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0.5 to 0.75
|
1.0
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(4) The design
of all facilities over karst shall include an evaluation of measures
to minimize adverse effects.
D. Stormwater hotspots.
Following is a list of examples of designated hotspots. If a site
is designated as a hotspot, it has important implications for how
stormwater is managed. First and foremost, untreated stormwater runoff
from hotspots shall not be allowed to recharge into groundwater where
it may contaminate water supplies. Therefore, the Rev requirement shall not applied to development sites that fit into
the hotspot category. (The entire WQv must still be treated.) Second,
a greater level of stormwater treatment shall be considered at hotspot
sites to prevent pollutant washoff after construction. The EPA's NPDES
stormwater program requires some industrial sites to prepare and implement
a stormwater pollution prevention plan.
(1) Examples
of hotspots:
(a) Vehicle
salvage yards and recycling facilities.
(b) Vehicle
fueling stations.
(c) Vehicle
service and maintenance facilities.
(d) Vehicle
and equipment cleaning facilities.
(e) Fleet
storage areas (bus, truck, etc.).
(f) Industrial
sites (based on Standard Industrial Codes).
(g) Marinas
(service and maintenance).
(h) Outdoor
liquid container storage.
(i) Outdoor
loading/unloading facilities.
(j) Public
works storage areas.
(k) Facilities
that generate or store hazardous materials.
(l) Commercial
container nursery.
(m) Other
land uses and activities as designated by an appropriate review authority.
(2) The following
land uses and activities are not normally considered hotspots:
(a) Residential
streets and rural highways.
(c) Institutional
development.
(f) Pervious
areas, except golf courses and nurseries [which may need an integrated
pest management (IPM) plan].
(3) While large
highways [average daily traffic volume (ADT) greater than 30,000]
are not designated as a stormwater hotspot, it is important to ensure
that highway stormwater management plans adequately protect groundwater.
E. Caution shall
be exercised where infiltration is proposed in source water protection
areas, as defined by the Township of Muhlenberg, a water authority,
or water company.
F. Infiltration
facilities shall be used in conjunction with other innovative or traditional
stormwater control facilities that are found within the PADEP BMP
Manual.
G. Caution shall
be exercised where salt or chloride (municipal salt storage) would
be a pollutant since soils do little to filter this pollutant and
it may contaminate the groundwater. The qualified design professional
shall evaluate the possibility of groundwater contamination from the
proposed infiltration facility and perform a hydrogeologic justification
study if necessary.
H. The infiltration
requirement in high quality or exceptional value waters shall be subject
to the DEP's Chapter 93 antidegradation regulations.
I. Dependant upon
certain land use or hotspots, 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 Township of Muhlenberg.
J. The Township
of Muhlenberg shall require the applicant to provide safeguards against
groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
K. For projects that disturb one acre or more, unless otherwise specified in Chapter
355, Zoning, of this Code, the following setbacks for infiltration facilities shall apply:
(1) One hundred
feet from water supply wells.
(2) Ten feet
downslope or 100 feet upslope from building foundations.
(3) Fifty feet
from septic system drainfields.
(4) Fifty feet
from a geologic contact with carbonate bedrock, unless a preliminary
site investigation is done in the carbonate bedrock to show the absence
of special geologic features within 50 feet of the proposed infiltration
area.
(5) One hundred
feet from the property line unless documentation is provided to show
all setbacks from wells, foundations and drainfields on the neighboring
property will be met.
The applicant shall comply with the following
water quality requirements of this article.
A. Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under §
297-15 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the infiltration volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site. To achieve this goal, the following criterion is established:
(1) The following
calculation formula is to be used to determine the water quality storage
volume (WQv) in acre-feet of storage for the Schuylkill River and
Maiden Creek watersheds:
Equation 297-16.1
|
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WQv = [(P)(Rv)(A)]/12
|
Where:
|
WQv = Water quality volume
(acre-feet).
|
P = 1 inch.
|
A = Total contributing drainage area to the
water quality BMP (acres).
|
Rv = 0.05 + 0.009(I)
where I is the percent of the area that is impervious surface [(impervious
area/A)*100].
|
(2) This volume
requirement can be accomplished by the permanent volume of a wet basin
or the detained volume from other BMPs.
(3) 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). The design
of the facility shall provide for protection from clogging and unwanted
sedimentation.
B. For areas within
defined special protection subwatersheds which include exceptional
value (EV) and high quality (HQ) waters and cold water fishery (CWF),
the temperature and quality of water and streams shall be maintained.
C. To accomplish
the above, the applicant shall use innovative or traditional stormwater
control facilities that are found within the PADEP State BMP Manual.
D. 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 appropriate native vegetation (reference to Appendix H of Pennsylvania
Handbook of Best Management Practices for Developing Area 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. This does
not include lakes or wetlands.
E. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Township of Muhlenberg. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of §
297-16A.