[Ord. No. 243, 3/19/2008]
(a)
Applicants proposing regulated activities in Amity Township which do not fall under the exemption criteria shown in § 25-402 shall submit a drainage plan consistent with this chapter to the Township for review. These criteria shall apply to the total proposed development even if development is to take place in stages.
(b)
The applicant is required to evaluate 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.
(c)
The drainage plan must be designed consistent with the sequencing provisions of § 25-304 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Part 3 of this chapter.
(d)
Existing points of concentrated drainage that discharge onto adjacent
property shall not be altered in any manner which could cause property
damage without permission of the affected property owner(s) and shall
be subject to any applicable discharge criteria specified in this
chapter.
(e)
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If diffused
drainage discharge is proposed to be concentrated and discharged onto
adjacent property, the applicant must document that adequate downstream
conveyance facilities exist to safely transport the concentrated discharge,
or otherwise prove that no erosion, sedimentation, flooding or other
impacts will result from the concentrated discharge.
(f)
Where a development site is traversed by existing watercourses, drainage
easements shall be provided conforming to the line of such watercourses.
The terms of the easement shall conform to the stream buffer requirements
contained in § 25-306(f) of this chapter.
(g)
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by PaDEP through the joint permit application
process, or, where deemed appropriate by PaDEP, the general permit
process. When there is a question whether wetlands may be involved,
it is the responsibility of the applicant or his agent to show that
the land in question cannot be classified as wetlands, otherwise approval
to work in the area must be obtained from PaDEP.
(h)
Any alteration that affects stormwater flow directly or indirectly
toward a PennDOT facility shall be subject to PennDOT regulations.
(i)
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., are encouraged, where soil
conditions permit, to reduce the size or eliminate the need for detention
facilities or other structural BMPs.
(j)
Roof drains shall not be connected to impervious surfaces in order
to promote overland flow and infiltration/percolation of stormwater
where advantageous to do so. When site conditions preclude infiltration/percolation,
then it shall be permitted on a case by case basis by the Township.
(k)
All stormwater runoff shall be treated for water quality.
(l)
Transference of runoff to or from an EV/HQ watershed is prohibited
unless otherwise authorized by DEP, DRBC or SRBC.
(m)
The Board may require that a landowner or developer provide reasonable
corrective measures to alleviate an existing off-site drainage problem
that may be affected by the proposed subdivision and/or land development.
It shall be the responsibility of the landowner or developer to obtain
all drainage easements on, over, or through other properties, and
the Township, its agents, workmen, servants and employees shall be
indemnified and held harmless from any liability.
[Ord. No. 243, 3/19/2008]
Permits must comply with any and all applicable local, county,
state and federal regulations.
[Ord. No. 243, 3/19/2008]
(a)
No regulated earth disturbance activities within the Township shall
commence until the Township receives an approval from the Conservation
District of an erosion and sediment control plan for construction
activities.
(b)
DEP has regulations that require an erosion and sediment control
plan for any earth disturbance activity of 5,000 square feet or more,
under 25 Pa. Code 102.4(b).
(c)
In addition, under 25 Pa. Code Chapter 92, a DEP "NPDES construction
activities" permit is required for regulated earth disturbance activities.
(d)
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the Township. The issuance of an NPDES construction permit (or permit coverage under the statewide general permit (PAG-2) satisfies the requirements § 25-303(a).
(e)
A copy of the Erosion and Sediment Control plan and any required
permit, as required by DEP regulations, shall be available at the
project site at all times.
(f)
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed
shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase to maintain
maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has achieved final stabilization.
[Ord. No. 243, 3/19/2008]
(a)
For projects disturbing one acre or more, the design of all regulated
activities shall include evaluation of 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.
(1)
An alternative is practicable if it is available and capable
of being done after taking into consideration cost, existing technology
and logistics in light of overall project purposes, and other Township
requirements.
(2)
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.
(b)
The applicant shall demonstrate that they designed the regulated
activities that disturb one acre or more included consideration of
the following issues:
(1)
Prepare an Existing Resource and Site Analysis Map (ERSAM),
showing environmentally sensitive areas including, but not limited
to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal
pools, flood plains, stream buffer zones, hydrologic soil groups A
and B (areas conducive to infiltration), special geologic features,
any existing recharge areas and any other requirements outlined in
the Township Subdivision and Land Development Ordinance.
(2)
Establish appropriate buffers for each of the delineated environmentally
sensitive areas per the Township Zoning Ordinance [see § 32-306(f)
for stream buffers and § 32-310(k) for special geologic
feature buffers.
(3)
Prepare a draft project layout avoiding sensitive areas identified in § 25-304(b)(1).
(4)
Identify site specific existing conditions drainage areas, discharge
points, recharge areas and hydrologic soil groups A and B.
(6)
Satisfy infiltration objective (§ 25-305) and provide for stormwater pretreatment prior to infiltration. Pretreatment may not be necessary for rooftop runoff which enters the infiltration facility directly from a roof leader.
(8)
Determine what management district the site falls into (Appendix
D)[1] and conduct an existing conditions runoff analysis.
[1]
Editor's Note: Appendix D is included as an attachment to this chapter.
(9)
Prepare final project design to maintain existing conditions
drainage areas and discharge points, to minimize earth disturbance
and impervious surfaces, and to the maximum extent possible, to ensure
the remaining site development has no surface or point discharge.
(10)
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 25-308).
(11)
Manage any remaining runoff through treatment prior to discharge,
as part of detention, bioretention, direct discharge or other structural
control.
[Ord. No. 243, 3/19/2008]
Maximizing the ground water recharge capacity of the area being developed is required. Design of the infiltration stormwater management facilities shall give consideration to providing ground water 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 § 25-103, 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 § 25-305(a)(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 § 25-305(a)(3)(i) or (ii), 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:
(i)
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
(ii)
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.
(iii)
The infiltration facility shall be capable of
completely infiltrating the required retention (infiltration) volume
within four days (96 hours).
(iv)
Pretreatment shall be provided prior to infiltration.
(3)
The size of the infiltration facility shall be based upon the
following volume criteria:
(i)
NRCS Curve Number equation.
The NRCS runoff equation shall be utilized to calculate infiltration
requirements (I) in inches.
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I (Infiltration requirement, in inches) = (200/ CN) - 2
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Eqn: 305.1
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Where:
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CN
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=
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SCS (NRCS) curve number of existing conditions contributing
to the infiltration facility.
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This equation is displayed graphically in, and the infiltration
requirement can be determined from Figure 305.1.
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It has been determined that infiltrating 0.46 inches of runoff
from the impervious areas will aid in maintaining the hydrologic regime
of the watershed. However, the rounded number 0.5 inches will be used.
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Figure 305.1. Infiltration requirement based upon NRCS
Curve Number.
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The retention (infiltration) volume (Rev) required to meet the infiltration requirement would therefore be
computed as:
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Rev = (0.5 or I, whichever is greater)*
impervious area (square feet)/(12 in/ft) = Cubic Feet
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Eqn: 305.2
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Where:
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I
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=
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Infiltration requirements (in inches)
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(ii)
Annual Recharge - Water Budget Approach. If the goals of § 25-305(a)(3)(i)a
cannot be achieved, then 0.5 inch of rainfall shall be infiltrated
from all impervious areas, up to a an existing site conditions curve
number of 81. Above a curve number of 81, Equation 305.1 or the curve
in Figure 305.1 should be used to determine the infiltration requirement.
The retention (infiltration) volume (Rev) required again would therefore be computed as:
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Rev = (0.5 or I, whichever is greater)
* impervious area (sq. ft.) / (12 in/ft) = cubic feet.
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(b)
Soils Testing. If on-lot infiltration is proposed as part of a project,
the applicant's design professional must demonstrate to the satisfaction
of the Township that the soils are conducive to infiltration at the
proposed location of infiltration facilities. Soils testing shall
be completed as follows:
(1)
Soils testing must be conducted by a qualified design professional
and at a minimum shall address depth to limiting zone, soil permeability,
and subgrade stability. Soils testing must be observed by a representative
of the Township.
(2)
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 sub-grade
stability; infiltration may not be ruled out without conducting these
tests.
(3)
Conduct field testing including: test pits to determine soil
horizons and depth to limiting zone and permeability tests, at the
elevation of the proposed infiltration facility surface, to determine
the appropriate hydraulic conductivity rate. Double ring infiltrometer
or hydraulic conductivity tests should be used to determine soil permeability
(percolation tests are not recommended for design purposes). Site
evaluation and soils testing should be conducted in accordance with
Appendix C of the Pennsylvania Stormwater Best Management Practices
Manual.
(4)
The proposed infiltration facilities shall be designed for the
required recharge (Rev) volume based on the
field determined capacity at the surface elevation of the proposed
infiltration facility.
(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 § 25-305(a). If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure B-1 in Appendix B.[1] However, the applicant is not required to use infiltration in carbonate areas even if the site falls into the "Recommended" range on Figure B-1 in Appendix B. If infiltration is not proposed, the calculated infiltration volume [§ 25-305(a)] shall be treated by an acceptable BMP.
Infiltration BMP loading rate percentages in Figure B-1 in Appendix
B shall be calculated as follows:
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The area tributary to the infiltration BMP shall be weighted
as follows:
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Area Description
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Weighting
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|---|---|
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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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Soil thickness is to be measured from the bottom of any proposed
infiltration BMP. The effective soil thickness in Figure B-1 in Appendix
B is the measured soil thickness multiplied by the thickness factor
based on soil permeability, as follows:
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Permeability Range
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Thickness Factor
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|---|---|
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6.0 to 12.0 inches/hr
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0.8
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2.0 to 6.0 inches/hr
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1.0
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1.0 to 2.0 inches/hr
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1.4
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0.75 to 1.0 inches/hr
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1.2
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0.5 to 0.75 inches/hr
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1.0
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The design of all facilities over Karst shall include an evaluation
of measures to minimize adverse effects.
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[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
(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. EPA's NPDES stormwater
program requires some industrial sites to prepare and implement a
stormwater pollution prevention plan.
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Examples of Hotspots:
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|---|---|
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•
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Vehicle salvage yards and recycling facilities
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•
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Vehicle fueling stations
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•
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Vehicle service and maintenance facilities
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•
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Vehicle and equipment cleaning facilities
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•
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Fleet storage areas (bus, truck, etc.)
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•
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Industrial sites (based on Standard Industrial Codes)
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•
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Marinas (service and maintenance)
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•
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Outdoor liquid container storage
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•
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Outdoor loading/unloading facilities
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•
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Public works storage areas
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•
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Facilities that generate or store hazardous materials
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•
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Commercial container nursery
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•
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Other land uses and activities as designated by an appropriate
review authority
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The following land uses and activities are not normally considered
hotspots:
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•
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Residential streets and rural highways
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•
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Residential development
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•
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Institutional development
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•
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Office developments
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•
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Non-industrial rooftops
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•
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Pervious areas, except golf courses and nurseries (which may
need an integrated pest management (IPM) plan).
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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.
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(e)
Caution shall be exercised where infiltration is proposed in Source
Water Protection Areas as defined by the Township or local Water Authority.
(f)
Infiltration facilities shall be used in conjunction with other innovative
or traditional stormwater control facilities that are found within
the PADEP State 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 Department'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.
(j)
The Township 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)
Infiltration Design Criteria.
(1)
All infiltration systems shall have appropriate positive overflow
controls to prevent storage within one foot of the finished surface
or grade.
(2)
All infiltration systems shall have a minimum setback of 15
feet from principal structures, 10 feet from property lines, 100 feet
from wells, 50 feet from septic system drain fields, and 50 feet from
a geologic contact with carbonate bedrock. Care should be taken to
prevent any seepage into subgrade structures.
(3)
Surface inflows shall be treated to prevent the direct discharge
of sediment and pollutants into the infiltration system; accumulated
sediment reduces stormwater storage capacity and ultimately clogs
the infiltration mechanism.
(4)
No sand or other particulate matter may be applied to a porous
paving surface for winter ice conditions.
(5)
During site construction, all recharge system components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Recharge areas shall be protected
from sedimentation. All areas designated for recharge shall not receive
runoff until the contributory drainage area has achieved final stabilization.
(6)
The following procedures and materials shall be required during
the construction of all subsurface facilities:
(i)
Excavation for the infiltration facility shall be performed
with equipment which will not compact the bottom of the seepage bed/trench,
or like facility.
(ii)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(iii)
Only clean aggregate, free of fines, shall be
allowed.
(iv)
The top and sides of all seepage beds, trenches, or like facilities
shall be covered with drainage filtration fabric. Fabric shall meet
the specifications of PennDOT Publication 408, Section 735, Construction
Class 1.
(v)
Perforated distribution pipes connected to centralized catch
basins and/or manholes with provision for the collection of debris
shall be provided in all facilities. The perforated pipes shall distribute
stormwater throughout the entire seepage bed/trench, or like facility.
(7)
All infiltration facilities which service more than one lot
and are considered a common facility shall have an easement provided
to the Township for future access if necessary.
(8)
No more than 50% of the required infiltration volume may be
provided in detention basin bottoms. The remaining 50% of infiltration
volumes shall be provided at or near the proposed impervious coverage.
[Ord. No. 243, 3/19/2008]
The applicant shall comply with the following water quality
requirements of this Part.
(a)
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 25-305 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:
The following calculation formula is to be used to determine
the water quality storage volume, (WQv), in acre-feet of storage for
Amity Township:
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WQv = [(P) (Rv)(A)]/12
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WQv = Water Quality Volume (acre-feet)
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P
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=
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1 inch
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A
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=
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Total contributing drainage area to the water quality BMP (acres)
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Rv
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=
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0.05 + 0.009(I) where I is the percent of the area that is impervious
surface ((impervious area/A)*100)
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This volume requirement can be accomplished by the permanent
volume of a wet basin or the detained volume from other BMPs.
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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.
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(b)
For areas within defined special protection subwatersheds which include
exceptional value (EV) and high quality (HQ) waters, 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.
[Ord. No. 243, 3/19/2008]
In addition to control of the 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 peak 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).
The minimum orifice size in the outlet structure to the BMP
shall be a three inch diameter orifice and a trash rack shall be installed
to prevent clogging. On sites with small contributing drainage areas
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.
[Ord. No. 243, 3/19/2008]
(a)
Amity Township has been divided into stormwater management districts
as shown on the Stormwater Management Districts Map in Appendix D.[1]
In addition to the requirements specified in Table 308.1 below, the groundwater recharge (§ 25-305), water quality (§ 25-306), and stream bank erosion control (§ 25-307), requirements shall be implemented.
Standards for managing runoff from each subarea for the two-year
through 100-year design storms are shown in Table 308.1. Development
sites located in each of the districts must control proposed conditions
runoff rates to existing conditions runoff rates for the design storms
in accord with Table 308.1.
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Table 308.1 — Water Quantity Requirements
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|---|---|---|---|
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Management District
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Proposed Condition Design Storm
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Existing Condition Design Storm
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B
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2-year
5-year
10-year
25-year
50-year
100-year
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Reduce To
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1-year
2-year
5-year
10-year
25-year
50-year
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C
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2-year
5-year
10-year
25-year
50-year
100-year
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Reduce To
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1-year
5-year
10-year
25-year
50-year
100-year
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[1]
Editor's Note: Appendix D is included as an attachment to this chapter.
(b)
General. Proposed condition rates of runoff from any regulated activity shall not exceed the peak release rates of runoff prior to development for the design storms specified on the Stormwater Management Districts Map ( Appendix D) and § 25-308, of this chapter.
(c)
District Boundaries. The boundaries of the stormwater management
districts are shown on in the Appendix D. The exact location of the
stormwater management district boundaries as they apply to a given
development site shall be determined by mapping the boundaries using
the two-foot topographic contours (or most accurate data required)
provided as part of the drainage plan.
(d)
Sites Located in More Than One District. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located, as indicated in § 25-308. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea.
(e)
Off-Site Areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
(f)
Site Areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area utilizing stormwater management measures shall
be subject to the management district criteria. In other words, unimpacted
areas bypassing the stormwater management facilities would not be
subject to the management district criteria.
[Ord. No. 243, 3/19/2008]
(a)
Stormwater runoff from all development sites with a drainage area
of greater than five acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS soil cover complex
method. Table 309-1 summarizes acceptable computation methods and
the method selected by the design professional shall be based on the
individual limitations and suitability of each method for a particular
site. The Township may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than five acres.
The Soil Complex Method shall be used for drainage areas greater than
five acres.
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Table 309-1
Acceptable Computation Methodologies For Stormwater Management
Plans
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|---|---|---|
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Method
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Method Developed By
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Applicability
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TR-20
(or commercial computer package based on TR-20)
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USDA NRCS
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Applicable where use of full hydrology computer model is desirable
or necessary.
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TR-55
(or commercial computer package based on TR-55)
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USDA NRCS
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Applicable for land development plans within limitations described
in TR-55.
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HEC-1 / HEC-HMS
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US Army Corps of Engineers
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Applicable where use of full hydrologic computer model is desirable
or necessary.
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PSRM
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Penn State University
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Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1.
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Rational Method or commercial computer package based on Rational
Method)
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Emil Kuichling (1889)
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For sites less than five acres and with time of concentration
less than 60 minutes (tc< 60 min), or as approved by the Township
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Other Methods
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Varies
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Other computation Methodologies approved by the Township.
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*Note: Successors to the above methods are also acceptable.
These successors include WinTR55 for TR-55 and WinTR20 for TR-20 and
SWMM
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(b)
All calculations consistent with this chapter using the soil cover
complex method shall use the appropriate design rainfall depths for
the various return period storms according to the region in which
they are located as presented in Table B-1 in Appendix B of this chapter.[1] If a hydrologic computer model such as PSRM or HEC-1 /
HEC-HMS is used for stormwater runoff calculations, then the duration
of rainfall shall be 24 hours. The rainfall distribution should reference
to NOAA Atlas 14.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
(c)
For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
Rational 'C' value (i.e., forest), as listed in Table B-2
or B-3 in Appendix B of this chapter.
(d)
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods from the NOAA Atlas 14 Precipitation-Frequency Atlas of the United States (2004, revised 2006). Times-of-concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times-of-concentration for channel and pipe flow shall be computed using Manning's equation. NRCS lag equation divided by 0.6 as acceptable method for Tc in undeveloped areas.
(e)
Runoff Curve Numbers (CN) for both existing and proposed conditions
to be used in the soil cover complex method shall be obtained from
Table B-2 in Appendix B of this chapter.
(f)
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational method shall be obtained from Table B-3 in
Appendix B of this chapter.
(g)
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's
roughness coefficient (n) shall be consistent with Table B-4 in Appendix
B of this chapter. Full flow shall be assumed for closed conduits.
(h)
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
(i)
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than five acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph (i.e. TR-20, TR-55, HEC-1, PSRM). The Township
may approve the use of any generally accepted full hydrograph approximation
technique that shall use a total runoff volume that is consistent
with the volume from a method that produces a full hydrograph.
[Ord. No. 243, 3/19/2008]
(a)
All wet basin designs shall incorporate biologic minimization controls
consistent with the West Nile Guidance found in Appendix F.[1]
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
(b)
Any stormwater management facility (i.e., detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the 100-year proposed
conditions and may be subject to PaDEP Chapter 105 regulations.
(c)
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), water encroachments, and
any work involving wetlands governed by PaDEP Chapter 105 regulations
(as amended or replaced from time to time by PaDEP), are subject to
PaDEP Chapter 105 regulations.
(d)
Any proposed roadway drainage facilities shall be designed according
to PennDOT Design Manual Part II.
(e)
Storm sewers must be able to convey proposed conditions runoff from
a fifty-year design storm without flooding inlets, where appropriate.
Roadway crossings must be able to convey runoff from a 100-year design
storm.
(f)
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge (DEP erosion and Erosion, Sediment
and Pollution Control Manual).
(g)
The Township reserves the right to disapprove any design that would
result in the construction in or continuation of a stormwater problem
area.
(h)
No stormwater detention facility shall be placed within 50 feet of
a special geologic feature. No stormwater conveyance facility shall
be constructed within 50 feet of a special geologic feature, unless
it is constructed of durable pipe utilizing watertight joints.
[Ord. No. 243, 3/19/2008]
(a)
The following criteria shall be used for the design of detention
and wet basins. Any reference to detention basin shall also include
wet basins.
(i)
When detention basins are provided, they shall be designed to
utilize the natural contours of the land wherever possible. When such
design is impracticable, the construction of the basin shall utilize
slopes as flat as possible to blend the structure into the terrain.
All basins shall have a maximum side slope of three horizontal to
one vertical (3:1).
(ii)
A routed hydrograph and calculations shall be furnished for
each storm through the detention basin.
(iii)
Emergency overflow facilities (i.e., emergency spillway) shall
be provided for all detention facilities which shall pass a 100-year
storm under orifice block conditions.
(b)
In the design of detention basins, the following items listed below
shall be submitted to the Township for review. Any reference to detention
basin shall also include retention basin.
(i)
Design computations for the sizing of the outlet device.
(ii)
A stage-storage curve for said detention/retention basin.
(iii)
Flood routing and/or storage requirement calculations.
(iv)
A plan showing the berm embankment and outlet structure. The
plan shall indicate top of berm elevations, width of the top of the
berm, side slopes, emergency spillway elevation, and elevations of
the outlet structure, including riser, dimensions and spacing of anti-seep
collars.
(v)
A cross section through the outlet structure, emergency spillway
and berm embankment.
(vi)
A detailed plan of the trash rack and anti-vortex device (if
required).
(vii)
The maximum side slope of earthen detention embankments shall
be three horizontal to one vertical (3:1). The top and/or toe (whichever
is closer to a property line) of any slope shall be located a minimum
of 25 feet from any property line. Whenever possible, the side slopes
and basin shape shall conform to the natural topography.
(viii)
The minimum top width of the detention basin
berm shall be 10 feet. A cutoff trench (key-way) of impervious material
shall be provided under all embankments that require fill material.
The cutoff trench shall be a minimum of eight feet wide, two feet
deep and minimum of two feet over the pipe, and have maximum side
slopes of one horizontal to one vertical (1:1).
(ix)
In order to insure proper drainage on the floor of the basin,
a minimum grade of 2% shall be maintained for areas of sheet flow.
For channel flow, a minimum grade of 1% shall be maintained. Under
certain circumstances, such as continuous seasonal flow, the Township
may require a low flow channel to be constructed. Wet basins may be
designed with a flat bottom upon approval by the Township Engineer.
(x)
All detention basin embankments shall be placed in a maximum
of eight inch lifts to a minimum of 95% of maximum dry density as
established by ASTM D-1557. Prior to proceeding to the next lift,
the compaction shall be checked by the Township Engineer. The developer's
contractor shall obtain the services of a qualified laboratory technician
to conduct compaction tests on the leading and the trailing edge of
the berm along with the top of the berm. All tests shall be furnished
to the Township for review.
(xi)
Whenever possible, the emergency spillway for detention basins
shall be constructed on undisturbed ground. Emergency spillways shall
be constructed of reinforced concrete checker blocks or other permanent
material approved by the Township Engineer. All emergency spillways
shall be constructed so that the detention basin berm is protected
against erosion. The minimum capacity of all emergency spillways shall
be the peak flow rate from the 100-year design storm after development.
The construction material of the emergency spillway shall extend along
the upstream and downstream berm embankment slopes. Construction material
on the upstream slope of the emergency spillway shall extend to a
minimum of two feet below the spillway crest elevation. The downstream
slope of the spillway shall as a minimum extend to the toe of the
berm embankment. The emergency spillway shall not discharge over earthen
fill and/or easily erodible material.
(xii)
A minimum freeboard of one foot shall be provided between the
100-year design stormwater elevation and the invert of the emergency
spillway, and between the design flow through the emergency spillway
and the top of the berm.
(xiii)
Anti-seep collars shall be installed around
the pipe barrel within the normal saturation zone of the detention
basin berms. The anti-seep collars and their connections to the pipe
barrel shall be watertight. The anti-seep collars shall extend a minimum
of two feet beyond the outside of the principal pipe barrel. The maximum
spacing between collars shall be 14 times the minimum projection of
the collar measured perpendicular to the pipe. A minimum of two anti-seep
collars shall be installed on each pipe outlet.
(xiv)
All outlet pipes through the basin berm shall be reinforced
concrete pipe having O-ring joints. All outlet structures shall be
concrete.
(xv)
Energy dissipating devices shall be placed at all basin outlets.
(xvi)
Easements of all basins shall be provided.
(xvii)
Permanent detention basins outlet structures
shall be designed to incorporate multiple stage outlet release devices.
(xviii)
A cross-section through the basin from the
proposed pipe termination into the basin to the termination of the
outlet pipe shall be provided.
(xix)
At the discretion of the Township, the perimeter of the detention/retention
pond shall be enclosed with a standard chain link security fence or
approved equal with a minimum height of four feet with locking gates.
Each basin shall be provided with two gates, one wide enough for maintenance
vehicles and a second gate for pedestrian access. The security fence
and gate shall be subject to the approval of the Township.
(xx)
Permanent plantings for wet ponds shall be designed by a wetland
biologist to have a mixture of plants that thrive in wet areas.
[Ord. No. 243, 3/19/2008]
(a)
Drainage easements shall be provided to accommodate all storm drainage
requirements and shall be a minimum of 20 feet in width. Easements
shall be provided for all watercourses and storm drainage piping that
are not located within street rights-of-way.
(b)
Storm sewers, as required, shall be placed in the cartway of curbed
streets and parallel to the roadway shoulders of streets without curbs.
When located in undedicated land with approval by the Board, storm
sewers shall be placed within an easement not less than 20 feet wide.
(c)
Storm Drainage Pipes. The minimum diameter of all storm drainage
pipes shall be 15 inches or an equivalent thereto. All storm drainage
piping shall be laid in a straight line. Storm drainage piping shall
not be permitted under buildings or structures. The minimum grade
of piping shall be 0.5%. All pipe shall be reinforced concrete with
O-rings joints, or smooth bore high density polyethylene pipe. All
structures exposed to the surface shall be reinforced concrete.
(d)
When proposed, manholes and inlets (catch basins) shall not be spaced
more than 400 feet apart for pipes of less than or equal to twenty-four-inch
diameter and 500 feet apart for pipes of greater than twenty-four-inch
diameter. Additional, manholes or inlets shall be placed at all changes
in alignment, grade or pipe size, and at all points of convergence
of two or more influent storm sewer lines. Inlets may be substituted
for manholes where they will serve a useful purpose. In addition,
the following standards shall apply:
(i)
Manholes and inlets must conform to the standards established
by PennDOT of Transportation and must be supplied by a PennDOT Bulletin
15 approved supplier. Such requirement shall be listed on the plan.
(ii)
At street intersections, inlets shall be placed to prevent the
flow of water across intersections.
(iii)
Inlets shall be spaced to limit the gutter spread to within
the parking lane, not more than eight feet during the ten-year storm.
Capacity of the inlets shall not exceed four cubic feet per second
(cfs) for four-foot inlets and five cfs for six-foot inlets in nonponding
areas.
(iv)
Inlets with a depth greater than five feet must be provided
with ladder rungs and noted on the plan as such.
(v)
When there is a change in pipe size in the inlet, the elevation
of the top of pipes shall be the same or the smaller pipe higher.
A minimum drop of two inches shall be provided at the inlet pipe invert
elevation and the outlet pipe elevation.
(vi)
Inlets shall contain a marker which discourages the discharge
of anything other than stormwater into the inlet.
(vii)
Inlets in paved areas shall be equipped with bicycle safe grates.
Inlets in nonpaved areas shall be equipped with standard grates.
(viii)
Manhole covers shall have the word "STORM" cast
on the top of the cover.
(e)
Properly designed, graded and lined drainage swales may be permitted
in lieu of storm sewers in commercial, industrial and residential
areas where approved by the Township. Swale lining must meet the Conservation
District design standards. All drainage channels shall have a maximum
side slope grade of three horizontal to one vertical (3:1). All drainage
swales shall be provided with a minimum six-inch freeboard, measured
from the top of the design storm flow to the top of the swale.
(f)
Curb requirements shall vary according to street hierarchy and intensity
of the proposed development. Curbing may also be required for any
one or more of the following reasons:
(g)
The maximum encroachment of water on the roadway pavement shall not
exceed four inches in depth at the curb line during a ten-year frequency
storm. Inlets shall be provided to control the encroachment of water
on the pavement.
(h)
Pipe underdrains and/or pavement base drains shall be provided in
areas delineated as having a "seasonal high water table" and in areas
deemed necessary by the Township Engineer during the construction
phase of the project. The installation of the underdrain system shall
be approved by the Township Engineer and paid for by the developer.
Pipe underdrains and pavement base drains shall be constructed in
accordance with PennDOT Pub. 408, Section 610, as amended.
(i)
Pipe end sections and/or head walls shall be utilized at all terminated
pipe segments.
(j)
All drainage structures located within a state highway right-of-way
shall be reviewed and approved by the PennDOT. A letter from PennDOT
indicating such approval shall be submitted to the Township prior
to Township approval.
(k)
Energy dissipaters shall be provided shall be provided at all pipe
end treatments.
(l)
At the discretion of the Township, in situations in which the design
standards and requirements do not apply to the site conditions, the
Township Engineer shall suggest or provide additional and/or alternative
design methods to meet the objectives of this Ordinance.
