The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the Municipality of the SWM site plan
and prior to commencement of any regulated activities, as applicable:
A. All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at 25 Pa. Code Chapter 102, or erosion
and sediment control requirements of the Municipality.
B. Work within natural drainageways subject to permit by PADEP under
25 Pa. Code Chapter 105.
C. Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under 25 Pa. Code Chapter 105.
D. Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way, or require access to or from a state highway
and be subject to approval by PennDOT.
E. Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under 25 Pa. Code
Chapter 105.
The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. For regulated activities with 10,000 or more square feet of proposed earth disturbance or 2,000 or more square feet of proposed impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in §
302-25C) that the design sequence, objectives and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:
A. First, identify and delineate all existing natural resources and natural and man-made hydrologic features listed in §
302-25B(8) that are located within the site, or receive discharge from, or may be impacted by the proposed regulated activity.
B. Second, provide a prioritized listing of these resources and features
to identify:
(1) Those to be incorporated into the site design in a manner that provides
protection from any disturbance or impact from the proposed regulated
activity;
(2) Those to be protected from further disturbance or impact but for
which the proposed regulated activity will provide improvement to
existing conditions;
(3) Those that can be incorporated into and utilized as components of
the overall site design in a manner that protects or improves their
existing conditions while utilizing their hydrologic function within
the limits of their available capacity (e.g., for infiltration, evapotranspiration,
or reducing pollutant loads, runoff volume or peak discharge rates,
etc.) to reduce the need for or size of constructed BMPs; and
(4) Those that may be considered for alteration, disturbance or removal.
C. Third, develop the site design to achieve the following:
(1) Recognize and incorporate the priorities identified in Subsection
B of this section as the basis for the proposed site layout, grading, construction, and permanent ground cover design;
(2) Minimize earth disturbance (both surface and subsurface);
(3) Maximize protection of or improvement to natural resources and special
management areas;
(4) Minimize the disturbance of natural site hydrology, in particular
natural drainage features and patterns, discharge points and flow
characteristics, natural infiltration patterns and characteristics,
and natural channel and floodplain conveyance capacity;
(5) Incorporate natural hydrologic features and functions identified in Subsection
B of this section into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6) Maximize infiltration and the use of natural site infiltration features,
patterns and conditions, and evapotranspiration features;
(7) Apply selective grading design methods to provide final grading patterns
or preserve existing topography in order to evenly distribute runoff
and minimize concentrated flows;
(8) Minimize the cumulative area to be covered by impervious surfaces
and:
(a)
Minimize the size of individual impervious surfaces;
(b)
Separate large impervious surfaces into smaller components;
(c)
Disconnect runoff from one impervious surface to another; and
(d)
Utilize porous materials in place of impervious wherever practicable.
(9) Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(11)
Locate infiltration and other BMPs:
(a)
At or as near to the source of generation as possible; and
(b)
At depths that are as shallow as possible.
(12)
Prioritize the selection and design of BMPs as follows:
(a)
Nonstructural and vegetation BMPs; then
(b)
Structural (surface and subsurface) BMPs.
(13)
For flow volumes requiring conveyance from the source of generation
to a BMP for management, give preference to open-channel conveyance
techniques that provide infiltration and water quality benefits and
landscaped-based management in common open space areas, where practicable;
and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of Subsections
B and
C of this section. Appendix B presents additional discussion of conservation design and low-impact development.
D. The procedures set forth above shall be utilized to the maximum extent practicable for the overall site design and selection, location and design of features and BMPs to be used to comply with the requirements of §§
302-17,
302-18,
302-19 and
302-20.
To control post-construction stormwater impacts from regulated
activities and meet State Water Quality Requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that post-construction stormwater
discharges do not degrade the physical, chemical or biological characteristics
of the receiving waters. The green infrastructure and low-impact development
(LID) practices provided in the PA BMP Manual, as well as the guidance
on green infrastructure, LID and conservation design (CD) provided
in Appendix B, shall be utilized for all regulated activities wherever
possible. The applicant shall comply with the following water quality
and runoff volume requirements for all regulated activities, including
all new development and redevelopment activities:
A. The post-construction total runoff volume shall not exceed the predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour duration precipitation (design storm). The water
quality and runoff volume to be managed shall consist of any runoff
volume generated by the proposed regulated activity over and above
the predevelopment total runoff volume and shall be captured and permanently
retained or infiltrated on the site. Permanent retention options may
include, but are not limited to, reuse, evaporation, transpiration,
and infiltration.
B. For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in §
302-21D of this chapter.
C. The design of the stormwater management facility outlet shall provide
for protection from clogging and unwanted sedimentation.
D. BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E. Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of §
302-18. The infiltration volume required under §
302-18 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F. Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide Pretreatment prior
to the runoff entering the stormwater management practice.
G. The Municipality may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in §
302-13P.
H. When the regulated activity contains or is divided by multiple drainage
areas, the water quality and runoff volume shall be separately addressed
for each drainage area.
I. Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
J. Areas located outside of the site (i.e., areas outside of the regulated
activity) may be excluded from the calculation of the water quality
and runoff volume requirements.
K. Water quality and volume control practices shall be selected and designed to meet the criteria of §
302-16C that apply to water quality and volume control.
L. Evapotranspiration may be quantified and credited towards meeting
volume requirements according to the PADEP Post Construction Stormwater
Management (PCSM) Spreadsheet and Instructions (December 2020) or
the most recent guidance from PADEP.
M. Infiltration which occurs during a rainfall event shall not be credited
toward meeting infiltration volume requirements.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A. For regulated activities involving both new development and redevelopment, infiltration should be designed to accommodate the entire water quality and runoff volume required in §
302-17. Infiltration BMPs should be consistent with design and infiltration period guidelines included in the PA BMP Manual or other PA DEP design guidance. If the runoff volume required by §
302-17 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PA DEP guidance, such as the Managed Release Concept, may be used to manage this volume with approval from the Municipal Engineer.
B. For regulated activities involving both new development and redevelopment, the volume as specified in §
302-17A from all regulated impervious surfaces shall be infiltrated.
C. If the requirements of Subsection §
302-18A or §
302-18B of this section cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Municipal Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated.
D. Only if a minimum infiltration of the first 0.5 inch of runoff volume cannot be physically accomplished on the site, shall a waiver from this §
302-18 be considered by the Municipality, in accordance with §
302-9, Waivers.
E. If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
F. When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
G. Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
H. A detailed soils evaluation of the site shall be conducted by a qualified
professional and, at a minimum, shall address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be conducted by a qualified licensed professional
and shall be consistent with the PA BMP Manual (as amended) (or other
guidance acceptable to the Municipal Engineer) and, in general, shall:
(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 other hydraulic
conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3) Design the infiltration facility for the required retention (infiltration)
volume based on field-determined infiltration capacity (and apply
safety factor as per applicable design guidelines) at the elevation
of the proposed infiltration surface.
(4) On-lot infiltration features are encouraged; however, it shall be
demonstrated to the Municipal Engineer that the soils are conducive
to infiltration on the identified lots.
(5) Tests shall be conducted at the proposed bottom elevation of an infiltration
BMP. At a minimum, two tests per infiltration BMP or one test per
1,000 square feet of infiltration area, whichever is greater, shall
be provided.
(6) Soil analysis and infiltration test methods and test locations shall
be submitted to the Township Engineer for review and approval prior
to testing. The Township Engineer shall observe all testing and will
require that specific test pits and percolation data be obtained in
order to approve test result data and ensure that the proposed infiltration
systems will function as designed.
I. Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(1) A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone. Additional depth may be required in areas underlain by Karst or carbonate geology (see §
302-18M).
(2) An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3) The infiltration facility shall completely drain the retention (infiltration)
volume within three days (72 hours) from the end of the design storm.
J. All infiltration practices shall:
(1) Be selected and designed to meet the criteria of §
302-16C that are applicable to infiltration;
(2) Be set back at least 25 feet from all buildings and features with
subgrade elements (e.g., basements, foundation walls, etc.), and at
least 10 feet from property lines or right-of-way boundaries, unless
otherwise approved by the Municipal Engineer;
(3) For any infiltration practice that collects runoff from shared or
multiple features and that is located within 50 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element.
K. Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1) When a hot spot is located in the area draining to a proposed infiltration
facility, an evaluation of the potential of groundwater contamination
from the proposed infiltration facility shall be performed, including
a hydrogeologic investigation (if necessary) by a qualified licensed
professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2) When located within a wellhead protection area of a public water
supply well, infiltration practices shall be in conformance with the
applicable approved source water protection assessment or source water
protection plan.
(3) The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
L. During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Municipal
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
M. Consideration of infiltration BMPs for areas underlain by Karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation are provided to the Municipality demonstrating that
the potential or likelihood of subsidence or sinkholes is minimal.
Evaluation of site conditions and infiltration design shall rely on
guidance in the PA BMP Manual (as amended) or other guidance acceptable
to the Municipal Engineer.
N. Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hotspots
(i.e., sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged though a water quality BMP(s) to remove pollutants
prior to infiltration.
O. Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
P. Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
Q. All infiltration practices shall have appropriate positive overflow
controls.
R. No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
S. The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1) Excavation for the infiltration facility shall be performed with
equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2) The bottom of the bed and/or trench shall be scarified prior to the
placement of aggregate.
(3) Only clean aggregate with documented porosity, free of fines, shall
be allowed.
(4) The tops, bottoms and sides of all seepage beds, trenches, or like
facilities shall be covered with drainage fabric. Fabric shall be
nonwoven fabric acceptable to the Municipal Engineer.
(5) Stormwater shall be distributed throughout the entire seepage bed/trench
or like facility, and provisions for the collection of debris shall
be provided in all facilities.
(6) 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) Recharge facilities shall be designed in accordance with infiltration
system guidelines of DEP BMP Manual Appendix C Protocol 2, as amended.
(8) The Municipal Engineer shall be permitted to inspect construction
methods. Shop drawings shall be submitted for all materials utilized
in recharge facilities.
T. 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.
For regulated activities involving new development with one
or more acres of earth disturbance, the applicant shall comply with
the following stream channel protection requirements to minimize stream
channel erosion and associated water quality impacts to the receiving
waters:
A. The peak flow rate of the post-construction two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour duration precipitation, using the
SCS Type II distribution.
B. To the maximum extent practicable, and unless otherwise approved
by the Municipal Engineer, the post-construction one-year, twenty-four-hour
storm flow shall be detained for a minimum of 24 hours and a maximum
not to exceed 72 hours from a point in time when the maximum volume
of water from the one-year, twenty-four-hour storm is stored in a
proposed BMP (i.e., when 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 orifice is at the invert of the proposed
BMP).
C. For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in §
302-21D of this chapter.
D. The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter unless otherwise approved by the Municipal
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the 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.
E. When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates subject
to the Municipal Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F. All proposed stormwater management facilities shall make use of measures
to extend the flow path and increase the travel time of flows in the
facility.
G. When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities including those
that involve new development and redevelopment.
A. Post-construction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design Storms specified in Table 308.1.
Table 308.1
Peak Rate Control Standards
|
---|
(Peak flow rate of post-construction design storm shall be reduced
to the peak flow rate of corresponding predevelopment design storm
shown in the table.)
|
---|
|
Predevelopment Design Storm
|
---|
Post-Construction Design Storm Frequency
(24-hour duration)
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
|
---|
1-year
|
75% (0.75) of the year
|
1-year
|
2-year
|
1-year
|
2-year
|
5-year
|
2-year
|
5-year
|
10-year
|
2-year
|
10-year
|
25-year
|
25-year
|
25-year
|
50-year
|
50-year
|
50-year
|
100-year
|
100-year
|
100-year
|
B. For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in §
302-21D of this chapter.
C. For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total regulated impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 308.1, using the redevelopment ground cover assumptions presented in §
302-21D. This design criterion for redevelopment is only permitted with approval of Municipal Engineer. It shall result in no impact on downstream properties.
D. Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E. Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F. When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G. The effect of structural and nonstructural stormwater management
Practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
Any design must also comply with any other existing Township ordinances, including, but not limited to, the West Vincent Township Subdivision and Land Development Ordinance (see Chapter
315 of the Township's Code).
A. Detention and retention facility design requirements.
(1) All detention/retention basins shall be designed to detain the peak
rate of water resulting from the site for all design storms up to
and including the fifty-year frequency rainfall. Design storms shall
be routed through the basin facilities, excluding in the design calculations,
the volume of the basin required for stormwater infiltration/recharge.
(2) The following setbacks are required for stormwater management facilities:
(a)
Stormwater retention or detention basins shall be located at
least 25 feet from any structure, whether existing or proposed.
(b)
Stormwater retention or detention basins shall be located at
least 25 feet from any property boundary or right-of-way.
(c)
Stormwater retention or detention basins shall be located at
least 50 feet from existing wetlands, or the banks of existing streams.
(d)
Recharge systems shall be located at least 25 feet from any
basement wall and 25 feet from wastewater treatment areas.
(e)
Any recharge system designed to handle runoff from any commercial
or industrial impervious parking or outside storage area shall be
a minimum of 50 feet from any water supply well or any wastewater
treatment area.
(3) Riser. A riser or other acceptable outfall shall be provided at the
outlet of all detention basins. The riser shall be constructed of
precast or poured-in-place concrete with controlled orifices. A trash
rack or similar appurtenance shall be provided to prevent debris from
entering the riser. All risers shall have a concrete base attached
with a watertight connection. The base shall be of sufficient weight
to prevent flotation of the riser.
(4) Landscaping. All stormwater control systems, whether existing or
proposed, shall be planted to effectively naturalize areas so as to
become an integral and harmonious element in the local landscape.
No trees shall be planted in the earthfill of berms or dams.
(5) Emergency spillway. Whenever possible, the emergency spillway for
detention basins shall be constructed on undisturbed ground. All emergency
spillways shall be constructed so that the detention basin berm is
protected against erosion with a permanent erosion control blanket.
The minimum capacity of all emergency spillways shall be such that
the capacity of the emergency spillway equals the peak flow rate from
the 100-year design storm. Emergency spillway linings shall extend
along the upstream and downstream berm embankment slopes. The upstream
edge of the emergency spillway lining shall extend to a minimum of
two feet below the spillway crest elevation. The downstream edge of
the spillway lining shall, at minimum, extend to a distance of 10
feet beyond the toe of the berm embankment. The emergency spillway
shall not discharge over earthen fill and/or easily erodible material.
(6) Anti-seep collars. Anti-seep collars shall be installed around the
principal 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.
(7) Freeboard. Any stormwater basin required or regulated by this chapter
designed to store runoff and requiring a berm or earthen embankment
shall be designed to provide an emergency spillway to safely convey
flow up to and including the 100-year proposed conditions. The height
of embankment shall provide a minimum 1.0 foot of freeboard above
the design flow depth of the emergency spillway. Should any BMP require
a dam safely permit under PA Chapter 105 regulations, the facility
shall be designed in accordance with and meet the regulations of PA
Chapter 105 concerning dam safety. PA Chapter 105 may require the
safe conveyance of storms larger than 100-year event.
(8) Slope of detention basin embankment. The maximum slope of earthen
detention basin embankments shall meet the requirements contained
in this subsection. Whenever possible, the side slopes and basin shape
shall be amenable to the natural topography. Straight side slopes
and rectangular basins shall be avoided.
(9) Width of berm. For dams less than 10 feet high, a minimum top width
of six feet is required, For dams 10 feet to 15 feet in height, a
top width of 10 feet is required. For dams greater than 15 feet in
height, the top width shall be designed by a professional engineer.
(10)
Slope of basin bottom. In order to ensure proper drainage of
the detention basin, a minimum grade of 2% shall be maintained for
all sheet flow. A minimum grade of 1% shall be maintained for all
channel flow. The Township Engineer may approve the use of a flat
(0%) bottom accompanied with appropriate water tolerant landscape
plantings to promote water quality and groundwater recharge.
(11)
Energy dissipaters. Energy - dissipating devices (rip-rap, end
sills. etc.) shall be placed at all basin outlets. Any pipe or other
component which discharges directly into the basin shall be equipped
with energy-dissipating devices and shall outlet into the bottom of
the basin.
(12)
Landscaping and grading of detention basin. All landscaping
and grading standards shall be as follows:
(a)
Cuts. No excavation shall be made with a cut face steeper than
three horizontal to one vertical, except under the condition that
the material in which the excavation is made is sufficiently stable
to sustain a slope of steeper than three horizontal to one vertical.
A written statement to that effect is required from an engineer and
must be submitted and approved by the Municipal Engineer. The statement
shall affirm that the site has been inspected and that the deviation
from the slope should not result in injury to persons or damage to
property. Retaining walls shall be required if a stable slope cannot
be maintained. Any retaining wall design must be signed and sealed
by a professional engineer. The toe of the slope or headwall of any
cut must be located a minimum of five feet from property lines.
(b)
Fills. No fills shall be made which creates any exposed surfaces
steeper in slope than three horizontal to one vertical, except where
the fill is located so that settlement, sliding, or erosion will not
result in property damage or be a hazard to adjoining property, streets,
or buildings. A written statement is required from an engineer certifying
that s/he has inspected the site and that any proposed deviation from
the slope specified above should not endanger any property or result
in property damage, and must be submitted to and approved by the Municipal
Engineer.
[1]
A concrete or stone masonry wall designed and constructed in
accordance with these specifications and standards may be required
to support the face of the fill where the above specified slopes are
exceeded.
[2]
The top of any fill or toe of the slope of any fill shall be
located 25 feet from any property line with the exception of a downstream
property line where the toe of the embankment shall be placed a sufficient
distance to allow for energy-dissipating devices, but in no case less
than 40 feet unless approved otherwise by the Township.
(c)
Planting requirements. All areas proposed for recreational use,
whether active or passive, shall be planted to effectively naturalize
the areas to become an integral and harmonious element in the natural
landscape.
(d)
Drainage channels and retention areas. All storm drainage channels
and retention areas, whether existing or proposed, shall be graded
and planted to effectively naturalize areas so as to become an integral
and harmonious part of the landscape by contour and type of plant
material employed.
(13)
Easements for all basins and storm pipes not located within the public street right-of-way shall be provided in accordance with §
302-37.
(14)
Any BMP intended to hold standing water for four days or longer
shall be designed to incorporate biologic controls consistent with
the West Nile Guidance found in PADEP document 363-0300-001 "Design
Criteria - Wetlands Replacement/Monitoring" (as amended), (or contact
the Pennsylvania State Cooperative Wetland Center or the Penn State
Cooperative Extension Officer for design information).
(15)
A profile shall be provided on the plan of the berm embankment
and outlet structure indicating the embankment top elevation, embankment
side slopes, top width embankment emergency spillway elevation, riser
dimensions, pipe barrel dimensions, and dimensions and spacing of
anti-seep collars.
B. Drainage and conveyance system design requirements.
(1) Design flow rate. The storm drain system shall be designed to carry
a twenty-five-year peak flow rate, and a fifty-year peak flow rate
at the sump area. The design twenty-five-year peak flow rate into
each inlet shall be indicated on the stormwater management plan. The
twenty-five-year flow rate shall be determined by the rational formula,
Q = CIA.
Where:
|
Q
|
=
|
Peak runoff rate, cubic feet per second (CFS);
|
C
|
=
|
Runoff coefficient equal to the ratio of the runoff rate to
the average rate of rainfall over a time period equal to the time
of concentration;
|
I
|
=
|
Average rainfall intensity to inches per hour for a time equivalent
to the time of concentration;
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A
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=
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Drainage area in acres.
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Approximate values for the runoff coefficient and runoff
intensity can be found in the Commonwealth of Pennsylvania, Department
of Transportation, PennDOT Drainage Manual Publication 584, as amended.
(2) Overflow system, An overflow system shall be provided to carry flow
to the detention basin when the capacity of the storm drain pipe system
is exceeded. The overflow system shall be of sufficient capacity to
carry the difference between the 100-year and the twenty-five-year
peak flow rates without creating additional damage to any drainage
facilities, structures or roadways.
(3) Inlet capacity. All inlets must be designed to accommodate the twenty-five-year
peak flow rate. The designer must submit capture/bypass calculations
for each inlet and include these calculations in the stormwater report.
As a minimum, the storm drainage system serving a street shall be
designed to collect water at any point where three cubic feet to five
cubic feet per second is accumulated, and at the bottom of all vertical
grades, and immediately upgrade of all street intersections. The system
shall discharge any collected water into the nearest practical drainage
channel or storm system. The capacity of each inlet shall be indicated
on the stormwater plan. The capacity of all C, M, or S type inlets
shall be determined from the following sources:
Commonwealth of Pennsylvania
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Department of Transportation
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Design Manual, Part 2
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Highway Design, Chapter 10
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(4) Summary table. A table shall be provided on the plan summarizing
storm sewer information including, inlet numbers, inlet pipes, elevation
of all grates and pipe inverts, pipe diameters and lengths, pipe material,
etc. A similar table shall be provided summarizing the design information
for all temporary and permanent drainage channels.
(5) Roadway crossings or structures located within designated floodplain
areas shall be able to convey runoff from a 100-year design storm
consistent with Federal Emergency Management Agency National flood
Insurance Program - floodplain management Requirements.
(6) Any facility located within a PennDOT right-of-way shall comply with
PennDOT minimum design standards and permit submission and approval
requirements.
(7) Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as
amended) and the PADEP Erosion and Sediment Pollution Control Program
Manual (Publication No. 363-2134-008, as amended), or other design
guidance acceptable to the Municipal Engineer.
(8) Straight pipe selections. Wherever possible, all storm drain pipes
shall be designed to follow straight courses. No angular deflections
of storm sewer pipe sections in excess of 5° shall be permitted.
No vertical curves shall be permitted in the storm drain pipe system.
(9) Minimum grade and size. All storm drain pipes shall be designed to
maintain a minimum grade of 1/2%. All storm pipes shall have a minimum
inside diameter of 15 inches, except that pipes under a twenty-five-foot-deep
or greater fill shall not be less than 24 inches, or a cross-sectional
area of 453 square inches.
(10)
Pipe material and thickness. All storm sewers shall meet PennDOT
standards for service life and proper class and thickness to support
the above fill material. Pipe type and joint specifications shall
be noted on the plans and shall be in accordance with PennDOT standards.
(11)
Pipe capacity. The capacity of all pipe culverts shall, as a
minimum, provide the required carrying capacity as determined by the
following source:
United States Department of Transportation
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Federal Highway Administration
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Hydraulic Design Series Number 5
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Hydraulic Design of Highway Culverts, 3rd Edition.
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(12)
Pipe arches. Where headroom is restricted, equivalent pipe arches
may be used in lieu of circular pipes.
(13)
Allowable headwater depth. At all inlets or manholes, the maximum
allowable headwater depth shall be one foot below the top of the inlet
grate of the manhole cover.
(14)
Horizontal pipe deflections. A manhole or inlet shall be provided
at all horizontal deflections in the storm pipe system exceeding 5°.
(15)
Inlet boxes shall conform to PennDOT Standard Drawings and Publication
408, latest edition. Inlets shall be permanently marked with the phrase
"No dumping - drains to creek" with metal medallion style signs or
other method approved by the Municipal Engineer.
(16)
Minimum and maximum cover. A minimum of 18 inches of cover shall
be maintained over all storm drain pipes, unless the pipe manufacturer
allows less cover for the specific location and expected loading.
The top of storm drain pipes shall be at least 1/2 foot below subgrade
elevation.
(17)
Culverts and drainage channels.
(a)
Design flow standards. All culverts and drainage channels shall
be designed to carry a flow rate equal to a fifty-year, twenty-four-hour
storm (NRCS, Technical Release No. 55).
(b)
Erosion prevention. All drainage channels shall be designed
to prevent the erosion of the bed and bank areas. The flow velocity
in all vegetated drainage channels shall not exceed three feet per
second to prevent erosion unless special provisions approved by the
Municipal Engineer are made to protect banks and channel bottoms against
erosion. Suitable bank stabilization shall be provided where required
to prevent erosion of the drainage channels. Where storm sewers discharge
into existing drainage channels at an angle greater than 30° from
parallel with the downstream channel flow, the far side bank shall
be stabilized by the use of rip-rap or masonry, and/or concrete walls.
The stabilization shall be designed to prevent erosion and frost heave
under and behind the stabilizing media.
(c)
Maximum side slope. Any vegetated drainage channel requiring
mowing of the vegetation shall have a maximum grade of three horizontal
to one vertical of those areas to be mowed.
(d)
Design standard. Because of the critical nature of the vegetated
drainage channels, the design of all vegetated channels shall, at
a minimum, conform to the design procedures outlines in the PADEP
manuals. Several acceptable sources outline procedures for non-vegetated
drainage channels, including the following:
Bureau of Public Roads
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Hydraulic Engineering Circular No. 5
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Hydraulic Charts for the Selection of Highway Culverts
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Federal Highway Administration
|
Hydraulic Engineering Circular No. 13
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Hydraulic Design of Improved Inlets for Culverts
|
(e)
Reference to publications and source documents in this section
shall be deemed to include any amendments and revisions thereof.
(18)
Easements for all drainage and conveyance systems, existing or proposed, not located within the public street right-of-way, shall be provided per §
302-43.
(19)
Residential subdivisions or development. Drainage channels shall
be provided to intercept stormwater along property lines at locations
where runoff from a lot would drain onto an adjacent lot. These channels
shall be designed to convey stormwater to a suitable discharge point
or the storm sewer system. Storm inlets shall be designed and located
in new residential developments to capture stormwater that drains
from one lot to another lot. Stormwater drainage channels shall not
convey stormwater across more than two adjacent lots without being
captured by a storm inlet and piping.