61.15.1.
A stormwater management site plan shall be required for each
subdivision, land development and regulated activity, unless exempted.
The stormwater management site plan shall demonstrate effective control
of stormwater runoff and compliance with the stormwater management
requirements, standards and design criteria of this chapter. The stormwater
management site plan shall be an integral part of each subdivision
and land development plan at the preliminary and final plan stage.
For the purposes of this section of this chapter, the following activities
are defined as regulated activities and shall be considered as land
development for regulation by this chapter:
1.
Construction of new or additional impervious surfaces (sidewalks,
driveways, parking lots, etc.); and
2.
Installation of new impervious surfaces where gravel exists; and
3.
Renovation of existing impervious surfaces which currently do not
have a stormwater management facility; and
4.
Construction of new buildings or additions to existing buildings;
and
5.
Diversion or piping of any natural or man-made stream channel; and
6.
Installation of stormwater management facilities or appurtenances
thereto; and
7.
Alteration of conditions that may increase or alter natural drainage
from the existing site.
61.16.1.
For all regulated activities, unless preparation of an SWM site
plan is specifically exempted:
1.
Preparation and implementation of an approved SWM site plan is required.
2.
No regulated activities shall commence until the Municipality issues
written approval of an SWM site plan which demonstrates compliance
with the requirements of this chapter.
3.
The Municipality must be notified of construction operations prior
to commencement so that an inspection can be completed.
4.
After the installation of the facility, the overseeing professional
engineer (design engineer) should submit a letter indicating that
the facility was installed per the approved plan.
61.16.2.
SWM site plans approved by the Municipality shall be on site
throughout the duration of the regulated activity.
61.16.3.
These standards apply to the landowner and any person engaged
in regulated activities.
61.16.4.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by SWM
BMPs or open channels consistent with this chapter.
61.16.5.
If diffused flow is proposed to be concentrated and discharged
onto adjacent property with permission from the receiving property
owner, the developer must document that adequate downstream conveyance
facilities exist to safely transport the concentrated discharge, or
otherwise prove that no erosion, sedimentation, flooding or other
harm will result from the concentrated discharge, and downstream easements
shall be established to provide drainage paths for concentrated discharge.
Maximum use shall be made of the existing on-site natural and man-made
stormwater management facilities.
61.16.6.
A concentrated discharge of stormwater to an adjacent property
shall be within an existing watercourse or otherwise an easement shall
be required. Pipe outlets shall also be provided with erosion-resistant
material or energy dissipaters to calm the anticipated velocity and
discharge of stormwater.
61.16.7.
Discharge points from any facility must be 15 feet from any
property line with adequate BMPs to ensure no channelizing of flows
onto neighboring property.
61.16.8.
For all regulated earth disturbance activities, erosion and
sediment control BMPs shall be designed, implemented, operated, and
maintained during the regulated earth disturbance activities (e.g.,
during construction) to meet the purposes and requirements of this
chapter and to meet all requirements under Title 25 of the Pennsylvania
Code and the Clean Streams Law. [1]Various BMPs and their design standards are listed in the
Erosion and Sediment Pollution Control Program Manual (E&S Manual),
No. 363-2134-008, as amended and updated.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
61.16.9.
Impervious areas:
1.
The measurement of impervious areas shall include all of the impervious
areas in the total proposed development, even if development is to
take place in stages;
2.
For development taking place in stages, the entire development plan
must be used in determining conformance with this chapter;
3.
For projects that add impervious area to a parcel, the total proposed
impervious area on the parcel is subject to the requirements of this
chapter.
4.
Any gravel or crushed stone surface existing prior to the issuance
of this chapter is to be assumed as pervious surface.
5.
Any areas proposed to be gravel or crushed stone shall be considered
impervious.
6.
The anticipated flooded surface area occurring within the site shall
be considered as impervious.
61.16.10.
Stormwater flows onto adjacent or downstream property shall
not be created, increased, decreased, relocated, impeded, or otherwise
altered without written notification of the affected property owner(s).
Notification shall include a description of the proposed development
and the stormwater flows that are being created, increased, decreased,
relocated, impeded, or otherwise altered. Adjacent property shall
at a minimum include any property having a shared boundary with the
subject property of the SWM site plan; however, if in the judgment
of the designated plan reviewer additional properties might be affected,
additional notifications may be required. Proof of notification (signed
postal receipt, for example) shall be included as part of the SWM
plan submission to the Municipality. Such stormwater flows shall be
subject to the requirements of this chapter.
61.16.11.
All regulated activities shall include such measures as necessary
to:
1.
Protect health, safety, and property.
2.
Meet the water quality goals of this chapter by implementing measures
to:
a.
Minimize disturbance to floodplains, wetlands, and wooded areas.
b.
Maintain or extend riparian buffers.
c.
Avoid erosive flow conditions in natural flow pathways.
d.
Minimize thermal impacts to waters of this commonwealth.
e.
Disconnect impervious surfaces by directing runoff to pervious areas,
wherever possible.
3.
Incorporate methods described in the Pennsylvania Stormwater Best
Management Practices Manual (BMP Manual). If methods other than green
infrastructure and LID methods are proposed to achieve the volume
and rate controls required under this chapter, the SWM site plan must
include a detailed justification, acceptable to the designated plan
reviewer, demonstrating that the use of LID and green infrastructure
is not practicable.
61.16.12.
Infiltration BMPs should be dispersed throughout the project
site at strategic locations, made as shallow as practicable, and located
to maximize use of natural on-site infiltration features while still
meeting the other requirements of this chapter. If infiltration is
utilized, appropriate percolation test results must be provided.
61.16.13.
Normally dry, open-top storage facilities should completely
drain both the volume control and rate control capacities over a period
of time not less than 24 and not more than 72 hours from the end of
the design storm.
61.16.14.
The design storm precipitation depths to be used in the analysis
of peak rates of discharge shall be as obtained in PennDOT's
Drainage Manual, Publication 584, Appendix 7A; or obtained from the
latest version of the Precipitation-Frequency Atlas of the United
States, National Oceanic and Atmospheric Administration (NOAA), National
Weather Service, Hydrometeorological Design Studies Center, Silver
Spring, Maryland. NOAA's Atlas 145 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
61.16.15.
61.16.16.
Various BMPs and their design standards are listed in the BMP
Manual.
61.16.17.
The Municipality may, after consultation with DEP, approve measures
for meeting the state water quality requirements other than those
in this chapter, provided that they meet the minimum requirements
of, and do not conflict with, state law, including, but not limited
to, the Clean Streams Law.[4]
[4]
Editor's Note: See 35 P.S. § 691.1 et seq.
61.16.18.
Stormwater management facilities which involve a state highway
shall be subject to the approval of PennDOT.
61.16.19.
Stormwater management facilities which involve a county highway
or a county facility shall be subject to the approval of Allegheny
County.
61.16.20.
Minimization of impervious surfaces and maximization of 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.
61.16.21.
Where a development site is traversed by watercourses other
than permanent streams, drainage easements shall be provided conforming
to the line of such watercourses. The terms of the easement shall
prohibit excavation, the placing of fill or structures, and any alterations
that may adversely affect the flow of stormwater within any portion
of the easement. Also, maintenance, including mowing of vegetation
within the easement, shall be required, except as approved by the
appropriate governing authority.
61.16.22.
The Pa. Code, Title 25, Chapter 105, rules and regulations apply
to the construction, modification, operation or maintenance of both
existing and proposed water obstructions and encroachments throughout
the watershed, including work in wetlands. Inquiries on permit requirements
or other concerns shall be addressed to the DEP, Bureau of Waterways
Engineering and Wetlands, in Harrisburg.
61.16.23.
When it can be shown that, due to topographic conditions, natural
drainageways on the site cannot adequately provide for drainage, open
channels may be constructed conforming substantially to the line and
grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by DEP through the joint permit application
process, or, where deemed appropriate by DEP, through the general
permit process.
61.16.24.
Stormwater resulting from regulated activities shall not be
discharged into sinkholes.
61.16.25.
To the maximum extent practicable, incorporate the techniques
for low-impact development practices described in the BMP Manual.
61.17.1.
Regulated activities that result in an increase in the proposed
impervious area less than 400 square feet, if there were no previous
impervious surface additions after May 1, 2017 (regardless if the
existing surface is impervious), are exempt from the requirements
of this chapter.
61.17.2.
Proposed impervious area between 400 square feet and 1,000 square feet of earth disturbance must submit an SWM site plan to the Municipality, which shall consist of the items and related supportive material needed to determine compliance with Section 61.18, Small projects.
61.17.3.
Agricultural activity is exempt from the SWM site plan preparation
requirements of this chapter, provided the activities are performed
according to the requirements of 25 Pa. Code Chapter 102.
61.17.4.
Forest management and timber operations are exempt from the
SWM site plan preparation requirements of this chapter, provided the
activities are performed according to the requirements of 25 Pa. Code
Chapter 102.
61.17.5.
Roadway resurfacing and maintenance projects which do not increase
or change the impervious area/type and underground infrastructure
projects are exempt from the provisions of this chapter, provided
the activities meet the requirements of all other municipal, state
and federal requirements.
61.17.6.
Exemptions from any provisions of this chapter shall not relieve the applicant from the requirements in Section 61.15.
61.17.7.
The Municipality may deny or revoke any exemption pursuant to
this section at any time for any project that the Municipality believes
may pose a threat to public health and safety or the environment.
61.17.8.
Voluntary green stormwater infrastructure (GSI) retrofit projects
that are solely intended to better manage runoff from existing development
and are not part of new development or redevelopment are exempt from
the stormwater management provisions of this chapter. This does not
exempt such projects from any other municipal, state, or federal regulation.
61.18.1.
Table 1 presents the Municipality's stormwater management
requirements for small projects. For projects that propose additional
impervious area to a parcel, the total proposed impervious area on
the parcel is subject to the requirements of this chapter. Impervious
area is described in Section 61.16.9.
|
Table 1 - Stormwater Management Requirements
| ||
|---|---|---|
|
No.
|
Square Feet of Proposed Impervious Surface
|
Stormwater Management Requirement
|
|
1
|
Less than 400
|
No requirements if there were no previous impervious surface
additions after May 1, 2017
|
|
2
|
400 to 1,000 (small project)
|
Capture and detain the first 2 inches of any storm event
|
|
3
|
Greater than 1,000
|
Comply with requirements of this chapter
|
61.18.2.
Stormwater management for small projects will consist of capturing
and detaining the first two inches of any storm event. For small projects,
consider the following:
1.
Two inches of rain results in the following volumes that need to
be captured and detained:
|
400 sq. ft. * 2 in./(12 in./ft.) * 7.48 gal/cu. ft. = 499 gal.,
approximately 500 gallons of detention storage.
|
|
1,000 sq. ft. * 2 in./(12 in./ft.) * 7.48 gal/cu. ft. = 1247
gal., approximately it 1,250 gallons of detention storage.
|
2.
Therefore, small project detention requirements range from 500 gallons
to 1,250 gallons.
61.18.3.
Capture and detention can be accomplished by a variety of methods,
including, but not limited to:
61.18.4.
In all cases, the detention method must be designed to drain
all accumulated runoff not less than 24 hours and not more than 72
hours after the end of the storm event. Appropriate soil percolation
conditions must be available for stone sumps and rain gardens, and
demonstrated by percolation tests.
61.18.5.
A maintenance agreement must be signed by the property owner
to ensure the proper, continual inspection and maintenance of the
capture and detention method.
61.18.6.
The property owner must submit the following stormwater management
information for review by the Municipality staff:
1.
A written description of the proposed project, including the dimensions
of all proposed impervious surfaces.
2.
A scale drawing showing existing and proposed features of the property.
3.
A written description of the proposed stormwater management methods.
4.
Dimensioned drawings of the proposed stormwater management methods
and their locations.
5.
A list of proposed impervious surfaces and their square footage.
6.
Supporting documentation, such as assumptions, calculations, rain
barrel size, stone size, percolation tests, etc.
7.
Draft operation and maintenance agreement.
61.18.7.
Construction of the proposed improvements will not begin prior
to receiving approval of the stormwater management plan from the Municipality.
61.19.1.
Stormwater management planning and stormwater management facilities
shall be designed and constructed in accordance with the following.
61.19.2.
General standards.
1.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices.
2.
All stormwater runoff flowing over the development site shall be
considered in the design of the stormwater management facilities.
3.
Runoff from impervious areas shall be drained to pervious areas of
the development site and the stormwater management control facilities.
4.
Stormwater runoff from a development site shall flow directly into
a natural watercourse, into an existing storm sewer system or onto
adjacent properties in a manner similar to the runoff characteristics
of the predevelopment flow.
5.
A concentrated discharge of stormwater to an adjacent property shall
be within an existing watercourse or storm sewer system and enclosed
within an easement. All discharges must be to stable areas or conveyances.
If the existing waterway or drainage system is not stable, a design
must be proposed that, when implemented, will ensure a stable conveyance.
Downstream easements are required to be established to provide drainage
paths for concentrated discharge. Design and construction shall preclude
erosion, sedimentation, flooding or similar damage.
6.
Where a development site is traversed by watercourses, drainage easements
shall be provided conforming to the line of such watercourses. The
terms of the easement shall prohibit excavation, the placing of fill
or structures, and any alterations that may adversely affect the flow
of stormwater within any portion of the easement. Also, maintenance,
including mowing of vegetation within the easement, shall be required,
except within BMP areas where mowing is not desired. The drainage
easement shall adequately contain the anticipated inundation associated
with the identified 100-year floodplain, or be depicted 50 feet from
the top of the channel bank for undefined floodplain areas.
7.
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 DEP. When there is a question whether
wetlands may be involved, it is the responsibility of the developer
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 DEP.
8.
Any stormwater management facilities regulated by this chapter that
would discharge to state highway rights-of-way shall be subject to
approval by the Pennsylvania Department of Transportation (PennDOT).
Stormwater detention basins, retention basins, infiltration basins
and similar structures shall maintain a ten-foot isolation from a
PennDOT right-of-way.
9.
Stormwater facilities that are not located within a street right-of-way
shall be centered within an adequate easement of no less than 20 feet
in width. Easements shall follow property lines where possible and
are to be provided with bearing and distance descriptions that tie
to known points and shall close on individual lots.
10.
A variety of methods for stormwater detention and retention are available
for use. These include surface detention, subsurface detention, use
of existing facilities (ponds, etc.) or a combination thereof. Subsurface
detention shall be utilized only where the subsurface is stable, the
area is not prone to sinkholes or landslides, and all underground
pipes are sealed to prevent leaks.
11.
Storage of equivalent stormwater runoff for a portion of a property
may be considered in lieu of storage of generated runoff, provided:
a.
The site is located so that it is physically impossible to detain
runoff from the proposed facilities or drainage problems exist upgrade
that would impact upon the site or downgrade properties.
b.
The impact of generated runoff discharging off site from the subdivision
or land development is determined by the Municipal Engineer to be
negligible and not detrimental to adjacent properties.
c.
Implementation of equivalent storage shall be determined applicable
and feasible by the Municipal Engineer.
12.
Storm sewer pipes, culverts, manholes, inlets, endwalls, end sections,
and other stormwater management facilities shall be designed and constructed
in accordance with the requirements of the Pennsylvania Department
of Transportation, Design Manual Part 2, Highway Design, Publication
13, including amendments thereto, unless specifications are otherwise
provided herein. Structures and their installation within or directly
connected to existing or intended public rights-of-way shall conform
to the current PennDOT Publication 72M and Publication 408, Roadway
Construction Standards.
13.
Headwalls and endwalls shall be used where stormwater runoff enters
or leaves the storm sewer horizontally from a natural or man-made
channel. PennDOT Type "DW" headwalls and endwalls shall be utilized.
Reinforced concrete flared end sections are also acceptable, but galvanized
metal or polyethylene flared end sections are prohibited. All headwalls
and endwalls and flared end sections with pipes of twelve-inch or
greater diameter shall be protected from child entry by placing removable
stainless steel bars (and compatible mounting hardware), spaced four
inches apart, across the opening. Periodic inspection and clearing
of debris from such structures is the responsibility of the owner.
61.20.1.
Any stormwater management facility designed to store stormwater
runoff and requiring a berm or earthen embankment (i.e., detention
or retention basin) shall be designed to provide an emergency spillway
to convey the 100-year post-development peak flow rate tributary to
the basin. The use of inlets and pipes are prohibited for the emergency
spillway. Stormwater runoff from existing natural swales and/or other
existing drainage conveyors shall not be directed towards or intercepted
by the stormwater management facilities. The height of embankment
must be set as to provide a minimum 1.0 foot of freeboard above the
maximum pool elevation computed when the entire 100-year peak flow
rate to the structure passes through the spillway. Rain gardens, with
a maximum water depth of 12 inches, shall be exempt from this requirement;
however, conveyance of the 100-year peak discharge must be shown to
be conveyed in a stable condition. The criteria for design and construction
of stormwater management facilities are not the same criteria that
are used in the permitting of dams under the DEP Dam Safety Program.
Depending upon the physical characteristics of a dam, a dam permit
may be required, and the design will have to meet the provisions of
Pa. Code, Title 25, Chapter 105. Depending on the physical characteristics
of a dam, the design could require that anywhere from a 100-year to
a probable maximum flood (PMF) storm event be considered. Retention
and detention basins shall be designed and constructed according to
the following minimum standards:
1.
The maximum water depth to the base of the emergency spillway shall
not exceed six feet in residential areas. Depths up to eight feet
are permitted in nonresidential projects, provided minimum six-foot-high
fencing is provided around the basin when depths exceed six feet.
2.
The minimum top width of dams shall be five feet for impoundments
draining five acres or less and eight feet minimum width for impoundments
with drainage areas of or exceeding five acres.
3.
The side slopes of earth fill dams shall not be less than three horizontal
to one vertical on both sides of the embankment. However, any portion
of the inside berm of an unfenced basin above a proposed water depth
of three feet shall have a maximum side slope of five horizontal to
one vertical. All pond areas, including bottoms of non-water-quality
ponds, side slopes and top of berms, shall be provided with a minimum
four inches of topsoil and shall be mulched and seeded with Formula
B in accordance with PennDOT Publication 408. All areas to receive
topsoil should be scarified 12 inches deep before topsoil placement.
Protect from erosion until vegetation is 70% established.
4.
All basins shall be structurally sound and shall be constructed of
sound and durable materials. The completed structure and the foundation
of all basins shall be stable under all probable conditions of operation.
5.
A cutoff or key trench of impervious material shall be provided under
all basin berms.
6.
All discharge control devices with appurtenances (except discharge
pipes) shall be made of reinforced concrete and stainless or hot dip
galvanized steel. Bolts/fasteners are to be stainless or galvanized
steel. Smooth lined corrugated polyethylene pipe may be used for discharge
piping.
7.
Concrete, polyethylene or welded galvanized steel anti-seep collars
compatible with the discharge pipe shall be placed around all basin
discharge pipes to increase the seepage length along the pipe by 15%
within the saturated zone of the pipe based on a 4:1 phreatic line.
The connection of the anti-seep collar to the discharge pipe shall
be completely watertight.
8.
Basin principal spillway outlet structures shall consist of stainless
steel orifice plates and mounting hardware, galvanized or reinforced
concrete riser and discharge pipe, and welded structural steel inlet
grates with a bituminous coating. Smooth lined corrugated polyethylene
pipe may be used for discharge piping. Principal spillways with riser
pipes where the designed water depth is deeper than 0.4 times the
diameter of the riser shall have an anti-vortex device to prevent
reduced capacity of the riser.
9.
All basin intake and discharge structures shall be protected from
frost-related movement by the construction of a concrete footer with
a minimum depth of 36 inches below finished grade level.
10.
Minimum floor elevations for all structures that would be affected
by a basin or storm conveyance system shall be two feet above the
100-year water surface elevation.
11.
An emergency spillway shall be provided to safely pass the peak flow
rate of the incoming 100-year storm, with one foot of freeboard between
the maximum pool elevation and the top of the embankment and in a
manner which will not damage the integrity of the facility and will
not create a downstream hazard. The maximum pool elevation shall be
established using the weir equation through the spillway while ignoring
discharge flows from the principal spillway and storage volume within
the basin. Routing calculations for rate control structures discharging
to the ground surface shall ignore exfiltration or infiltration discharges
from the structure.
12.
Where practical, the emergency spillway shall be constructed in undisturbed
ground. An easement for inspection and repair shall be provided when
the conveyance structure crosses property boundaries. Downstream drainage
easements from the emergency spillway may be required.
13.
Except for retention basins, permanent ponds and wetlands, all basins
shall have dewatering features such as low flow channels or tile fields.
Low flow channels shall be provided with reinforced matting extending
two feet beyond the channel. All detention basins shall include an
outlet structure designed to completely drain the basin within 24
hours following the end of the design rainfall. However, basins containing
groundwater recharge and/or water quality storage shall include an
outlet structure designed to fully drain the recharge and/or water
quality volume in no less than 24 hours or more than 72 hours.
14.
Basin discharges to proposed or existing conveyance systems shall
require evidence of adequate capacity in the receiving facility.
15.
The minimum slope of any detention basin bottom surface shall be
1% positive grade toward the outlet, along all flow paths except those
basins specifically designed to provide infiltration of stormwater
in conformance with the water quality standards.
16.
Detention basin length or basin flow path length shall be at least
two times the basin width to facilitate water treatment and infiltration.
17.
In areas of carbonate geology, retention and detention basins shall:
a.
Be placed at least 100 feet from the rim of any sinkhole or closed
depression; and
b.
Be placed a minimum of 25 feet from rock outcroppings or pinnacles;
and
c.
Not discharge into a sinkhole; and
d.
Be designed and located to prevent groundwater contamination and
sinkhole formation, including the use of impermeable liners (see specifications
below) where deemed necessary to avoid or abate such problems. The
construction of clay liners shall not qualify as an impermeable liner.
e.
Be constructed under the supervision of a professional engineer or
professional geologist licensed by the commonwealth. A professionally
sealed as-built construction report, and plan including relative dates,
name(s) of contractor(s), methods of construction, and verification
of conformance to plan specifications and good engineering practices
shall be provided to the Municipality prior to bond reduction requests
related to the basin(s).
18.
The impermeable liner shall be constructed to meet the following
minimum requirements.
a.
The minimum liner required is 30 mil high-density polyethene (HDPE)
and must be UV-resistant. Actual individual liner specifications shall
be provided by the manufacturer for each individual pond.
b.
The liner must be placed on a layer of fine-grained soil that has
been rolled with a smooth drum roller in both directions to produce
a smooth level base for the liner. The soil may not contain sharp
angular rock or other debris that could puncture the liner, and must
meet all manufacturers' specifications for a liner bedding. All
vegetation, roots, and grass must be removed, and any cracks or voids
shall be filled.
c.
If rock is encountered in the bedding area, this rock must be excavated
to a depth of six inches below the liner and backfilled with a fine-grained
soil. This area should then be covered with geotextile fabric, extending
three feet beyond the limits of the rock outcrop before placing the
pond liner.
d.
Installation of the liner may only take place when the ambient temperature
is within the manufacturer's specifications. The number of field
seams shall be minimized by requiring factory fabrication of large
panels. Any field seams performed must be in accordance with the manufacturer's
specifications.
e.
All structures (i.e., headwalls, pipes, outlet structures) which
come in contact with the liner must have a waterproof seal installed
to prevent leaks around the structure. These seals shall be installed
per the manufacturer's recommendations.
f.
A minimum of 12 inches of earth cover shall be placed over the lining.
Soil containing sharp jagged rocks, roots, debris, or any other material
that may puncture the liner shall not be used as cover material.
g.
The liner must be installed to a minimum height of the 100-year floodwater
elevation in the facility.
19.
Basins shall not be divided by a property line.
20.
Vertical pipes, inlets, and other surface water receiving structures
shall be installed with trash racks.
61.21.1.
Pipes, curbs, gutters, manholes, inlets, headwalls, endwalls,
streets, and other stormwater conveyance facilities shall be designed
and constructed in accordance with the following:
1.
Pipe trenching and backfilling shall be in accordance with the requirements
of the Pennsylvania Department of Transportation, Publication 408
and Roadway Construction Drawings, current edition, unless the manufacturer's
specifications and the Municipal Engineer authorize alternative procedures.
All trench backfill within existing or proposed dedicated street right-of-way
areas shall be 100% crushed aggregate backfill to the subbase elevation
of the street paving. Such crushed aggregate backfill shall be supplied
and placed in accordance with PennDOT specifications, Publication
408.
2.
Pipe sizes and type shall be in accordance with the approved drawings.
Minimum pipe size for drainage facilities to be dedicated for municipal
ownership shall be 15 inches in diameter, and the type shall be smooth
lined high-density polyethylene pipe (HDPEP), or reinforced concrete
pipe (RCP) or approved equivalent.
3.
Pipes shall be constructed and set to line and grade as shown on
approved drawings. All pipes shall be laid on straight runs between
drainage structures. Minimum pipe slope shall be 1.0%.
4.
Pipes shall be provided with a minimum of 12 inches of cover from
the top of pipe barrel to bottom of base course. In unpaved areas,
one foot of cover is required to the finished grade.
5.
Hauling shall not be permitted over pipe with less than four feet
of cover.
6.
All pipe outlets shall be discharged to natural or man-made waterways
and shall be provided with reinforced concrete headwalls or pipe end
sections. Pipe outlets shall also be provided with an erosion-resistant
material or energy dissipators to calm the anticipated velocity and
discharge of stormwater. All pipe outlet structures shall be protected
from frost-related movement by the construction of a concrete footer
with a minimum depth of 30 inches below finished grade level.
7.
Underdrains, pavement base drains, or combination storm sewer and
underdrains shall be provided at low points, cut sections, poorly
drained areas and other areas which are required to provide adequate
subsurface drainage to protect the integrity of the street.
8.
All storm sewers which cross a street shall be perpendicular to the
street center line or within 30° of perpendicular. Vertical and
horizontal isolation conflicts with other utilities shall be avoided.
Storm sewers within a street shall not cross underneath a curb, especially
at curb radii locations.
9.
A concentrated discharge of stormwater to an adjacent property shall
be within an existing watercourse or otherwise an easement shall be
required. Pipe outlets shall also be provided with erosion-resistant
material or energy dissipaters to calm the anticipated velocity and
discharge of stormwater.
10.
For contributing watersheds of less than 100 acres, a twenty-five-year
design storm shall be the minimum used for all constructed facilities,
including roadside gutters or swale. For contributing watersheds of
100 acres or more, a 100-year design storm (at least) or storm as
required by the Pennsylvania Department of Environmental Protection,
Division of Dams and Encroachment Regulations, whichever is more restrictive,
shall be used for all constructed facilities, such as culverts, piping
and major swales or water courses.
61.22.1.
Inlet tops shall be precast concrete top units conforming to
the Pennsylvania Department of Transportation, Publication 408, current
edition, and as detailed on the Roadway Construction Standards RC-45M
drawings, providing an eight-inch curb reveal from the gutter grade
point. Type "C" inlets with ten-inch hoods that provide a two-inch
sump condition may be used with approval from the Director of Engineering.
Type "M" inlets shall be used in off-street conditions.
61.22.2.
The maximum allowable spread of water shall be 1/2 of a through
travel lane. Design calculations are required to document the capacity
and spacing. Inlets shall be analyzed for collection efficiency, and
bypass flows from upstream structures shall be accounted for in inlet
spacing design.
61.22.3.
All inlets over four feet in depth shall be provided with steps
for accessibility. Inlets shall be placed along the curbline, gutter
line, or edge of paving.
61.22.4.
All inlets in paved areas shall have vane grates.
61.22.5.
Inlets shall be located at tangents on the uphill side of street
intersections.
61.22.6.
All inlets shall be constructed with concrete flow channels
cast in place in the bottom of each inlet.
61.22.7.
Inlets and manholes shall not be spaced more than 300 feet apart.
Manholes or inlets are required at all points of horizontal or vertical
pipe deflection.
61.22.8.
Manhole covers shall have a minimum of thirty-inch diameter
entryway clearance.
61.23.1.
All channels shall be lined with adequate channel lining material,
regardless of the designed velocity or shear stress. Maximum permitted
channel velocities are:
1.
Three feet per second where only sparse vegetation can be established.
2.
Four feet per second under normal conditions where vegetation is
to be established by seeding or sodding.
3.
Velocities may not exceed four feet per second for newly constructed
grass channels, unless appropriately designed and approved by the
Allegheny County Conservation District.
4.
The following conditions shall be met for all swales:
a.
Swales shall be designed using Manning's equation. Vegetated
swales shall be designed based upon accepted "n" factors for the anticipated
degree of vegetative retardance. The maximum allowable velocity for
an established grass swale is four feet per second.
b.
The "n" factors to be used for paved or riprap swales or gutters
shall be based upon accepted engineering design practices as specified
by PennDOT or approved by the Engineer.
c.
Swale side slopes shall be 2:1 minimum. Side slopes for grass-covered
swales which will be mowed shall be 3:1 minimum.
d.
All swales shall be designed to concentrate low flows to minimize
siltation and meandering.
e.
All vegetated swales shall have a minimum longitudinal slope of 2%
unless approved by the Municipal Engineer.
f.
Swales shall be centered within a minimum twenty-foot-wide easement.
61.24.1.
Provisions for stormwater volume reductions are required for
areas being developed. Design of the stormwater BMPs shall give consideration
to providing groundwater recharge to compensate for the reduction
in the percolation that occurs when the ground surface is paved and
roofed over. These groundwater recharge measures are required wherever
feasible. Soils used for the construction of basins shall have low-erodibility
factors ("K" factors).
61.24.2.
The low-impact development practices provided in the Pennsylvania
Stormwater BMP Manual shall be utilized for all regulated activities
to the maximum extent practicable using one of the following methods:
1.
The Design Storm Method (CG-1 in the BMP Manual) is applicable as
a method to any size of regulated activity. This method requires detailed
modeling based on site conditions. The following shall be incorporated
into the CG-1 method:
a.
Reduce the post-development total runoff volume by 10% of predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour duration precipitation.
b.
At least the first one inch of runoff from the net increase in impervious
surfaces shall be permanently removed from the runoff flow, i.e.,
it shall not be released into the surface waters of this commonwealth.
Removal options include reuse, evaporation, transpiration, and infiltration.
If the developer provides justification that the listed removal options
are not feasible, and the designated plan reviewer agrees, runoff
shall be detained in a facility designed for a twenty-four- to seventy-two-hour
dewatering time in an area with a dedicated stormwater system.
2.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
a.
Stormwater facilities shall capture at least the first two inches
of runoff from the net increase in impervious surfaces.
b.
At least the first one inch of runoff from the net increase in impervious
surfaces shall be permanently removed from the runoff flow, i.e.,
it shall not be released into the surface waters of this commonwealth.
Removal options include reuse, evaporation, transpiration, and infiltration.
If the developer provides justification that the listed removal options
are not feasible, and the designated plan reviewer agrees, runoff
shall be detained in a facility designed for a twenty-four-hour dewatering
time in an area with a dedicated stormwater system.
c.
Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
61.25.1.
Stormwater runoff calculations for all development sites and
regulated activities shall be calculated in accordance with the following
computation methodologies:
|
Method
|
Accepted Calculation Methods Applicability
|
|---|---|
|
Soil Cover Complex Method (formerly SCS)
|
Acceptable for all watersheds. Preferred for watersheds > 3
acres and for use with detention basin routings.
|
|
Rational Method
|
Acceptable for small watersheds and residential underground
absorption systems. Recommended for watersheds < 3 acres.
|
|
TR-20, USDA Soil Conservation Service
|
Acceptable for all watersheds, especially where full hydrologic
computer model is desired.
|
|
HEC-1 U.S. Army Corps of Engineers
|
Acceptable for all watersheds, especially where full hydrologic
computer model is desired.
|
61.25.2.
Selection of the method of calculation by the design professional
shall be based upon the limitations and suitability of each method
for the development site. The Municipal Engineer should be consulted
for method alternatives and applicability.
61.25.3.
Stormwater event criteria.
1.
All runoff calculations shall be completed in accordance with the
standard guidelines for the selected method of calculation.
2.
Rational Method. The PennDOT Storm Intensity-Duration-Frequency Chart
shall be used. Where stormwater runoff hydrographs are produced using
the Rational Method, the provided storage volume shall be verified
with a critical duration analysis that investigates similar storm
occurrences with extended durations and applicable intensities to
determine the anticipated maximum elevation to occur in the storage
facility for each required storm event.
3.
Soil Cover Complex Method. The Soil Conservation Service Type II,
twenty-four-hour rainfall distribution shall be used in conjunction
with the rainfall depths from NOAA Atlas 14.
a.
Stormwater runoff control criteria.
[1]
Stormwater management shall be accomplished by controlling post-development
runoff rates to 90% predevelopment runoff rates for the two-year,
five-year, ten-year, twenty-five-year, and 100-year storms.
b.
Assumptions and criteria.
[1]
Runoff calculations shall include a hydrologic and hydraulic
analysis indicating volume and velocities of flow and the grades,
sizes, and capacities of water-carrying structures, sediment basins,
retention and detention structures and sufficient design information
to construct such facilities. Runoff calculations shall also indicate
both predevelopment and post-development rates for peak discharge
of stormwater runoff from the development site.
[2]
All predevelopment calculations, unless in woodland, shall be
based upon the assumption of meadow in good hydrologic condition.
Wooded areas shall utilize forest/woodland cover coefficients. Where
the site contains existing impervious surface, up to 50% of the impervious
area may be considered as an existing predevelopment condition. Runoff
coefficients and curve numbers shall be selected from the tables in
the appendix.
[3]
Drainage areas tributary to sinkholes or closed depressions
shall be excluded from the modeled analysis defining predevelopment
flows. If left undisturbed during construction, activities in areas
draining to closed depressions may also be removed from peak runoff
rates and post-development analysis. New additional contributing runoff
shall not be directed to existing sinkholes or closed depressions.
[4]
Runoff calculations for the predevelopment and post-development
comparison shall evaluate all stormwater events listed within the
stormwater runoff control criteria.
[5]
Post-development analysis shall consider open areas of possible
water impoundments receiving rainfall (such as detention basins, rain
gardens, etc.) to have an impervious cover area of the maximum surface
storage area of the water surface correlating to the storm event of
interest.
[6]
Design of stormwater facilities shall be verified by routing
the storm event hydrographs through the facilities using the Storage
Indication Method. Routings of Rational Method hydrographs shall determine
the critical duration corresponding to the highest peak water surface
elevation in the detention facility based on extended runoff inflows
for each control storm event, or consist of a methodology approved
by the Municipal Engineer. The combination of Rational Method hydrographs
based on timing shall be prohibited.
[7]
Predevelopment and post-development time-of-concentration paths
shall be delineated on topographic mapping with the end and beginning
of segments clearly identified, as well as the lengths and corresponding
end elevations used for each segment. The post-development time of
concentration shall never be greater than the predevelopment time
of concentration for any watershed or subwatershed. Times of concentration
shall be based upon the following:
[a]
The maximum length of overland sheet flow shall be 100 feet before shallow concentrated or open channel flow develops and shall be justified using the methodology presented in Chapter 3 of the NRCS Technical Release 55 (TR-55) to include the following.
[b]
Travel time for shallow concentrated flows shall utilize the
NRCS methodology.
[c]
Overland flows which are concentrated within field depressions,
swales, gutters, curbs or pipe collection systems shall be designed
using Manning's Equation for time of concentration criteria for
open channel conditions between these design points using Manning's
n values per the current edition of PennDOT Publication 584.
[8]
Storm sewer pipes, culverts, gutters, inlets, outlets and swales
shall be designed and constructed in accordance with the standards
of the PennDOT Design Manual, Part 2, Highway Design (latest edition),
including the following requirements:
|
Facility
|
Minimum Post-Development Peak Discharge Requirement
|
|---|---|
|
Pipes, gutters and swales, culverts, streets, cross drains and
low points
|
25-year storm (Area < 100AC) and 100-year storm (Area > 100AC)
|
|
Bridges and stream crossings
|
100-year storm
|
[9]
All stormwater collection facilities shall investigate and account
for bypass scenarios.
[11]
Swales shall be designed utilizing Manning's Equation to
insure adequate capacity, control of velocity and swale stability.
Calculations shall support swale stability for initial conditions
(prior to established vegetation) and long-term conditions for the
required peak flow rate. Vegetated swales shall have a minimum longitudinal
slope of 2% and maximum side slopes of 3:1, unless specifically designed
as a BMP structure. Culvert design shall investigate, at a minimum,
inlet, barrel and outlet control conditions.
c.
Calculation process.
[1]
A general procedure recommended for site evaluation and stormwater
design is as follows:
[a]
Evaluate factors influencing stormwater runoff, with a goal
of limiting earth disturbance, minimizing grading, and reducing or
dispersing impervious surfaces.
[b]
Satisfy the volume control requirements.
[c]
Meet water quality (BMP) standards, preferably with BMPs near
the source of the runoff.
[d]
Calculate and satisfy peak runoff objective, considering all
measures other than detention basins.
[e]
Size detention basins accordingly.
[f]
Demonstrate compliance with pre-development hydrograph requirements.
61.26.1.
Goals and objectives.
1.
Preserve existing natural features, especially those which store,
infiltrate or filter water runoff.
2.
Infiltrate rainfall to recharge the groundwater table.
3.
Use physical (structural) and biological or vegetative (nonstructural)
filtration of water runoff to reduce pollutants and remove sediment.
4.
Moderate water runoff velocities to minimize erosion and damage to
downstream aquatic habitat.
5.
Integrate BMPs into the site layout to perform a water quality function
and complement the developed use of the site.
6.
Enhance site aesthetics through the use of a variety of BMP techniques
and components.
7.
Maximize collection and treatment of small storm event (first flush)
stormwater runoff which contains the highest concentration of pollutants.
8.
Utilize a system of BMP facilities and groundwater recharge devices
throughout the site.
61.26.2.
General standards.
1.
Water quality shall be maintained through the requirement for BMP
design components for all subdivisions, land developments and regulated
activities, except where other provisions of this chapter provide
for plan or stormwater design exemptions. Revision or expansion projects
requiring land development approval with stormwater design shall include
measures to retrofit the site with BMPs to maintain or improve the
water quality of the stormwater discharges.
2.
The required volume control, specified elsewhere herein, shall be
detained and treated within BMPs for all areas of the site to be developed.
3.
Site designs shall minimize earth disturbance and the generation
of stormwater runoff while maximizing pervious areas for treatment
of stormwater.
4.
All BMPs shall be sized to capture the required control volume, designed
according to the BMP criteria within this chapter, constructed properly
and maintained regularly.
5.
Stormwater runoff which is directly discharged to wetlands, streams,
ponds, high-quality or exceptional value watersheds or which originates
from land uses or activities with higher potential for pollutant loadings
(such as auto salvage yards, vehicle service areas, loading/unloading
areas, truck centers, etc.) require the use of additional or specific
structural BMPs for pollution prevention and maintenance of water
temperatures and quality.
6.
Place BMPs near the source of stormwater runoff and treat runoff
from impervious surfaces before mixing with runoff from less contaminated
sources.
7.
Use native vegetation and water-tolerant plants. Trees and shrubs
shall not be planted on stormwater facility embankments or in other
areas where roots may endanger pipes, embankments, headwalls, endwalls,
spillway structures or other structural facilities.
8.
All impervious area runoff shall be directed to BMPs. The volume
control shall be provided within each watershed or subwatershed to
provide the intended treatment for upstream runoff. Volume control
credit will not be allowed toward noncontributing runoff areas.
9.
BMP categories used within these regulations are 1) ponds, 2) wetlands,
3) infiltration systems, 4) filtering systems, and 5) open channels.
The design criteria subsection listed hereafter provides specific
descriptions of the BMPs within these classifications. Where effectiveness
can be demonstrated, alternative BMP designs and concepts may be utilized.
10.
Site designs shall include measures to reduce stormwater velocities
and collect sediment near the source of the water runoff so that BMPs
can be effective in treating water quality and maintenance can be
reduced. Recommended facilities are forebays, energy dissipators,
outlet stabilization structures, inlet protection devices, level spreaders,
and flow splitters.
11.
Carbonate geology (limestone) areas require careful evaluation for
appropriate BMP design. Facility depths should be minimized and impermeable
liners will be required.
12.
Forebays and micropools are recommended for ponds and required for
wetlands. Forebays and micropools should each contain approximately
10% of the required water volume. Forebays should be at least 10 feet
long and be baffled from the main basin with a berm of riprap or similar
material, to a depth of one foot below the water quality volume level,
to insure an indirect flow path. Additionally, when forebays are used,
a minimum of 90% of the discharge into the facility shall be directed
into the forebays.
13.
All ponds and wetlands shall be surrounded by a riparian buffer strip
of a minimum 25 feet in width. Streams shall be bordered by a riparian
buffer strip, a minimum of 25 feet or the width of the floodplain,
whichever is greater. All PA DEP standards for buffer strips are required.
14.
Planting of wetland plants is required within created wetlands and
encouraged in ponds and other applicable BMPs. Fringe wetland plants
may be used on aquatic benches or within shallow pools, while emergent
wetlands vegetation should be planted along the side slopes and facility
edges.
15.
Infiltration, filtering or other BMP systems which are designed to
treat the required control volume from small storms shall be preceded
by a flow splitter or equivalent bypass device to route larger water
volumes around the system.
16.
All underground stone and sand BMP systems shall be lined with geotextile
fabric on the sides, bottom and top (double layer on top), have a
flat bottom, be underlain by a minimum of two feet of soil or sand
above the seasonal high-water table and be placed a minimum of 10
feet horizontally from building foundation walls.
17.
Infiltration systems with stone shall utilize AASHTO No. 1 (No. 4)
stone and assume a maximum of 40% voids area.
18.
Grass swales should be designed with a flat channel bottom at least
two feet in width, with a longitudinal slope of 1% to 2%. If grass
swale slopes exceed 4%, check dams or similar water velocity modifiers
should be used.
61.26.3.
Design criteria.
1.
The following site factors should be considered in selecting and
designing the appropriate BMPs:
2.
The following factors should be evaluated when determining the suitability
of BMPs for a development site:
a.
Peak discharge and required volume control.
b.
Stream bank erosion.
c.
Efficiency of the BMPs to mitigate potential water quality problems.
d.
The volume of runoff that will be effectively treated.
e.
The nature of the pollutant being removed.
f.
Maintenance requirements.
g.
Creation/protection of aquatic and wildlife habitat.
h.
Recreational value.
i.
Enhancement of aesthetic and property value.
61.27.1.
All subdivision and land development plans which propose earthmoving
activity shall include erosion and sediment pollution control design
to satisfy the requirements of Chapter 102, Pennsylvania Department
of Environmental Protection, 25 Pa. Code Chapter 92 and/or 102, and
the Pennsylvania Clean Streams Law (35 P.S. § 691.1 et seq.),
and to prevent soil erosion, sediment and other pollutants from entering
streams, lakes and neighboring properties.
61.28.1.
In order to protect and improve water quality, a riparian corridor
easement shall be created and recorded as part of any subdivision,
land development or stormwater management plan that encompasses a
riparian corridor.
61.28.2.
Except as otherwise required by 25 Pa. Code Chapter 102, the
riparian corridor easement shall be measured to be the greater of
the limit of the 100-year floodplain or 35 feet from the top of stream
bank (on each side).
61.28.3.
Minimum management requirements for riparian corridors.
1.
Existing native vegetation shall be protected and maintained within
the riparian corridor easement.
2.
Whenever practicable, invasive vegetation shall be actively removed,
and the riparian corridor easement shall be planted with native trees,
shrubs and other vegetation to create a diverse native plant community
appropriate to the intended ecological context of the site.
61.28.4.
The riparian corridor easement shall be enforceable by the Engineer
and shall be recorded in the Allegheny County Recorder of Deeds Office,
so that it shall run with the land and shall limit the use of the
property located therein. The recorded easement shall be superior
to all liens of record. The easement shall allow for the continued
private ownership and shall count toward the minimum lot area as required
by zoning.
61.28.5.
Any permitted use within the riparian corridor easement shall
be conducted in a manner that will maintain the extent of the existing
100-year floodplain, improve or maintain the stream stability, and
preserve and protect the ecological function of the floodplain.
61.28.6.
The following conditions shall apply when public and/or private
recreation trails are permitted within riparian corridors:
61.28.7.
Septic drain fields and sewage disposal systems shall not be
permitted within the riparian corridor easement and shall comply with
setback requirements established under 25 Pa. Code Chapter 73.
61.28.8.
Stormwater drainage pipes shall be permitted within the riparian
buffer easement, but they shall cross the easement in the shortest
practical distance. Other structural stormwater management facilities
are not permitted within the riparian buffer easement.
61.28.9.
New sanitary sewage conveyance pipes shall be permitted within
the riparian buffer easement, but the easement shall not be used as
a linear utility easement. Pipes shall cross the easement in the shortest
practical distance.
61.28.10.
Existing utilities may remain and be maintained as required.
