A.
Regulated activities are subject to the following permitting requirements:
(1)
Regulated activities that involve a proposal of less than 2,000 square feet of impervious surface, where the total cumulative impervious added since January 18, 2017, is less than 2,000 square feet, and where the disturbed area is less than one acre, may be exempted from portions of Article III (Stormwater Management Standards), including § 214-15 (Volume controls) and § 214-16 16 (Rate controls), and Article IV [Stormwater Management (SWM) Site Plan Requirements], including § 214-30 (Financial security) of this chapter. The exemption is subject to review by the Township to verify compliance with Subsection F listed below. Regulated activities that involve a proposal of between 2,000 and 7,500 square feet of impervious are considered small projects. For these activities, the applicant shall complete the requirements as defined by the Small Project Approach in Appendix D.[1] The Township's Small Project Approach shall be amended
from time to time by resolution of the Board of Commissioners. The
regulated activity shall be subject to all other requirements of this
chapter other than related requirements for which an exemption or
exemptions have been authorized.
[1]
Editor's Note: Appendix D is included as an attachment to this chapter.
(2)
Regulated activities that propose greater than 7,500 square feet of new impervious area shall be required to follow the requirements set forth in Article IV [Stormwater Management (SWM) Site Plan Requirements].
(3)
No regulated activities shall commence until the municipality issues
written approval of an SWM site plan or other applicable authorization
which demonstrates compliance with the requirements of this chapter.
B.
SWM site plans approved by the municipality, in accordance with § 214-28 (Authorization to construct; term of validity), shall be on-site throughout the duration of the regulated activity.
C.
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.[2]
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
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)
beginning with initial topsoil removal and through the conversion
and operation of all post-construction stormwater management 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.
Various BMPs and their design standards are listed in the Erosion
and Sediment Pollution Control Program Manual (E&S Manual[3]), No. 363-2134-008, as amended and updated.
E.
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 impervious area on the parcel is subject to the requirements of this chapter, except that the volume controls in § 214-15 and the peak rate controls of § 214-16 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
F.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written approval
of the adjacent property owner(s). Such stormwater flows shall be
subject to the requirements of this chapter.
G.
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[4]). 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
demonstrating that the use of LID and green infrastructure is not
practicable.
H.
The design of all facilities over karst geology shall include an
evaluation of measures to minimize adverse effects.
I.
Infiltration BMPs should be spread out such that impervious BMP loading
ratios are less than or equal to 5:1, 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.
J.
Unless designed as a retention pond with permanent ponding, storage
facilities should completely drain both the volume control and rate
control capacities over a period of time not more than 72 hours from
the end of the design storm.
K.
The design storm precipitation depth to be used in the analysis of
peak rates of discharge should be 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/.
L.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this chapter and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law,[5] and the Storm Water Management Act.[6] Various BMPs and their design standards are listed in
the BMP Manual.[7]
A.
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.
B.
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.
C.
The use of land for domestic (i.e., for a single individual dwelling
unit) gardening and landscaping is exempt from specific approval and
permitting under this chapter so long as such activities comply with
all other applicable ordinances and statutes.
E.
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.
Green infrastructure and low impact development practices provided
in the BMP Manual[1] shall be utilized for all regulated activities wherever
possible. Water volume controls shall be implemented using the Design
Storm Method in Subsection A or the Simplified Method in Subsection
B below. This chapter establishes that only the Design Storm Method
may be used for regulated activities that add greater than 1/2 acre
of new impervious area or disturb greater than one acre of total area.
A.
The Design Storm Method (CG-1 in the BMP Manual[2]) is applicable to any size of regulated activity. This
method requires detailed modeling based on site conditions, as verified
through design phase soil infiltration testing.
(1)
Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year, twenty-four-hour duration
precipitation.
(2)
For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
Twenty percent of existing impervious areas to be disturbed
by the project, when present, shall be considered meadow in good condition
in the model for existing conditions. Regulated activities not requiring
an NDPES permit are exempt from this requirement.
B.
The Simplified Method (CG-2 in the BMP Manual[3]) provided below is independent of site conditions and
should be used if the Design Storm Method is not followed. This method
is not applicable for regulated activities that add greater than 1/2
acre of new impervious area. For new impervious surfaces:
(1)
Stormwater facilities shall capture and store the first two inches
of runoff from all new impervious surfaces.
(2)
At least the first one inch of runoff from new 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.
(3)
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.
(4)
Volume not infiltrated or otherwise permanently removed (up to one
inch) shall be detained using BMPs and released at a controlled rate.
A.
For the computation of predevelopment peak discharge rates, 20% of
existing impervious areas to be disturbed by the project, when present,
shall be considered meadow. Regulated activities not requiring an
NDPES permit are exempt from this requirement.
B.
The municipality has been divided into two stormwater management
districts as shown on the Upper Allen Township Release Rates Map in
Appendix B.[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
C.
Standards for managing runoff from each subarea for the two-, five-,
ten-, twenty-five-, fifty- and 100-year design storms are shown below.
Development sites located in each of the districts must control post-development
runoff rates to predevelopment runoff rates for the design storms
as follows:
|
District
|
Control Criteria
|
|---|---|
|
100%
|
Post-development peak discharge for all design storms must be
no greater than predevelopment peak discharges
|
|
75%
|
Post-development peak discharge for all design storms must be
no greater than 75% of the predevelopment peak discharges
|
D.
General. Post-development rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff prior to development
for the design storms specified on the Stormwater Runoff Peak Rate
Districts Map, Appendix B,[2] and § 214-16C of this chapter.
[2]
Editor's Note: Appendix B is included as an attachment to this chapter.
E.
District boundaries. The boundaries of the stormwater management
districts are shown on an official map, which is available for inspection
at the municipal office. A copy of the official map at a reduced scale
is included in the adoption of this chapter as Appendix B.[3] The exact location of the stormwater management district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot topographic contours
(or most accurate data required) provided as part of the SWM site
plan.
[3]
Editor's Note: Appendix B is included as an attachment to this chapter.
F.
Sites located in more than one district. For a proposed development
site located within two or more release category subareas, the peak
discharge rate from any subarea shall be the predevelopment peak discharge
for that subarea multiplied by the applicable release rate. The calculated
peak discharges shall apply regardless of whether the grading plan
changes the drainage area by subarea. An exception to the above may
be granted if discharges from multiple subareas recombine in proximity
to the site. In this case, peak discharge in any direction may be
a 100% release rate, provided that the overall site discharge meets
the weighted average release rate.
A.
If a riparian buffer is proposed as part of a plan as a BMP intended
to protect and improve water quality, a riparian buffer easement shall
be created and recorded.
C.
Minimum Management Requirements for Riparian Buffers.
(1)
Existing native vegetation shall be protected and maintained within
the riparian buffer easement.
(2)
Whenever practicable, invasive vegetation shall be actively removed,
and the riparian buffer 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.
D.
The riparian buffer easement shall be enforceable by the municipality
and shall be recorded in the appropriate County Recorder of Deeds
office so that it shall run with the land and shall limit the use
of the property located therein. The easement shall allow for the
continued private ownership and shall count toward the minimum lot
area required by zoning unless otherwise specified in the Municipal
Zoning Ordinance.[2]
E.
Any permitted use within the riparian buffer 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.
A.
Off-site areas. Off-site areas which drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site for the
100-year storm.
B.
On-site areas. On-site areas proposed to remain undisturbed as part
of the regulated activity, including previously developed areas, shall
be considered as existing conditions.
C.
Downstream hydraulic capacity analysis. The downstream capacity hydraulic
analysis shall be conducted in accordance with this chapter.
(1)
All downstream facilities impacted by the total site area of the
regulated facility shall be studied to determine if the facility has
adequate capacity to handle existing and proposed flows. An impacted
downstream facility is one to which the runoff from the total site
area of the regulated activity comprises more than 50% of the total
flow to such a facility. The study shall end at a perennial stream
unless directed otherwise by the Municipal Engineer. Downstream facilities
include, but are not limited to, man-made or natural swales and open
channels, pipes, inlets, culverts, bridges, and roadways.
(2)
If any facility is found to be undersized, the applicant shall be
responsible for evaluating the impact of the regulated activity and
mitigating any negative impacts.
D.
Regional detention alternatives. For certain areas within the study
area, it may be more cost-effective to provide one control facility
for more than one development site than to provide an individual control
facility for each development site. The initiative and funding for
any regional runoff control alternatives are the responsibility of
prospective developers. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined on a case-by-case
basis using the hydrologic model of the watershed consistent with
the protection of the downstream watershed areas. "Hydrologic model"
refers to the calibrated model as developed for the stormwater management
plan.
E.
Capacity improvements may also be provided as necessary to implement
any regional or subregional detention alternatives.
F.
Watershed integrity. Stormwater runoff shall not be transferred from
one watershed to another unless the watersheds are subwatersheds of
a common watershed that join together within the perimeter of the
property. The transfer of watersheds may be permitted in the event
the transfer does not alter the peak discharge onto downstream lands
or drainage easements are acquired from the affected landowners.
G.
Design standards: collection systems.
(1)
Storm sewer pipes, other than those used for street subbase underdrains,
shall have a minimum diameter of 15 inches. Structural calculations
that address the actual design requirements will be required where
installation conditions merit.
(2)
Storm sewer pipes and culverts shall be installed with a minimum
slope of 0.5%.
(3)
Allowable pipes, culvert, and bridge materials shall be as outlined
in the municipality's construction and material specifications.
(4)
All storm sewer crossings and culvert crossings of streets shall
not deviate by more than 15° from perpendicular to the street
center line.
(5)
Design storm frequency to be used for:
(a)
All storm sewer pipes, culverts, bridges, gutters, and swales
(excluding outfall structures from stormwater management facilities)
conveying water originating only from within the boundaries of the
project site shall be designed for a twenty-five-year storm event.
(b)
All storm sewer pipes, culverts and bridges, gutters, and swales
conveying water originating from off-site shall be designed for a
fifty-year storm event with provisions to pass the 100-year storm
safely through the site.
(c)
Culverts under sole access drives or streets to a developed
property shall be designed to convey the 100-year storm event without
roadway overtopping.
(d)
Drainage and access easements shall be provided to encompass
the water surface limits of the 100-year storm event throughout the
project site and to provide access from a public street to the stormwater
facility. Easements shall begin at the furthest upstream property
line of the proposed development in the watershed.
(e)
When a pipe or culvert is intended to convey the discharge from
a stormwater management facility, its required capacity shall be computed
by the Rational Method and compared to the peak outflow from the stormwater
facility for the 100-year storm. The greater flow shall govern the
design of the pipe or culvert.
(f)
When a pipe is part of a storm sewer system and crosses the
roadway, it shall be designed as a storm sewer with the same design
storm as the remainder of the drainage system.
(g)
A 100-year storm frequency may be required for the design of
the stormwater collection system to ensure that the runoff from the
post-development storm is directed into the intended management facility.
(h)
Proposed channels or swales must be able to convey the increased
runoff associated with a proposed 100-year return period event within
their banks at velocities consistent with protection of the channels
from erosion. Acceptable velocities shall be based upon criteria included
in the PADEP Erosion and Sediment Pollution Control Program Manual
(E&S Manual[1]).
(6)
All storm sewer pipes and culverts shall be laid to a minimum depth
of 12 inches from finished subgrade to the crown of the pipe in paved
and grassed areas, or more if specified by the manufacturer.
(7)
Curves or angle points in pipes or box culverts without the use of
an inlet or manhole are prohibited unless preapproved by the municipality.
Tee joints, elbows, and wyes shall be limited for use in constructing
underground detention facilities, underground retention facilities,
underdrain systems, and roof leader collection systems.
(8)
Pipe connections to box structures shall not penetrate through the
corner of the box.
(9)
Manholes, inlets, headwalls, endwalls, and end sections shall conform
to the requirements of PennDOT Publication 408.
(10)
Inlets shall be:
(a)
Placed on both sides of the street at low spots;
(b)
At a maximum of 600 feet apart along a storm sewer pipe run
and at points of abrupt changes in the horizontal or vertical directions
of storm sewers;
(c)
Shall normally be along the curbline at or beyond the curb radius
points;
(d)
Within the street right-of-way;
(e)
Designed with a maximum gutter spread (based on the twenty-five-year
storm event) no greater than 1/2 of the travel lane width and have
a maximum depth of three inches at the curbline; a parking lane shall
not be considered as part of the travel lane;
(f)
Designed at intersections and access driveways to allow the
depth of flow to be less than 1 1/2 inches (for the twenty-five-year
storm event);
(g)
In general, spaced such that, based upon the Rational Method,
tc = five minutes and ten-year rainfall intensity, and the area contributing
to the inlet shall not produce a peak runoff of greater than four
cfs. Also, inlets shall be spaced so that their efficiency, based
upon efficiency curves published by the Pennsylvania Department of
Transportation, is not less than 65%.
(11)
Flow velocities from any storm sewer may not result in erosion
of the receiving channel. Adequate erosion protection shall be provided
along all open channels and at all points of discharge.
(12)
All other storm sewer manholes and inlets shall have smooth
flow lines grouted.
(13)
Headwalls and endwalls shall be used where stormwater runoff
enters or leaves the storm sewer horizontally from a natural or man-made
channel. In all cases where drainage is picked up by means of a headwall,
the pipe shall be designed as a culvert. Inlet and outlet conditions
shall be analyzed. PennDOT type "dw" headwalls and endwalls shall
be utilized for pipes 15 inches and larger in diameter. End sections
shall be utilized for pipes smaller than 15 inches in diameter. An
entrance condition analysis must be performed according to PennDOT
Publication 13M, Chapter 10, Drainage Design and Related Procedures.
(14)
Guards shall be provided on all intake and outfall structures
as well as outlet structures. The guard bars shall be one-half-inch-diameter
galvanized bars on six-inch centers attached to the structure with
three-eighth-inch-diameter stainless steel anchors. Guards shall also
be provided for any pipe opening 18 inches in diameter or larger.
(15)
For all swales, capacities and velocities shall be computed
using the Manning Equation. The following design considerations shall
be met:
(a)
Two analyses of channel velocity and stability shall be provided
with each swale design. One analysis shall be based upon the swale
in an unvegetated state with control matting, and the second shall
consider the channel in permanent, designed conditions.
(b)
All swales shall have a minimum slope of 1% unless otherwise
approved by the Municipal Engineer.
(16)
Placement or use of BMPs within the public street right-of-way
shall be prohibited.
H.
Design standards: general BMP regulations.
(1)
The design of all BMP facilities shall incorporate best engineering
practices. The design engineer shall utilize all available design
criteria in the BMP Manual to meet the requirements of this chapter
and shall provide all the necessary backup documentation with their
submittal.
(2)
The BMPs must be designed to protect and maintain existing uses and
maintain the level of water quality necessary to protect those uses
in all streams, and to protect and maintain water quality in special
protection streams, as required by statewide regulations at 25 Pa.
Code Chapter 93.
(3)
No regulated earth disturbance activities within the municipality
shall commence until approval by the municipality of a plan that ensures
post-construction stormwater discharges do not degrade the physical,
chemical, or biological characteristics of the receiving waters.
I.
Design standards: subsurface infiltration facilities.
(1)
Positive overflow must be provided in the subsurface infiltration
facility. Overflow structures or pipes must be designed to convey
the inflow capacity of the facility. Volume control credits may not
be used above the lowest invert of the positive overflow structure
or pipe.
(2)
The minimum allowable distance between a subsurface infiltration
facility and structure/building/retaining wall is 10 feet.
(3)
The subsurface infiltration facility must be located at least two
feet above any limiting zone, i.e., seasonal high groundwater table,
bedrock, or poorly infiltrating soils.
(4)
Soils with rates exceeding 10 inches per hour require soil amendments.
During construction, upon reaching the subgrade of the infiltration
facility, a two-foot-thick layer of soil amendments must be spread
across the entire facility bottom area, below the designed bottom
of the facility. Soil media infiltration rate must be provided upon
design.
(5)
The infiltration facility must include an acceptable form of pretreatment
before stormwater enters the facility.
(6)
Subsurface infiltration facilities may not use a geotextile liner
to separate the storage area from the subgrade. Geotextile liners
may only be used on the sides and top. AASHTO Class 1 or Class 2 geotextile
is recommended.
(7)
Subsurface infiltration facilities must contain a cleanout or observation
well at the end of any portion of underdrain. The observation well
or cleanout must be placed at the invert of the stone bed and extend
up to grade. Adequate inspection and maintenance access to the observation
well or cleanout must be provided.
(8)
In subsurface infiltration facilities that include chamber, pipe,
or other storage systems, a sufficient number of access features must
be provided to efficiently inspect and maintain the infiltration area.
(9)
Perforated distribution pipes must contain a bedding of at least
four inches separating the pipe and subgrade.
(10)
Infiltration testing standards set forth in Appendix C[2] shall be followed when designing a subsurface infiltration
facility.
[2]
Editor's Note: Appendix C is included as an attachment to this chapter.
J.
Design standards: detention and retention basins.
(1)
Permanent detention and retention basins shall be designed to meet
the following standards:
(a)
The maximum permitted depth for detention or retention basins
shall be six feet, measured from the bottom of the emergency spillway
to the lowest point in the basin.
(b)
The minimum top width of all basin embankments shall be eight
feet.
(c)
The maximum permitted inside side slopes for detention or retention
basins shall be four to one vertical. Three to one vertical inside
side slopes may be approved upon furnishing an appropriate planting
schedule. The maximum permitted outside side slopes shall be three
to one vertical and utilize a seed mix designed for embankments. Areas
for vehicle access shall be provided and encompassed by a twenty-foot-wide
easement and shall have slopes no greater than five to one vertical.
The proposed vegetation shall be low-maintenance varieties.
(d)
Any stormwater management facility (i.e., detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the 100-year, twenty-four-hour
design storm at post-development conditions, assuming the principal
outlet structure to be clogged. The height of the embankment must
be set as to provide a minimum of one foot of freeboard above the
maximum elevation computed. Should any stormwater management facility
require a dam safety permit under PADEP 25 Pa. Code Chapter 105, the
facility shall be designed in accordance with PADEP 25 Pa. Code Chapter
105 and meet the regulations of PADEP 25 Pa. Code Chapter 105 concerning
dam safety, which may be required to pass storms larger than the 100-year
event.
(e)
A cutoff trench composed of impervious material, four feet minimum
in width, shall be provided within all basin embankments. The top
of the cutoff trench should be six inches from the spillway elevation.
(f)
Where a basin embankment is constructed using fill on an existing
15% or greater slope, the basin must be keyed into the existing grade.
(g)
All outlet structures and emergency spillways shall include
a nonerosive means of energy dissipation at its outlet to assure conveyance
and flow without endangering the safety and integrity of the basin
and the downstream drainage area.
(h)
Plans for infiltration must show the locations of existing and
proposed septic tank infiltration areas and wells. A minimum twenty-five-foot
separation from on-lot disposal systems (OLDS) infiltration areas,
including replacement areas, is desired and will be evaluated by the
municipality on a case-by-case basis. However, the separation shall
not be less than the PADEP required 10 feet. Infiltration rates shall
be based upon perc and probe tests conducted at the site of the proposed
facility.
(i)
Aboveground infiltration facilities shall provide a means of
emergency dewatering of the facilities to the bottom elevation. If
amended soils are used in the facility bottom, an underdrain shall
be placed below or at the bottom of the amended soils to dewater the
amended soils in emergency situations.
K.
Design standards: rain gardens/bioretention facilities.
(1)
A rain garden/bioretention facility is an excavated shallow surface
depression or storage area created by an earthen embankment in which
amended soils are planted with specific native vegetation to treat
and capture runoff. Rain gardens shall meet the following design standards.
(Note: Any SWM facility exceeding these design standards shall be
considered a retention/detention basin and subject to their design
standards):
(a)
The maximum ponding depth shall be 12 inches.
(b)
Infiltration testing standards set forth in Appendix C[3] shall be followed when designing a rain garden facility.
[3]
Editor's Note: Appendix C is included as an attachment to this chapter.
(c)
The bottom of the rain garden storage area must be located at
least two feet above any limiting zone, i.e., seasonal high groundwater
table, bedrock, or poorly infiltrating soils.
(d)
Pretreatment shall be used in the design of the facility. Pretreatment
can include structures such as sumped and trapped inlets, sediment/grit
chambers or separators, media filters, inlet inserts, or other appropriate
prefabricated or proprietary designs to remove sediment, floatables,
and/or hydrocarbons from stormwater runoff prior to being conveyed
to a rain garden/bioretention basin. A facility may be exempt from
pretreatment at the discretion of the Municipal Engineer.
(e)
Maximum side slopes for surface storage areas shall be 4(H):1(V).
(f)
Any stone storage systems incorporated into the rain garden
design must be separated from the soil media by a geotextile liner.
A geotextile liner shall not be used to separate the stone storage
system from the subgrade. Stone storage systems shall have a level
bottom or use a terraced system if installed along a slope.
(g)
In the event a sand layer is to be used as part of the underground
filtration/storage system, the sand must be placed between the soil
medium and stone storage. All sides of the sand must be separated
by a geotextile liner.
(h)
The planting soil medium must have a minimum depth of 18 inches.
Planting soil shall be a loam soil capable of supporting a healthy
vegetative cover. Soils shall be amended with a composted organic
material. At a minimum, the organic amended soil shall be combined
with 20% to 30% organic material (compost) and 70% to 80% soil base
(topsoil) free of clay. Amended soils shall be spread throughout the
bottom floor of the facility.
(i)
Underdrains must be provided for all rain gardens and extend
throughout the entire rain garden bottom. Underdrains must be surrounded
by a stone layer with a minimum of four inches above and below the
pipe.
(j)
Rain gardens that are used for volume control credits shall
have an underdrain with a valve that is to remain closed at all times
unless dewatering the facility to perform maintenance.
(k)
A sufficient number of cleanouts must be provided to access
underdrains to allow for maintenance and inspection of the underdrain
pipe.
(l)
Native plants shall be utilized and capable of supporting the
proposed ponding depth. The designer shall refer to the PADEP BMP
Manual for a list of potential native plant species.
A.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
B.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by PADEP through the joint permit application
process or, where deemed appropriate by PADEP, the general permit
process. When there is a question of 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 PADEP.
C.
Any stormwater management facility located within the vicinity of
a floodplain shall be subject to approval in accordance with PADEP
25 Pa. Code Chapter 106, Floodplain Management, of PADEP's rules
and regulations and the municipal floodplain management regulations.[1]
D.
The design of all stormwater management facilities shall incorporate
good engineering principles and practices. The municipality shall
reserve the right to disapprove any design that would result in the
occupancy or continuation of adverse hydrologic or hydraulic conditions
within the watershed.
E.
The existing points of concentrated drainage that discharge onto
adjacent property shall not be altered without permission of the adjacent
property owner(s) and shall be subject to any applicable discharge
criteria specified in this chapter. New proposed discharge points
shall be no closer than 10 feet to the property line in which they
are located.
F.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If the
diffused flow is proposed to be concentrated and discharged onto adjacent
property, 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.
G.
Where a development site is traversed by watercourses, twenty-foot-wide
minimum drainage easements (10 feet from top of the bank out) 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, maintaining
of vegetation in a natural state within the easement shall be required,
except as approved by the appropriate governing authority.
H.
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 PADEP through the joint permit application
process or, where deemed appropriate by PADEP, through the general
permit process.
I.
Roof drains and sump pumps shall discharge to lawns, infiltration
or vegetative BMPs, or pervious area to promote overland flow and
infiltration/percolation wherever/whenever possible. If the above
discharge criteria cannot be met or when it is more advantageous to
connect directly to streets, storm sewers or other stabilized conveyance
designations, then the discharge will be permitted on a case-by-case
basis by the municipality. All discharges shall be conveyed in a manner
so as to not cause water problems on adjoining property owners without
written approval.
J.
Special requirements for areas falling within defined exceptional
value and high-quality subwatersheds. The temperature and quality
of water and streams that have been declared as exceptional value
and high quality are to be maintained as defined in 25 Pa. Code Chapter
93, Water Quality Standards, Pennsylvania Department of Environmental
Protection rules and regulations. Temperature-sensitive BMPs and stormwater
conveyance systems are to be used and designed with storage pool areas
and supply outflow channels and should be shaded with trees. This
will require modification of berms for permanent ponds and the relaxation
of restrictions on planting vegetation within the facilities, provided
that capacity for volumes and rate control is maintained. At a minimum,
the southern half of pond shorelines shall be planted with shade or
canopy trees within 10 feet of the pond shoreline. In conjunction
with this requirement, the maximum slope allowed on the berm area
to be planted is 10 to 1. This will lessen the destabilization of
berm soils due to root growth. A long-term maintenance schedule and
management plan for the thermal control BMPs are to be established
and recorded for all development sites within defined exceptional
value and/or high-quality subwatersheds.
K.
The use of soil amendment, or amended soil, shall not be utilized
within the yard area or building envelope of a lot unless approved
by the Municipal Engineer or unless the soil amendment is a component
of the BMP facilities design, such as the use of soil amendment within
infiltration basins or rain gardens.
L.
No SWM facilities shall be installed over existing utility mains
or services.
A.
Stormwater runoff from all development sites shall be calculated
using the Rational Method, Modified Rational Method, or a Soil Cover
Complex methodology.
(1)
Any stormwater runoff calculations involving drainage areas greater
than 200 acres, including on- and off-site areas, shall use a generally
accepted calculation technique that is based on the NRCS Soil Cover
Complex Method. It is assumed that all methods will be selected by
the design professional based on the individual limitations and suitability
of each method for a particular site.
(2)
The municipality may allow the use of the Rational Method or Modified
Rational Method to estimate peak discharges from drainage areas that
contain less than 200 acres. When using the Rational Method, an ascending
and descending limb factor of three and seven shall be used, respectively.
(3)
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths. If
a hydrologic computer model such as PSRM or HEC-RAS is used for stormwater
runoff calculations, then the duration of rainfall shall be 24 hours.
The SCS Rainfall Type II curve shall be used for the rainfall distribution.
Those projects proposing infiltration structures shall utilize this
method.
(4)
When routing a detention basin, the Rational Method or SCS Method
may be used to determine peak rates through the primary outlet structure.
Only the SCS Method may be used to determine the peak water surface
elevation during the 100-year, twenty-four-hour design storm, dictating
the height of the embankment.
(5)
Underground storage facilities that solely receive stormwater and
are designed to capture and infiltrate the entire 100-year, twenty-four-hour
SCS runoff volume may have their drainage areas removed from the overall
post-development drainage area(s).
(6)
For the purposes of predevelopment flow rate determination, undeveloped
land, including disturbed areas, shall be considered as meadow in
good condition, unless the natural ground cover generates a lower
curve number or rational C value (i.e., forest), as listed in Tables
A-1 and A-2,[1] respectively.
[1]
Editor's Note: See Appendix A, included as an attachment to this chapter.
(7)
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods. Times of concentration for overland flow shall
be calculated using the methodology presented in Chapter 3 of Urban
Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced
from time to time by NRCS). Time of concentration for channel and
pipe flow shall be computed using Manning's Equation.
(8)
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table A-1.[2]
[2]
Editor's Note: See Appendix A, included as an attachment to this chapter.
(9)
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table A-2.[3]
[3]
Editor's Note: See Appendix A, included as an attachment to this chapter.
(10)
Where uniform flow is anticipated, the Manning Equation shall
be used for hydraulic computations such as the capacity of open channels,
pipes, and storm sewers. Values for Manning's roughness coefficient
(n) shall be consistent with Table A-3.[4]
[4]
Editor's Note: See Appendix A, included as an attachment to this chapter.
(11)
The design of any stormwater detention facilities intended to
meet the performance standards of this chapter shall be verified by
routing the design storm hydrograph through these facilities, using
either manual methods or computerized routing. Routing shall be based
upon the Modified PULS Method; other routing methodologies shall be
subject to the approval of the Municipal Engineer.
(12)
The stormwater collection system shall be designed using the
peak discharge computed using the Rational Formula.
In areas of carbonate geology, a geologist shall certify to
the following:
A.
No stormwater management facility will be placed in, over, or immediately
adjacent to the following features:
(1)
Closer than 100 feet from sinkholes;
(2)
Closer than 100 feet from closed depressions;
(3)
Closer than 100 feet from caverns, intermittent lakes, or ephemeral
streams;
(4)
Closer than 50 feet from lineaments in carbonate areas;
(5)
Closer than 50 feet from fracture traces;
(6)
Closer than 25 feet from bedrock pinnacles (surface or subsurface).
B.
Stormwater resulting from regulated activities shall not be discharged
into sinkholes.
C.
If the developer can prove through analysis that the project site is an area underlain by carbonate geology, and such geologic conditions may result in sinkhole formations, then the project site is exempt from recharge requirements as described in § 214-15, Volume controls. However, the project site shall still be required to meet all other standards found in this chapter.
D.
It shall be the developer's responsibility to verify if the
project site is underlain by carbonate geology. The following note
shall be attached to all stormwater management plans and signed and
sealed by the developer's registered professional: "I, __________,
certify that the proposed stormwater management facility (circle one)
is/is not underlain by carbonate geology."
E.
Whenever a stormwater management facility will be located in an area
underlain by carbonate geology, a geological evaluation of the proposed
location by a geologist shall be conducted to determine susceptibility
to sinkhole formation. The evaluation may include the use of impermeable
liners to reduce or eliminate the separation distances listed in the
BMP Manual. Additionally, the evaluation shall, at a minimum, address
soil permeability, depth to bedrock, seasonally high groundwater table,
susceptibility for sinkhole formation, suitability of stormwater management
facilities, subgrade stability, and maximum infiltration capacity
in depth of water per unit area.
F.
A detailed soils evaluation of the project site shall be performed
to determine the suitability of recharge facilities. The evaluation
shall be performed by a qualified professional and, at a minimum,
address soil permeability, depth to bedrock, susceptibility to sinkhole
formation, and subgrade stability. The general process for designing
the infiltration BMP shall be:
(1)
Site evaluation to determine general areas of suitability for infiltration
practices.
(2)
Provide field percolation tests throughout the area proposed for
development to determine appropriate percolation rate and/or hydraulic
conductivity. At least one infiltration test must be included in each
soil group, and at least one infiltration test must be conducted for
every five lots proposed for development. Infiltration tests must
be taken at the location and depth of all proposed infiltration structures.
(3)
Design infiltration structure for required storm volume based on
all available data.
G.
Extreme caution shall be exercised where infiltration is proposed
in geologically susceptible areas such as strip mine or limestone
areas. It is also extremely important that the design professional
evaluate the possibility of groundwater contamination from the proposed
infiltration/recharge facility and recommend a hydrogeologic study
be performed if necessary. Whenever a basin is located in an area
underlain by limestone, a geological evaluation of the proposed location
shall be conducted to determine susceptibility to sinkhole formations.
The design of all facilities over carbonate formations shall include
measures to prevent groundwater contamination and, where necessary,
sinkhole formation. The infiltration requirement in the high-quality/exceptional
waters shall be subject to the Department's 25 Pa. Code Chapter
93 and antidegradation regulations. A detailed hydrogeologic investigation
may be required by the municipality, and, where appropriate, the municipality
may require the installation of an impermeable liner in detention
basins.
A.
As required in § 214-13D, whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Pa. Code Title 25, Environmental Protection, Part I, Subpart C, Article II, Chapter 102, Erosion and Sediment Control, and in accordance with the County Conservation District.
B.
It is extremely important that strict erosion and sedimentation control
measures be applied surrounding infiltration structures during installation
to prevent the infiltrative surfaces from becoming clogged. Additional
erosion and sedimentation control design standards and criteria must
be applied where infiltration BMPs are proposed and shall include
the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase so as to maintain their
maximum infiltration capacity.
D.
The developer shall demonstrate that the post-development hydrograph flows during erosion and sedimentation control phase are less than or equal to the predevelopment hydrograph flows to assure the rate and volume of runoff leaving the site is controlled for the two-, five-, and ten-year frequency storms. All calculation methodology shall be in accordance with §§ 214-15 through 214-21.
E.
In the event a regulated activity falls under the Chapter 102 thresholds
for requiring a written erosion and sedimentation plan and/or NPDES
permit, the applicant is still required to provide suitable erosion
and sedimentation best management practices to prevent an illicit
discharge caused by erosion during a precipitation event.
