A.
For all regulated activities, unless preparation of a stormwater management site plan is specifically exempted in § 247-302:
(1)
Preparation and implementation of an approved stormwater management
site plan is required.
(2)
No regulated activities shall commence until Perkiomen Township
issues written approval of a stormwater management site plan which
demonstrates compliance with the requirements of this chapter. Written
approvals may come from the Township Code Enforcement Officer, and
the Township Engineer, if requested by the Code Enforcement Officer
due to the complexity of the proposed improvements.
B.
Stormwater management site plans approved by Perkiomen Township shall
be on-site throughout the duration of the regulated activity.
C.
Perkiomen Township may 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.
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)
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), 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 not exempt under § 247-302 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 § 247-303 and the peak rate controls of § 247-304 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
F.
The flow of stormwater onto adjacent property shall not be created, increased, or otherwise altered without providing written approval from the affected adjacent property owner to the Township, unless the activity meets the exemption criteria set forth in § 247-302.
G.
If flow is proposed to be concentrated and discharged onto adjacent property, the applicant must document to the Township, in accordance with § 247-301F, that adequate downstream conveyance exists to safely transport the concentrated discharge, or the applicant must obtain drainage easements from affected downstream property owners and provide the facilities to safely convey the flow.
H.
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).
I.
The design of all facilities over karst geology shall include an
evaluation of measures to minimize adverse effects by a qualified
professional.
J.
Infiltration BMPs should be spread out, 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.
K.
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, unless otherwise approved.
L.
The design storm volumes 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 14 can be accessed
at: https://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
For all regulated activities, stormwater 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, and the Storm Water Management
Act.
N.
Various BMPs and their design standards are listed in the PA BMP
Manual and within this chapter. If the BMP Manual and this chapter
have conflicting requirements, the more restrictive requirements shall
apply.
O.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If diffused
flow is proposed to be concentrated and discharged onto adjacent property,
the applicant must document that adequate downstream conveyance facilities
exist to safely transport the concentrated discharge or otherwise
prove that no erosion, sedimentation, flooding or other harm will
result from the concentrated discharge.
P.
Whenever a watercourse is located within a development site, it shall
remain open in its natural state and location and should not be piped,
impeded or altered (except for road and driveway crossings). It is
the responsibility of the developer to stabilize existing eroded stream/channel
banks.
Q.
Any stormwater management facilities regulated by this chapter that
would be located on or discharge into state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
R.
Where a development site is traversed by watercourses, conservation easements shall be provided. Conservation easements shall be measured to be the greater of the limit of the 100-year floodplain, or a minimum of 50 feet from the top of the stream bank (on each side), and a minimum of 25 feet surrounding a wetland, or as further required by the Riparian Corridor Conservation District (Article XXVI of Chapter 310, Zoning, as amended), where more strict requirements may apply. 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. Residential accessory structures, up to no more 225 square feet in size, are exempt from the limitations associated with a conservation easement, subject to the approval of the Code Enforcement Officer.
A.
Requirements for exempt activities.
(1)
An exemption from any requirement of this chapter shall not
relieve the applicant from implementing all other applicable requirements
of this chapter or from implementing such measures as are necessary
to protect public health, safety, and welfare, property and water
quality.
(2)
An exemption shall not relieve the applicant from complying
with the requirements for state-designated special protection waters
designated by PADEP as high quality (HQ) or exceptional value (EV)
waters, or any other current or future state or municipal water quality
protection requirements.
(3)
An exemption under this chapter shall not relieve the applicant
from complying with all other applicable municipal ordinances or regulations.
B.
General exemptions.
(1)
Involve less than 1,500 square feet of new proposed impervious
surfaces and less than 5,000 square feet of earth disturbance in the
aggregate since May 3, 2005; or
(2)
Are listed in § 247-302C; are exempt from those (and only those) requirements of this chapter that are included in the sections and articles listed in Table 302.1. Exemptions are for the items noted in Table 302.1 only, and shall not relieve the landowner from other applicable requirements of this chapter. Exemption shall not relieve the applicant from implementing such measures as are necessary to protect health, safety, and welfare, property, and water quality.
(3)
Any regulated activity that meets the exception criteria in the following table is exempt from the stormwater management site plan submission requirement of this chapter (Article IV). This criterion shall apply to the total development even if development is to take place in phases. May 3, 2005, shall be the starting point from which impervious area computations shall be cumulatively considered. Impervious areas existing on the subject tract prior to May 3, 2005, shall not be considered in cumulative impervious area calculations for exemption purposes. An exemption shall not relieve the applicant from implementing such measures as are necessary to protect health, safety and property. This exemption shall not relieve the applicant from meeting the special requirements for watersheds drainage to high quality (HQ) or exceptional value (EV) waters [§ 247-302A(2)].
|
Table 302.1
Thresholds for Regulated Activities that Are Exempt from the
Provisions of this Chapter as Listed Below (see Notes below)
| ||||
|---|---|---|---|---|
|
Chapter Article/Section
|
Activities Listed in § 247-302D
|
<1,500 sq. ft. of Proposed Impervious Surfaces AND <5,000
sq. ft. of Proposed Earth Disturbance
|
1,500 sq. ft. to 3,000 sq. ft. of Proposed Impervious Surfaces
AND <10,000 sq. ft. of Proposed Earth Disturbance
|
>3,000 sq. ft. of Proposed Impervious Surfaces
|
|
Article I
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
|
Article II
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
| |
|
Exempt
|
Exempt
|
Exempt
|
Not Exempt
| |
|
Article IV
|
Exempt
|
Exempt
|
Exempt
|
Not Exempt
|
|
Article V
|
Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
|
Article VI
|
Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
|
Article VII
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
|
Article VIII
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
Not Exempt
|
|
Other Erosion, Sediment and Pollution Control Requirements
|
Must comply with Title 25, Chapter 102, of the Pa. Code and
other applicable state and municipal codes, including the Clean Streams
Law.
| |||
|
Table 302.1 Notes:
| |
|
•
|
Specific activities listed in § 247-302D are exempt from the indicated requirements, regardless of size.
|
|
•
|
A proposed regulated activity must be less than both the proposed
impervious surfaces and proposed earth disturbance thresholds to be
eligible for exemption from the requirements listed in this table.
|
|
•
|
"Proposed Impervious Surface" - as defined in this chapter.
|
|
•
|
"Exempt" - Regulated activities are exempt from the requirements
of listed section(s) only; all other provisions of this chapter apply.
|
|
•
|
"Modified Requirements" - Regulated activities listed within the subsections of this chapter noted in Table 302.1 are eligible for exemption only from the indicated sections and subsections of this chapter and only if the modified requirements of § 247-302D are met to the satisfaction of the Township; all other provisions of this chapter apply.
|
C.
Exemptions shall be at the discretion of Perkiomen Township upon
review of site conditions, topography, soils and other factors as
deemed appropriate by the Code Enforcement Officer and Township Engineer
based upon comprehensive municipal goals and strategies for local
development. If a project meets the exemption criteria of this chapter,
it does not guarantee an exemption will be granted by the Township.
D.
Exemptions for specific activities. The following specific regulated
activities are exempt, as shown in Table 302.1, unless otherwise noted
below. All other conveyance and system design standards established
by the Township in other codes or ordinances shall be required, and
all other provisions of this chapter shall apply.
(1)
Emergency exemption. Emergency maintenance work performed for
the protection of public health, safety and welfare. This exemption
is limited to repair of the existing facility; upgrades, additions
or other improvements are not exempt. A written description of the
scope and extent of any emergency work performed shall be submitted
to Perkiomen Township within two calendar days of the commencement
of the activity. A detailed plan shall be submitted no later than
30 days following commencement of the activity. If Perkiomen Township
finds that the work is not an emergency, then the work shall cease
immediately and the requirements of this chapter shall be addressed
as applicable.
(2)
Maintenance. Any maintenance to an existing stormwater management
system, facility, BMP or conveyance made in accordance with plans
and specifications approved by the Township Engineer or Township.
(3)
Existing landscaping. Use of land for maintenance, replacement,
or enhancement of existing landscaping.
(4)
Gardening. Use of land for gardening for home consumption.
(6)
Forest management. Forest management operations, which are consistent with a sound forest management plan as filed with the Township and which comply with the Pennsylvania Department of Environmental Protection's management practices contained in its publication "Soil Erosion and Sedimentation Control Guidelines for Forestry" (as amended or replaced by subsequent guidance). Such operations are required to have an erosion and sedimentation control plan which meets the requirements of 25 Pa. Code Chapter 102 and meets the erosion and sediment control standards of § 247-301 of this chapter.
(7)
Maintenance of existing paved surfaces. Replacement of existing paved surfaces shall meet the erosion and sediment control requirements of 25 Pa. Code Chapter 102 and Article III of this chapter, and is exempt from all other requirements of this chapter listed in § 247-302D above. Resurfacing of existing paved surfaces is exempt from the requirements of this chapter listed above. Construction of new or additional impervious surfaces shall comply with all requirements of this chapter as indicated in Table 302.1.
(8)
Municipal roadway shoulder improvements. Shoulder improvements
conducted within the existing roadway cross-section of municipal owned
roadways, unless an NPDES permit is required, in which case the proposed
work must comply with all requirements of this chapter.
(9)
In-place replacement of residential dwelling unit. The replacement
in the exact footprint of an existing one- or two-family dwelling
unit.
(10)
In-place replacement, repair, or maintenance of residential
impervious surfaces. The replacement of existing residential patios,
decks, driveways, pools, garages, and/or sidewalks that are accessory
to an existing one- or two-family dwelling unit in the exact footprint
of the existing impervious surface.
E.
Modified requirements for small projects.
(1)
Regulated activities that involve 1,500 to 3,000 square feet
of proposed impervious surfaces and 10,000 or less square feet of
proposed earth disturbance may apply the modified requirements presented
in the "Simplified Approach to Stormwater Management for Small Projects"
(Simplified Approach) (Appendix C[1]) to comply with the requirements of Table 302.1. The applicant
shall first contact the Township or their designee to confirm that
the proposed project is eligible for use of the Simplified Approach
and is not otherwise exempt from these provisions; to determine what
components of the proposed project are to be considered as impervious
surfaces; and to determine if other known site or local conditions
exist that may preclude the use of any techniques included in the
Simplified Approach. Appendix C includes instructions and procedures
for preparation, submittal, review and approval of documents required
when using the Simplified Approach and shall be adhered to by the
applicant. All other provisions of this chapter shall apply.
[1]
Editor's Note: Appendix C is included as an attachment
to this chapter.
(2)
Projects that qualify for the Simplified Approach will be administered
by and may be approved by the Township Code Enforcement Officer, and
the Township Engineer, if requested by the Code Enforcement Officer
due to the complexity of the proposed improvements. The use of the
Simplified Approach shall be at the discretion of Township Code Enforcement
Officer upon review of site conditions, topography, soils and other
factors as deemed appropriate based upon comprehensive municipal goals
and strategies for local development. If a project meets the criteria
of the Simplified Approach of this chapter, it does not guarantee
the Simplified Approach will be permitted to be used, unless granted
by the Township Code Enforcement Officer.
(3)
The Board of Supervisors hereby authorizes the Chairperson to execute all agreements, contracts and covenants associated with the small projects set forth in § 247-302E(1), provided same have been reviewed by and approved by the Township Code Enforcement Officer, Township Manager, Township Engineer, and Township Solicitor.
The green infrastructure and low impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in § 247-303A or the Simplified Method in § 247-303B below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology and other factors.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(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.
B.
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:
(1)
Stormwater facilities shall capture at least 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.
A.
Post-development discharge rates shall not exceed the predevelopment
discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-,
and 100-year, twenty-four-hour storm events. If it is shown that the
peak rates of discharge indicated by the post-development analysis
are less than or equal to the peak rates of discharge indicated by
the predevelopment analysis for one-, two-, five-, ten-, twenty-five-,
fifty-, and 100-year, twenty-four-hour storms, then the requirements
of this section have been met. Otherwise, the applicant shall provide
additional controls as necessary to satisfy the peak rate of discharge
requirement.
A.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Regulated activities will be required to recharge (infiltrate) a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be recharged shall be determined from § 247-305A(1)(b), depending upon demonstrated site conditions.
(a)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
[1]
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
[2]
An infiltration and/or percolation rate sufficient to accept
the additional stormwater load and drain completely, as determined
by field tests conducted by the applicant's design professional.
[3]
The recharge facility shall be capable of completely infiltrating
the recharge volume within three days (72 hours).
[4]
Pretreatment shall be provided prior to infiltration.
[5]
The requirements for recharge are applied to all disturbed areas,
even if they are ultimately to be an undeveloped land use such as
grass, since studies have found that compaction of the soils during
disturbance reduces their infiltrative capacity.
(b)
The size of the infiltration facility shall be based upon the
net Two-Year Volume Approach, where the recharge (infiltration) volume
(Rev) to be captured and infiltrated shall be the volume difference
between the predevelopment two-year, twenty-four-hour storm event
and post- development two-year, twenty-four-hour storm event. The
recharge volume calculated using this section is the minimum volume
the applicant must control through an infiltration BMP facility.
(2)
The recharge values derived from these methods are the minimum
volumes the applicant must control through an infiltration/recharge
BMP facility. However, if a site has areas of soils where additional
volume of infiltration can be achieved, the applicant is encouraged
to recharge as much of the stormwater runoff from the site as possible.
B.
The general process for designing the infiltration BMP shall be:
(1)
A detailed soils evaluation of the project site shall be required
to determine the suitability of recharge facilities. The evaluation
shall be performed by a qualified applicant and, at a minimum, address
soil permeability, depth to bedrock, and subgrade stability.
(a)
Analyze hydrologic soil groups as well as natural and man-made
features within the watershed to determine general areas of suitability
for infiltration practices.
(b)
Provide field tests, such as double-ring infiltration tests
at the level of the proposed infiltration surface to determine the
appropriate hydraulic conductivity rate.
(c)
Design the infiltration facilities for the required storm volume
based on field-determined capacity at the level of the proposed infiltration
surface.
(d)
Where the recharge volume requirement cannot be physically accomplished
due to the results of the field soils testing, supporting documentation
and justification shall be supplied to Perkiomen Township with the
stormwater management site plan. Alternate methods to address volume
control and water quality requirements may be approved by the Township
Engineer, or by the PA DEP. Means and methods approved by the PA DEP,
such as the Managed Release Concept, can be utilized to meet volume
control requirements.
(e)
If on-lot infiltration facilities are proposed by the applicant's
design professional, it must be demonstrated to Perkiomen Township
that the soils are conducive to infiltrate on the lots identified.
(f)
No infiltration should be proposed within 10 feet of a building,
structure, or property line.
(g)
Infiltration should not be proposed in locations of known hot
spots.
(h)
Infiltration should not be proposed in areas which contain karst
material which may lead to an increased potential for sinkholes to
occur. If BMPs are proposed in karst areas, they must contain a bottom
impermeable liner, or other approved method to prevent infiltration.
The liner shall be sandwiched between two layers of geotextile fabric
to prevent puncture of the liner.
C.
Perkiomen Township shall require the applicant to provide safeguards
against groundwater contamination for uses which may cause groundwater
contamination should there be a mishap or spill. Facilities which
have a high potential for a contaminated spill shall incorporate a
shutoff valve within the stormwater system such that the spill can
be contained and remediated on-site, rather than downstream.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil-Cover-Complex methodology.
A.
Any stormwater runoff calculations shall use a generally accepted calculation technique that is based on the NRCS Soil-Cover-Complex Method. The table below in § 247-306I summarizes acceptable computation methods. It is assumed that all methods will be selected by the applicant based on the individual limitations and suitability of each method for a particular site. Perkiomen Township may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 20 acres.
B.
All calculations consistent with this chapter using the Soil-Cover-Complex
Method shall use the appropriate design rainfall depths for the various
return period storms according to the latest version of the Precipitation-Frequency
Atlas of the United States, National Oceanic and Atmospheric Administration
(NOAA).
C.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil-Cover-Complex Method shall be obtained from
Table B-1 in Appendix B of this chapter.[1] For the purposes of modeling existing conditions (predevelopment),
nonforested pervious areas must be considered meadow in good condition,
and 20% of existing impervious area, when present, shall be considered
meadow in good condition in the model for existing conditions. For
the purposes of modeling proposed conditions (post-development), actual
land cover conditions shall be applied, in accordance with the requirements
in Appendix B.
[1]
Editor's Note: Appendix B is included as an attachment
to this chapter.
D.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods from the latest version of the Precipitation-Frequency
Atlas of the United States, National Oceanic and Atmospheric Administration
(NOAA). Times of concentration for overland flow shall be calculated
using the methodology presented in Chapter 3 of Urban Hydrology for
Small Watersheds, NRCS, TR-55 (as amended or replaced from time to
time by NRCS). Times of concentration for channel and pipe flow shall
be computed using Manning's equation.
E.
The designer shall consider that the runoff from proposed sites graded
to the subsoil will not have the same runoff conditions as the site
under existing conditions even if topsoiled and seeded. The designer
shall increase their proposed condition CN or "c" value to reflect
proposed soil conditions by using the CN or "c" value of one HSG lower
than existing site soils.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table B-2 in
Appendix B of this chapter. For the purpose of estimating runoff,
a 3:7 ascending/receding limb factor shall be applied.
G.
Where uniform flow is anticipated, Manning's equation shall
be used for hydraulic computations and to determine the capacity of
open channels, pipes and storm sewers. Values for Manning's roughness
coefficient (n) shall be consistent with Table B-3 in Appendix B of
this chapter.
H.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
I.
Acceptable computation methodologies for the design and preparation
of stormwater management site plans are indicated the following table:
|
Acceptable Computation Methodologies for Stormwater Management
Plans
| ||
|---|---|---|
|
Method
|
Method Developed By
|
Applicability
|
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
|
HEC-1, HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
|
Rational Method (or commercial computer package based on Rational
Method)*
|
Emil Kuichling (1889)
|
For sites less than 20 acres, or as approved by Perkiomen Township
and/or Perkiomen Township Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by Perkiomen Township
and/or Perkiomen Township Engineer
|
|
NOTES:
| |
|
*
|
Use of the Rational Method to estimate peak discharges from drainage areas that contain more than 20 acres must be approved by the Perkiomen Township Engineer. The Rational Method shall not be used to satisfy the requirements of Article III without the consent of the Perkiomen Township Engineer.
|
A.
Standards for conventional basins shall meet requirements listed
in this section.
(1)
Basins shall be installed prior to any earthmoving or land disturbances
that they will serve. The phasing of their construction shall be noted
in the erosion and sedimentation control narrative and on the E&S
plan. Permanent vegetation shall be established prior to denuding
any other land, unless the basin functions as an E&S device.
(2)
Energy dissipaters and/or level spreaders shall be installed
at points where pipes or drainageways discharge from basins. Multiple-outlet
structures and multiple-outlet piping from the basin may be required
by Perkiomen Township to reduce the impact of point discharges.
(3)
The following slope restrictions shall apply to basins:
(a)
Exterior slopes of compacted soil shall not exceed one foot
vertical for three feet horizontal and may be further reduced if the
soil has unstable characteristics.
(b)
Interior slopes of the basin shall not exceed one foot vertical
in three feet horizontal except with approval of Perkiomen Township,
and:
[1]
Where maximum water depth will not exceed three feet; or
[2]
When a two-inch rainfall in one hour will not exceed the capacity
of the basin in one hour; or
[3]
Where concrete, stone or brick walls are used with side slopes
proposed to be steeper than one foot vertical in three feet horizontal,
in which case the basin shall be fenced by a permanent fence 42 inches
in height and a ramp of durable, nonslip materials for maintenance
vehicles shall be provided for access into the basin.
(c)
The maximum bottom slope shall be 3% for grass, and 1% may be
used for grass if an underdrain system is provided.
(4)
Basins shall also be designed to meet the following requirements:
(a)
The minimum top of berm width shall be eight feet.
(b)
Outlet pipes shall have a minimum diameter of 15 inches.
(c)
Properly spaced anti-seep collars shall be installed on all
basin outlet pipes. Design calculations shall be provided.
(d)
All basins shall be constructed with a compacted relatively
impervious (unified soil classification CL-ML or CL) key trench and
core. The key trench shall extend at least two feet into undisturbed
subsoil (below topsoil layer). The minimum bottom width of the trench
shall be six feet, and the minimum top width of the core shall be
four feet. The side slopes of the compacted core and trench shall
not exceed one horizontal to one vertical, and the top elevation of
the core shall be set at or above the twenty-five-year design water
elevation. Perkiomen Township may require an impermeable liner to
be installed up to the 100-year design water surface elevation.
(5)
Basin outlet structures and emergency spillways.
(a)
Outlet structures within basins which will control peak discharge
flows and distribute the flows by pipes to discharge areas shall be
constructed of reinforced vegetative or stone material and shall have
childproof, nonclogging trash racks over all design openings, except
those openings designed to carry perennial stream flows. Trash rack
material should be epoxy-coated, galvanized or stainless steel or
high-density polyethylene. Other materials are subject to the approval
of Perkiomen Township.
(b)
Six inches of freeboard shall be provided between the crest
of the primary outlet structure and the invert of the emergency spillway.
(c)
Emergency spillways shall be constructed in undisturbed earth
wherever possible. When constructed in fill, sod, precast concrete
paving blocks, concrete or permanent erosion control matting shall
be used. Design calculations shall be submitted indicating that the
specified material can withstand velocities based on the 100-year
design storm event. When using sod, it shall be applied along the
inside slope above the twenty-five-year water surface elevation, along
the face and sides of the spillway and down the outside slope to existing
grade. Emergency spillways shall be designed to safely convey the
100-year basin inflow hydrograph through the basin, assuming the principal
outlet is completely blocked and the basin water surface elevation
is equal to the spillway invert elevation.
(6)
Basin inlet and outlet structures should be located at maximum
distances from one another. Perkiomen Township may require a rock
filter berm or rock-filled gabions between inlet and outlet areas
when the distance is deemed insufficient for sediment trappings.
(7)
Permanent grasses or stabilization measures shall be established
on the sides of all earthen basins within five days of initial construction
(or conversion from sediment basin or sediment trap). Perkiomen Township
may require erosion control matting to be installed inside the basin
or on the basin embankment.
(8)
Stormwater runoff shall discharge to a suitable natural drainage course (except where prohibited by riparian buffer area regulations of this chapter and Chapter 310, Zoning) or storm sewer system. Where not possible or not permitted, level spreading devices or other suitable facilities (i.e., swale) shall be designed with sufficient capacity to convey the 100-year storm event without creating any safety, flooding or property hazard. Securing of necessary drainage easements for this purpose shall be the sole responsibility of the developer.
(9)
Perkiomen Township may require soil samples from the site to
be analyzed to determine if these soils are suitable for berm embankment
construction. If in the opinion of Perkiomen Township the soils are
found to be unsuitable, the developer shall import suitable soils
for constructing the basin.
(10)
Where basin storage depths will exceed two feet of ponding stormwater,
the Township at its discretion may require fencing around the basin.
B.
Swale design.
(1)
Grass swales not specifically designed as BMP devices shall
have a minimum bottom slope of 2%, and their depth shall be deep enough
to convey the twenty-five-year storm event with a minimum of 1.0 foot
of freeboard. Swale linings shall be designed based on the ten-year
velocity. Swales shall have sufficient freeboard to convey the 100-year
storm discharge without creating any safety or property hazard.
(2)
Swales, when located outside of Perkiomen Township right-of-way,
shall be located within an easement not less than 20 feet wide but
of sufficient width to allow access for maintenance and to convey
all storms up to and including the 100-year storm. A note on the plan
shall indicate that the easement allows Perkiomen Township the right,
though not the responsibility, to perform needed maintenance and/or
repairs and back charge the maintainer or lien the property of the
maintainer.
C.
Storm sewer design.
(1)
Where storm sewer pipe and inlets are required, they shall be
placed immediately in front of the curb or road edge within the right-of-way.
Any storm sewer pipe which outlets on private residential property
may only discharge to the side and rear yards and must be a minimum
of 50 feet beyond habitable structures, unless other means and methods
such as level spreaders are used. Any discharges in the front yard
must be approved by the Township.
(2)
Storm sewers shall have a minimum diameter of 15 inches, and
only reinforced cement concrete pipe or smooth-bore corrugated polyethylene
pipe shall be used. The minimum grade of the pipe shall be 0.5%. Corrugated
metal pipe will not be permitted within Perkiomen Township no matter
who has the maintenance responsibility.
(3)
All storm sewers within the public right-of-way and/or beneath
a paved surface shall be bedded and backfilled with PennDOT No. 2A
stone. This backfill shall be placed at a minimum, unless Perkiomen
Township approves an adjustment in the field, in six-inch lifts and
solidly compacted to the satisfaction of Perkiomen Township.
(4)
Watertight pipe connections are required, and appropriate specifications
shall be specified on the plans.
(5)
Headwalls, endwalls or end sections shall be required on all
open pipes, shall be of concrete construction and shall be set on
a minimum of 12 inches of AASHTO No. 57 (PennDOT 2B) coarse aggregate.
(6)
All storm sewers shall be constructed per PennDOT specifications
as outlined in Publication 408, Design Manual, Part 2, and the Highway
Design and Standards for Roadway Construction, RC-Series, unless otherwise
dictated by Perkiomen Township.
(7)
Any changes in alignment (vertical or horizontal) shall be straight
sections connected by inlets or manholes.
(8)
Precast inlet tops and boxes shall meet the requirements of
PennDOT Publication 408. PennDOT type "C" precast concrete inlet tops
are to be provided with a five-inch by twenty-four-inch cast iron
"Dump No Waste - Drains To Waterway" (with 1/2 inch raised lettering)
plate with trout logo as manufactured by E. Jordan Iron Works, or
approved equal. Bottom of tops (grates) are to be clearly marked with
grade of iron (ASTM A48, Class 35B), product number and date of manufacture.
All other PennDOT inlets are to be provided with either a painted
stenciled logo on the roadway or another acceptable marking approved
by Perkiomen Township.
(9)
The words "Dump No Waste - Drains To Waterway" in 1 1/4
inch raised letters with bass logo shall be cast or stamped into the
storm sewer manhole covers as manufactured by E. Jordan Iron Works,
or approved equal. Bottoms of covers are to be clearly marked with
grade of iron (ASTM A48, Class 35B), product number and date of manufacture.
(10)
When there is a change in pipe size through an inlet, the top
inside elevation of the outlet pipe shall be at or below the top inside
elevations of all incoming pipes.
(11)
Storm sewer sizes shall be determined based upon the following
design storm frequencies:
(a)
Twenty-five years in all subdivisions or land developments,
unless otherwise specified by Perkiomen Township.
(12)
The design of storm sewer systems within the drainage area of
detention or retention facilities must be analyzed for adequacy during
the 100-year storm, including the effects of the control facility
tailwater. This may require a hydraulic grade-line analysis. When
approved by Perkiomen Township, overflow swales may be provided at
low points in streets to safely convey the full 100-year peak flow
to the control facility, in lieu of providing the full capacity in
the storm sewer.
(13)
Storm sewer design shall be based upon PennDOT design methods.
Inlet efficiency and bypass flow shall be determined for all inlets,
and the gutter flow spread shall not exceed 1/2 the travel lane width
or to a maximum of eight feet where parking is permitted. Perkiomen
Township may require that a hydraulic grade line analysis be performed
on storm sewer systems.
(14)
Culverts shall be evaluated for inlet and outlet control restrictions.
(15)
Rainfall intensity curves and other hydraulic design data from
NOAA shall be used for design purposes.
(16)
Manholes and/or inlets shall not be more than 300 feet apart
on pipe sizes up to 24 inches and not more than 400 feet apart on
larger sizes.
(17)
Inlets, manholes, covers and frames shall conform to Pennsylvania
Department of Transportation specifications. At street intersections,
every attempt should be made to place inlets in the tangent and not
in the curved portion of the curbing.
(18)
When precast concrete inlets or manholes are used within a street,
either concrete adjustment units or rubber ring adjustment units shall
be placed to bring the grate or cover to proper elevation.
D.
Roof drains.
(1)
Stormwater roof drains shall not discharge water directly over
a sidewalk or into any sanitary sewer line.
(2)
Proposed roof drains and collector locations shall be shown
on the stormwater management site plans. Roof drains and collectors
shall meet all appropriate Perkiomen Township codes.
(3)
Gutters shall have gutter guards or similar devices to prevent
debris from entering any underground drains associated with a roof.
(4)
Underdrain pipes associated with roof drains or downspouts are
permitted to be a diameter no smaller than four inches and shall be
comprised of smooth-lined high-density polyethylene pipe or schedule-40
PVC pipe. All underdrains shall contain an overflow tee or similar
outlet at the ground surface.
E.
Design of bridges and culverts.
(1)
Bridges and culverts shall have ample waterway to carry the
design flows, based on a minimum storm frequency of 100 years, unless
a larger design flow is required by PA DEP. One-hundred-year water
depths shall not exceed six inches above the roadway center-line elevation.
Bridge and/or culvert construction shall be in accordance with the
PennDOT specifications and shall meet the requirements of the PA DEP.
The appropriate permits and approvals must be acquired by the applicant
prior to final plan approval.
(2)
Culverts shall be provided with wing walls and constructed for
the full width of the right-of-way. If Perkiomen Township determines
that the character of the road is expected to change for future planning,
the cartway of the bridge shall be made to anticipate this condition.
On each side of the bridge cartway, the bridge railing must be set
back from the edge of the final cartway, and this area may be used
to place sidewalks, bike trails, etc.
(3)
Permanent easements must be provided to the Township for all
bridges or culverts used for vehicular access. The easements shall
include the entire bridge or culvert, provide additional areas for
future staging which may be necessary for future maintenance, and
an access area no less than 20 feet wide. Easement sizes should be
prepared through coordination with the Township and Township Engineer
to ensure an adequate sized easement is offered to the Township.
F.
Stone and pipe underground infiltration beds.
(1)
The plans shall note: Areas proposed for infiltration BMPs shall
be protected from sedimentation and compaction during the construction
phase so as to maintain their maximum infiltration capacity.
(2)
All stormwater runoff shall be pretreated for water quality
prior to discharge to an infiltration facility or BMP. This shall
consist of a water quality device (i.e., snout, water quality filter,
water quality structure, etc.) upstream of the infiltration facility.
(3)
Pipes shall be made of perforated high-density polyethylene
pipe, or an approved equivalent. Any pipes within the infiltration
bed shall be a minimum of six inches off of the bed bottom and shall
be underlain with clean stone. Pipes may be substituted with arched
storm chambers if constructed in accordance with the manufacturer's
recommendations.
(4)
The minimum pipe separation between parallel pipes and the pipes
to the outer limits of the basin bed shall be 18 inches, or as specified
by the manufacturer.
(5)
Infiltration beds shall be wrapped in nonwoven geotextile fabric
on all sides. Fabric should overlap a minimum of 12 inches where applicable.
Where pipes puncture the geotextile fabric, a pipe boot, or similar
fitting, should be used to secure the pipe to the geotextile fabric.
The bottom of the bed may substitute a combination of sand, pea gravel,
and fine stone in lieu of a geotextile fabric to create separation.
(6)
A minimum of one foot of natural cover over the top of the infiltration
bed shall be provided in nonvehicular areas. In vehicular areas, a
minimum of 12 inches of natural materials shall be provided beneath
any pavement (Example: An infiltration bed under a parking lot with
a four-inch pavement section would require top of pipe depth to be
16 inches from paving surface.)
(7)
Observation ports shall be provided for the underground pipe
network, as required by the Township Engineer. For pipe diameters
24 inches and greater, a manhole shall be provided which would allow
access to the pipe network.
(8)
All underground infiltration facilities shall contain an overflow
pipe outlet such that water can be released if the bed were to become
filled with stormwater. Surcharging of any structure, including inlets
and outlet structures, is prohibited.
(9)
Stone and pipe underground infiltration beds shall have a flat
bottom.
(10)
The stone portion of the bed shall be comprised of clean washed
stone, typically AASHTO #57 stone, AASHTO #1 stone, or AASHTO #3 stone,
which contains a 40% void space, unless otherwise approved.
G.
Rain gardens.
(1)
The plans shall note: Areas proposed for infiltration BMPs shall
be protected from sedimentation and compaction during the construction
phase so as to maintain their maximum infiltration capacity.
(2)
Rain gardens shall contain no greater than a twelve-inch ponding
depth, unless otherwise approved.
(3)
An outlet overflow control device shall be provided within the
rain garden which would allow for water to leave the facility in the
event the rain garden becomes full. This device shall be in the form
of an outlet structure with grate or domed pipe riser. The riser top
elevations shall be six inches below the top of the berm.
(4)
Rain garden interior and exterior slopes shall be no steeper
than a 3:1 grade.
(5)
The maximum loading ratio permitted is 5:1.
(6)
The planting soil within the rain garden shall be no less than
a depth of 18 inches and be comprised of amended soils.
(7)
Native plant species susceptible to increased amounts of water
should be utilized to plant the rain garden bottom.
(8)
A minimum three-inch depth of mulch or similar natural material
shall be placed at the rain garden's surface.
(9)
When underdrain pipes are proposed within a rain garden, they
shall be within a stone trench that is a minimum width of four feet
wide and shall contain a minimum of six inches of stone above and
below the underdrain pipe. The stone trench shall be lined on all
sides with nonwoven geotextile fabric. Fabric should overlap a minimum
of 12 inches where applicable. Where pipes puncture the geotextile
fabric, a pipe boot, or similar fitting, should be used to secure
the pipe to the geotextile fabric. The bottom of the bed may substitute
a combination of sand, pea gravel, and fine stone in lieu of a geotextile
fabric to create separation. All pipes should be level. Cleanouts
and/or observation ports shall be included within any underdrain network.
(10)
All underground infiltration facilities shall contain an overflow
pipe outlet such that water can be released if the bed were to become
filled with stormwater. Surcharging of any structure, including inlets
and outlet structures, is prohibited.
H.
Existing facilities.
(1)
If proposed improvements rely on existing stormwater management
facilities, a detailed study and or investigation of the facilities
may be required, to the extent necessary by the Township Engineer,
to determine the conditions and stormwater capacity of the system.
In the event the existing stormwater facility system is not adequate
based on existing conditions, the stormwater facility may be required
to be upgraded or replaced, at the applicant's expense, in order
to rely on the existing system.
A.
Procedures for protecting soils or geologic structures with water
supply potential from contamination by surface water or other disruption
by construction activity shall be established in consultation with
Perkiomen Township, and such areas shall include, at minimum, those
underlain by carbonate limestone formations. Perkiomen Township may
require pollution control facilities to be provided on existing or
proposed stormwater management systems within or adjacent to the project
site.
B.
Provisions for protecting existing wells or other water supplies
shall be established.
C.
Any graded slopes shall not be steeper than three horizontal units
to one vertical unit.
E.
Any approvals required by the Floodplain Conservation District (Chapter 172, as amended) shall be secured prior to earthmoving or stripping of vegetation.
G.
A minimum of four inches of topsoil shall be provided on all disturbed
areas prior to final seeding and mulching.
H.
Mature trees with a twelve-inch caliper (i.e., diameter) or greater,
and other significant existing vegetation, within the limits of earth
disturbance, shall be located in the field and on the stormwater management
site plan and shall be retained and protected to the extent feasible.
The filling of soil over the roots of trees to be preserved is prohibited.
Roots are presumed to extend out from the tree as far as the tree's
branches extend outward.
Where more than five mature trees (i.e., twelve-inch caliper
or greater) are removed, all mature trees removed shall be replaced
at a rate of one two-inch caliper tree per twelve-inch caliper tree
removed, unless otherwise approved by the Township. Replacement trees
shall be guaranteed for a period of 18 months from the date of installation,
and such guarantee should be in the form of a financial security,
which should be released after the guaranteed period, upon certification
of the Township Engineer or Code Enforcement Officer.
Nonviable and dead trees do not require replacement. Photo evidence
of each nonviable and/or dead tree should be provided for review and
be certified by a registered consulting arborist or certified arborist,
or as otherwise approved by the Township or their representatives.
A.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Pennsylvania Code Title 25, Environmental Protection, Part I, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, and Chapter 102, Erosion and Sediment Control, and in accordance with the Montgomery County Conservation District.
B.
Additional erosion and sedimentation control design standards and
criteria that must be applied where infiltration BMPs are proposed
shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase so as to
maintain their maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has received final stabilization.
C.
The applicant shall submit evidence to Perkiomen Township that they
have received approval for any project in which the earth disturbance
will exceed one acre or more in totality of disturbance from the Montgomery
County Conservation District.
