A.
Any person or entity intending to engage in a regulated activity shall, prior to commencing the regulated activity, apply for and obtain a stormwater management permit and comply with the requirements of this chapter. An application for a stormwater management permit and a SWM site plan approval shall be submitted on a form prescribed by the Township accompanied by the required fee. Prior to the commencement of any of the regulated activities in the Township set forth under § 94-105 which are not exempt under § 94-106, a stormwater management permit and a stormwater management site plan (SWM site plan) must be approved in writing by the Township. SWM site plans approved by the Township shall be on site throughout the duration of the regulated activity.
B.
The stormwater management and runoff control criteria and standards
in this chapter shall apply to the total proposed regulated activity,
even if it is to take place in stages. The measurement of impervious
surfaces shall include all of the impervious surfaces in the total
proposed regulated activity even if the development is to take place
in stages.
C.
No regulated activity within the Township shall commence until:
(1)
The Township issues the stormwater management permit and approval
of a SWM site plan, which demonstrates compliance with the requirements
of this chapter; and
(2)
The applicant has received a letter of adequacy or approval for the
erosion and sediment control plan review by the Township and the Conservation
District (if required) and has received all other local, state and
federal permit approvals required for the project involving the regulated
activity.
D.
Neither submission of an SWM site plan under the provisions herein
nor compliance with the provisions of this chapter shall relieve any
person from responsibility for damage to any person or property otherwise
imposed by law.
E.
The applicant shall design the site to minimize disturbances to land, site hydrology, and natural resources and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in § 94-304 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.
F.
To the maximum extent practicable, postconstruction stormwater shall
be discharged within the drainage area of the same stream or water
body receiving the runoff prior to construction of the proposed regulated
activity.
G.
Existing drainage peak rate discharges up to and including the one-hundred-year
storm onto or through adjacent property(ies) or downgradient property(ies),
including diffuse drainage discharge, shall not be altered in any
manner without written permission from, and, where applicable as determined
by the Township, an easement and agreement with, the affected landowner(s)
for conveyance of discharges onto or through their property(ies).
Such discharge shall be subject to any applicable discharge criteria
specified in this chapter.
H.
Areas located outside of the site (i.e., areas outside of the regulated activity) that drain through a proposed site are not subject to water quality and volume control, infiltration, stream channel protection, or peak flow rate control requirements (as presented in §§ 94-305, 94-306, 94-307 and 94-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
I.
If site conditions preclude capture of runoff from limited portions
of the disturbed area for achieving water quality volume control standards,
stream channel protection standards, and the two-, five-, and ten-year
storm event peak runoff rate reduction standards for new development
required by this chapter, the applicant shall propose alternate methods
to mitigate the bypass of the BMPs, subject to the approval of the
Township Engineer. In no case shall resulting peak rate be greater
than the predevelopment peak rate for the equivalent design storm.
J.
For all regulated activities, erosion and sediment control BMPs shall
be designed, implemented, operated, and maintained during the regulated
activities (i.e., during construction) as required to meet the purposes
and requirements of this chapter, to meet the erosion and sediment
control requirements of the Township, if applicable, and to meet all
requirements under Title 25 of the Pa. Code and the Clean Streams
Law.
K.
For all regulated activities, permanent BMPs and conveyances 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.
L.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the Township. The Township reserves
the right to disapprove any design that would result in construction
in an area affected by existing stormwater problem(s) or continuation
of an existing stormwater problem(s).
M.
Existing wetlands, either on the site or on an adjacent property,
shall not be used to meet the minimum design requirements for stormwater
management or stormwater runoff quality treatment. Stormwater discharges
to existing wetlands shall not degrade the quality or hydrologic integrity
of the wetland.
N.
Hotspot runoff controls. Specific structural or pollution prevention
practices may be required, as determined to be necessary by the Township
Engineer, to pretreat runoff from hotspots prior to infiltration.
Following is a list of examples of hotspots:
(1)
Vehicle salvage yards and recycling facilities;
(2)
Vehicle fueling stations;
(3)
Vehicle service and maintenance facilities;
(4)
Vehicle and equipment cleaning facilities;
(5)
Fleet storage areas (bus, truck, etc.);
(6)
Industrial sites based on Standard Industrial Classification codes;
(7)
Marinas (service and maintenance areas);
(8)
Outdoor liquid container storage;
(9)
Outdoor loading/unloading facilities;
(10)
Public works storage areas;
(11)
Facilities that generate or store hazardous materials;
(12)
Commercial container nursery;
(13)
Contaminated sites/brownfields;
(14)
Other land uses and activities as designated by the Township.
O.
Contaminated and brownfield sites. Where BMPs may contribute to the
migration of contaminants in groundwater, the water quality and runoff
volume, stream channel protection, and peak rate control standards
shall be met; however, at the Township Engineer's discretion, the
minimum infiltration requirement may be reduced or eliminated commensurate
with the contaminated area, and the required water quality and runoff
control measures may be increased to mitigate the reduced infiltration
requirement for the contaminated area.
P.
Additional water quality requirements. The Township may require additional
stormwater control measures for stormwater discharges to special management
areas, including, but not limited to:
(1)
Water bodies listed as "impaired" by PADEP.
(2)
Any water body or watershed with an approved total maximum daily
load (TMDL).
(3)
Areas of known existing flooding problems.
(4)
Critical areas with sensitive resources (e.g., state-designated special
protection waters, cold-water fisheries, carbonate geology or other
groundwater recharge areas that may be highly vulnerable to contamination,
drainage areas to water supply reservoirs, etc.).
Q.
Applicants shall utilize the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual), as amended, or other sources acceptable to the Township Engineer, for testing and design standards for BMPs, and where there is a conflict with the provisions of this chapter, the most restrictive applies.
R.
For areas underlain by karst or carbonate geology that may be susceptible to the formation of sinkholes and other karst features, the location, type, and design of infiltration BMPs shall be based on a site evaluation conducted by a qualified licensed professional and based on the PA BMP Manual or other design guidance acceptable to the Township Engineer.
S.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with § 115-42 of the Zoning Ordinance and shall be designed to maintain the flood-carrying capacity of the floodway such that the base flood elevations are not increased, either upstream or downstream. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.
T.
Disturbance of existing ground cover during construction of the proposed
regulated activity is prohibited within 50 feet of top-of-bank of
all perennial and intermittent waterways, water bodies (lakes, ponds,
etc.) and wetlands, except for activities otherwise approved by state
or local agencies (e.g., stream restoration projects, road crossings,
subsurface utility projects, etc.). At the Township Engineer's discretion,
and with Conservation District and PADEP approval where necessary,
the nondisturbance buffer may be reduced because of setback or other
site constraints, but never be less than 10 feet.
The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the Township of the SWM site plan
and prior to commencement of any regulated activities, as applicable:
A.
All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at 25 Pa. Code Chapter 102 or erosion and
sediment control requirements of the Township.
B.
Work within natural drainageways subject to permit by PADEP under
25 Pa. Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under 25 Pa. Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a state highway right-of-way or require access to or from a state highway and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area, and any facility which
may constitute a dam subject to permit by PADEP under 25 Pa. Code
Chapter 105.
A.
No regulated activity within the Township shall commence until:
(1)
The Township receives documentation that the applicant has received:
(a)
A "letter of adequacy" from the Conservation District or other
approval from PADEP in compliance with Title 25, Chapter 102, of the
Pennsylvania Code of an erosion and sediment control plan for construction
activities, if applicable;
(c)
Evidence of any other permit(s) or approvals required for the
regulated activities; and
(2)
An erosion and sediment control plan has been approved by the Township,
if required.
B.
A copy of the erosion and sediment control plan and any required
permit(s), as required by PADEP regulations, shall be available on
the site at all times.
The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall demonstrate in its SWM site plan (as required in § 94-402C) that the design sequence, objectives and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:
A.
First, identify and delineate all existing natural resources and natural and man-made hydrologic features listed in § 94-402B(8) that are located within the site or receive discharge from or may be impacted by the proposed regulated activity.
B.
Second, provide a prioritized listing of these resources and features
to identify:
(1)
Those to be incorporated into the site design in a manner that provides
protection from any disturbance or impact from the proposed regulated
activity;
(2)
Those to be protected from further disturbance or impact but for
which the proposed regulated activity will provide improvement to
existing conditions;
(3)
Those that can be incorporated into and utilized as components of
the overall site design in a manner that protects or improves their
existing conditions while utilizing their hydrologic function within
the limits of their available capacity (e.g., for infiltration, evapotranspiration,
or reducing pollutant loads, runoff volume or peak discharge rates,
etc.) to reduce the need for or size of constructed BMPs; and
(4)
Those that may be considered for alteration, disturbance or removal.
C.
Third, develop the site design to achieve the following:
(1)
Recognize and incorporate the priorities identified in § 94-304B as the basis for the proposed site layout, grading, construction, and permanent ground cover design;
(2)
Minimize earth disturbance (both surface and subsurface);
(3)
Maximize protection of or improvement to natural resources and special
management areas;
(4)
Minimize the disturbance of natural site hydrology, in particular
natural drainage features and patterns, discharge points and flow
characteristics, natural infiltration patterns and characteristics,
and natural channel and floodplain conveyance capacity;
(5)
Incorporate natural hydrologic features and functions identified in § 94-304B into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6)
Maximize infiltration and the use of natural site infiltration features,
patterns and conditions, and evapotranspiration features;
(7)
Apply selective grading design methods to provide final grading patterns
or preserve existing topography in order to evenly distribute runoff
and minimize concentrated flows;
(8)
Minimize the cumulative area to be covered by impervious surfaces
and:
(9)
Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(13)
For flow volumes requiring conveyance from the source of generation to a BMP for management, give preference to open-channel conveyance techniques that provide infiltration and water quality benefits, and landscaped-based management in common open space areas, where practicable; and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of Subsections B and C. Appendix A presents additional discussion of natural hydrology site design and sources of information for "conservation design," "low-impact design" and "sustainable design."[1]
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
To control postconstruction stormwater impacts from regulated
activities and meet state water quality requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that postconstruction stormwater
discharges do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The applicant shall comply with the following
water quality and runoff volume requirements for all regulated activities,
including all new development and redevelopment activities:
A.
The postconstruction total runoff volume shall not exceed the predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour-duration precipitation (design storm). The water
quality and runoff volume to be managed shall consist of any runoff
volume generated by the proposed regulated activity over and above
the predevelopment total runoff volume and shall be captured and permanently
retained or infiltrated on the site. Permanent retention options may
include, but are not limited to, reuse, evaporation, transpiration,
and infiltration.
B.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 94-309C of this chapter.
C.
The design of the facility outlet shall provide for protection from
clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters. Applicants shall fulfill
the requirements of the PADEP "Thermal Impact Analysis" for the "PAG-02
Stormwater Discharges Associated with Construction Activities, NOI
for Coverage under General or Individual Permit" if they cannot meet
the volume control requirements.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 94-306. The infiltration volume required under § 94-306 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F.
Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide pretreatment prior
to the runoff entering the stormwater management practice.
G.
The Township may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 94-301P.
H.
When the regulated activity contains or is divided by multiple drainage
areas, the water quality and runoff volume shall be separately addressed
for each drainage area.
I.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
J.
Areas located outside of the site (i.e., areas outside of the regulated
activity) may be excluded from the calculation of the water quality
and runoff volume requirements.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A.
Infiltration shall be designed to accommodate the entire water quality and runoff volume required in § 94-305.
B.
For regulated activities involving new development, the volume of
a minimum of one inch of runoff from all proposed impervious surfaces
shall be infiltrated.
D.
If the requirements of Subsection B or C cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Township Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however in all cases at least the first 0.5 inch of runoff volume shall be infiltrated.
E.
Only if a minimum of at least 0.5 inch infiltration requirement cannot be physically accomplished on the site shall a waiver from § 94-306 be considered by the Township.
F.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
G.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
H.
Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
I.
A detailed soils evaluation of the site shall be conducted by a qualified professional and at a minimum shall address soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be conducted by a qualified licensed professional and shall be consistent with the PA BMP Manual (as amended) (or other guidance acceptable to the Township Engineer) and in general shall:
(1)
Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
(2)
Provide field tests such as double ring infiltrometer or other hydraulic
conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention (infiltration)
volume based on field-determined infiltration capacity (and apply
safety factor as per applicable design guidelines) at the elevation
of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall be
demonstrated to the Township Engineer that the soils are conducive
to infiltration on the identified lots.
J.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(2)
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3)
The infiltration facility shall completely drain the retention (infiltration)
volume within three days (72 hours) from the end of the design storm.
K.
All infiltration practices shall:
(2)
Be set back at least 25 feet from all buildings and features with
subgrade elements (e.g., basements, foundation walls, etc.), unless
otherwise approved by the Township Engineer.
(3)
Be set back at least 10 feet from property lines and right-of-way
boundaries.
(4)
For any infiltration practice that collects runoff from shared or
multiple features and that is located within 25 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element.
L.
Infiltration facilities shall be located to avoid introducing contaminants
to groundwater by adhering to the following practices:
(1)
When a hotspot is located in the area draining to a proposed infiltration
facility, an evaluation of the potential of groundwater contamination
from the proposed infiltration facility shall be performed, including
a hydrogeologic investigation (if necessary) by a qualified licensed
professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a "wellhead protection area" of a public water
supply well, infiltration practices shall be in conformance with the
applicable approved source water protection assessment or source water
protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
M.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Township
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
N.
Consideration of infiltration BMPs for areas underlain by karst or carbonate geology is encouraged, but only where the design, supporting calculations, results of soils or other site investigations or other documentation is provided to the Township demonstrating that the potential or likelihood of subsidence or sinkholes is minimal. Evaluation of site conditions and infiltration design shall rely on guidance in the PA BMP Manual (as amended) or other guidance acceptable to the Township Engineer.
O.
Groundwater quality of the carbonate aquifer shall be protected from infiltration of pollutants. At a minimum, stormwater runoff from hotspots (i.e., sources of significant pollutant runoff) shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration. Where soil characteristics are insufficient to provide removal of pollutants from sources other than hotspots, stormwater runoff shall first be discharged through a water quality BMP(s) to remove pollutants prior to infiltration.
P.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
Q.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
R.
All infiltration practices shall have appropriate positive overflow
controls.
S.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
T.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed with
equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior to the
placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines, shall
be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or like
facilities shall be covered with drainage fabric. Fabric shall be
nonwoven fabric acceptable to the Township Engineer.
(5)
Stormwater shall be distributed throughout the entire seepage bed/trench
or like facility, and provisions for the collection of debris shall
be provided in all facilities.
For regulated activities involving new development with one
or more acres of earth disturbance, the applicant shall comply with
the following stream channel protection requirements to minimize stream
channel erosion and associated water quality impacts to the receiving
waters:
A.
The peak flow rate of the postconstruction two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour-duration precipitation, using the
SCS Type II distribution.
B.
To the maximum extent practicable, and unless otherwise approved by the Township Engineer, the postconstruction one-year, twenty-four-hour storm flow shall be detained for a minimum of 24 hours and a maximum not to exceed 72 hours from a point in time when the maximum volume of water from the one-year, twenty-four-hour storm is stored in a proposed BMP (i.e., when the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the orifice is at the invert of the proposed BMP).
C.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 94-309C of this chapter.
D.
The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter unless otherwise approved by the Township Engineer, and a trash rack shall be installed to prevent clogging. For sites with small drainage areas contributing to the BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition.
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates subject
to the Township Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F.
All proposed stormwater facilities shall make use of measures to
extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities, including those
that involve new development and redevelopment:
A.
Postconstruction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design storms specified in Table 94-308.1.
|
Table 94-308.1
Peak Rate Control Standards
| |||
|---|---|---|---|
|
(Peak flow rate of the postconstruction design storm shall
be reduced to the peak flow rate of the corresponding predevelopment
design storm shown in the table.)
| |||
|
Predevelopment Design Storm
| |||
|
Postconstruction Design Storm Frequency (24-Hour Duration)
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| |
|
2-year
|
1-year
|
2-year
| |
|
5-year
|
5-year
|
5-year
| |
|
10-year
|
10-year
|
10-year
| |
|
25-year
|
25-year
|
25-year
| |
|
50-year
|
50-year
|
50-year
| |
|
100-year
|
100-year
|
100-year
| |
B.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 94-309C of this chapter.
C.
For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total proposed impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 94-308.1, using the redevelopment ground cover assumptions presented in § 94-309C.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
A.
Stormwater runoff from all regulated activity sites with a drainage
area of greater than five acres shall be calculated using a generally
accepted calculation technique(s) that is based on the NRCS Soil Cover
Complex Method. Table 94-309.1 summarizes acceptable computation methods.
The method selected for use shall be based on the individual limitations
and suitability of each method for a particular site. The use of the
Rational Method to estimate peak discharges for drainage areas greater
than five acres shall be permitted only upon approval by the Township
Engineer.
|
Table 94-309.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for SWM Site Plan
| |||
|
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 where limitations described
in TR-55 are met
| |
|
HEC-1/HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township
| |
B.
All calculations using the Soil Cover Complex Method shall use the
appropriate design rainfall depths for the various return period storms
consistent with this chapter. Rainfall depths used shall be obtained
from NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or infiltration, water quality and runoff volume functions, the duration
of rainfall shall be 24 hours.
C.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
(1)
For regulated activities involving new development, the following
ground cover assumptions shall be used:
(2)
For regulated activities involving redevelopment, the following ground
cover assumptions shall be used:
(a)
For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(b)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(c)
For areas that are impervious surfaces, predevelopment calculations
shall assume at least 20% of the existing impervious surface area
to be disturbed as "meadow" ground cover.
(3)
The applicant shall determine which stormwater standards apply to
the proposed regulated activity as follows:
(a)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(b)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(c)
At the discretion of the Township Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this chapter, may either:
[1]
Apply the stormwater standards (redevelopment or new development)
that are associated with the activity that involves the greatest amount
of land area; or
[2]
Apply the redevelopment and new development stormwater standards
to the corresponding redevelopment and new development portions of
the proposed regulated activity.
D.
Runoff curve numbers (CN) for both predevelopment and proposed (postconstruction) conditions to be used in the Soil Cover Complex Method shall be obtained from Table B-1 in Appendix B of this chapter.[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
E.
Runoff coefficients (C) for both predevelopment and proposed (postconstruction) conditions for use in the Rational Method shall be obtained from Table B-2 in Appendix B of this chapter.[2]
[2]
Editor's Note: Appendix B is included as an attachment to this chapter.
F.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
G.
Hydraulic computations to determine the capacity of pipes, culverts, and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series No. 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular No. 15 (Publication No. FHWA-NH1-05-114 HEC 15, as amended). Values for Manning's roughness coefficient (n) shall be consistent with Table B-3 in Appendix B of the chapter.[3]
[3]
Editor's Note: Appendix B is included as an attachment to this chapter.
A.
Any BMP intended to hold standing water for four days or longer shall be designed to incorporate biologic controls consistent with the West Nile guidance found in Appendix C,[1] PADEP document 363-0300-001, "Design Criteria - Wetlands
Replacement/Monitoring" (as amended) (or contact the Pennsylvania
State Cooperative Wetland Center or the Penn State Cooperative Extension
Office for design information).
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
B.
Any stormwater basin required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to safely convey flow up to and including the one-hundred-year proposed conditions. The height of embankment shall provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year proposed conditions inflow. Should any BMP require a dam safety permit under PA Chapter 105 regulations, the facility shall be designed in accordance with and meet the regulations of PA Chapter 105 concerning dam safety.[2] PA Chapter 105 may require the safe conveyance of storms
larger than the one-hundred-year event.
[2]
Editor's Note: See 25 Pa. Code Chapter 105.
C.
Any drainage conveyance facility and/or channel not governed by PA
Chapter 105 regulations shall be designed to convey, without damage
to the drainage facility or roadway, runoff from the twenty-five-year
storm event. Larger storm events (fifty- and one-hundred-year storms)
shall also be safely conveyed in the direction of natural flow without
creating additional damage to any drainage facilities, nearby structures,
or roadways.
D.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
E.
Roadway crossings or structures located within designated floodplain
areas shall be able to convey runoff from a one-hundred-year design
storm consistent with Federal Emergency Management Agency National
Flood Insurance Program - Floodplain Management Requirements.
F.
Any facility located within a PennDOT right-of-way shall comply with
PennDOT minimum design standards and permit submission and approval
requirements.
G.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular No. 11 (Publication No. FHWA-IP-89-016, as amended)
and the PADEP Erosion and Sediment Pollution Control Program Manual
(Publication No. 363-2134-008, as amended), or other design guidance
acceptable to the Township Engineer.
A.
The Township has obtained a Phase II individual national pollutant
discharge elimination system permit for operation of small municipal
separate storm sewer systems (MS4) from the Pennsylvania Department
of Environmental Protection (PADEP). In the event of design criteria
conflict between this chapter and the PADEP ordinance, the more stringent
requirement shall apply, and in the event of a dispute in this regard,
the Township Engineer's determination shall be final.
(1)
While some minor modification of internal drainage areas will be
considered in the development process, the relocation of substantial
areas from one internal drainage area to another will not be acceptable.
(2)
The stormwater management calculations for postdeveloped conditions
(runoff curve number concentration times, etc.) shall assume the following:
(a)
Residential subdivision by right (conventional):
[1]
The maximum impervious coverage allowed by zoning will be constructed
on each individual lot if the lot is less than one acre in area. If
the lot is greater than or equal to one acre in area, the lot development
shall be assumed to be the maximum permitted by zoning for a one-acre
lot.
[2]
The maximum amount of woodland removal allowed by zoning will
be from each lot and converted to lawn or impervious surfaces as proposed.
[3]
The woodland underbrush and forest litter are removed from all
woodland areas not permitted to be removed on each lot.
(b)
Residential subdivision by conditional use (open space development
option):
[1]
The maximum impervious coverage allowed by zoning will be constructed
on the entire original tract(s).
[2]
The maximum amount of woodland removal permitted by zoning will
be removed from the original tract(s) and converted to lawn or impervious
surfaces as proposed.
[3]
The woodland underbrush and forest litter will be removed from
all woodland areas not permitted to be removed that are also located
on individual postdeveloped lots.
(c)
Commercial, industrial, institutional and other types of nonresidential
development:
(3)
Basins.
(a)
Basins shall be installed prior to any earthmoving or land disturbance
which they will serve. The phasing of their construction shall be
noted in the narrative and on the plan. Allowance must be made in
designs for sediment storage and sediment removal during and after
construction. Stormwater management facilities must be returned to
a one-hundred-percent capacity when construction activities are complete.
(b)
Detention basins shall be designed to utilize the natural contours
of the land. When such design is not feasible, the construction of
the basin shall utilize slopes as shallow as possible to blend the
structures into the existing topography. The use of multiple-detention
facilities, which are small and less intrusive on the site, shall
be used to meet the requirements of this subsection.
(c)
Stormwater detention basins shall be located a minimum 50 feet
from any structure, whether existing or proposed, or any property
boundary or ultimate right-of-way line and a minimum 50 feet from
existing wetlands and the banks of existing streams.
(d)
Interior slopes in basins shall have a maximum grade of one
to three, while exterior grades shall be a maximum grade of one to
four wherever possible.
(e)
The top of a basin berm shall be no less than 10 feet in width.
(f)
A clay core keyed into the situ soil shall be required.
(g)
An access ramp with a slope no greater than 20% and an access
easement 20 feet in width shall be provided to each basin to facilitate
cleanout.
(4)
Spillways.
(a)
Basins shall be designed with an emergency spillway to allow
passage of the predevelopment one-hundred-year peak runoff rate flow
over the berm with a minimum one-foot freeboard. The spillway shall
be designed according to soil conservation methods, sized assuming
that outlet structures are plugged.
(b)
The direct discharge of concentrated stormwater flow from a
basin across adjacent properties shall not be permitted, except when
stormwater has entered a stream or drainage structure.
(c)
Outfall pipes from basins shall have a minimum diameter of 18
inches and shall be fitted with trash racks.
(d)
Outlet facilities, including all piping and riser structures,
shall be constructed of reinforced concrete or shall be standard concrete
structures meeting Pennsylvania Department of Transportation standards.
(5)
Conveyance of stormwater.
(a)
Except where slopes exceed 6%, properly designed, graded and
turfed drainage swales shall be required rather than storm sewers
in residential areas and in commercial and industrial areas, except
as otherwise specified by the Township Engineer. Where swales are
used to convey stormwater flows, they shall be broad and flat in cross
section. Particular care shall be given to points of discharge from
gutter lines at the end of a cut road construction where water will
discharge over banks with the possibility of creating erosion channels.
(b)
All natural streams, channels, swales or drainage systems existing
prior to development shall be maintained in their existing conditions.
Alterations to the existing natural conditions of the site shall be
kept to a minimum; where necessary, storm sewers, culverts, bridges
and related installations shall be provided to:
(c)
Culverts under roadways shall be designed for a fifty-year return
storm and shall have a natural stream bottom, except when under a
single-access road. For a single-access street, a one-hundred-year
return storm shall be used. A culvert for a driveway shall be designed
for a ten-year storm. The ten-year return storm shall be used to design
inlets and pipes in nonsump conditions. For sump conditions and thereafter,
the one-hundred-year return storm shall be used. Culverts shall be
provided with wing walls and constructed for the full width of the
right-of-way. Bridges shall be provided with a paved flow line and
deep aprons and wing walls at each end. The cartway area over the
bridge shall be as wide as the widest road connecting with the bridge,
or if 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 either side of the cartway, the bridge railing shall be at least
five feet back from the edge of the final cartway width, and this
five feet may be used to place sidewalks, present or future, if sidewalks
are to be run up to and connected with the bridge on either end or
either side. Encroachment permits shall be obtained from the Pennsylvania
Department of Environmental Protection for stream crossing under the
jurisdiction of that Department before construction begins.
(d)
All data and information used shall be compatible with the Pennsylvania
Department of Transportation Design Manual, Part 2, Highway Design,
October 1976, or the latest revision thereof.
(e)
For preliminary design of the inlets and pipes, Manning's Method
may be implemented. The final design shall require an inlet/outlet
hydraulic analysis be performed with a maximum headwater height of
one foot below the top of grate elevation.
(f)
All stormwater inlet casings, manhole covers and frames, piping
and other appurtenances shall conform to the Pennsylvania Department
of Transportation Specifications, Publication 408, dated 1991, or
latest revision thereof. All stormwater piping shall be reinforced
concrete; the class of piping shall be determined by Pennsylvania
Department of Transportation design standards.
(g)
Inlet spacing, not to exceed 300 feet, shall be so arranged
that at least 95% of the flow will be captured. In addition, the spacing
from any high point to an inlet shall not exceed 400 feet.
(h)
When designing the capture rate of inlets, a plugging factor
of 0.05 shall be assumed.
(i)
Submission plans shall include a detail of grates (with associated
design curves) and trench details for open conveyance systems.
(j)
Storm sewers and culverts shall be constructed of reinforced
concrete and have a minimum fifteen-inch diameter and minimum grade
of 1/2%.
(k)
Changes in alignment shall be made with straight sections connected
by inlets and/or manholes.
(l)
Manholes and/or inlets shall not be more than 300 feet apart
for pipes less than or equal to 24 inches in diameter nor greater
than 450 feet apart for pipes larger in diameter.
(m)
Stormwater management and/or drainage designs shall not rely
on the ability of roof gutter collection systems to convey runoff
to stormwater management facilities. Overland flow or underground
piping systems shall be designed downslope of the proposed building
to convey roof gutter overflows to the stormwater facility. All drainage
piping systems, drainage swales, channels, conveyance systems, etc.,
shall be designed for the one-hundred-year-return-frequency design
storm and shall meet other minimum requirements of the Township Code.
A stormwater easement a minimum of 10 feet in width from each side
of the drainage facility and an access strip 20 feet in width from
the public right-of-way to the facility shall be provided for any
such systems, described by metes and bounds. Such easements shall
be deducted from gross lot areas as required by the Township Code.
