A.
Applicants proposing regulated activities in the municipality which are not exempt under § 101-106 shall submit a stormwater management site plan (SWM site plan) to the municipality for review and approval in accordance with Articles III and IV. SWM site plans approved by the municipality 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 municipality shall commence until:
(1)
The municipality issues 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 municipality
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 a 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 § 101-304 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, Post-construction 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 and the volume of runoff up to and including the two-year storm
onto or through adjacent property(ies) or downgradient property(ies),
including diffuse drainage discharge, shall not be altered in any
manner by regulated activities under this chapter without written
permission from, and, where applicable as determined by the municipality,
an easement and agreement with, the affected landowner(s) for conveyance
of discharges onto or through their property(ies). Altered stormwater
discharges 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 §§ 101-305, 101-306, 101-307, and 101-308). 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-year, five-year 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 Municipal 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 municipality, 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 municipality. The municipality
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.
Hotspots runoff controls. Specific structural or pollution prevention
practices may be required, as determined to be necessary by the Municipal
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; and
(14)
Other land uses and activities as designated by the municipality.
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 Municipal 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 municipality 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.).
(5)
In the portion of the Township located in the Chester Creek
Watershed, if a perennial or intermittent stream passes through the
site, the applicant shall create a riparian buffer extending a minimum
of 30 feet to either side of the top of the bank of the channel. The
buffer area shall be maintained with appropriate native vegetation.
If the applicable side or rear yard setback is less than 30 feet,
the buffer width may be reduced to 25% of the setback, to a minimum
width of 10 feet. If an existing buffer is legally prescribed (e.g.,
deed covenant, easement, etc.) and it exceeds the requirement of this
section, the existing buffer shall be maintained.
Q.
Applicants shall utilize the Pennsylvania Storm Water Best Management
Practices Manual (PA BMP Manual), as amended, or other sources acceptable
to the Municipal 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 (as amended) or other design guidance acceptable to the
Municipal Engineer.
S.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with Article XIII of Chapter 122, Zoning, 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.
The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the municipality of the SWM site plan
and prior to commencement of any regulated activities, as applicable:
A.
All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at Title 25 Pennsylvania Code Chapter 102,
or erosion and sediment control requirements of the municipality.
B.
Work within natural drainage ways subject to permit by PADEP under Title 25 Pennsylvania 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 Title 25 Pennsylvania 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.
A.
No regulated activity within the municipality shall commence until:
(1)
The municipality 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 for projects where the area of disturbance exceeds one acre, where pond dredging is involved, or when the disturbance is associated with activities described under Title 25 Chapter 105 of the Pennsylvania Code permits;
(b)
A PADEP NPDES Permit for Stormwater Discharges Associated with
Construction Activities as required under Title 25 Pennsylvania Code
Chapter 92.a, if applicable; and
(c)
Evidence of any other permit(s) or approvals required for the
regulated activities.
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 § 101-402C) 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 § 101-402B(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 § 101-304B 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 § 101-304B 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 § 101-304B and C. Appendix B presents additional discussion of conservation design and low-impact development.[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
To control post-construction stormwater impacts from regulated
activities and meet state water quality requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that post-construction stormwater
discharges do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The green infrastructure and low-impact development
(LID) practices provided in the PA BMP Manual, as well as the guidance
on green infrastructure, LID and conservation design (CD) provided
in Appendix B, shall be utilized for all regulated activities wherever
possible. The applicant shall comply with the following water quality
and runoff volume requirements for all regulated activities, including
all new development and redevelopment activities:
A.
The post-construction total runoff volume shall not exceed the predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour duration precipitation (design storm). The water
quality and runoff volume to be managed shall consist of any runoff
volume generated by the proposed regulated activity over and above
the predevelopment total runoff volume and shall be captured and permanently
retained or infiltrated on the site. Permanent retention options may
include, but are not limited to, reuse, evaporation, transpiration,
and infiltration.
B.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 101-309D of this chapter, as appropriate.
C.
The design of the stormwater management facility outlet shall provide
for protection from clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 101-306. The infiltration volume required under § 101-306 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 municipality may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 101-301P.
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.
K.
Water quality and volume control practices shall be selected and designed to meet the criteria of § 101-304C that apply to water quality and volume control.
L.
Evapotranspiration may be quantified and credited towards meeting
volume requirements according to the PADEP Post-Construction Stormwater
Management (PCSM) Spreadsheet and Instructions (December 2020) or
the most recent guidance from PADEP.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A.
For regulated activities involving both new development and redevelopment, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 101-305. Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual or other PA DEP design guidance. If the runoff volume required by § 101-305 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PA DEP guidance, such as the managed release concept, may be used to manage this volume with approval from the Municipal Engineer.
B.
For regulated activities involving both new development and redevelopment,
the volume of a minimum of one inch of runoff from all regulated impervious
surfaces shall be infiltrated.
C.
If the requirements of § 101-306A or B cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Municipal Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however in all cases at least the first 1/2 inch of runoff from all proposed impervious surfaces shall be infiltrated.
E.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
F.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
G.
Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
H.
A detailed soils evaluation of the site shall be conducted by a qualified
professional and at a minimum shall address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be conducted by a qualified licensed professional
and shall be consistent with the PA BMP Manual (as amended) (or other
guidance acceptable to the Municipal Engineer) and in general shall:
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(2)
Provide field tests such as double ring infiltrometer or other
hydraulic conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention
(infiltration) volume based on field-determined infiltration capacity
(and apply safety factor as per applicable design guidelines) at the
elevation of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall
be demonstrated to the Municipal Engineer that the soils are conducive
to infiltration on the identified lots.
I.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(1)
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone. Additional depth may be required in areas underlain by karst or carbonate geology (see § 101-306M).
(2)
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3)
The infiltration facility shall completely drain the retention
(infiltration) volume within three days (72 hours) from the end of
the design storm.
J.
All infiltration practices shall:
(1)
Be selected and designed to meet the criteria of § 101-304C that are applicable to infiltration;
(2)
Be set back at least 10 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.),
and 15 feet from property or right-of-way lines, unless otherwise
approved by the Municipal Engineer;
(3)
For any infiltration practice that collects runoff from shared
or multiple features and that is located within 20 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element.
K.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1)
When a 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 requested by the Township
Engineer) 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 well head protection area of a public
water supply well, infiltration practices shall be in conformance
with the applicable approved source water protection assessment or
source water protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
L.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Municipal
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
M.
Consideration of infiltration BMPs for areas underlain by karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation are provided to the municipality demonstrating that
the potential or likelihood of subsidence or sinkholes is minimal.
Evaluation of site conditions and infiltration design shall rely on
guidance in the PA BMP Manual (as amended) or other guidance acceptable
to the Municipal Engineer.
N.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hotspots
(i.e., sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged through a water quality BMP(s) to remove pollutants
prior to infiltration.
O.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
P.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
Q.
All infiltration practices shall have appropriate positive overflow
controls.
R.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
S.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
(4)
The tops and sides of all seepage beds, trenches, or like facilities
shall be covered with drainage fabric. Where seepage beds, trenches,
or like facilities are located where they will be subject to vehicular
traffic, the Township Engineer may require the bottom of the facility
to be covered with drainage fabric as well. Fabric shall be nonwoven
fabric acceptable to the Municipal Engineer.
(5)
Stormwater shall be distributed throughout the entire seepage
bed/trench or like facility and provisions for the collection of debris
shall be provided in all facilities.
For regulated activities involving new development with one
or more acres of earth disturbance, the applicant shall comply with
the following stream channel protection requirements to minimize stream
channel erosion and associated water quality impacts to the receiving
waters:
A.
The peak flow rate of the post-construction two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour duration precipitation, using the
SCS Type II distribution.
B.
To the maximum extent practicable, and unless otherwise approved
by the Municipal Engineer, the post-construction one-year, twenty-four-hour
storm flow shall be detained for a minimum of 24 hours and a maximum
not to exceed 72 hours from a point in time when the maximum volume
of water from the one-year, twenty-four-hour storm is stored in a
proposed BMP (i.e., when the maximum water surface elevation is achieved
in the facility). Release of water can begin at the start of the storm
(i.e., the invert of the orifice is at the invert of the proposed
BMP).
C.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 101-309D of this chapter, as appropriate.
D.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter unless otherwise approved by the Municipal
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the BMP that do
not provide enough runoff volume to allow a twenty-four hour attenuation
with the three-inch orifice, the calculations shall be submitted showing
this condition.
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates subject
to the Municipal Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F.
All proposed stormwater management facilities shall make use of measures
to extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities including those
that involve new development and redevelopment that are not located
in the Conestoga River, Chester Creek, Darby Creek, (East) Valley
Creek, or Crum Creek Watershed(s).
A.
Post-construction peak flow rates from any regulated activity not
located in the Chester Creek Watershed shall not exceed the predevelopment
peak flow rates as shown for each of the design storms specified in
Table 101-308.1. For projects located in the Chester Creek Watershed,
peak rate control shall be provided in accordance with Table 101-308.1.
|
Table 101-308.1
| ||
|---|---|---|
|
Peak Rate Control Standards for Projects Not Located in the
Chester Creek Watershed
| ||
|
(Peak flow rate of the post-construction design storm shall
be reduced to the peak flow rate of the corresponding predevelopment
design storm shown in the table)
| ||
|
Post-Construction Design Storm Frequency
(24-Hour Duration)
|
Predevelopment Design Storm
| |
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| |
|
2-Year
|
1-Year
|
2-Year
|
|
5-Year
|
2-Year
|
5-Year
|
|
10-Year
|
2-Year
|
10-Year
|
|
25-Year
|
25-Year
|
25-Year
|
|
50-Year
|
50-Year
|
50-Year
|
|
100-Year
|
100-Year
|
100-Year
|
B.
For projects not located in the Chester Creek Watershed, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 101-309D of this chapter. For projects that are located in the Chester Creek Watershed, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 101-309E of this chapter.
C.
For regulated activities, outside the Chester Creek Watershed, involving only redevelopment, no peak flow rate controls are required when and only if the total regulated impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 101-308.1, using the redevelopment ground cover assumptions presented in § 101-309D.
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.
H.
Regulated activities located within the Chester Creek Watershed shall
achieve the applicable peak flow release rate control requirements
presented in the approved PA Act 167 Plan for that watershed and as
presented in Table 101-308.2 below, and as presented in the Chester
Creek Watershed Release Rate Map in Appendix F of this chapter.[1]
|
Table 101-308.2
| |
|---|---|
|
District
|
Release Rate
|
|
100%
|
Post-development peak discharge for all design storms must be
no greater than the predeveloped peak discharge
|
|
75%
|
Post-development peak discharge for all design storms must be
no greater than 75% of the predeveloped peak discharge
|
|
50%
|
Post-development peak discharge for all design storms must be
no greater than 50% the predeveloped peak discharge
|
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
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 101-309.1 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 Municipal Engineer.
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Table 101-309.1
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|---|---|---|
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Acceptable Computation Methodologies for SWM site Plan
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|
Method
|
Developed By
|
Applicability
|
|
TR-20
(or commercial computer package based on TR-20)
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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
|
US Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites up to five acres, or as approved by the municipality
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
|
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 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 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.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration (duration) and
storm events with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates, or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times-of-concentration shall
be calculated based on the methodology recommended in the respective
model used. Times of concentration for channel and pipe flow shall
be computed using Manning's equation.
D.
Outside of the Chester Creek watershed, 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 Municipal 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.
E.
For projects in the Chester Creek Watershed, all undeveloped land shall be considered to be meadow good condition, Type B soils (CN=58, C=0.12), unless the natural ground cover generates a lower CN or C value (i.e., forest). If a proposed development meets the definition of redevelopment as defined in Article II of this chapter, the applicant may adjust the predevelopment CN or C value based on the curves provided in Appendix C.[1] Runoff characteristics for off site areas draining through
the project site shall be based on actual existing conditions, and
shall be assumed to not have any controls implemented on future development
(i.e., no release rate restrictions).
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
F.
Runoff curve numbers (CN) for both predevelopment and proposed (post-construction)
conditions to be used in the Soil Cover Complex Method shall be obtained
from Table C-1 in Appendix C of this chapter.
G.
Runoff coefficients (C) for both predevelopment and proposed (post-construction)
conditions for use in the Rational Method shall be obtained from Table
C-2 in Appendix C of this chapter.
H.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
I.
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
Number 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 Number
15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for
Manning's roughness coefficient (n) shall be consistent with
Table 3-3 in Appendix C of 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 D, 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]
[1]
Editor's Note: Appendix D 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 of one 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. Pa. Chapter 105 may require the safe conveyance of storms larger than one-hundred-year event.
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-year 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 one-hundred-year
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 stormwater management 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 Number 11 (Publication No. FHWA-IP-89-016, as
amended) and the PADEP Erosion and Sediment Pollution Control Program
Manual (Publication No. 363-2134-008, as amended), or other design
guidance acceptable to the Municipal Engineer.
A.
Design standards for stormwater management facilities.
(1)
Construction of detention or retention basins.
(a)
Basins shall be installed prior to any earthmoving or land disturbances
which they will serve. The phasing of their construction shall be
noted in the narrative and on the SWM site plan.
(b)
Basins shall be located only on soils acceptable for such use
as designated in Appendix G, Soil Use Guide.[1]
[1]
Editor's Note: Appendix G is included as an attachment to this chapter.
(c)
Soils used for the construction of basins should be stable soils.
(d)
Detention basins designed to hold the excess volume of runoff from a two-year storm should be located on soils with a high percolation rate, in order to encourage rapid recharge and reduce potential for stagnant water conditions. Where soils with high percolation rates are found at higher elevations of a site, such detention facilities may require incorporation into building lot layout. Drainage easements and deed restrictions against the alteration of such detention areas shall be provided in compliance with Article VII and Appendix E of this chapter.
(e)
Energy dissipators and/or level spreaders shall be installed
at points where pipes or drainageways discharge to or from basins.
Energy dissipators may be used where pipes discharge to a stable channel
of conveyance, such as a stream or lined swale. Level spreaders must
be used whenever a stable channel of conveyance is not available to
receive pipe discharge.
(f)
The following slope restrictions shall be applied to basins:
[1]
Exterior slopes of compacted soil shall not exceed one foot
vertical for 2.5 feet horizontal and may be further reduced if the
soil has unstable characteristics as noted in the Soil Use Guide.
[2]
Interior slopes of the basin shall not exceed one foot vertical
in three feet horizontal, except:
[a]
Where maximum water depth will not exceed three
feet.
[b]
When a two-inch rainfall in one hour will not fill
the basin in one hour.
[c]
Where interior slopes are proposed to be steeper
than one to three, the basin shall be surrounded by a permanent fence
48 inches in height, with individual link spacing not more than four
inches in diameter.
(g)
Outlet structures within basins which will control peak discharge
flows and distribute the flows by pipes to discharge areas shall be
constructed of concrete, and shall have childproof, nonclogging trash
racks over all openings exceeding 12 inches in diameter, except those
openings designed to carry perennial stream flows. Where spillways
will be used to control peak discharges, the control weirs shall be
constructed of concrete of sufficient mass and structural stability
to withstand the pressures of impounded waters and outlet velocities.
Concrete outlet aprons shall be designed as level spreaders and shall
extend at a minimum to the toe of the basin slope. The incorporation
of large stones found on the site into the concrete apron to provide
a more natural appearance is suggested. Construction shall comply
with Pennsylvania Department of Transportation Form 408 specifications.
(h)
Inlet and outlet structures shall be located at maximum distances
from one another. The Municipal Engineer may require a rock filter
berm or rock-filled gabions between inlet and outlet areas when the
distance is deemed insufficient for sediment trapping. All inlets
shall either be located above the bottom of the basin or shall discharge
to areas of the basin which slope downward to lower elevations of
the basin.
(i)
Emergency spillways shall be provided and shall be designed
to provide maximum protection against erosion due to overtopping.
The Municipal Engineer may require the use of concrete lattice blocks,
stone riprap, turf reinforcement matting, or concrete spillways when
slopes will exceed one to four or spillway velocities might exceed
Natural Resources Conservation Service standards for the particular
soils involved.
(j)
Temporary and permanent grasses or stabilization measures shall
be established on the sides of all earthen basins within 15 days of
construction.
(2)
Building site excavation and surface runoff.
(a)
If temporary or permanent diversion channels or berms have not
been established during general site preparation diversion channels
or berms may be required whenever slopes exceed 10% above or below
proposed excavation areas. Installation shall occur prior to excavations
or other earthmoving on the uphill or downhill sides of the building
location and any other areas to be disturbed. This requirement may
be waived if it would result in the destruction of trees and shrubs.
In all cases, hay bales or silt fence shall be installed and maintained
downhill of all excavations until the diversion channels or berms
required by the Municipal Engineer have been stabilized.
(b)
All exposed earth shall be stabilized with appropriate grasses
or other materials no later than 15 days after disturbance.
(c)
A single access route to a development site for contractors'
vehicles should be established. During construction, vehicles should
be parked on the street or road, except when the vehicle is specifically
required in construction or for delivery of materials, when distances
to the construction area would be unreasonable or where roadway parking
would constitute a traffic hazard.
(3)
Storm sewers, drainage swales and perennial streams.
(a)
General design.
[1]
Storm sewer systems for conveyance of stormwater shall be designed
to accommodate a ten-, twenty-five, or fifty-year storm, depending
on the grade of the path of any overflows from inlets (0-8%, 8-15%,
15-25%, respectively), and the judgment of the Municipal Engineer.
All perennial stream crossings will require one-hundred-year design
for culverts and/or bridges.
[2]
Open ditches per se shall be avoided, but properly designed,
graded and turfed drainage swales shall be permitted in lieu of storm
sewers in residential areas and, where approved by the Township Engineer,
in commercial and industrial areas.
[3]
All storm sewers or drainage swales shall discharge to a detention
or retention basin for the control of peak runoff flows, except as
otherwise provided in the plan.
(b)
Specific design criteria for storm sewers.
[1]
Storm sewers, as required, shall be placed under or immediately
in front of the curb, when parallel to a public street and within
the right-of-way. When located in undedicated land, they shall be
placed within an easement not less than 20 feet wide as approved by
the Municipal Engineer.
[2]
Storm sewers shall be constructed of reinforced concrete pipe
or smooth lined corrugated polyethylene pipe, with gasketed joints.
No mortar joints will be allowed.
[3]
Storm sewers shall have a minimum diameter of 15 inches. All
pipe materials shall meet or exceed Pennsylvania Department of Transportation
Publication 408 standards and shall have been approved for use by
the Pennsylvania Department of Transportation Materials Testing Division.
[4]
Minimum grade shall be 0.5%.
[5]
Changes in alignment shall be by straight sections connected
by inlets or manholes.
[6]
Rainfall intensity curves and other hydrologic design data,
provided by the Pennsylvania Department of Transportation and/or the
shall be used for design purposes.
[7]
Manholes shall be not more than 300 feet apart for diameters
up to 24 inches, and not more than 450 feet apart for greater diameters.
Inlets may be substituted for manholes on approval by the Municipal
Engineer.
[8]
Inlets, manholes, covers and frames shall conform to Pennsylvania
Department of Transportation specifications. All manhole lids and
inlet grates shall have the phrase "No dumping, drains to streams"
cast into them.
[9]
At street intersections, inlets shall be placed in the tangent
and not in the curved portion of the curbing.
[10]
The Township shall be granted a twenty-five-foot-wide
drainage easement for the distance from the right-of-way of any dedicated
road to all detention and retention basins, to provide unobstructed
access to Township personnel for purposes of maintenance and repair.
(c)
Specific design criteria for drainage swales and perennial streams.
[1]
Where drainage swales are used in lieu of or in addition to
storm sewers, they shall be designed to carry the required discharge
without excessive erosion, and also to increase the time of concentration,
reduce the peak discharge and velocity and permit the water to percolate
into the soil.
[2]
Deed restrictions shall be required on property(ies) containing
drainage swales and/or perennial streams. These deed restrictions
shall specify that no property owner may obstruct or alter any drainage
swale or perennial stream identified in the stormwater management
plan if such action would alter the rational course of runoff and/or
negatively impact abutting landowners.
