A.
Applicants proposing regulated activities in the municipality which are not exempt under § 152-106 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 a 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 § 152-304 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.
For regulated activities with one acre or more of proposed earth
disturbance, existing drainage peak rate discharges up to and including
the 100-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 §§ 152-305, 152-306, 152-307, and 152-308). 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
municipality. 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, 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 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 municipality,
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 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 municipality'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.).
Q.
Applicants shall utilize the Pennsylvania Stormwater Best Management
Practices Manual (PA BMP Manual), as amended, or other sources acceptable
to the municipality, 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
municipality.
S.
All regulated activities located within a Special Flood Hazard Area
designated by the Federal Emergency Management Agency (FEMA) shall
comply with Zoning Ordinance, Article XIII, Flood Hazard District,
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 75 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.).
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.
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 Title 25 Pennsylvania
Code Chapter 105.
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, 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
municipality, 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 § 152-402C) 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 § 152-402B(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 § 152-304B 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 § 152-304B 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
lands caped-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 § 152-304B and C. Appendix B[1] presents additional discussion of "conservation design"
and "low-impact development."
[1]
Editor's Note: The appendix 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 § 152-309D of this chapter.
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 § 152-306. The infiltration volume required under § 152-306 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 § 152-301P.
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 § 152-304C 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 § 152-305. 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 § 152-305 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 § 152-306A or B cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the municipality 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.
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 municipality) 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 municipality 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 § 152-306M).
(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 § 152-304C that are applicable to infiltration;
(2)
Be set back at least 15 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.);
(3)
Setbacks from property lines and rights-of-way are to be consistent
with accessory structures in the Zoning Ordinance, for the applicable
zoning district;
(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.
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 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 "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 municipality
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 municipality.
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, 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 municipality.
(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 municipality, 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 § 152-309D 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 municipality,
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 municipality'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.
A.
Post-construction 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 308.1.
|
Table 308.1 Peak Rate Control Standards
(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
|
1-Year
|
|
5-Year
|
2-Year
|
2-Year
|
|
10-Year
|
2-Year
|
2-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 § 152-309D of this chapter.
C.
For regulated activities 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 308.1, using the redevelopment ground cover assumptions presented in § 152-309D. This design criterion for redevelopment is only permitted with approval of Municipal Engineer. It shall result in no impact on downstream properties.
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 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 municipality.
|
Table 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
|
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.
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 municipality, 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.
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[1] of this chapter.
[1]
Editor's Note: The appendix is included as an attachment to this chapter.
F.
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.
G.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
H.
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 B-3 in Appendix B[2] of this chapter.
[2]
Editor's Note: The appendix 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 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.
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 100-year proposed conditions. The height of embankment
shall provide a minimum one foot of freeboard above the maximum pool
elevation computed when the facility functions for the 100-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
100-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 100-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 100-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.
Storm drainage requirements and facility design standards.
A.
Stormwater management system required. A stormwater management system
designed to achieve the objectives of this section, including a storm
sewer system and all appurtenances where necessary, shall be required
to be constructed by the owner or subdivider consistent with the provisions
of this section and shall, in particular, be designed to avoid any
situation where surface or subsurface drainage could impair public
safety or cause physical damage to adjacent lands or public property.
The system, after proper management in accordance with this chapter,
shall discharge to the nearest practical subsurface location in order
to recharge groundwater, to a natural drainage channel, and/or to
an existing stormwater management system. Applicant shall show evidence
of consideration of all discharge options, including documentation
as to why the chosen discharge location(s) was/were selected.
B.
Off-site improvements. Any increase in runoff off-site, measured
both in total volume and peak rate between pre- and post-development
conditions shall require necessary corrective measures deemed appropriate
by the Board of Supervisors to alleviate any off-site drainage problem
affected by the subdivision or land development. The costs for such
measures will be borne entirely by the developer or subdivider, including
deed of easement, wherever applicable.
C.
Total watershed evaluation. The future use of undeveloped lands upstream
of a subject property, as are shown on the Future Land Use Plan of
the adopted Township Comprehensive Plan, shall be taken into account
in the design of all stormwater management systems. The capacity and
maximum anticipated present flow of the system receiving the proposed
discharge shall be evaluated to determine whether it is capable of
receiving additional flow generated by proposed and potential future
development. This evaluation shall also include the anticipated runoff
from other existing or proposed developments within the same watershed.
Upon the recommendation of the Township Engineer, stormwater management
facilities, designed in accordance with this chapter, may serve more
than one property or development in the same watershed. Consultation
with the Township is required prior to the design of such facilities.
D.
Underdrains. All building foundations, grade slabs, and cellar floors
located in soils that have a community development limitation degree
of moderate to severe seasonal high water table (as defined in the
Chester County Soil Survey) shall be provided with an underdrain system.
This system shall provide for drainage of the enclosed volume above
the slab, and relief of subsurface water to a depth of not less than
18 inches below the slab or foundation bottom. The system shall consist
of a pipe field of the herringbone or gridiron configuration in course,
gravel-filled trenches that are in direct contact with the slab or
foundation subbase. The excavation shall provide a minimum of 0.05
foot/foot slope to the gravel-filled trenches. The underdrain system(s)
shall discharge to location(s) sufficiently downgradient of the building
being drained and other nearby buildings in order so as not to exacerbate
high water table problems.
E.
Contribution to comprehensive drainage plan. In addition to any other
requirement of this chapter, the developer may be required to participate
in off-site storm drainage improvements in the drainage basin within
which the proposed development is located. The specific off-site drainage
improvements required shall be those specified by the Township Board
of Supervisors.
F.
Storm drain pipes and inlets. Except where alternative design approaches
are recommended by the Township Engineer, a storm drain system shall
be designed to collect water at any point where three to five cubic
feet per second accumulates, at the bottom of all vertical grades,
and immediately upgrade from all street intersections.
(1)
Design flow rate. Storm drain system(s), where provided, shall
be designed to carry a twenty-five-year peak flow rate, and a fifty-year
peak flow rate at the sump area. The design twenty-five-year peak
flow rate into each inlet shall be indicated on the stormwater drainage
plan.
(2)
Overflow system. An overflow system shall be provided to carry
storm flow when the capacity of the storm drainpipe system is exceeded.
The overflow system shall be of sufficient capacity to carry the difference
between the 100-year and the ten-year peak flow rates.
(3)
Inlet capacity.
(a)
All inlets must be designed to accommodate the twenty-five-year
peak flow rate. The capacity of all CM or S type inlets shall be determined
from the following source:
Commonwealth of Pennsylvania
Department of Transportation
Design Manual, Part 2
Highway Design
(b)
The capacity of each inlet shall be indicated on the stormwater
drainage plan.
(c)
All inlet boxes shall be prefabricated concrete. The pipe locations
shall be precut.
(d)
All inlet grates shall be bicycle-safe grates.
(e)
All inlet grates shall be flush with the top of the wearing
course.
(f)
All inlet risers shall be prefabricated concrete.
G.
Pipe.
(1)
All proposed piping material shall be either reinforced concrete
or high density polyethylene. (HDPE)
(2)
Straight pipe sections. Wherever possible, all storm drain pipes
shall be designed to follow straight courses. No angular deflections
of storm sewer pipe sections in excess of 5° shall be permitted.
No vertical curves shall be permitted in the storm drain pipe system.
(3)
Minimum grade and size. All storm drain pipes shall be designed
to maintain a minimum grade of 1/2%. All storm pipes shall have a
minimum inside diameter of 15 inches, except that pipes under 25 feet
or more of fill, where permitted, shall not be less than 24 inches
or a cross-sectional area of 453 square inches.
(4)
Pipe capacity. The capacity of all pipe culverts shall, as a
minimum, provide the required carrying capacity as determined by the
Manning's Equation and the following sources:
United States Department of Commerce
Bureau of Public Roads
Hydraulic Engineering Circular No. 5
Hydraulic Charts for the Selection of Highway Culverts
United States Department of Commerce
Bureau of Public Roads
Hydraulic Engineering Circular No. 10
Capacity Charts for the Hydraulic Design of Highway Culverts
(5)
Pipe arches. Where headroom is restricted, equivalent pipe arches
may be used in lieu of circular pipes.
(6)
Allowable headwater depth. At all inlets or manholes, the maximum
allowable headwater depth shall be one foot below the top of the inlet
grate of the manhole cover.
(7)
Horizontal pipe deflections. A manhole or inlet shall be provided
at all horizontal deflections in the storm pipe system exceeding 5°.
(8)
Minimum and maximum cover. A minimum of 18 inches of cover shall
be maintained over all storm drain pipes. The top of storm drain pipes
shall be at least 1/2 foot below subgrade elevation. The maximum cover
over storm drainpipes shall be 10 feet except where, due to specific
site conditions, approval of greater cover (fill) is recommended by
the Township Engineer.
(9)
Diversion or runoff. All storm drain pipes shall be designed
to carry stormwater runoff into detention, retention or infiltration
facilities utilized to control the rate and volume of stormwater runoff.
(10)
The top of all inlet grating shall be designed to be sumped
two inches from the road wearing surface.
H.
Detention and retention facilities:
(1)
Maximum depth of detention or retention basins. The maximum
depth of water in a detention or retention basin shall be two feet
as a result of a two-year, twenty-four-hour storm event and five feet
as a result of a 100-year, twenty-four-hour storm event.
(2)
An acceptable outfall shall be provided at the outlet of all
detention basins and shall not be permitted to discharge onto adjacent
properties.
(3)
Emergency spillway. All stormwater flow into any permanent detention,
retention or infiltration facility exceeding design capacity shall
be handled by an emergency spillway. Whenever possible, any emergency
spillway shall be constructed on undisturbed ground. Emergency spillways
for facilities which may attenuate more than three feet of water,
shall be paved with concrete monoslab pavers which shall run off the
sides and down each side of the berm. All emergency spillways shall
be constructed so that any earthen berms are protected against erosion.
The minimum capacity of all emergency spillways shall be such that
the combined capacity of the emergency spillway and the principal
outfall provide for the passage of the peak flow rate from the 100-year,
twenty-four-hour storm event with a flow depth across the spillway
not greater than one foot. The dimensions of the emergency spillways
can be determined from the Environmental Protection Handbook - Chester
County Conservation District. Emergency spillways shall extend along
the upstream and downstream berm embankment slopes. For facilities
which may attenuate more than three feet of water, the top of the
basin berm shall be set at an elevation a minimum of one foot above
the proposed flow depth through the spillway. The downstream slope
of the spillway shall, as a minimum, extend to the toe of any affected
embankment. The emergency spillway shall not discharge directly over
earthen fill and/or other easily erodible material.
(4)
Anti-seep collars. Anti-seep collars shall be installed around
the principal pipe barrel within the normal saturation zone of the
detention basin berms. The anti-seep collars and their connections
to the pipe barrel shall be watertight. The anti-seep collars shall
extend a minimum of two feet beyond the outside of the principal pipe
barrel. The maximum spacing between collars shall be 14 times the
minimum projection of the collar measured perpendicular to the pipe.
(5)
Slope of detention basin embankment. The maximum slope of earthen
detention basin embankments shall be three to one. The top or toe
of any slope shall be located a minimum of five feet from any property
line. Whenever possible, the side slopes and basin shape shall be
amendable to the natural topography. Straight side slopes and rectangular
basins shall be avoided whenever possible.
(6)
Width and construction of berm. The minimum top width of detention
basin berms which may attenuate greater than three feet of water,
shall be 10 feet. Berms shall be constructed of clay and shall have
a key cut into virgin soil.
(7)
Slope of basin bottom. In order to insure proper drainage of
the detention basin, except where infiltration is proposed within
the floor of the basin, a minimum grade of 2% shall be maintained.
(8)
Energy dissipators. Energy dissipating devices (rip-rap, end
sills, etc.) shall be placed at all basin outlets.
I.
Grading standards for stormwater management facilities.
(1)
Cuts. No excavation shall be made with a cut face steeper than
three horizontal to one vertical, except under the condition that
the materials in which the excavation is made is sufficiently stable
to sustain a slope of steeper than three horizontal to one vertical.
A written statement to that effect is required from a civil engineer,
licensed by the Commonwealth of Pennsylvania, having experience in
soils engineering and must be submitted to the Township Engineer and
approved by him. The statement shall affirm that the site has been
inspected and that the deviation from the slope should not result
in injury to persons or damage to property. Retaining walls will be
required if a stable slope cannot be maintained. Any retaining wall
design must be approved by the Township Engineer. The toe of the slope
or headwall of any cut must be located a minimum of five feet from
property lines.
(2)
Fills. No fill shall be made which creates any exposed surfaces
steeper in slope than three horizontal to one vertical, except where
the fill is located so that settlement, sliding, or erosion will not
result in property damage or be a hazard to adjoining property, streets,
or buildings. A written statement is required from a civil engineer
licensed by the Commonwealth of Pennsylvania having experience in
soils engineering certifying that he has inspected the site and that
any proposed deviation from the slope specified above should not endanger
any property or result in property damage, and must be submitted to
and approved by the Township Engineer.
(3)
A concrete or stone masonry wall designed and constructed in
accordance with these specifications and standards may be required
to support the face of the fill where the above specified slopes are
exceeded.
(4)
The top of any fill or toe of the slope of any fill shall be
located five feet from any property line.
(5)
Landscaping of storm drainage, retention and detention areas.
(6)
All storm drainage channels, retention areas, and detention
basins, whether existing or proposed, shall be graded and planted
with permanent vegetative cover acceptable to the Township and in
a manner which effectively naturalizes such areas as an integral and
harmonious part of the surrounding landscape.
(7)
Fence or screening. A fence or suitable vegetative screen shall
be provided around all detention basins upon the recommendation of
the Township Engineer. Fencing shall be required around all basins
where water may attenuate greater than three feet. All fencing shall
be at least 3 1/2 feet in height. Where provided in lieu of fencing,
vegetative screening shall be composed of shrubs intended to provide
a barrier to access into the detention basin area, including, for
example, Barberry (Barberis species), Eleagnus (Eleagnus species),
Firethorn (Pyracantha species), or Rose (Rose species). This fencing
or vegetative screening requirement shall be waived only with the
expressed consent of the Board of Supervisors.
(8)
A minimum of six inches of topsoil material shall be placed
on all areas affected by construction of detention basins (bottom
of basin, side, slopes, top of berm, etc.). The material must meet
the requirements of the Pennsylvania Department of Transportation,
Form 408 Specification, as amended.
J.
Culverts and drainage channels. Culverts shall generally be required
at points of vehicular or pedestrian crossing over drainage channels
receiving drainage from an area in excess of 2 1/2 acres.
(1)
Design flow standard. All culverts and drainage channels shall
be designed to carry the flow rate resulting from a fifty-year, twenty-four-hour
storm event (Soil Conservation Service, Technical Release No. 55).
(2)
Erosion prevention. All drainage channels shall be designed
to prevent the erosion of the bed and bank areas. The flow velocity
in all vegetated drainage channels shall not exceed three feet per
second to prevent erosion. Suitable bank stabilization shall be provided
where required to prevent erosion of the drainage channels. Where
storm sewers discharge into existing drainage channels at an angle
greater than 30° from parallel with the downstream channel flow,
the far side bank shall be stabilized by the use of rip-rap or masonry,
and/or concrete walls. The stabilization shall be designed to prevent
erosion and frost heave under and behind the stabilizing media.
(3)
Maximum side slope. Any vegetated drainage channel requiring
mowing of the vegetation shall have a maximum grade of three horizontal
to one vertical on those areas to be mowed.
(4)
Design standard. Because of the critical nature of vegetated
drainage channels, the design of all vegetated channels shall, as
a minimum, conform to the design procedures outlined in the Chester
County Conservation District Environmental Protection Handbook. Several
acceptable sources outline procedures for nonvegetated drainage channels,
including the following:
Bureau of Public Roads
Hydraulic Engineering Circular No. 5
Hydraulic Charts for the Selection of Highway Culverts
Federal Highway Administration
Hydraulic Engineering Circular No. 13
Hydraulic Design of Improved Inlets for Culverts
