A.
Applicants proposing regulated activities in the municipality which are not exempt under § 246-106 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 an 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 an SWM site plan under the provisions herein
nor compliance with the provisions of this chapter shall relieve any
person from responsibility for damage to any person or property otherwise
imposed by law.
E.
The applicant shall design the site to minimize disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall apply the procedures set forth in § 246-304 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.
(1)
For regulated activities with one acre or less proposed earth
disturbance, the applicant shall provide written notification to the
affected landowner(s) describing the proposed regulated activity and
proposed discharge(s), unless otherwise required by the municipality.
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 §§ 246-305, 246-306, 246-307, and 246-308). 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, if applicable, and to meet
all requirements under Title 25 of the Pa. Code and the Clean Streams
Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
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.[2]
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
and 32 P.S. § 680.1 et seq., respectively.
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 an 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;
(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.).
Q.
Applicants shall utilize the Pennsylvania Stormwater 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 § 287-12 of the Zoning Ordinance and shall be designed to maintain the flood-carrying capacity of the floodway such that the base flood elevations are not increased, either upstream or downstream. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.
T.
Disturbance of existing ground cover during construction of the proposed
regulated activity is prohibited within 50 feet of top-of-bank of
all perennial and intermittent waterways, water bodies (lakes, ponds,
etc.) and wetlands, except for activities otherwise approved by state
or local agencies (e.g., stream restoration projects, road crossings,
subsurface utility projects, etc.). At the Municipal Engineer's
discretion, and with Conservation District and PADEP approval where
necessary, the nondisturbance buffer may be reduced because of setback
or other site constraints, but never be less than 10 feet.
The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the 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 25 Pa. Code Chapter 102, or erosion
and sediment control requirements of the municipality.
B.
Work within natural drainageways subject to permit by PADEP under
25 Pa. Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under 25 Pa. Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way or require access to or from a state highway
and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under 25 Pa. Code
Chapter 105.
A.
No regulated activity within the 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 25 Pa. Code Chapter 92a,
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. For regulated activities with 10,000 or more square feet of proposed earth disturbance or 2,000 or more square feet of regulated impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in § 246-402C) 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 § 246-402B(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 Subsection B 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 Subsection B into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6)
Maximize infiltration and the use of natural site infiltration
features, patterns and conditions, and evapotranspiration features;
(7)
Apply selective grading design methods to provide final grading
patterns or preserve existing topography in order to evenly distribute
runoff and minimize concentrated flows;
(8)
Minimize the cumulative area to be covered by impervious surfaces
and:
(9)
Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(13)
For flow volumes requiring conveyance from the source of generation
to a BMP for management, give preference to open channel conveyance
techniques that provide infiltration and water quality benefits, and
landscaped-based management in common open space areas, where practicable;
and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of Subsections B and C. Appendix B[1] presents additional discussion of conservation design
and low impact development.
[1]
Editor's Note: Appendixes A through E are included as attachments to this chapter.
To control postconstruction stormwater impacts from regulated
activities and meet state water quality requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that 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,[1] 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) or a minimum
of 1.5 inches of runoff from all regulated impervious surfaces shall
be managed. Whichever volume is greater shall be managed. 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 § 246-309D 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. The applicant shall fulfill
the requirements of the PADEP "Thermal Impact Analysis" for the "PAG-02
Stormwater Discharges Associated with Construction Activities, NOI
for Coverage under General or Individual Permit" if they cannot meet
the volume control requirements.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 246-306. The infiltration volume required under § 246-306 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 § 246-301P.
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 § 246-304C 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.
[1]
Editor's Note: Appendixes A through E are included as attachments to this chapter.
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 § 246-305. 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 § 246-305 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PADEP 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 Subsection A 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.
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 § 246-306M).
(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 § 246-304C 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.),
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 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.
(4)
All infiltration practices shall be set back at least 10 feet
from a property line and right-of-way line.
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 wellhead protection area of a public water
supply well, infiltration practices shall be in conformance with the
applicable approved source water protection assessment or source water
protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
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, 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 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 § 246-309D 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 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.
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 246-308.1.
|
Table 246-308.1
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
|
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 modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 246-309D 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 246-308.1, using the redevelopment ground cover assumptions presented in § 246-309D. 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 246-309.1 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 246-309.1
Acceptable Computation Methodologies For SWM Site Plan
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Method
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Developed By
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Applicability
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TR-20
(or commercial computer package based on TR-20)
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USDA NRCS
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Applicable where use of full hydrology computer model is desirable
or necessary
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TR-55
(or commercial computer package based on TR-55)
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USDA NRCS
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Applicable for land development plans where limitations described
in TR-55 are met
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HEC-1/HEC-HMS
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U.S. Army Corps of Engineers
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Applicable where use of a full hydrologic computer model is
desirable or necessary
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Rational Method
(or commercial computer package based on Rational Method)
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Emil Kuichling (1889)
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For sites up to 5 acres, or as approved by the municipality
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Other Methods
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Varies
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Other computation methodologies approved by the municipality
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B.
All calculations using the Soil Cover Complex Method shall use the
appropriate design rainfall depths for the various return period storms
consistent with this chapter. Rainfall depths used shall be obtained
from NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or infiltration, water quality and runoff volume functions, the duration
of rainfall shall be 24 hours.
C.
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 25% 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.
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.[1]
[1]
Editor's Note: Appendixes A through E are included as attachments 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.[2]
[2]
Editor's Note: Appendixes A through E are included as attachments to 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 C-3 in Appendix C of this chapter.[3]
[3]
Editor's Note: Appendixes A through E are included as attachments 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,[1] PADEP document 363-0300-001 "Design Criteria - Wetlands
Replacement/Monitoring" (as amended), or contact the Pennsylvania
State Cooperative Wetland Center or the Penn State Cooperative Extension
Office for design information.
[1]
Editor's Note: Appendixes A through E are included as attachments 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 100-year proposed conditions. The height of embankment
shall provide a minimum 1.0 foot of freeboard above the maximum pool
elevation computed when the facility functions for the 100-year proposed
conditions inflow. Should any BMP require a dam safety permit under
25 Pa. Code Chapter 105 regulations, the facility shall be designed
in accordance with and meet the regulations of 25 Pa. Code Chapter
105 concerning dam safety. 25 Pa. Code Chapter 105 may require the
safe conveyance of storms larger than a 100-year event.
C.
Any drainage conveyance facility and/or channel not governed by 25
Pa. Code 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.
H.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. Downingtown Borough reserves
the right to disapprove any design that would result in construction
in or continuation of a stormwater problem area.
A.
The design of stormwater management collection and conveyance facilities
that service drainage areas within the site shall be based upon a
twenty-five-year storm frequency event. Stormwater management facilities
that convey off-site stormwater through the site must be designed
to convey a fifty-year event. For storm events greater, the overflow
must drain to the appropriate BMP.
B.
All developments must also include design provisions that allow for
the overland conveyance of the post-100-year-storm flows through the
site without damage to any private or public property.
C.
Cross-pipes, box culverts, and bridges shall be designed to convey
a 100-year storm event.
D.
Any drainage facility crossing a state highway shall conform to all
applicable PennDOT design standards.
E.
When the design of the overall stormwater management plan requires
a transfer of watershed, the design shall illustrate that the facilities
utilized to accomplish the transfer can safely convey the 100-year
storm event.
F.
The capacities of the pipes, gutters, inlets, culverts, outlet structures
and swales shall consider all possible hydraulic conditions. The following
minimum design standards have been established by the Borough:
(2)
Permanent open swales shall be designed to convey a 100-year
storm event computed from Manning's Equation.
(3)
Open swales shall be stabilized with vegetation or other materials
in accordance with 25 Pa. Code Chapter 102 rules and regulations of
the Pennsylvania Department of Environmental Protection. Slopes for
swale banks shall not exceed one foot vertical for every four feet
horizontal.
(4)
The velocity to be used in the design of any piped stormwater
conveyance system shall be based on the maximum velocity obtainable.
The capacity shall be based upon full flow conditions. Stormwater
pipe systems shall be designed to produce a minimum velocity of 3.0
feet per second when flowing full. The maximum permissible velocity
shall be 10.0 feet per second. Pipe slopes shall not be less than
0.5%.
(5)
Inlets, culverts and basin discharge systems shall be designed
for the worst case condition. Inlet tops shall contain a Trout Logo
Plate with text "Dump No Waste" and "Drains To Waterway" in accordance
with the Borough-approved detail. Inlet capacity shall be based on
design data provided by the manufacturers and accepted by the Borough
Engineer. If acceptable information is not available, inlets in nonponding
areas shall be designed for a maximum capacity of 3.5 cubic feet per
second (cfs). Where ponding occurs, inlet capacity shall be based
on accepted engineering design practices. Culvert design shall consider
either inlet/outlet control or a combination of hydraulic losses through
the system, whichever is greater. Basin discharge systems shall be
designed to the same standards as culverts. If it cannot be readily
determined which hydraulic condition controls, the basin discharge
rate shall be based on the highest possible discharge rating curve
with the basin capacity sized to store the excessive storm runoff
based on the lowest possible discharge rating curve.
G.
Inlets shall be located at curb tangents on the uphill side of street
intersections and at intervals along the curbline to control the maximum
amount of encroachment of runoff on the roadway pavement so that it
does not exceed a width of four feet during the design storm event.
Design and location of inlets shall be approved by the municipality.
The maximum amount of flow through an intersection may not exceed
one inch.
H.
Manholes and inlets, when proposed, shall not be spaced more than
300 feet apart. Additionally, manholes shall be placed at points of
abrupt changes in the horizontal or vertical direction of storm sewers.
Inlets shall be substituted for manholes where they will serve a useful
purpose.
I.
Curves in pipes or box culverts without an inlet or manhole are prohibited.
Tee joints, elbows and wyes are also prohibited.
J.
Stormwater management pipe collection and conveyance systems shall
have a minimum diameter of 15 inches and shall be made of reinforced
concrete pipe, double wall HDPE, or approved equivalent. Where installation
conditions merit, structural calculations that address the actual
design requirements will be required.
K.
Open end pipes must be fitted with concrete end walls and wing walls
in accordance with PennDOT standards.
L.
All storm sewer pipe and culverts shall be laid to a minimum depth
of one foot from finished subgrade to the crown of pipe in paved areas
and one foot from finished grade to the crown of pipe in grassed areas.
M.
All storm sewer pipes, culverts, manholes, inlets, end walls and
end sections shall be constructed in accordance with Pennsylvania
Department of Transportation Form 408 and PennDOT Publication 72,
RC Standard, in effect at the time the design is submitted, as modified
by the adopted Borough construction standards.
N.
All inlets used along existing and proposed curbed streets shall
include PennDOT Type C top units or approved equivalent.
O.
Where storm sewers exceed 15% slope, properly spaced concrete anchors
will be used.
P.
Stormwater roof drains and pipes, wherever possible, shall discharge
water into a stormwater runoff dispersion and infiltration control
device and not into storm sewers or street gutters.
Q.
All storm sewer crossings of streets shall be perpendicular to the
street center line unless otherwise approved by the Borough Engineer.
R.
All storm pipes, other than culverts for existing channels, which
discharge from residential lots to a street or from a street to residential
lots shall extend from the street right-of-way a minimum distance
of 2/3 the length of the longest adjacent lot dimension.
S.
The proposed stormwater discharge at the perimeter of the site shall
not be beyond the capacity of any existing, immediately contiguous,
stormwater management facility into which it flows.
T.
End walls and end sections shall be used where stormwater runoff
enters or leaves the storm sewer horizontally from a natural or man-made
channel.
U.
A minimum of a twenty-foot-wide access and maintenance easement shall
be provided for and centered on all storm sewers and open swales not
located within the public right-of-way.
