A.
Applicants proposing regulated activities in the Township which are not exempt under § 388-6 shall submit a stormwater management site plan (SWM site plan) to the Township for review and approval in accordance with Articles III and IV. SWM site plans approved by the Township shall be on site throughout the duration of the regulated activity.
[Amended 12-15-2014 by Ord. No. 424-14]
B.
The stormwater management and runoff control criteria and standards
in this chapter shall apply to the total proposed regulated activity,
even if it is to take place in stages. The measurement of impervious
surfaces shall include all of the impervious surfaces in the total
proposed regulated activity even if the development is to take place
in stages.
C.
No regulated activity within the Township shall commence until:
[Amended 12-15-2014 by Ord. No. 424-14]
(1)
The Township issues approval of a SWM site plan, which demonstrates
compliance with the requirements of this chapter; and
(2)
The applicant has received a letter of adequacy or approval for the
erosion and sediment control plan review by the Township and the Conservation
District (if required) and has received all other local, state and
federal permit approvals required for the project involving the regulated
activity.
D.
Neither submission of 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.
[Amended 10-17-2022 by Ord. No. 452-22]
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 § 388-16 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 or more acre 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
Township, 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.
[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]
(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 Township.
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 §§ 388-17, 388-18, 388-19 and 388-20). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
I.
If site conditions preclude capture of runoff from limited portions
of the disturbed area for achieving water quality volume control standards,
stream channel protection standards, and the two-, five- and ten-year
storm event peak runoff rate reduction standards for new development
required by this chapter, the applicant shall propose alternate methods
to mitigate the bypass of the BMPs, subject to the approval of the
Township Engineer. In no case shall resulting peak rate be greater
than the predevelopment peak rate for the equivalent design storm.
[Amended 12-15-2014 by Ord. No. 424-14]
J.
For all regulated activities, erosion and sediment control BMPs shall
be designed, implemented, operated, and maintained during the regulated
activities (i.e., during construction) as required to meet the purposes
and requirements of this chapter, to meet the erosion and sediment
control requirements of the Township, if applicable, and to meet all
requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law.[1]
[Amended 12-15-2014 by Ord. No. 424-14]
[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,[2] and the Storm Water Management Act.[3]
L.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the Township. The Township reserves
the right to disapprove any design that would result in construction
in an area affected by existing stormwater problem(s) or continuation
of an existing stormwater problem(s).
[Amended 12-15-2014 by Ord. No. 424-14]
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 Township
Engineer, to pretreat runoff from hotspots prior to infiltration.
Following is a list of examples of hotspots:
[Amended 12-15-2014 by Ord. No. 424-14]
(1)
Vehicle salvage yards and recycling facilities;
(2)
Vehicle fueling stations;
(3)
Vehicle service and maintenance facilities;
(4)
Vehicle and equipment cleaning facilities;
(5)
Fleet storage areas (bus, truck, etc.);
(6)
Industrial sites based on Standard Industrial Classification Codes;
(7)
Marinas (service and maintenance areas);
(8)
Outdoor liquid container storage;
(9)
Outdoor loading/unloading facilities;
(10)
Public works storage areas;
(11)
Facilities that generate or store hazardous materials;
(12)
Commercial container nursery;
(13)
Contaminated sites/brownfields;
(14)
Other land uses and activities as designated by the Township.
O.
Contaminated and brownfield sites. Where BMPs may contribute to the
migration of contaminants in groundwater, the water quality and runoff
volume, stream channel protection, and peak rate control standards
shall be met; however, at the Township Engineer's discretion, the
minimum infiltration requirement may be reduced or eliminated commensurate
with the contaminated area, and the required water quality and runoff
control measures may be increased to mitigate the reduced infiltration
requirement for the contaminated area.
[Amended 12-15-2014 by Ord. No. 424-14]
P.
Additional water quality requirements. The Township may require additional
stormwater control measures for stormwater discharges to special management
areas, including, but not limited to:
[Amended 12-15-2014 by Ord. No. 424-14]
(1)
Water bodies listed as impaired by PADEP.
(2)
Any water body or watershed with an approved total maximum daily
load (TMDL).
(3)
Areas of known existing flooding problems.
(4)
Critical areas with sensitive resources (e.g., state-designated special
protection waters, cold water fisheries, carbonate geology or other
groundwater recharge areas that may be highly vulnerable to contamination,
drainage areas to water supply reservoirs, etc.).
Q.
Applicants shall utilize the Pennsylvania Stormwater Best Management
Practices Manual (PA BMP Manual), as amended, or other sources acceptable
to the Township Engineer, for testing and design standards for BMPs,
and where there is a conflict with the provisions of this chapter,
the most restrictive applies. If methods other than green infrastructure,
LID, and CD methods are proposed to achieve the volume and rate controls
required under this chapter, the SWM site plan must include a detailed
justification demonstrating that the use of LID, green infrastructure,
and CD are not practicable.
[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]
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
Township Engineer.
[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]
S.
All regulated activities located within an identified floodplain area shall comply with Chapter 220, Floodplains, of this Code. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.
[Amended 5-15-2017 by Ord. No. 428-17]
T.
Riparian buffer zones.
[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]
(2)
For regulated activities within the Darby Creek, Trout Creek, and/or
Crum Creek Watersheds (refer to Appendix F), the riparian buffer shall
be planted with native vegetation and maintained in a vegetated state
(refer to Appendix B, Native Plant List, contained in the PA BMP Manual).
If an existing buffer is legally prescribed (i.e., deed, covenant,
easement, etc.) and it exceeds the requirements of this chapter, the
existing buffer shall be maintained.
[Amended 12-15-2014 by Ord. No. 424-14]
The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the Township of the SWM site plan
and prior to commencement of any regulated activities, as applicable:
A.
All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at 25 Pa. Code Chapter 102, or erosion
and sediment control requirements of the Township.
B.
Work within natural drainageways subject to permit by PADEP under
25 Pa. Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under 25 Pa. Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way or require access to or from a state highway
and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under 25 Pa. Code
Chapter 105.
A.
No regulated activity within the Township shall commence until:
[Amended 12-15-2014 by Ord. No. 424-14]
(1)
The Township receives documentation that the applicant has received:
(a)
A letter of adequacy from the Conservation District or other
approval from PADEP in compliance with Title 25, Chapter 102, of the
Pennsylvania Code of an erosion and sediment control plan for construction
activities, 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;
[Amended 10-17-2022 by Ord. No. 452-22]
(b)
A PADEP NPDES permit for stormwater discharges associated with
construction activities as required under 25 Pa. Code Chapter 92a,
if applicable;
[Amended 6-2-2014 by Ord. No. 422-14; 10-17-2022 by Ord. No. 452-22]
(c)
Evidence of any other permit(s) or approvals required for the
regulated activities; and
(2)
An erosion and sediment control plan has been approved by the Township,
in the case of projects with less than one acre of earth disturbance.
The erosion and sediment control plan shall be included with the stormwater
management site plan. Measures to minimize soil erosion and sedimentation
shall meet the standards and specifications contained in the Pennsylvania
Department of Environmental Protection "Soil Erosion and Sediment
Pollution Control Manual," as amended, and the Pennsylvania Clean
Streams Law, Chapter 102, Erosion and Sedimentation Control Rules
and Regulations, as amended, and the specifications contained therein.
The Township Engineer shall ensure compliance with the appropriate
standards and specifications.
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 § 388-25C) 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 § 388-25B(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 presents additional discussion of "conservation design" and "low-impact development."[1]
[Amended 10-17-2022 by Ord. No. 452-22]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]
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 amended, 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 § 388-21D 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 § 388-18. The infiltration volume required under § 388-18 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F.
Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide pretreatment prior
to the runoff entering the stormwater management practice.
G.
The Township may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 388-13P.
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 § 388-16C 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.
Wherever possible, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 388-17. 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 Section 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.
[Added 10-17-2022 by Ord. No. 452-22]
B.
For regulated activities involving new development, the volume of
a minimum of one inch of runoff from all proposed impervious surfaces
shall be infiltrated.
D.
If the requirements of Subsection B cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Township Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however, in all cases, at least the first 0.5 inch of runoff volume shall be infiltrated.
[Amended 12-15-2014 by Ord. No. 424-14; 10-17-2022 by Ord. No. 452-22]
F.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
G.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
H.
Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
I.
A detailed soils evaluation of the site shall be conducted by a qualified
professional and at a minimum shall address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be conducted by a qualified licensed professional
and shall be consistent with the PA BMP Manual (as amended) (or other
guidance acceptable to the Township Engineer) and in general shall:
[Amended 12-15-2014 by Ord. No. 424-14]
(1)
Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
(2)
Provide field tests such as double-ring infiltrometer or other hydraulic
conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention (infiltration)
volume based on field-determined infiltration capacity (and apply
safety factor as per applicable design guidelines) at the elevation
of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall be
demonstrated to the Township Engineer that the soils are conducive
to infiltration on the identified lots.
J.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(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 Subsection N.)
(2)
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3)
The infiltration facility shall completely drain the retention (infiltration)
volume within three days (72 hours) from the end of the design storm.
K.
All infiltration practices shall:
(2)
Be set back at least 25 feet from all buildings and features with
subgrade elements (e.g., basements, foundation walls, etc.), unless
otherwise approved by the Township Engineer;
[Amended 12-15-2014 by Ord. No. 424-14]
(3)
For any infiltration practice that collects runoff from shared or
multiple features and that is located within 50 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the bottom elevation
of the subgrade element;
L.
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.
M.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Township
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
[Amended 12-15-2014 by Ord. No. 424-14]
N.
Consideration of infiltration BMPs for areas underlain by karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation are provided to the Township demonstrating that the
potential or likelihood of subsidence or sinkholes is minimal. Evaluation
of site conditions and infiltration design shall rely on guidance
in the PA BMP Manual (as amended) or other guidance acceptable to
the Township Engineer.
[Amended 12-15-2014 by Ord. No. 424-14]
O.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hotspots
(i.e., sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged through a water quality BMP(s) to remove pollutants
prior to infiltration.
P.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
Q.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
R.
All infiltration practices shall have appropriate positive overflow
controls.
S.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
T.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed with
equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior to the
placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines, shall
be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or like
facilities shall be covered with drainage fabric. Fabric shall be
nonwoven fabric acceptable to the Township Engineer.
[Amended 12-15-2014 by Ord. No. 424-14]
(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 Township 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).
[Amended 12-15-2014 by Ord. No. 424-14]
C.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 388-21D of this chapter.
D.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter unless otherwise approved by the Township
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the BMP that do
not provide enough runoff volume to allow a twenty-four-hour attenuation
with the three-inch orifice, the calculations shall be submitted showing
this condition.
[Amended 12-15-2014 by Ord. No. 424-14]
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates subject
to the Township Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
[Amended 12-15-2014 by Ord. No. 424-14]
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.
[Amended 10-17-2022 by Ord. No. 452-22]
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities, including those
that involve new development and redevelopment that are not located
in the Darby Creek or Crum Creek Watersheds:
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 388-20.1.
Table 388-20.1
Peak Rate Control Standards
| ||
---|---|---|
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
|
NOTE: 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.
|
B.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 388-21D of this chapter.
C.
For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the 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 388-20.1, using the redevelopment ground cover assumptions presented in § 388-21D. This design criterion for redevelopment is only permitted with approval of the Township Engineer. It shall result in no impact on downstream properties.
[Amended 10-17-2022 by Ord. No. 452-22]
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
H.
Regulated activities located within the Darby Creek Watershed (refer
to Appendix F[1]) shall achieve the applicable peak flow release rate control
requirements presented in the approved PA Act 167 plan for that watershed
and as presented in Table 388-20.2 below:
Table 388-20.2
Peak Rate Control Standards for Darby Creek Watershed
| ||
---|---|---|
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
|
5-year
|
5-year
|
10-year
|
10-year
|
10-year
|
25-year
|
25-year
|
25-year
|
50-year
|
50-year
|
50-year
|
100-year
|
100-year
|
100-year
|
NOTE: 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.
|
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
I.
Regulated activities located within the Crum Creek Watershed (refer
to Appendix F[2]) shall achieve the applicable peak flow release rate control
requirements presented in the approved PA Act 167 plan for that watershed
and as presented in Tables 388-20.3 and 388-20.4 below, and as identified
in the Crum Creek Management Districts Map in Appendix G of this chapter
(the map is Appendix A of the Crum Creek Act 167 Plan Model Ordinance,
and "Appendix A" therefore appears in the title block of the map,
although it is contained in Appendix G of this chapter).[3]
Table 388-20.3
Peak Rate Control Standards for Crum Creek Watershed,
Management District A
| ||
---|---|---|
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
|
5-year
|
5-year
|
10-year
|
10-year
|
10-year
|
25-year
|
25-year
|
25-year
|
50-year
|
50-year
|
50-year
|
100-year
|
100-year
|
100-year
|
NOTE: 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.
|
Table 388-20.4
Peak Rate Control Standards for Crum Creek Watershed,
Management District B
| ||
---|---|---|
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
|
5-year
|
5-year
|
25-year
|
10-year
|
10-year
|
50-year
|
25-year
|
25-year
|
100-year
|
100-year
|
100-year
|
NOTE: 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.
|
[2]
Editor's Note: Appendix F is included as an attachment to this chapter.
[3]
Editor's Note: Appendix G is included as an attachment to this chapter.
[Amended 12-15-2014 by Ord. No. 424-14]
A.
Stormwater runoff from all regulated activity sites shall be calculated
using a generally accepted calculation technique(s) that is based
on the NRCS Soil Cover Complex Method. Table 388-21.1 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.
Table 388-21.1
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
|
United States Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township
|
B.
All calculations using the Soil Cover Complex Method shall use the
appropriate design rainfall depths for the various return period storms
consistent with this chapter. Rainfall depths used shall be obtained
from 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.
[Amended 10-17-2022 by Ord. No. 452-22]
C.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
(1)
For regulated activities involving new development, the following
ground cover assumptions shall be used:
(2)
For regulated activities involving redevelopment, the following ground
cover assumptions shall be used:
(a)
For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(b)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(c)
For areas that are impervious surfaces, predevelopment calculations
shall assume all impervious areas as "meadow" ground cover, except
for those existing impervious surfaces which are served by stormwater
management facilities designed and constructed in conformance with
the current ordinance.
[Amended 1-15-2018 by Ord. No. 433-18]
(3)
The applicant shall determine which stormwater standards apply to
the proposed regulated activity as follows:
(a)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(b)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(c)
At the discretion of the Township Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this chapter, may either:
[1]
Apply the stormwater standards (redevelopment or new development)
that are associated with the activity that involves the greatest amount
of land area; or
[2]
Apply the redevelopment and new development stormwater standards
to the corresponding redevelopment and new development portions of
the proposed regulated activity.
D.
Runoff curve numbers (CN) for both predevelopment and proposed (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: Appendix C is included as an attachment to this chapter.
E.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
F.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
No. 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended). Hydraulic
computations to determine the capacity of open channels shall be consistent
with methods and computations contained in the Federal Highway Administration
Hydraulic Engineering Circular No. 15 (Publication No. FHWA-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.
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 Virus Design 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: Appendix D is included as an attachment to this chapter.
B.
Any stormwater basin required or regulated by this chapter designed
to store runoff and requiring a berm or earthen embankment shall be
designed to provide an emergency spillway to safely convey flow up
to and including the one-hundred-year proposed conditions. The height
of embankment shall provide a minimum one foot of freeboard above
the maximum pool elevation computed when the facility functions for
the one-hundred-year proposed conditions inflow. Should any BMP require
a dam safety permit under 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 the one-hundred-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 one-hundred-year
storms) shall also be safely conveyed in the direction of natural
flow without creating additional damage to any drainage facilities,
nearby structures, or roadways.
D.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
E.
Roadway crossings or structures located within identified 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 and Chapter 220, Floodplains.
[Amended 5-15-2017 by Ord. No. 428-17]
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.
[Amended 10-17-2022 by Ord. No. 452-22]
G.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular No. 11 (Publication No. FHWA-IP-89-016, as amended)
and the PADEP Erosion and Sediment Pollution Control Program Manual
(Publication No. 363-2134-008, as amended), or other design guidance
acceptable to the Township Engineer.
[Amended 12-15-2014 by Ord. No. 424-14]