A.
Applicants proposing regulated activities in the Township which are not exempt under § 345-106 shall submit a SWM site plan to the Township for review and approval in accordance with Articles III and IV. Stormwater management site plans approved by the Township shall be on site throughout the duration of the regulated activity.
B.
The stormwater management and runoff control criteria and standards
in this chapter shall apply to the total proposed regulated activity,
even if it is to take place in stages. The measurement of impervious
surfaces shall include all of the impervious surfaces in the total
proposed regulated activity even if the development is to take place
in stages.
C.
No regulated activity within the Township shall commence until:
(1)
The Township issues 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.
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 § 345-304 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.
Adjacent property notification and/or permission:
(1)
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 properties or downgradient
properties, 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 properties. Altered stormwater
discharges shall be subject to any applicable discharge criteria specified
in this chapter.
(2)
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 §§ 345-305, 345-306, 345-307, and 345-308). 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-storm-event
peak runoff rate reduction standards for new development required
by this chapter, the applicant shall propose alternate methods to
mitigate the bypass of the BMPs, subject to the approval of the Township
Engineer. In no case shall resulting peak rate be greater than the
predevelopment peak rate for the equivalent design storm.
J.
For all regulated activities, erosion and sediment control, BMPs
shall be designed, implemented, operated, and maintained during the
regulated activities (i.e., during construction) as required to meet
the purposes and requirements of this chapter, to meet the erosion
and sediment control requirements of the Township, if applicable,
and to meet all requirements under Title 25 of the Pennsylvania 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 Act
167.
L.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the Township. The Township reserves
the right to disapprove any design that would result in construction
in an area affected by existing stormwater problem(s) or continuation
of an existing stormwater problem(s).
M.
Existing wetlands, either on the site or on an adjacent property,
shall not be used to meet the minimum design requirements for stormwater
management or stormwater runoff quality treatment. Stormwater discharges
to existing wetlands shall not degrade the quality or hydrologic integrity
of the wetland.
N.
Hotspot runoff controls. Specific structural or pollution prevention
practices may be required, as determined to be necessary by the Township
Engineer, to pretreat runoff from hotspots prior to infiltration.
Following is a list of examples of hotspots:
(1)
Vehicle salvage yards and recycling facilities;
(2)
Vehicle fueling stations;
(3)
Vehicle service and maintenance facilities;
(4)
Vehicle and equipment cleaning facilities;
(5)
Fleet storage areas (bus, truck, etc.);
(6)
Industrial sites based on Standard Industrial Classification
Codes;
(7)
Marinas (service and maintenance areas);
(8)
Outdoor liquid container storage;
(9)
Outdoor loading/unloading facilities;
(10)
Public works storage areas;
(11)
Facilities that generate or store hazardous materials;
(12)
Commercial container nursery;
(13)
Contaminated sites/brownfields;
(14)
Other land uses and activities as designated by the Township.
O.
Contaminated and brownfield sites. Where BMPs may contribute to the
migration of contaminants in groundwater, the water quality and runoff
volume, stream channel protection, and peak rate control standards
shall be met; however, at the Township Engineer's discretion,
the minimum infiltration requirement may be reduced or eliminated
commensurate with the contaminated area and the required water quality
and runoff control measures may be increased to mitigate the reduced
infiltration requirement for the contaminated area.
P.
Additional water quality requirements. The Township may require additional
stormwater control measures for stormwater discharges to special management
areas, including, but not limited to:
(1)
Water bodies listed as "impaired" by PADEP.
(2)
Any water body or watershed with an approved total maximum daily
load (TMDL).
(3)
Areas of known existing flooding problems.
(4)
Critical areas with sensitive resources (e.g., state-designated
special protection waters, cold-water fisheries, carbonate geology
or other groundwater recharge areas that may be highly vulnerable
to contamination, drainage areas to water supply reservoirs, etc.).
Q.
Applicants shall utilize the Pennsylvania Stormwater Best Management
Practices Manual (PADEP BMP Manual), as amended, or other sources
acceptable to the Township Engineer, for testing and design standards
for BMPs, and where there is a conflict with the provisions of this
chapter, the most restrictive applies.
R.
For areas underlain by karst or carbonate geology that may be susceptible
to the formation of sinkholes and other karst features, the location,
type, and design of infiltration BMPs shall be based on a site evaluation
conducted by a qualified licensed professional and based on the PADEP
BMP Manual (as amended) or other design guidance acceptable to the
Township Engineer.
S.
All regulated activities located within a special flood hazard area
designated by the Federal Emergency Management Agency (FEMA) shall
comply with § 399-13F(2)(a) 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.
All regulated activity is subject to the riparian buffer regulations as found in § 399-13.1C(1)(a) of the East Brandywine Township Zoning Code.
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.
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.
A.
No regulated activity within the Township shall commence until:
(1)
The Township receives documentation that the applicant has received:
(a)
A letter of adequacy from the Conservation District or other approval from PADEP in compliance with 25 Pa. Code Chapter 102 for 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 25 Pa. Code Chapter 105 permits;
(b)
A PADEP NPDES permit for stormwater discharges associated with
construction activities as required under 25 Pa. Code Chapter 92.a,
if applicable;
(c)
Evidence of any other permit(s) or approvals required for the
regulated activities; and
(2)
An erosion and sediment control plan has been approved by the
Township, if required.
B.
A copy of the erosion and sediment control plan and any required
permit(s), as required by PADEP regulations, shall be available on
the site at all times.
The applicant shall design the site to minimize the disturbances to land, site hydrology, and natural resources, and to maintain the natural hydrologic regime, drainage patterns and flow conditions. The applicant shall demonstrate in its SWM site plan (as required in § 345-402C) 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 § 345-402B(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 § 345-304B as the basis for the proposed site layout, grading, construction, and permanent ground cover design;
(2)
Minimize earth disturbance (both surface and subsurface);
(3)
Maximize protection of or improvement to natural resources and
special management areas;
(4)
Minimize the disturbance of natural site hydrology, in particular
natural drainage features and patterns, discharge points and flow
characteristics, natural infiltration patterns and characteristics,
and natural channel and floodplain conveyance capacity;
(5)
Incorporate natural hydrologic features and functions identified in § 345-304B into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6)
Maximize infiltration and the use of natural site infiltration
features, patterns and conditions, and evapotranspiration features;
(7)
Apply selective grading design methods to provide final grading
patterns or preserve existing topography in order to evenly distribute
runoff and minimize concentrated flows;
(8)
Minimize the cumulative area to be covered by impervious surfaces
and:
(9)
Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(13)
For flow volumes requiring conveyance from the source of generation
to a BMP for management, give preference to open channel conveyance
techniques that provide infiltration and water quality benefits, and
landscaped-based management in common open space areas, where practicable;
and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of § 345-304B and C. Appendix A[1] presents additional discussion of conservation design
and low-impact development.
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
To control post-construction stormwater impacts from regulated
activities and meet state water quality requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that post-construction stormwater
discharges do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The green infrastructure and low-impact development
(LID) practices provided in the PA BMP Manual, as well as the guidance
on green infrastructure, LID and conservation design (CD) provided
in Appendix B,[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). 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 § 345-309D of this chapter.
C.
The design of the stormwater management facility outlet shall provide
for protection from clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 345-306. The infiltration volume required under § 345-306 may be included as a component of the water quality volume. If the calculated water quality and runoff volume is greater than the volume infiltrated, then the difference between the two volumes shall be managed for water quality and runoff volume control through other techniques or practices but shall not be discharged from the site.
F.
Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide pretreatment prior
to the runoff entering the stormwater management practice.
G.
The Township may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 345-301P.
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 § 345-304C 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: Appendix B is included as an attachment 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 § 345-305. Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual or other PADEP design guidance. If the runoff volume required by § 345-305 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 § 345-306A or 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.
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 PADEP BMP Manual (as amended) (or
other guidance acceptable to the Township Engineer) and in general
shall:
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(2)
Provide field tests such as double-ring infiltrometer or other
hydraulic conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention
(infiltration) volume based on field-determined infiltration capacity
(and apply safety factor as per applicable design guidelines) at the
elevation of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall
be demonstrated to the Township Engineer that the soils are conducive
to infiltration on the identified lots.
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 § 345-306M.)
(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 § 345-304C that are applicable to infiltration;
(2)
Be set back at least 25 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.) and
be set back at least 10 feet from property lines or right-of-way,
unless otherwise approved by the Township Engineer;
(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 elevation of the
subgrade element;
(4)
All infiltration practices shall be installed with a minimum
of 50 feet from water supply well and septic disposal areas.
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 hydrogeological 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 Township
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
M.
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.
N.
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.
O.
All infiltration practices shall have appropriate positive overflow
controls.
P.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
Q.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or
like facilities shall be covered with drainage fabric. Fabric shall
be nonwoven fabric acceptable to the Township Engineer.
(5)
Stormwater shall be distributed throughout the entire seepage
bed/trench or like facility and provisions for the collection of debris
shall be provided in all facilities.
For regulated activities involving new development with one
or more acres of earth disturbance, the applicant shall comply with
the following stream channel protection requirements to minimize stream
channel erosion and associated water quality impacts to the receiving
waters:
A.
The peak flow rate of the 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).
C.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 345-309D 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 Township
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the BMP that do
not provide enough runoff volume to allow a twenty-four-hour attenuation
with the three-inch orifice, the calculations shall be submitted showing
this condition.
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates subject
to the Township provided to ensure that 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 according to the following
regulations:
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 345-308.1.
|
TABLE 345-308.1
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)
| ||
|
Predevelopment Design Storm
| ||
|
Post-Construction Design Storm Frequency (24-Hour Duration)
|
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 § 345-309D 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 345-308.1, using the redevelopment ground cover assumptions presented in § 345-309D of this chapter.
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 345-309.1 summarizes acceptable computation
methods. The method selected for use shall be based on the individual
limitations and suitability of each method for a particular site.
The use of the Rational Method to estimate peak discharges for drainage
areas greater than five acres shall be permitted only upon approval
by the Township Engineer.
|
Table 345-309.1
Acceptable Computation Methodologies for SWM Site Plan
| ||
|---|---|---|
|
Method
|
Developed by
|
Applicability
|
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans where limitations described
in TR-55 are met
|
|
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites up to 5 acres, or as approved by the municipality
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
|
B.
All calculations using the Soil Cover Complex Method shall use the
appropriate design rainfall depths for the various return period storms
consistent with this chapter. Rainfall depths used shall be obtained
from the latest version of the Precipitation-Frequency Atlas of the
United States, National Oceanic and Atmospheric Administration (NOAA),
National Weather Services, Hydrometeorological Design Studies Center,
Silver Spring, Maryland (NOAA Atlas 14) values consistent with a partial
duration series. When stormwater calculations are performed for routing
procedures or infiltration, water quality and runoff volume functions,
the duration of rainfall shall be 24 hours.
C.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration (duration) and
storm events, with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times of concentration shall
be calculated based on the methodology recommended in the respective
model used. Times of concentration for channel and pipe flow shall
be computed using the Manning's Equation.
D.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
(1)
For regulated activities involving new development, the following
ground cover assumptions shall be used:
(2)
For regulated activities involving redevelopment, the following
ground cover assumptions shall be used:
(a)
For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(b)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(c)
For areas that are impervious surfaces, predevelopment calculations
shall assume at least 20% of the existing impervious surface area
to be disturbed as "meadow" ground cover.
(3)
The applicant shall determine which stormwater standards apply
to the proposed regulated activity as follows:
(a)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(b)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(c)
At the discretion of the 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.
E.
Runoff CN for both predevelopment and proposed (post-construction)
conditions to be used in the Soil Cover Complex Method shall be obtained
from Table B-1 in Appendix B of this chapter.[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
F.
Runoff coefficients (C) for both predevelopment and proposed (post-construction)
conditions for use in the Rational Method shall be obtained from Table
B-2 in Appendix B of this chapter.
G.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
H.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended).
Hydraulic computations to determine the capacity of open channels
shall be consistent with methods and computations contained in the
Federal Highway Administration Hydraulic Engineering Circular Number
15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for
Manning's roughness coefficient (n) shall be consistent with
Table B-3 in Appendix B of this chapter.
A.
Any BMP intended to hold standing water for four days or longer shall
be designed to incorporate biologic controls consistent with the West
Nile Guidance found in Appendix C;[1] PADEP document 363-0300-001, "Design Criteria - Wetlands
Replacement/Monitoring" (as amended); or contact the Pennsylvania
State Cooperative Wetland Center or the Penn State Cooperative Extension
Office for design information.
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
B.
Any stormwater basin required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to safely convey flow up to and including the 100-year proposed conditions. The height of embankment shall provide a minimum of two feet 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. In addition, 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 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 Township Engineer.
In any application that is subject to the regulations of a subdivision
and land development application under the Township regulations, stormwater
management design shall incorporate and provide for stormwater management
controls that consider the maximum impervious coverage allowed for
on the entire project site by the Township's Zoning Ordinance.[1]
A.
Stormwater conveyance: swales.
(1)
Open ditches shall be avoided, but properly designed, graded,
and turfed drainage swales shall be permitted in lieu of storm sewers
in commercial and industrial areas and, where approved by the Township
Engineer, in residential areas.
(2)
Swales shall be designed not only to carry the required discharge
without excessive erosion but also to increase the time of concentration,
reduce peak discharge and velocity, and permit the water to percolate
into the soil. Permanent open swales shall be designed to convey a
100-year storm event computed from the Manning's Equation.
(3)
Open swales shall be stabilized with vegetation or other materials
in accordance with Title 25, 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.
B.
Stormwater conveyance: closed conveyance, including inlets, manholes,
etc.
(1)
Storm sewers, as required, shall be placed under or immediately
at the back of the shoulder, when parallel to the street within the
right-of-way. When located in undedicated land, they shall be placed
within an easement not less than 20 feet wide as approved by the Township
Engineer.
(2)
Storm sewers shall have a minimum diameter of 15 inches for
reinforced concrete pipe, or 18 inches for aluminum corrugated or
polymer-coated, galvanized-steel corrugated pipe, and a minimum grade
of 1/2%. Other pipe-coating material (e.g., bituminous) commonly in
use and acceptable to the Township Engineer also may be used. High-density
polyethylene pipe (HDPE) may be proposed and installed under manufacturer's
specifications and with specific permission by the Township Engineer.
All pipe may be subject to video inspection upon completion of installation
and prior to acceptance of public improvements. Changes in alignment
shall be by straight sections connected by inlets or manholes.
(3)
Storm sewer sizes shall be selected in accordance with storm
intensity frequency of 25 years in all residential subdivisions or
land developments unless otherwise specified by the Township Engineer.
All storm piping shall be in accordance with PennDOT specifications.
Piping shall be saw-cut at ends, as needed, and not hammered or broken.
(4)
Inlets and manholes shall be spaced at intervals not exceeding
300 feet and shall be located wherever branches are connected or sizes
are changed and wherever there is a change in alignment or grade.
For drainage lines of at least 36 inches diameter, inlets and manholes
may be spaced at intervals of 450 feet.
(5)
Inlets, manholes, covers, and frames shall conform to Pennsylvania
Department of Transportation specifications. At street intersections,
inlets shall be placed in the tangent and not the curved portion of
the curbing.
(6)
When precast concrete inlets or manholes are used, a minimum
of two courses of brick masonry or grade ring shall be placed to bring
the grate or cover to proper elevation.
(7)
Curb 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 same does not exceed a width of four feet during
the design storm event. Design and location of curb inlets shall be
approved by the Township. The maximum amount of flow through an intersection
may not exceed one inch.
(8)
The capacity of all stormwater pipes shall be calculated utilizing
the Manning's Equation for open channel flow as applied to closed
conduit flow. The Manning's roughness coefficient shall be 0.13
for all concrete pipe. In cases where pressure flow may occur, the
hydraulic grade line shall be calculated throughout the storm sewer
system to verify that at least one foot of freeboard will be provided
in all inlets and manholes for the design storm event.
(9)
The capacity of all inlets shall be based on a maximum surface
flow to the inlet of 3.5 cfs, calculated based on the design storm
event. The maximum flow to inlets located in low points (such as sag
vertical curves) shall include the overland flow directed to the inlet
as well as all bypass runoff from upstream inlets. The bypass flow
from upstream inlets shall be calculated using inlet efficiency curves
included in PennDOT Design Manual, Part 2, latest edition. If the
surface flow to an inlet exceeds 3.5 cfs, additional inlets shall
be provided upstream of the inlet to intercept the excessive surface
flow.
(10)
Stormwater 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%.
(11)
Open end pipes must be fitted with concrete endwalls and wing
walls in accordance with PennDOT standards.
(12)
All storm sewer pipes shall be laid to a minimum depth of one
foot from subgrade to crown of pipe.
(13)
Trench backfill within any paved area, including the Township
right-of-way, Township cartway or private roadways, must be backfilled
to the base of the road section with 2A stone per PennDOT 408 backfill
and compaction standards. Trenches within open space or lawn areas
may be backfilled with select fill beyond the proper pipe bedding.
Alternative pipe bedding may be approved in areas of excessive moisture
or if proposed by the applicant according to the manufacturer's specifications.
All pipe backfill details must be approved by the Township Engineer.
(14)
Stormwater pipes must be oriented at right angles to electric,
water, sanitary sewer, and gas utilities when crossing above or beneath
same. Crossing angles of less than 90° will only be permitted
at the discretion of East Brandywine Township. When skewed crossings
are permitted, interior angles between alignment of the storm sewer
pipe and utility may not be less than 45°. Vertical and horizontal
design of the storm sewer must be linear.
(15)
Modified inlet boxes shall be used when required in accordance
with the PennDOT RC standards. Pipes shall not be allowed to enter
the corner of boxes. The crown of the pipe shall be at least 12 inches
below the top of the precast inlet or manhole box.
(16)
Storm drainage pipes shall be extended to storm basins, if used,
or to a point of at least 100 feet beyond any home site.
C.
Bridges and culverts shall have ample waterway to carry expected
flows, based on a minimum storm frequency of 100 years. Bridge and/or
culvert construction shall be in accordance with the Pennsylvania
Department of Transportation specifications and shall meet the requirements
of the Pennsylvania Department of Environmental Protection. Culverts
shall be provided with wing walls and constructed for the full width
of the right-of-way. The cartway area over the bridge shall be 24
inches wider, on either side, than the road connecting with the bridge,
or if the character of the road is expected to change for future planning,
the cartway of the bridge shall be made to anticipate this condition.
On either side of the bridge cartway, the bridge railing must be set
back from the edge of the final cartway a minimum of eight feet, and
this area may be used to place sidewalks, present or future.
D.
Detention or retention basins for the control of stormwater peak
discharges shall meet the following requirements:
(1)
Basins shall be installed prior to any earthmoving or land disturbances
which they will serve. The phasing of their construction shall be
noted in the narrative and on the plan.
(2)
Soils used for the construction of basins shall have low-erodibility
factors ("K" factors).
(3)
Detention basins designed to hold the excess volume of runoff
from a two-year storm should be located on soils with a moderate to
high percolation rate in order to encourage recharge and reduce potential
for stagnant water conditions. Where soils with moderate to high percolation
rates are found at higher elevations of a site, such detention facilities
may require incorporation into building lot layout. Drainage easements
and deed restrictions against the alteration of such detention areas
shall be executed to the satisfaction of the Board.
(4)
Energy dissipaters and/or level spreaders shall be installed
at points where pipes or drainage ways discharge to or from basins.
Generally, outlet pipes designed to carry the predevelopment, two-year
storm flow will be permitted to discharge to a stream with only an
energy dissipater. Storms of a ten-year or greater intensity should
be spread across floodplains by level spreaders; rock material found
on the site is suggested for their construction.
(5)
The following slope restrictions shall apply to basins:
(a)
Exterior slopes of compacted soil shall not exceed one foot
vertical for three feet horizontal and may be further reduced if the
soil has unstable characteristics.
(b)
Interior side slopes of the basin shall not exceed one foot
vertical in three feet horizontal except, with approval of the Township:
[1]
Where maximum water depth will not exceed three feet; or
[2]
When a two-inch rainfall in one hour will not fill the basin
in one hour; or
[3]
Where concrete, stone, or brick walls are used with side slopes
proposed to be steeper than one foot vertical in three feet horizontal,
in which case the basin shall be fenced by a permanent wire fence
42 inches in height and a ramp of durable, nonslip materials for maintenance
vehicles shall be provided for access into the basin.
[4]
Minimum basin interior slope shall be 2% unless designed as
an infiltration basin.
(6)
Outlet structures within basins which will control peak discharge
flows and distribute the flows by pipes to discharge areas shall be
constructed of concrete, coated (e.g., polymer, bituminous) galvanized
steel, or aluminum and shall have childproof, nonclogging trash racks
over all design openings exceeding 12 inches in diameter, except those
openings designed to carry perennial stream flows. Where spillways
will be used to control peak discharges in excess of the ten-year
storm, the control weirs shall be constructed of concrete of sufficient
mass and structural stability to withstand the pressures of impounded
waters and outlet velocities. Concrete outlet aprons shall be designed
as level spreaders and shall extend at a minimum to the toe of the
basin slope. The incorporation of large stones found on the site into
the concrete apron to provide a more natural appearance is suggested.
Construction shall comply with PennDOT Form 408 specification.
(7)
Inlet and outlet structures will be located at maximum distances
from one another. The Township Engineer may require a rock filter
berm or rock-filled gabions between inlet and outlet areas when the
distance is deemed insufficient for sediment trapping. All inlets
shall either be located above the bottom of the basin or shall discharge
to areas of the basin which slope downward to lower elevations of
the basin.
(8)
Detention or retention basin berms shall have a minimum ten-foot
top width.
(9)
Detention or retention basin berms shall be provided with an
impervious clay core and shall be keyed into the existing ground surface
at least 1 1/2 feet.
(10)
Detention basin outlet pipes shall be provided with at least
two concrete anti-seep collars per pipe. Detention basin outlet pipes
shall be reinforced-concrete pipe.
(11)
Detention basin emergency spillways shall be constructed of
a stabilized, nonerodible material with maximum 3:1 side slopes.
(12)
The discharge (or outfall) as well as the emergency spillway,
dam breast areas, or water storage area of a detention basin shall
be located at least a minimum distance from the original property
line of the parcel being developed, or any new property lines that
are created, according to the following table:
|
Drainage Area
|
Distance From Property Line
|
|---|---|
|
1 acre
|
25 feet
|
|
2 to 4 acres
|
50 feet
|
|
4 to 10 acres
|
75 feet
|
|
Over 10 acres
|
100 feet
|
(13)
Storm retention basins, if used, shall be described by metes
and bounds, which area shall be known as an easement for maintenance
and access and shall be deed restricted against removal or modification
without the express consent of East Brandywine Township.
(14)
Temporary and permanent grasses or stabilization measures shall be established on the sides of all earthen basins within 15 days of initial construction. Their maintenance and repair shall be the direct responsibility of the developer during the site preparation process, prior to a formal maintenance guarantee being established as per § 345-703 of this chapter.
E.
The grading of driveway entrances shall be so designed that normal
stormwater flow along the street gutter or roadside swale shall not
be interrupted thereby and that stormwater runoff from driveways shall
not be discharged directly into the cartway.
F.
When utilizing the SCS Method for East Brandywine Township, use the
following rainfall depths for a twenty-four-hour, Type II, storm.
G.
Erosion and sediment control during regulated earth disturbance activities.
(1)
No regulated earth disturbance activities within East Brandywine
Township shall commence until the Township receives an approval from
the Conservation District of an erosion and sediment control plan
for construction activities.
(2)
DEP has regulations that require an erosion and sediment control
plan for any earth disturbance activity of 5,000 square feet or more,
under 25 Pa. Code § 102.4(b).
(3)
In addition, under 25 Pa. Code Chapter 92, a DEP NPDES construction
activities permit is required for regulated earth disturbance activities.
(4)
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or Chester Conservation District must be provided to East Brandywine Township. The issuance of an NPDES construction permit (or permit coverage under the statewide general permit PAG-2) satisfies the requirements of § 345-403A.
(5)
A copy of the erosion and sediment control plan and any required
permit, as required by DEP regulations, shall be available on the
project site at all times.
(6)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase to maintain
maximum infiltration capacity.
(7)
Infiltration BMPs shall not be constructed or receive runoff
until the entire drainage area contributory to the infiltration BMP
has achieved final stabilization.
