A.
Applicants proposing regulated activities in the Township which are not exempt under § 170-106 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.
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 § 170-304 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.
F.
To the maximum extent practicable, post-construction stormwater shall
be discharged within the drainage area of the same stream or water
body receiving the runoff prior to construction of the proposed regulated
activity.
G.
Existing drainage peak rate discharges up to and including the 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.
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 §§ 170-305, 170-306, 170-307, and 170-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
I.
If site conditions preclude capture of runoff from limited portions
of the disturbed area for achieving water quality volume control standards,
stream channel protection standards, and the two-year, five-year and
ten-year storm event peak runoff rate reduction standards for new
development required by this chapter, the applicant shall propose
alternate methods to mitigate the bypass of the BMPs, subject to the
approval of the 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 Pa. Code and the Clean Streams
Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
K.
For all regulated activities, permanent BMPs and conveyances shall
be designed, implemented, operated, and maintained to meet the purposes
and requirements of this chapter and to meet all requirements under
Title 25 of the Pennsylvania Code, the Clean Streams Law,[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).
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.
Hot spots runoff controls. Specific structural or pollution prevention
practices may be required, as determined to be necessary by the Township
Engineer, to pretreat runoff from hot spots prior to infiltration.
Following is a list of examples of hot spots:
(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 (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.
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.
S.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with § 200-55, Flood hazard districts, of the East Whiteland Township Zoning Ordinance[4] and shall be designed to maintain the flood-carrying capacity
of the floodway such that the base flood elevations are not increased,
either upstream or downstream. The natural conveyance characteristics
of the site and the receiving floodplain shall be incorporated into
the stormwater management practices proposed for the site.
T.
Disturbance of existing ground cover during construction of the proposed
regulated activity is prohibited within 50 feet of top-of-bank of
all perennial and intermittent waterways, water bodies (lakes, ponds,
etc., and wetlands, except for activities otherwise approved by state
or local agencies (e.g., stream restoration projects, road crossings,
subsurface utility projects, etc.). At the Township Engineer's discretion,
and with conservation district and PADEP approval where necessary,
the nondisturbance buffer may be reduced because of setback or other
site constraints, but never be less than 10 feet.
The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the 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 Title 25 Pennsylvania Code Chapter 102,
or erosion and sediment control requirements of the Township.
B.
Work within natural drainage ways subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way, or require access to or from a state highway
and be subject to approval by PennDOT.
A.
No regulated activity within the 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 Title 25, Chapter 102, of the Pennsylvania Code of an erosion and sediment control plan for construction activities or projects where the area of disturbance exceeds one acre, where pond dredging is involved, or when the disturbance is associated with activities described under Title 25, Chapter 105, of the Pennsylvania Code permits;
(b)
A PADEP NPDES permit for stormwater discharges associated with
construction activities as required under Title 25 Pennsylvania Code
Chapter 92.a, if applicable;
(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 § 170-402C) 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 § 170-402B(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 § 170-304B 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 § 170-304B 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 § 170-304B and C. Appendix B[1] presents additional discussion of conservation design
and low-impact development.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
To control post-construction stormwater impacts from regulated
activities and meet state water quality requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that post-construction stormwater
discharges do not degrade the physical, chemical, or biological characteristics
of the receiving waters. The green infrastructure and low-impact development
(LID) practices provided in the PA BMP Manual, as well as the guidance
on green infrastructure, LID and Conservation Design (CD) provided
in Appendix B,[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 § 170-309D 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.
(1)
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 § 170-306. The infiltration volume required under § 170-306 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 § 170-301P.
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 § 170-304C 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 § 170-305. Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual or other PA DEP design guidance. If the runoff volume required by § 170-305 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 Township 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 § 170-306A 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 PA 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 § 170-306M).
(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 § 170-304C that are applicable to infiltration;
(2)
Be set back at least 15 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.),
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 15 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element;
(4)
All infiltration practices shall be set back at least 20 feet
from a property line and right-of-way line.
K.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1)
When a hot spot is located in the area draining to a proposed
infiltration facility, an evaluation of the potential of groundwater
contamination from the proposed infiltration facility shall be performed,
including a hydrogeologic investigation (if necessary) by a qualified
licensed professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a wellhead protection area of a public water
supply well, infiltration practices shall be in conformance with the
applicable approved source water protection assessment or source water
protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
L.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Township
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
M.
Consideration of infiltration BMPs for areas underlain by karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation are provided to the 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.
N.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hot
spots (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 hot spots,
stormwater runoff shall first be discharged through a water quality
BMP(s) to remove pollutants prior to infiltration.
O.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
P.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include, but are not limited to, leaf traps, gutter guards and cleanouts.
Q.
All infiltration practices shall have appropriate positive overflow
controls.
R.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
S.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or
like facilities shall be covered with drainage fabric. Fabric shall
be nonwoven fabric acceptable to the 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 § 170-309D 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 Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F.
All proposed stormwater management facilities shall make use of measures
to extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities, including those
that involve new development and redevelopment that are not located
in the (East) Valley Creek watershed.
A.
Post-construction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design storms specified in Table 308.1.
|
Table 308.1
Peak Rate Control Standards
(Peak flow rate of the post-construction design storm shall
be reduced to the peak flow rate of the corresponding predevelopment
design storm shown in the table)
| ||
|---|---|---|
|
Post-Construction Design Storm Frequency (24-Hour Duration)
|
Predevelopment Design Storm
| |
|
New Development regulated activities
|
Redevelopment regulated activities
| |
|
2-year
|
1-year
|
90% of 2-year
|
|
5-year
|
2-year
|
90% of 5-year
|
|
10-year
|
2-year
|
90% of 10-year
|
|
25-year
|
90% of 25-year
|
90% of 25-year
|
|
50-year
|
90% of 50-year
|
90% of 50-year
|
|
100-year
|
90% of 100-year
|
90% of 100-year
|
B.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 170-309D of this chapter.
C.
For regulated activities involving only redevelopment, no peak flow rate controls are required when and only if the total regulated impervious surface area is at least 20% less than the total existing impervious surface area to be disturbed by the regulated activity. In all cases where this requirement is not met, the redevelopment regulated activity shall achieve the peak flow rate controls presented in Table 308.1, using the redevelopment ground cover assumptions presented in § 170-309D.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
H.
Regulated activities located within the (East) Valley Creek Watershed
shall achieve the applicable peak flow release rate control requirements
presented in the approved PA Act 167 Plan for that watershed and as
presented in Table 308.1.
A.
Stormwater runoff from all regulated activity sites with a drainage
area of greater than five acres shall be calculated using a generally
accepted calculation technique(s) that is based on the NRCS Soil Cover
Complex Method. Table 309.1 summarizes acceptable computation methods.
The method selected for use shall be based on the individual limitations
and suitability of each method for a particular site. The use of the
Rational Method to estimate peak discharges for drainage areas greater
than five acres shall be permitted only upon approval by the Township
Engineer.
|
Table 309.1
Acceptable Computation Methodologies for SWM Site Plan
| ||
|---|---|---|
|
Method
|
Developed By
|
Applicability
|
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans where limitations described
in TR-55 are met
|
|
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites up to five acres, or as approved by the Township
|
|
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.
C.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration (duration) and
storm events with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates, or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times of concentration shall
be calculated based on the methodology recommended in the respective
model used. Times of concentration for channel and pipe flow shall
be computed using Manning's equation.
D.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
(1)
For regulated activities involving new development, the following
ground cover assumptions shall be used:
(2)
For regulated activities involving redevelopment, the following
ground cover assumptions shall be used:
(a)
For areas that are woods (as defined in Article II of this chapter), predevelopment calculations shall assume ground cover of woods in good condition.
(b)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of meadow.
(c)
For areas that are impervious surfaces, predevelopment calculations
shall assume at least 20% of the existing impervious surface area
to be disturbed as meadow ground cover.
(3)
The applicant shall determine which stormwater standards apply
to the proposed regulated activity as follows:
(a)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(b)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(c)
At the discretion of the 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 curve numbers (CN) for both predevelopment and proposed (post-construction)
conditions to be used in the Soil Cover Complex Method shall be obtained
from Table C-1 in Appendix C[1] of this chapter.
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
F.
Runoff coefficients (C) for both predevelopment and proposed (post-construction)
conditions for use in the Rational Method shall be obtained from Table
C-2 in Appendix C[2] of this chapter.
[2]
Editor's Note: Appendix C is included as an attachment to this chapter.
G.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
H.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended).
Hydraulic computations to determine the capacity of open channels
shall be consistent with methods and computations contained in the
Federal Highway Administration Hydraulic Engineering Circular Number
15 (Publication No. FHWA-NHI- 05-114 HEC 15, as amended). Values for
Manning's roughness coefficient (n) shall be consistent with Table
C-3 in Appendix C[3] of the ordinance.
[3]
Editor's Note: Appendix C is included as an attachment to this chapter.
A.
Open vegetated channels.
(1)
Open vegetated channels are conveyance systems that are engineered
to also perform as water quality and infiltration practices. Such
systems can be used for the conveyance, retention, infiltration and
filtration of stormwater runoff.
(2)
Open vegetated channels primarily serve a water quality function
(WQv), they also have the potential to augment infiltration. Examples
of such systems include, but are not limited to: dry swales, wet swales,
grass channels, and biofilters. Open vegetated channels are primarily
applicable for land uses such as roads, highways, residential developments
(dry swales only) and pervious areas.
(3)
Open vegetated channels shall be designed to meet the following
minimum standards:
(a)
The channel shall be designed to safely convey the ten-year
frequency storm event with a freeboard of at least six inches. Freeboard
is the difference between the elevation of the design flow in the
channel and the top elevation of the channel.
(b)
The peak velocity of the runoff from the ten-year storm shall
be nonerosive for the soil and ground cover provided in the channel.
(c)
The longitudinal slope shall be no greater than 4%.
(d)
Channels shall be trapezoidal in cross section. The minimum
bottom width shall be two feet. The maximum bottom width shall be
eight feet.
(e)
Channels shall be designed with moderate side slopes of four
horizontal to one vertical. Flatter side slopes may be necessary under
certain circumstances.
(f)
The maximum allowable ponding time in the channel shall be less
than 48 hours.
(g)
Channels (for example, dry swales) may require an underdrain
in order to function and dewater.
(h)
Accumulated sediment within the channel bottom shall be removed
when 25% of the original WQv volume has been exceeded.
(i)
Check dams along the channel length may be warranted.
(j)
The bottom of dry swales shall be situated at least two feet
above the seasonal high-water table.
(4)
Additional design information for open vegetated channels is
available in Design of Stormwater Filtering Systems (CWP, 1996).
B.
Retention basins.
(1)
Retention basins shall be designed to create a healthy ecological community with sufficient circulation of water to prevent the growth of unwanted vegetation and mosquitoes. Care should be taken to landscape retention basins in accordance with § 170-310E of this chapter.
(2)
The retention basin shall be of sufficient size to allow the
appropriate aquatic community needed to maintain healthy pond ecology
and avoid mosquitoes capable of carrying West Nile Virus and other
diseases. The Chester County Health Department, Pennsylvania Fish
and Boat Commission, the Natural Resource Conservation Service, the
Pennsylvania Extension Service, or other qualified professional consultant
shall be consulted during the design of these facilities in order
to ensure the health of aquatic communities and minimize the risk
of creating mosquito breeding areas.
(3)
An outlet structure shall be designed to allow complete drainage
of the pond for maintenance.
(4)
The design of a retention basin shall include the determination
of the proposed site's ability to support a viable permanent pool.
The design shall take into account such factors as the available and
required rate and quality of dry weather inflow, the stormwater inflow,
seasonal and longer-term variations in groundwater table, and impacts
of potential pollutant loadings.
(5)
Sediment storage volume equal to at least 20% of the volume
of the permanent pool shall be provided.
(6)
A sediment forebay with a hardened bottom shall be provided
at each inlet into the retention basin. The forebay storage capacity
shall at minimum be 10% of the permanent pool storage. The forebay
shall be designed to allow for access by maintenance equipment for
periodic cleaning.
(7)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a 100-year storm. The maximum
velocities in vegetated spillways excavated in otherwise undisturbed
soil shall be analyzed based upon the velocity of peak flow in the
emergency spillway during an assumed clogged primary outlet condition.
Where maximum velocities exceed design standards contained in the
Engineering Field Manual for Conservation Practices (USDA, SCS, July
1984) suitable lining shall be provided. All emergency spillways placed
on fill materials shall be lined. Lining for emergency spillways shall
incorporate native colors and materials where possible, including
mono slab revetments, grass pavers and native stone.
(8)
Existing ponds or permanent pool basins can be used for stormwater
management, provided that it can be demonstrated that the ponds are
structurally sound and meet the design requirements herein.
(9)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the retention basin.
(10)
Retention basins shall be designed to provide a length-to-width
ratio of at least 3L:1W as measured in plan view (for example, a ratio
of 4L:1W is too narrow).
(11)
The retention basin depth shall average three feet to six feet
with no area shallower than three feet. In residential areas, ponds
shall be equipped with management practices that reduce the potential
or unauthorized entry and use of the pond by the general public. Preference
shall be given to split rail fences equipped with mesh wire or other
such practices that are both functional and attractive. A securable
gate shall be provided to allow for periodic maintenance equipment/vehicle
access. Any fence or barrier around a retention basin shall be no
less than 42 inches in height or as otherwise required by local building
codes or ordinances.
(12)
An aquatic bench/shelf at least 10 feet wide and with a gentle
slope not exceeding 10H:1V shall be provided along the entire perimeter
of the retention basin.
(13)
Any side slopes below the permanent water surface level shall
not exceed 5H:1V. Side slopes above the permanent water surface level
shall not exceed 3H:1V.
(14)
Stabilization. Proper stabilization structures, including stilling
basins, energy dissipaters, and channel lining, shall be constructed
at the outlets of all retention basins and emergency spillways. The
stabilization structures shall control water to: avoid erosion; reduce
velocities of released water and direct water so that it does not
interfere with downstream activities.
(15)
Energy dissipaters and level spreaders. Energy dissipaters and/or
level spreaders shall be installed to prevent erosion and/or initiate
sheet flow at points where pipes or drainage ways discharge to or
from basins. Energy dissipaters shall comply with criteria in Hydraulic
Engineering Circular No. 15 Design for Stable Channels with Flexible
Linings (USDOT, FHWA, 1986) or the Engineering Field Manual for Conservation
Practices (USDA, SCS, July 1984). Such facilities shall be both functional
and harmonious with the surrounding environment; for example, native
rock shall be used in constructing dissipaters where practical.
(16)
Discharge points. The minimum distance between a proposed basin
discharge point (including the energy dissipater, etc.) and a downstream
property boundary shall in no case be less than 15 feet. Where there
is discharge onto or through adjacent properties prior to release
to a stream, designers shall demonstrate how downstream properties
are to be protected. The Township Engineer may require that the setback
distance be increased based upon factors such as topography, soil
conditions, the size of structures, the location of structures, and
discharge rates. A drainage easement may also be required.
(17)
Outlet structures. Outlet structures shall meet the following
specifications:
(a)
To minimize clogging and to facilitate cleaning and inspecting,
outlet pipes shall have an internal diameter of at least 18 inches
and a minimum grade of 1%.
(b)
Antiseep collars shall be provided on all outlet pipes within
a constructed berm.
(c)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(d)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
(e)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets in conformance with AASHTO M170, M198 and M207.
(f)
Basin outlet structures shall have childproof nonclogging trash
racks overall design openings exceeding 12 inches in diameter except
those openings designed to carry perennial stream flows. Periodic
cleaning of debris from trash racks shall be included in the operation
and maintenance plan.
(g)
Antivortex devices, consisting of a thin vertical plate normal
to the basin berm, shall be provided at the top of all circular risers
or stand pipes.
C.
Detention basins.
(1)
Detention basins are generally discouraged as a stormwater management practice and should only be used as a last resort where no other management facility is practical. Detention basins typically collect and quickly release runoff from a site in a manner that is contrary to the principles, goals and standards presented within this chapter. The landscape standards of § 170-310E of this chapter shall apply.
(2)
The maximum inside side slopes shall not exceed three horizontal: to one vertical (3H:1V). The minimum required slope for the basin bottom is 2%. A level bottom is acceptable, provided the designer demonstrates to the Township Engineer's satisfaction that the basin bottom will be landscaped with appropriate wetland vegetation pursuant to § 170-310E. In addition, detention basins of sufficient size and slope may serve other functions as well, including recreational uses which do not hinder or conflict with the function of the detention basin.
(3)
Inlet structures. The inlet pipe invert into a basin shall be
six inches above the basin floor or lining so that the pipe can adequately
drain after rainstorms. Inlets shall discharge into areas of the basin
that slope toward the outlet structure.
(4)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the retention basin.
(5)
Low-flow channels. Low-flow channels constructed of concrete
or asphalt are not permitted. Where low-flow channels are necessary,
they shall be composed of a natural or bioengineered material. Low-flow
channels shall be designed to promote water quality and slow the rate
of flow through the basin. Low-flow channels may also be designed
to infiltrate where practical.
(6)
Outlet structures. Outlet structures shall meet the following
specifications:
(a)
To minimize clogging and to facilitate cleaning and inspection,
outlet pipes shall have an internal diameter of at least 18 inches
and a minimum grade of 1%.
(b)
Antiseep collars shall be provided on all outlet pipes within
a constructed berm.
(c)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(d)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
(e)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets in conformance with AASHTO M170, Mi98 and M207.
(f)
Energy dissipation practices that convert concentrated flow
to uniform shallow sheet flow shall be used where appropriate.
(g)
Basin outlet structures shall have childproof nonclogging trash
racks overall design opening exceeding 12 inches in diameter except
those openings designed to carry perennial stream flows.
(h)
Antivortex devices, consisting of a thin vertical plate normal
to the basin berm, shall be provided at the top of all circular risers
or stand pipes.
(7)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a 100-year storm. The maximum
velocities in vegetated spillways excavated in otherwise undisturbed
soil shall be analyzed based upon the velocity of peak flow in the
emergency spillway during an assumed clogged primary outlet condition.
Where maximum velocities exceed design standards contained in the
Engineering Field Manual for Conservation Practices (USDA, SCS, July
1984) suitable lining shall be provided. In general, emergency spillways
should not be located in fill areas; all such practices placed on
fill materials shall be lined. Lining for emergency spillways shall
incorporate native colors and materials where possible, including
mono slab revetments, grass pavers and native stone.
(8)
Freeboard. Freeboard is the difference between the elevation
of the design flow in the emergency spillway (usually the 100-year
peak elevation) and the top elevation of the settled basin embankment
(that is, top of berm). The minimum freeboard shall be one foot.
(9)
Energy dissipaters and level spreaders. Energy dissipaters and/or
level spreaders shall be installed to prevent erosion and/or initiate
sheet flow at points where pipes or drainage ways discharge to or
from basins. Energy dissipaters shall comply with criteria in Hydraulic
Engineering Circular No. 15 Design for Stable Channels with Flexible
Linings (USDOT, FHWA, 1986) or the Engineering Field Manual for Conservation
Practices (USDA, SCS, July 1984). Such facilities shall be both functional
and attractive; for example, native rock shall be used in constructing
dissipaters where practical.
(10)
Stabilization. Proper stabilization structures, including stilling
basins, energy dissipaters, and channel lining, shall be constructed
at the outlets of all basins and emergency spillways. The stabilization
structures shall control water to: avoid erosion, reduce velocities
of released water and direct water so that it does not interfere with
downstream activities.
(11)
Discharge points. The minimum distance between a proposed basin
discharge point (including the energy dissipater, etc.) and a downstream
property boundary shall in no case be less than 15 feet. Where there
is discharge onto or through adjacent properties prior to release
to a stream, designers shall demonstrate how downstream properties
are to be protected. The Township engineer may require that the setback
distance be increased based upon factors such as topography, soil
conditions, the size of structures, the location of structures, and
discharge rates. A drainage easement may also be required.
(12)
A sediment forebay with a hardened bottom shall be provided
at each inlet into the detention basin. The forebay storage capacity
shall at minimum be 10% of the permanent pool storage. The forebay
shall be designed to allow for access by maintenance equipment for
periodic cleaning.
D.
Conveyance systems (open channels, drainageways, and storm sewers).
(1)
Applicants are encouraged to design conveyance systems that
encourage infiltration and improve water quality wherever practicable.
(2)
Wherever conveyance channels are necessary, drainage shall be
maintained by an open channel with landscaped banks designed to carry
the ten-year, twenty-four-hour stormwater runoff from upstream contributory
areas. The Township Engineer may increase the design storm, as conditions
require. All open channels shall be designed with one foot of freeboard
above the design water surface elevation of the design runoff condition.
(3)
Flood relief channels shall be provided and designed to convey
the runoff from the 100-year, twenty-four-hour storm, such that a
positive discharge of this runoff to an adequate receiving stream
or conveyance system occurs without allowing this runoff to encroach
upon other properties.
(4)
Open channels along existing roadways may be required to be
enclosed by the Township if PennDOT standards for safety and maintenance
cannot be satisfied. All drainage structures shall conform to the
latest edition of Form 408, PennDOT specifications.
(5)
Manholes and/or inlets shall not be spaced more than 300 feet
apart for pipe sizes up to 24 inches in diameter and not more than
450 feet apart for larger pipe sizes.
(6)
Where drainage swales are used in lieu of or in addition to storm sewers, they shall be designed to carry the required runoff without erosion and in a manner not detrimental to the properties they cross. Drainage swales shall provide a minimum grade of 2% but shall not exceed a grade of 9%. Drainage swales used strictly for conveyance are not the same as open vegetated channels. Design standards for open vegetated channels are provided under § 170-310A of this chapter.
(7)
Street curbing for the purpose of stormwater conveyance is discouraged.
On streets that must contain curbing, storm sewers shall be placed
in front of the curbing. To the greatest extent possible, storm sewers
shall not be placed directly under curbing. At curbed street intersections,
storm inlets shall be placed in the tangent section of the road.
(8)
Use of grassed swales or open vegetated swales in lieu of curbing
to convey, infiltrate and/or treat stormwater runoff from roadways
is encouraged. Inlets shall be placed at the center of the shoulder
swale draining the street and shall be located no closer than four
feet from the edge of the cartway.
(9)
The Township shall be granted a minimum twenty-foot-wide drainage
easement over all storm sewers, drainage swales, channels, etc., that
are a component of the stormwater management system when located within
undedicated land. All permanent detention basins and/or other stormwater
management facilities providing stormwater control for other than
a single residential lot shall be located within a defined drainage
easement that allows proper legal access and maintenance vehicle access
by Township personnel if the need arises for such access.
(10)
No property owner shall obstruct or alter the flow, location
or carrying capacity of a stream, channel or drainage swale to the
detriment of any other property owner, whether upstream or downstream.
All subdivision and/or land development plans containing streams,
channels, drainage swales, storm sewers or other conveyance systems
that cross property boundaries, existing or proposed, or whose discharge
crosses such boundaries shall contain a note stating the above.
(11)
Water quality inlets. Storm drainage systems that collect runoff
from parking areas and/or loading areas exceeding 10,000 square feet
of impervious coverage and discharge to stormwater management systems,
including surface or subsurface infiltration systems, shall have a
minimum of one water quality inlet per each acre of drainage area.
The purpose of water quality inlets is to remove oil, grease, and
heavy particulates or total suspended solids, hydrocarbons and other
floating substances from stormwater runoff. Methods other than water
quality inlets may be permitted if the applicant demonstrates to the
Township Engineer's satisfaction that any such alternative will be
as effective and as easily maintained. Periodic cleaning of these
systems shall be addressed in the operation and maintenance plan submitted
to the Township.
E.
Landscaping of stormwater management practices. Stormwater management
practices shall be landscaped in accordance with the following standards.
Landscape plans shall be prepared by a professional landscape architect
licensed in the Commonwealth of Pennsylvania.
(1)
Landscaping shall be required in and around all constructed
stormwater management practices with a minimum surface area of 1,000
square feet for the purposes of:
(a)
Assisting in the management of stormwater;
(b)
Stabilizing the soil within such facilities to minimize and
control erosion;
(c)
Enhancing the visual appearance of such facilities; and
(d)
Mitigating maintenance problems commonly associated with the
creation of such facilities.
(e)
A fence or suitable vegetative screen shall be provided around
all detention and retention basins as required by the Township Engineer.
(f)
All fencing shall be at least 3 1/2 feet in height and
shall be composed of the following shrubs: 1) barberry (Beris species);
2) eleagnus (eleagnus species); 3) firethorn (pyracantha species);
or 4) rose (rose species).
(g)
When vegetative screening is utilized a wire, mesh fencing shall
also be installed on the reservoir side of the basin behind the vegetative
screening so as to provide a barrier to prevent entrance to the basin
area.
(2)
A planting plan and planting schedule shall be submitted in
accordance with the following:
(a)
Wet meadows, including floors of stormwater management practices.
[1]
Wet meadows and floors of stormwater management practices shall
be planted with wildflowers and nonaggressive grasses, the intent
being to create a mixed meadow of such plantings, where appropriate.
Selection of plantings shall be based on whether the area in question
is usually well drained or permanently wet and whether the area will
be used for recreation purposes. No woody plants shall be planted
within the saturated zone (phreatic line) of a stormwater management
practice or on a berm constructed for impounded water.
[2]
Seeding by drills, corrugated rollers, cyclone or drop seeders
or hand seeding of such areas is preferred; however, hydroseeding
followed by hydromulching can be used on wet ground and steep slopes.
[3]
Fertilizers, as a nutrient supplement, shall not be used unless
it is documented that soil conditions warrant such use and nutrient
applied does not exceed plant uptake. Soil for planting of wildflowers
shall contain not less than 3% or more than 10% organic matter, as
determined by an agricultural chemist, with certification of the test
before planting.
[4]
Seeding shall take place either between April 1 and May 15 or
between September 1 and October 15. Planting areas shall be soaked
to maintain a consistent level of moisture for at least four to six
weeks after planting.
[5]
Once established, a single annual mowing when plants are dormant
should be sufficient to maintain a wet meadow and/or floor of a stormwater
management practice.
(b)
Wet edges that remain wet all or most of the year shall be planted
with wildflowers, grasses and shrubs. Plants to be located on rims
or banks, which remain dry most of the year, shall be planted with
species tolerant of dry soil conditions.
(c)
Wooded areas.
[1]
Where stormwater management practices adjoin wooded areas, trees
and shrubs shall be selected and planted outside the practice so as
to blend with existing surroundings.
[2]
Plantings in such areas shall be of sufficient density to eliminate
the need for mowing.
[3]
It is recommended that clusters of trees and shrubs be planted
around stormwater management facilities but well away from outfalls
and any constructed berms, where applicable, to provide for wildlife
habitat, wind control and buffering and screening.
[4]
Vegetation shall be planted during appropriate times of the
year, predominantly between late March and mid-May or from early October
until evidence of ground freezing, depending upon the species selected.
Most deciduous trees and shrubs can be planted in either spring or
fall. Evergreens are best planted in late summer or early fall.
(d)
Slopes.
[1]
Where slopes are gentle, a mixture of meadow grasses and wildflowers
(for wet meadows) shall be planted.
[2]
On slopes exceeding 15%, dense spreading shrubs (shrubs tolerant
of dry soils) shall be planted. Heavy mat mulch shall be used during
the period of establishment.
[3]
No woody plant materials or trees shall be located on a constructed
or natural berm acting as the impoundment structure of a stormwater
management practice. Trees shall be located downstream of an impoundment
berm a sufficient distance from the toe of the constructed slope to
assure that the toe of the slope is outside the dripline of the species
planted at maturity but in no case less than 15 feet.
[4]
In cases where stormwater management practices are to be located
in proximity to wetlands or waterways, the applicant's planting plan
and schedule shall consider the sensitive conditions existing therein
and be modified accordingly to reflect existing flora.
[5]
Stormwater management practices shall be screened in a manner
which complements the existing landscape and provides sufficient access
for maintenance.
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 100-year proposed conditions. The height of embankment shall provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the 100-year proposed conditions inflow. Should any BMP require a dam safety permit under PA Chapter 105 regulations, the facility shall be designed in accordance with and meet the regulations of PA Chapter 105 concerning dam safety. PA Chapter 105 may require the safe conveyance of storms larger than 100-year event.
C.
Any drainage conveyance facility and/or channel not governed by PA Chapter 105 regulations shall be designed to convey, without damage to the drainage facility or roadway, runoff from the twenty-five-year storm event. Larger storm events (fifty-year and 100-year storms) shall also be safely conveyed in the direction of natural flow without creating additional damage to any drainage facilities, nearby structures, or roadways.
D.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
E.
Roadway crossings or structures located within designated floodplain
areas shall be able to convey runoff from a 100-year design storm
consistent with Federal Emergency Management Agency National Flood
Insurance Program - Floodplain Management Requirements.
F.
Any stormwater management facility located within a PennDOT right-of-way
shall comply with PennDOT minimum design standards and permit submission
and approval requirements.
G.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as
amended) and the PADEP Erosion and Sediment Pollution Control Program
Manual (Publication No. 363-2134-008, as amended), or other design
guidance acceptable to the Township Engineer.
A.
In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as part of any subdivision
or land development that encompasses a stream to aid in the establishment
of a riparian buffer.
B.
Except as required by Chapter 102, the riparian buffer easement shall
be measured to be the greater of the limit of the 100-year floodplain
or a minimum of 100 feet from the top of the streambank (on each side).
C.
Minimum management requirements for riparian buffers.
(1)
Existing native vegetation shall be protected and maintained
within the riparian buffer easement.
(2)
Whenever practicable, invasive vegetation shall be actively
removed and the riparian buffer easement shall be planted with native
trees, shrubs, and other vegetation to create a diverse native plant
community appropriate to the intended ecological context of the site.
D.
The riparian buffer easement shall be enforceable by the Township
and shall be recorded in the Chester County Recorder of Deeds office,
so that it shall run with the land and shall limit the use of the
property located therein. The easement shall allow for the continued
private ownership and shall count toward the minimum lot area as required
by zoning, unless otherwise specified in the Township's Zoning Ordinance.[1]
E.
Any permitted use within the riparian buffer easement shall be conducted
in a manner that will maintain the extent of the existing 100-year
floodplain, improve or maintain the stream stability, and preserve
and protect the ecological function of the floodplain.
G.
Septic drainfields and sewage disposal systems shall not be permitted
within the riparian buffer easement and shall comply with setback
requirements established under 25 Pa. Code Chapter 73.
