A.
Any person or entity intending to engage in a regulated activity shall, prior to commencing the regulated activity, apply for and obtain a stormwater management permit and comply with the requirements of this chapter. An application for stormwater management permit shall be submitted on a form prescribed by the Township accompanied by the required fee. Applicants proposing regulated activities in the Township which are not exempt under § 270-6 shall submit a stormwater management site plan (SWM site plan) to the Township for review and approval in accordance with Articles III and IV. SWM site plans approved by the Township shall be on site throughout the duration of the regulated activity.
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 the stormwater management permit and 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 reviewed 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 § 270-18 for the overall site design and for selection, location and design of features and BMPs to be used to comply with the requirements of this chapter.
F.
To the maximum extent practicable, post-construction stormwater shall
be discharged within the drainage area of the same stream or water
body receiving the runoff prior to construction of the proposed regulated
activity.
G.
For regulated activities with one acre or more of proposed earth
disturbance, existing drainage peak rate discharges up to and including
the 100-year storm and the volume of runoff up to and including the
two-year storm onto or through adjacent property(ies) or downgradient
property(ies), including diffuse drainage discharge, shall not be
altered in any manner by regulated activities under this chapter without
written permission from, and, where applicable as determined by the
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 §§ 270-19, 270-20, 270-21 and 270-22). 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 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.
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, and the
Stormwater Management Act.[2]
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
and 32 P.S. § 680.1 et seq., respectively.
L.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the 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 (PA BMP Manual), as amended, or other sources acceptable
to the Township for soil testing and design standards for BMPs, and
where there is a conflict with the provisions of this chapter, the
most restrictive applies as determined by the Township.
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.
S.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with Chapter 325, Article XIII, Floodplain Conservation, 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.
Riparian buffer area (RBA).
(1)
Areas immediately adjacent to the Township's perennial streams,
intermittent streams, and waterways are defined as the riparian buffer
areas (RBA). In the RBA, special requirements as set forth in this
section shall apply for the purpose of maintaining natural functions.
These requirements are established due to:
(2)
The RBA includes three subzones, Zones 1 through 3, extending landward
from the top of the stream bank where different requirements are imposed.
(a)
Zone 1: a minimum twenty-five-foot setback, plus wetlands, measured
from the top of the stream bank. No disturbance of vegetation or soil
shall be permitted except for restoration or reforestation activities.
The purpose of this limitation is to promote shading of the stream
with natural vegetation, to provide a source of organic inputs to
the aquatic system, to anchor the stream bank and floodplain area,
and to remove nitrogen, sediment, and other substances that can adversely
affect stream systems. Where prohibitive slopes (25% and steeper)
are located within 35 feet of the stream bank, Zone 1 shall extend
the entire distance of this sloped area.
(b)
Zone 2: a 125-foot buffer zone extending outward from Zone 1;
the combined width of Zone 1 and Zone 2 shall not be less than 150
feet. Within Zone 2, disturbance of natural vegetative cover shall
be limited to selective logging (not to exceed 25%) and other activities
that minimally disrupt the existing tree and soil cover. The purpose
of this limitation is to maximize filtering and overall physical removal
of particulate-form pollutants from runoff generated upgradient and
to promote subsurfance vegetative uptake of nitrogen and other nonparticulate
elements from stormwater generated upgradient.
[1]
Where the Zone 2 riparian buffer is not wooded, it shall be
maintained as a filter strip of dense grass and forbs or other features
that will provide sediment filtering, nutrient uptake, and will convert
concentrated flow to uniform, shallow, sheet flow. When a land development
is proposed where there is no established vegetated or wooded buffer
(such as in areas previously cultivated for agriculture), Zone 1 and
Zone 2 combined shall be not less than 150 feet wide and shall be
established and maintained in accordance with the following guidelines:
[a]
Forested and unforested vegetation shall be established
through natural succession. Selective planting shall be incorporated
on sites devoid of vegetation to stimulate native species and to discourage
invasive species.
[b]
Plant selection and planting shall be consistent
with Chester County Conservation District, PADEP, or USDA riparian
forested buffer guidance.
[2]
Reforestation is a natural BMP and is encouraged; it may be
considered in the stormwater design calculations for stormwater quantity
and quality control.
(c)
Zone 3: a zone of varying width extending outward from Zone
2, defined as upslope areas adjacent to Zone 2 that are being disturbed
during the land development process and where direct discharge of
stormwater would otherwise occur. Zone 3 is intended to include level
spreading or similar devices as necessary to ensure that any direct
discharge flows are properly distributed as sheet flow and channelization
and point source discharges are avoided. Zone 3 shall include a minimum
twenty-five-foot setback, measured from Zone 2, from impervious surfaces.
(3)
The RBA may be included in net density calculations with uses permitted
in the West Whiteland Township Zoning Ordinance of 1998,[3] unless this RBA is required to be subtracted out as the
result of other applicable regulations of the said ordinance, such
as regulation of the floodplain.
[3]
Editor's Note: See 35 P.S. § 691.1 et seq.
and 32 P.S. § 680.1 et seq., respectively.
(4)
An RBA adjacent to "high-quality waters" and "exceptional-value waters"
designated under PADEP, Chapter 93, Rules and Regulations, shall be
subject to the provisions of the PADEP Special Protection Waters Implementation
Handbook and its amendments.
(5)
RBAs shall be restricted to the following uses:
(a)
Zone 1 (very restricted): flood control; utility rights-of-way;
pervious footpaths.
(b)
Zone 2 (restricted): pervious surfaces; passive recreational
uses; natural stormwater BMPs; tree removal by permit.
(c)
Zone 3 setback (limited restrictions): lawn and garden areas;
sheds and similar structures covering less than 150 square feet; composting
of yard wastes; nonstructural stormwater BMPs.
(6)
RBAs shall be preserved or restored with native vegetation that can
be maintained through the delineation, plan review, construction,
and occupancy stages of development.
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 drainageways subject to permit by PADEP under
Title 25 Pennsylvania Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under Title 25 Pennsylvania Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way, or require access to or from a state highway
and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under Title 25 Pennsylvania
Code Chapter 105.
A.
No regulated activity within the 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, if applicable;
(b)
A PADEP NPDES Construction Activities Permit as required under
Title 25 Pennsylvania Code Chapter 92,[1] if applicable;
[1]
Editor's Note: Chapter 92 of Title 25 of the Pennsylvania
Code was reserved 10-8-2010, effective 10-9-2010. It was replaced
by Chapter 92a, National Pollutant Discharge Elimination System (NPDES)
Permitting, Monitoring and Compliance.
(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. For regulated activities with 10,000 or more square feet of proposed earth disturbance or 2,000 or more square feet of proposed impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in § 270-32C) 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 § 270-32B(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 § 270-18B 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 § 270-18B 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 § 270-18B and C. Appendix B[1] presents additional discussion of natural hydrology site
design and sources of information for conservation design, low-impact
development, and sustainable design.
[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 § 270-23D of this chapter.
C.
The design of the stormwater management facility outlet shall provide
for protection from clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters. The applicant shall fulfill
the requirements of the PADEP "Thermal Impact Analysis" for the "PAG-02
Stormwater Discharges Associated with Construction Activities, NOI
for Coverage under General or Individual Permit" if they cannot meet
the volume control requirements. 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 § 270-20. The infiltration volume required under § 270-20 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 § 270-15P.
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 § 270-18C 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: Said appendix 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 is § 270-19.
(1)
Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual, other PADEP design guidance or as noted elsewhere in this chapter. If the runoff volume required by § 270-19 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 § 270-20A 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; however in all cases at least the first 0.5 inch of runoff volume shall 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
licensed 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 § 270-20M).
(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:
(2)
Be set back at least 20 feet from all buildings and features with
subgrade elements (e.g., basements, foundation walls, etc.) and 20
feet from all property lines or right-of-way boundaries unless otherwise
approved by the Township;
(3)
For any infiltration practice that collects runoff from shared or
multiple features and that is located within 20 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element.
K.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1)
When a hotspot is located in the area draining to a proposed infiltration
facility, an evaluation of the potential of groundwater contamination
from the proposed infiltration facility shall be performed, including
a hydrogeologic investigation (if necessary) by a qualified licensed
professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a "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.
N.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. Stormwater runoff from hotspots (i.e.,
sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged through a water quality BMP(s) to remove pollutants
prior to infiltration.
O.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
P.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
Q.
All infiltration practices shall have appropriate positive overflow
controls.
R.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
S.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed with
equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior to the
placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines, shall
be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or like
facilities shall be covered with drainage fabric. Fabric shall be
nonwoven fabric acceptable to the 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 § 270-23D 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 facilities shall make use of measures to
extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities including those
that involve new development and redevelopment.
A.
Post-construction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design storms specified in Table 22.1.
|
Table 22.1
Peak Rate Control Standards
|
|---|
|
The limits of each district (watershed) are illustrated on the
Official Stormwater Management Release Rate Map, provided in Appendix
D.[1]
|
|
For the Chester Creek District, the peak post-development runoff
rates for the two-, five-, ten-, twenty-five-, fifty-, and 100-year
design storms shall be no greater than 50% of the predevelopment peak
discharges. For all other districts, the peak flow rate of the post-construction
design storm shall be reduced to the peak flow rate of the corresponding
predevelopment design storm as shown in the table below.
|
|
Post-Construction Design Storm Frequency
(24-Hour Duration)
|
Predevelopment Design Storm
| |
|---|---|---|
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| |
|
2-year
|
1-year
|
2-year
|
|
5-year
|
5-year
|
5-year
|
|
10-year
|
10-year
|
10-year
|
|
25-year
|
25-year
|
25-year
|
|
50-year
|
50-year
|
50-year
|
|
100-year
|
100-year
|
100-year
|
[1]
Editor's Note: Appendix D is included as attachment to
this chapter.
B.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 270-23D 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 22.1, using the redevelopment ground cover assumptions presented in § 270-23D. This design criterion for redevelopment is only permitted with approval of Municipal Engineer. It shall result in no impact on downstream properties.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
A.
Stormwater runoff from all regulated activity sites with a drainage
area of greater than five acres shall be calculated using a generally
accepted calculation technique(s) that is based on the NRCS Soil Cover
Complex Method. Table 23.1 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 23.1
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 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)
Stormwater standards for new development shall apply to regulated
activities that involve a combination of both new development and
redevelopment activities, as defined in this chapter.
E.
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.
F.
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.
G.
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)
Stormwater standards for new development shall apply to regulated
activities that involve a combination of both new development and
redevelopment activities, as defined in this chapter.
H.
Runoff coefficients and curve numbers shall be consistent with those
provided in Appendix C, or as otherwise approved by the Township Engineer.
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 PADEP document 363-0300-001"Design Criteria
— Wetlands Replacement/Monitoring" (as amended), or contact
the Penn State Cooperative Wetland Center or the Penn State Cooperative
Extension Office for design information.
B.
Any stormwater basin required or regulated by this chapter designed
to store runoff and requiring a berm or earthen embankment shall be
designed to provide an emergency spillway to safely convey flow up
to and including the 100-year proposed conditions. The height of embankment
shall provide a minimum one foot of freeboard above the maximum pool
elevation computed when the facility functions for the 100-year proposed
conditions inflow. Should any BMP require a dam safety permit under
PA Chapter 105 regulations,[1] 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.
[1]
Editor's Note: See 25 Pa. Code Chapter 105.
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.
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.
E.
Any stormwater management facility located within a PennDOT right-of-way
shall comply with PennDOT minimum design standards and permit submission
and approval requirements.
F.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular Number 11 (Publication No. FHWA-IP-89-016, as
amended) and the PADEP Erosion and Sediment Pollution Control Program
Manual (Publication No. 363-2134-008, as amended), or other design
guidance acceptable to the Municipal Engineer.
A.
Any stormwater management facility (i.e., detention basin) designed
to store runoff regulated by this chapter shall be designed to provide
an emergency spillway to handle flow up to and including the 100-year
post development conditions. The height of the embankment (top of
berm) must be set so as to provide a minimum one foot of freeboard
above the maximum pool elevation computed when the facility functions
for the 100-year post development inflow. Should any stormwater management
facility require a dam safety permit under PADEP Chapter 105, the
facility shall be designed in accordance with Chapter 105[1] and meet the regulations of Chapter 105 concerning dam
safety which may be required to pass storms larger than the 100-year
event. Any inspections of this dam required by any regulatory agency
shall be prepared by competent consultants and at the cost of the
maintainer of the facility.
[1]
Editor's Note: See 25 Pa. Code Chapter 105.
B.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures) and any work involving wetlands
as directed in PADEP Chapter 105 regulations (as amended or replaced
from time to time by PADEP) shall be designed in accordance with Chapter
105[2] and will require a permit from PADEP. Any other drainage
conveyance facility that does not fall under Chapter 105 regulations
shall be designed to convey, without damage to the drainage structure
or roadway, runoff from a minimum twenty-five-year design storm. The
Township may require design based on a larger storm event. Any facility
that constitutes a dam as defined in PADEP Chapter 105 regulations
may require a permit under dam safety regulations. Any facility located
within a PennDOT right-of-way must meet PennDOT minimum design standards
and permit submission requirements. If the primary drainage facilities
do not have capacity for future flows, then a safe drainage path must
be provided to convey up to the 100-year design storm without impacting
structures.
[2]
Editor's Note: See 25 Pa. Code Chapter 105.
C.
Storm sewers must be able to convey post development runoff from
a minimum twenty-five-year design storm without surcharging inlets.
D.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
E.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Township shall reserve
the right to reject any design that would result in the occurrence
or continuation of an adverse hydrologic, hydraulic, or water quality
condition within the watershed.
F.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
G.
The existing points of concentrated drainage that discharge onto
adjacent property shall not be altered without permission of the adjacent
property owner(s) and shall be subject to applicable discharge criteria
specified in this chapter.
H.
Areas of existing diffused drainage discharge shall be subject to applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the applicant must document to the Township in accordance with § 270-25I that adequate downstream conveyance exists to safely transport the concentrated discharge, or the applicant must obtain drainage easements from affected downstream property owners and provide the facilities to safely convey the flow.
I.
Downstream hydraulic capacity analysis. Any downstream capacity hydraulic
analysis conducted in accordance with this chapter shall use the following
criteria for determining adequacy for accepting increased peak flow
rates:
(1)
Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year return period event within
their banks at velocities consistent with protection of the channels
from erosion. Acceptable velocities shall be based upon criteria included
in the PADEP Erosion and Sediment Pollution Control Program Manual.
(2)
Natural or man-made channels or swales must be able to convey the
increased twenty-five-year return period runoff without creating any
hazard or adverse effects to adjacent or downstream persons or property.
(3)
Culverts, bridges, storm sewers, and all other facilities that must
pass or convey flows from the tributary area must be designed in accordance
with PADEP Chapter 105 regulations (if applicable) and, at a minimum,
pass the increased twenty-five-year return period runoff.
J.
Where a development site is traversed by watercourses, riparian buffers shall be provided conforming to the line of such watercourses. The width of the buffers shall be determined as set forth in § 270-15T, here above. Excavating, placing of fill, building structures, or making any alterations that may adversely affect the flow of stormwater within any portion of the riparian buffer shall be prohibited unless the proposed work is associated with a regulated wetlands mitigation program. The buffer must be defined and restricted by a restrictive covenant.
K.
When it can be shown that, due to topographic conditions, natural
drainage ways on the site cannot adequately provide for drainage,
open channels may be constructed conforming substantially to the line
and grade of such natural drainage ways. Work within natural drainage
ways shall be subject to approval by PADEP through the joint permit
application process, or, where deemed appropriate by PADEP, through
the general permit process.
L.
Any stormwater management facilities regulated by this chapter that
would be located in wetlands or within the floodway (as determined
by FEMA or 50 feet from top of bank) of waters of the commonwealth
shall be subject to approval by PADEP through the joint permit application
process, or, where deemed appropriate by PADEP, the general permit
process. When there is a question as to whether wetlands may be involved,
it is the responsibility of the applicant or his agent to show that
the land in question cannot be classified as wetlands; otherwise,
approval to work in the area must be obtained from PADEP.
M.
Any stormwater management facilities regulated by this chapter located
on or flowing into state highway rights-of-way or stormwater systems
shall be subject to approval by PennDOT.
N.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, or other BMPs are required, where
soil conditions permit, to reduce the size or eliminate the need for
detention facilities.
O.
In order to promote overland flow and infiltration/percolation of
stormwater, roof drains must discharge into an accepted BMP providing
infiltration and filtering of the stormwater.
A.
All existing and proposed 100-year floodplains shall be delineated on the drainage plan (see § 270-32). If the 100-year floodplain is not mapped by the Federal Emergency Management Agency as part of the National Flood Insurance Program, the horizontal and vertical limits of the floodplain shall be determined utilizing the standard step method (i.e., HEC-RAS or similar approved computer model). If the HEC-RAS model is used, the applicant shall submit a computer disc containing all input files for the calculations, in order to expedite the floodplain review. If the drainage area is less than 100 acres, the Manning's equation shall be used.
B.
The methods below shall be used to compute the design flow(s) in
the drainage course although other methods may be used with approval
of the Township. A conservative average of two methods shall be used,
and the design flow is subject to approval by the Township.
(1)
The graphical and tabular methods in TR-55. The graphical method
may be used for streams whose drainage area at the point of interest
is no larger than 2,000 acres, and the tabular hydrograph method may
be used for drainage areas up to 20 square miles (12,800 acres).
(2)
The Rational Method may be used for streams whose drainage area at
the point of interest is no larger than 320 acres.
(3)
The method in Water Resources Bulletin Number 13, Floods in Pennsylvania,
issued by the Pennsylvania Department of Environmental Resources (now
PADEP), may be used for streams whose drainage area at the point of
interest is larger than two square miles.
(4)
The Procedure PSU-IV for Estimating Design Flood Peaks on Ungaged
Pennsylvania Watersheds.
(5)
The Penn State Runoff Model (PSRM).
C.
All development activity within a special flood hazard area designated
by the Federal Emergency Management Agency shall comply with Article
XIII of the West Whiteland Township Zoning Ordinance[1] and this subsection. All development 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.
A.
Infiltration devices shall be selected based on suitability of soils
and site conditions. Suitability of soils shall be determined by soil
infiltration testing, with suitability defined as having minimum percolation
rates of 0.5 inch per hour at the elevation of the bottom of the facility;
lower rates may be acceptable if approved by the Township.
(1)
Soil infiltration testing shall be performed for all proposed infiltration
areas; soil testing shall include evaluation of appropriate soil horizons
with deep pits and percolation measurements, making sure to assess
percolation rates at the proposed infiltration device bed bottom.
Soil testing, including the methodology, frequency, and locations
of the tests, shall be based on the type of proposed BMP and on the
known geology of the area and be reviewed and approved by the Township
as advised by the Township Engineer. The design soil infiltration
rate shall be the average infiltration rate measured at each proposed
area. Soil infiltration tests shall be performed within the soil horizon
in which the bottom elevation of the proposed infiltration areas lie.
The location and number of test pits and percolation holes shall be
determined based on the type(s) of stormwater management practices
being designed in consultation with and guidance from the Township
Engineer.
(2)
The Township Engineer shall be given a minimum of three working days'
notice of when the testing will be completed.
B.
The lowest elevation of the infiltration area shall be at least two
feet above the seasonal high-water table ("SHWT") and bedrock, except
in areas of carbonate geology, in which case the distance shall be
four feet.
C.
All roof drains that discharge to infiltration systems shall have
appropriate measures to prevent clogging by vegetation and to prevent
sinkhole formation.
D.
All infiltration systems shall have appropriate positive overflow
controls within one foot of the finished surface or grade.
E.
All infiltration systems shall have a setback of 15 feet from all
residential structures and property lines. Negative impact to below-ground
structures shall be prevented.
F.
All infiltration systems shall be designed to infiltrate the stored
volume within 48 hours or whatever longer period may be specified
by the Township.
G.
All surface inflows shall be designed to minimize the discharge of
sediment into the infiltration system in order to prevent sediment
accumulation, which reduces stormwater storage capacity and ultimately
clogs the infiltration mechanism.
H.
Special provisions are required when using infiltration BMPs in carbonate
areas in order to avoid groundwater contamination and the formation
of solution channels and sinkholes.
(1)
Because the potential for these problems to develop increases as
soil thickness decreases, the soil mantle should be determined to
be a minimum of four feet in thickness, in order both to remove pollutants
and to disperse uniformly groundwater movement, which is important
to avoid the formation of solution channels.
(2)
In carbonate areas, BMPs that disperse stormwater over the largest
feasible area shall be used (e.g., subtle berms and level spreaders)
so as not to modify the natural hydrologic regime significantly. Use
of infiltration BMPs that result in significant increases (more than
100%) in the predevelopment rate of infiltration per unit area shall
be avoided in carbonate areas, unless a detailed geologic evaluation
(see below) demonstrates that the potential for sinkhole formation
is minimal.
(3)
Stormwater runoff from significant pollutant-producing sources ("hotspots," as defined in § 270-15N) shall be filtered and/or pretreated using a water quality BMP before being discharged in carbonate areas.
(4)
In these cases, the Township may require that a detailed geologic
evaluation of the project site be performed to determine the suitability
for recharge, including both the potential for groundwater contamination
and potential for sinkhole formation. The evaluation shall be performed
by a registered professional geologist licensed in the Commonwealth
of Pennsylvania and/or any other Township-approved professional, and
shall, at a minimum, address soil permeability, depth to bedrock,
susceptibility to sinkhole formation, and subgrade stability.
I.
The following procedures and materials shall be required during the
construction of all subsurface facilities and shall be noted on the
plans:
(1)
The area of the subsurface facilities shall be protected by orange
safety fence prior to the start of construction; equipment and traffic
shall be prohibited from traveling over the proposed location(s) of
the facilities.
(2)
Excavation shall be performed with equipment that will not compact
the bottom of the bed. Construction equipment shall not be allowed
on excavated areas of the seepage beds at any time during its construction.
(3)
The bottom of the bed shall be scarified immediately prior to the
placement of the geotextile fabric on the bottom of the bed.
(4)
The bed shall be protected during construction. Sediment shall not
be allowed to be washed back into the bed both during the time when
the bottom of the bed is open and once the stone is in place.
(5)
Only uniformly graded, clean aggregate, free of fines, slate, shale,
clay, silt, and vegetative material shall be used. The porosity of
the stone must meet the plan specification, and the supplier of the
stone shall provide certification of such for the stone delivered
to the site. The design engineer shall verify that the porosity meets
or exceeds the requirements of the design and submit such verification
to the Township.
(6)
Compaction of the infiltration bed is prohibited. Excavation of the
final two feet of the bed shall be completed from outside the bed
or by other means approved by the Township to prevent compaction of
the bed floor.
(7)
Construction equipment no heavier than light-tracked vehicles shall
be allowed on the top of the stone only after it has been placed to
its full depth and solely for the purpose of completion of the placement
of the stone and backfill.
(8)
Drainage filter fabric shall be placed in accordance with manufacturer's
directions, including pipe penetrations, and shall overlap a minimum
of 18 inches.
(9)
During site construction, all facility components shall be protected from sedimentation using storm inlet protection in conformance with the PADEP Chapter 102 regulations, as amended, and the Erosion and Sediment Pollution Control Manual, as amended. Inlet protection shall remain until the contributory drainage area has achieved full stabilization. If sediment enters the system, the contractor shall be required to clean out the sediment to the satisfaction of the Township; this may require the reconstruction of the infiltration facility in whole or in part.
(10)
The geotextile fabric around the infiltration bed shall be PennDOT
nonwoven Class 1 and shall completely enclose the stone (all sides,
top and bottom). Other materials may be used if approved by the Township;
documentation supporting the use of such other materials shall be
submitted to the Township for review.
(11)
Upon completion, the applicant shall be responsible for post-construction
testing of the facilities sufficient to demonstrate that such facilities
function as intended, such testing shall be coordinated with the Township
Engineer and shall be included in any developer agreement required
for a land development.
J.
BMP design.
(1)
The applicant shall submit designs for water quality facilities to
the Township for review and approval. Such designs shall achieve the
water quality objectives through a combination of BMPs. The use of
multiple, small, unconnected BMPs on a site, rather than one large
stormwater management facility, shall be required.
(2)
All BMPs, including structural and nonstructural BMPs, shall be designed
to assure that they function in accordance with the standards set
forth in the latest version of the Pennsylvania Stormwater BMP Manual.
The specifications and construction of BMPs shall be in accordance
with the 2000 Maryland Stormwater Design Manual.
(3)
In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
(a)
A total contributing area.
(b)
Permeability and infiltration rate of the site soils.
(c)
Slope and depth to bedrock.
(d)
Seasonal high-water table.
(e)
Proximity to building foundations and wellheads.
(f)
Erodibility of soils.
(g)
Land availability and configuration of the topography.
(h)
Consistency with approved watershed and stormwater management
plans or regulations.
(i)
Impact on neighboring and downstream properties and facilities.
(4)
The following additional factors shall be considered when evaluating
the suitability of BMPs used to control water quality at a given development
site: Peak discharge and required volume control.
(a)
Peak discharge and required volume control.
(b)
Stream bank erosion.
(c)
Efficiency of the BMPs to mitigate potential water quality problems.
(d)
The volume of runoff that will be effectively treated.
(e)
The nature of the pollutant being removed.
(f)
Maintenance requirements.
(g)
Creation or protection of aquatic and wildlife habitat.
(h)
Recreational value.
(i)
Enhancement of aesthetic and property value.
(j)
Impact on neighboring and downstream properties and facilities.
Basins, storm sewers, culverts, bridges and other structural
installations shall be provided to accommodate stormwater safely where
natural nonstructural practices are not feasible and where stormwater
flows otherwise would have adverse impact on the environment and the
general welfare of the Township and its citizens in order to:
A.
Maintain natural hydrologic regimes of streams and watercourses.
Such flows may be redirected as required, subject to the approval
of PADEP.
B.
Promote drainage of all low points along the line of streets. Overflow
swales shall be designed to convey the full 100-year storm flows away
from all street low points. These swales shall be located to prevent
flooding of the downslope lots.
C.
Intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area drained, and to prevent
substantial flow of water across intersections or flooding of intersections
during the design storm stipulated elsewhere in this section.
D.
Insure adequate and unimpeded flow of stormwater under driveways
in, near, or across natural watercourses or drainage swales. Suitable
pipes or other waterway openings shall be provided as necessary.
A.
All detention and retention basins shall meet the requirements of
this section.
(1)
Basins shall be installed prior to any earthmoving or land disturbances
that they will serve. The phasing of their construction shall be noted
in the erosion and sediment pollution control ("E&S") plan and
accompanying narrative. Permanent vegetation shall be established
prior to denuding any other land, unless the basin functions as an
E&S device.
(2)
Energy dissipaters and/or level spreaders shall be installed at points
approved by the Township where pipes or drainageways discharge from
basins. Multiple outlet structures and multiple outlet piping from
the basin may be required by the Township to reduce the impact of
point discharges.
(3)
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 slopes of the basin shall not exceed one foot vertical
in three feet horizontal.
(c)
Concrete, stone, and brick walls are prohibited as elements
of a basin.
(4)
Basins shall also be designed to satisfy the following requirements:
(a)
The minimum top of berm width shall be eight feet.
(b)
Outlet pipes shall have a minimum diameter of 12 inches. For
pipe lengths exceeding 100 feet, the minimum diameter shall be 15
inches.
(c)
Properly spaced anti-seep collars shall be installed on all
basin outlet pipes. Design calculations shall be provided, which shall
demonstrate the adequacy of the design.
(d)
All basins shall be constructed with a compacted relatively
impervious (Unified Soil Classification CL-ML or CL) key trench and
core. The key trench shall extend at least two feet into undisturbed
subsoil (below topsoil layer). The minimum bottom width of the trench
shall be six feet and the minimum top width of the core shall be four
feet. The side slopes of the compacted core and trench shall not exceed
one horizontal to one vertical, and the top elevation of the core
shall be set at or above the twenty-five-year design water elevation.
The Township may require an impermeable liner to be installed up to
the 100-year design water surface elevation.
(5)
Basin outlet structures and emergency spillways.
(a)
Outlet structures designed to control peak discharge flows and
to distribute the flows by pipes to discharge areas shall be constructed
primarily of concrete or masonry material and shall have childproof,
nonclogging trash racks over all design openings, except those openings
designed to carry perennial stream flows. The preferred trash rack
material shall be epoxy-coated, or stainless steel; other materials
may be permitted if approved by the Township.
(b)
Six inches of freeboard shall be provided between the crest
of the primary outlet structure and the invert of the emergency spillway.
(c)
Emergency spillways shall be constructed in undisturbed earth
wherever possible. When constructed in fill, sod, precast concrete
paving blocks, concrete, or permanent erosion control matting shall
be used. Design calculations shall be submitted demonstrating that
the specified material can withstand velocities based on the 100-year
design storm event. When using sod, it shall be applied along the
inside slope above the twenty-five-year water surface elevation, along
the face and sides of the spillway, and down the outside slope to
the existing grade. Emergency spillways shall be designed to convey
safely the 100-year basin inflow hydrograph through the basin assuming
the principal outlet is completely blocked and the basin water surface
elevation is equal to the spillway invert elevation.
(d)
Emergency spillways shall be designed with a minimum of one
foot of freeboard and shall be designed to convey the 100-year storm
event considering that the stormwater management facility is full
up to the spillway invert elevation.
(e)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(f)
Facilities shall be both functional and harmonious in design
with the surrounding environment. The use of architecturally treated
concrete, stucco, or stone facade treatment shall be utilized for
enhancing the outlet structure.
(6)
An earthen berm between inlet and outlet areas shall be required
when the distance is deemed by the Township to be insufficient for
sediment trappings.
(7)
Permanent grasses or stabilization measures shall be established
on the sides of all earthen basins by hydroseeding within five days
of initial construction (or conversion from sediment basin or sediment
trap). The Township may require jute or erosion control matting to
be installed inside the basin or on the basin embankment.
(8)
Stormwater runoff shall discharge to a suitable natural drainage course (except where prohibited by the riparian buffer area regulations of § 270-15T, hereinabove) or storm sewer system. Where not possible or not permitted, level spreading devices or other suitable facilities (i.e., swale) shall be designed with sufficient capacity to convey the 100-year storm event without creating any safety, flooding, or property hazard. Securing of necessary drainage easements for this purpose shall be the sole responsibility of the developer.
(9)
The developer shall test soil samples from the site to determine
if the soils are suitable for berm embankment construction. If in
the opinion of the Township the soils are found to be unsuitable,
the developer shall import suitable soils for constructing the basin
berm.
(10)
Basin inlet and outlet structures shall be located at maximum
distances from one another.
(11)
In residential areas, ponds shall be equipped with management
practices that reduce the potential for 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 access by maintenance equipment. Such barriers
shall have a minimum height of 42 inches.
(12)
The minimum distance between a proposed basin discharge point
(including the energy dissipater) and a downstream property boundary
shall in no case be less than 20 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 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
shall also be required.
B.
Detention basins shall also meet the following requirements. The
minimum bottom slope shall be 2% unless the basin is designed to allow
for infiltration or with an underdrain system. Slight variation in
elevation (six to 12 inches) is encouraged to promote plant diversity.
C.
Retention basins shall also meet the following requirements:
(1)
The retention basin depth shall average three feet to six feet with
no area shallower than three feet.
(2)
An aquatic bench or shelf at least 10 feet wide and with a gentle
slope not exceeding 10H:IV shall be provided along the entire perimeter
of the retention basin.
(3)
Any side slopes below the permanent water surface level shall not
exceed 5H:IV.
D.
Swale design.
(1)
Grass swales not specifically designed as BMP devices shall have
a minimum bottom slope of 2% and their depth shall be deep enough
to convey the twenty-five-year storm event with a minimum of one foot
of freeboard. Swale linings shall be designed based on the ten-year
velocity. Swales shall have sufficient capacity to convey the 100-year
storm discharge without creating any safety or property hazard.
(2)
Swales, when located outside of the Township right-of-way, shall
be located within an easement of sufficient width to allow access
for maintenance and to convey all storms up to and including the 100-year
storm; in no case shall such a right-of-way be less than 20 feet wide.
A note on the plan shall indicate that the easement allows the Township
the right, though not the responsibility, to perform needed maintenance
and/or repairs and to invoice the property owner for such work. The
note shall further state that, in the event that the property owner
fails to pay such invoice, the Township shall have the right to lien
the subject property.
(3)
Cross country swales shall be designed as bio-swales and include
check dams to remove sediment.
E.
Storm sewer design.
(1)
Where storm sewer pipe and inlets are required, they shall be placed
immediately in front of the curb within the right-of-way. Any storm
sewer pipe which outlets on private residential property shall only
discharge to the rear yard and must be a minimum of 50 feet from habitable
structures.
(2)
When storm sewers are placed outside of the right-of-way of a street,
they shall be placed within an easement not less than 20 feet unless
otherwise approved by the Township. Larger easements may be required
if site conditions dictate additional area is required as determined
by the Township. Open ditches shall be avoided. Diversion terraces
and swales can be considered if conditions are warranted (soil erodibility,
slope and liner type) and approved by the Township Engineer and/or
Conservation District, as applicable.
(3)
Storm sewers shall have a minimum diameter of 15 inches and only
reinforced concrete or smooth lined corrugated polyethylene pipe shall
be used. The minimum grade of the pipe shall be 1/2%. Smooth bore
corrugated polyethylene pipe up to a maximum diameter of 48 inches
may be used. Smaller diameter storm sewers may be permitted for BMPs
in accordance with DEP guidelines if approved by the Township Engineer.
(4)
All storm sewer utilities beneath a paved surface shall be bedded
and backfilled with PennDOT No. 2A stone. This backfill shall be placed
in six-inch lifts and solidly compacted to the satisfaction of the
Township.
(5)
In carbonate areas, watertight pipe connections are required and
appropriate specifications shall be indicated on the plans.
(6)
Headwalls, endwalls, or end sections shall be required on all open
pipes, shall be of concrete construction, and shall be set on a minimum
of 12 inches of AASHTO No. 57 (PennDOT 2B) coarse aggregate.
(7)
All storm sewers shall be constructed per PennDOT specifications as outlined in Publication 408 Design Manual, Part 2, Highway Design and Standards for Roadway Construction, RC-Series unless otherwise dictated by the Township Code.
(8)
Any changes in alignment shall be straight sections connected by
inlets or manholes.
(9)
When there is a change in pipe size through an inlet, the top inside
elevation of the outlet pipe shall be at or below the top inside elevations
of all incoming pipes.
(10)
Storm sewer sizes shall be determined based upon the following
design storm frequencies:
(a)
Twenty-five years, unless otherwise specified by the Township.
(b)
The design of storm sewer systems within the drainage area of
detention or retention facilities must be analyzed for adequacy during
the 100-year storm, including the effects of the control facility
tailwater. This may require a hydraulic grade line analysis. When
approved by the Township, overflow swales may be provided at low points
in streets to convey safely the full 100-year peak flow to the control
facility, in lieu of providing the full capacity in the storm sewer.
(11)
Storm sewer design shall be based upon PennDOT design methods.
Inlet efficiency and bypass flow shall be determined for all inlets,
and the gutter flow spread shall not exceed 1/2 the travel lane width
or, where on-street parking is permitted, a maximum of eight feet.
The Township may require that a hydraulic grade line analysis be performed
on storm sewer systems.
(12)
Culverts shall be evaluated for inlet and outlet control restrictions.
(13)
Rainfall intensities 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).
(14)
Manholes and/or inlets shall not be more than 300 feet apart
on pipe sizes up to 24 inches and not more than 400 feet apart on
larger sizes.
(15)
Inlets, manholes, covers, and frames shall conform to PennDOT
specifications. At street intersections, inlets shall be placed along
the tangent and not in the curved portion of the curbing.
(16)
When precast concrete inlets or manholes are used within a street,
a minimum of two courses of brick masonry or a grade ring shall be
placed to bring the grate or cover to proper elevation. If brick is
used, every third vertical joint shall be left open; if grade ring(s)
are used, the horizontal joint(s) shall be left open to facilitate
drainage of the base course. Geotextile fabric shall be wrapped around
the outside of the brick masonry or grade ring(s) to prevent fines
from washing into the structure.
F.
Roof drains.
(1)
Stormwater roof drains shall not discharge water directly over a
sidewalk or into any sanitary sewer line.
(2)
Except for single-family homes, proposed roof drains and collector
locations shall be shown on the storm drainage plans. Roof drains
and collectors shall meet all appropriate Township Code requirements.
G.
Design of bridges and culverts.
(1)
Bridges and culverts shall have ample waterway opening to carry the
design flows, based on a minimum storm frequency of 100 years. One-hundred-year
water depths shall not exceed six inches above the roadway center
line elevation. Bridge and culvert construction shall be in accordance
with PennDOT specifications and shall also meet PADEP requirements.
The appropriate permits and approvals must be acquired by the applicant
prior to final plan approval.
(2)
Culverts shall be provided with wing walls and constructed for the
full width of the right-of-way. If the Township determines that the
character of the road is expected to change in the future, the cartway
of the bridge shall be made to anticipate this condition. On each
side of the bridge cartway, the bridge railing must be set back from
the edge of the final cartway for the accommodation of sidewalks,
and/or bicycle trails.
A.
Grading shall not redirect or concentrate surface water onto an adjacent
property.
B.
Lots shall be laid out and graded to provide positive drainage away
from buildings.
C.
Procedures for protecting soils or geologic structures with water
supply potential from contamination by surface water or other disruption
by construction activity shall be established in consultation with
the Township. Such areas shall include, at minimum, those underlain
by carbonate formations. The Township shall require pollution control
facilities to be provided on existing or proposed stormwater management
systems within or adjacent to the project site.
D.
Provisions for protecting existing wells and other water supplies
shall be established.
E.
Graded slopes shall not be steeper than three horizontal units to
one vertical unit.
G.
A minimum of four inches of topsoil shall be provided on all disturbed
areas prior to final seeding and mulching.
H.
Mature healthy trees of at least six inches diameter at breast height
along with other significant existing vegetation within the limits
of earth disturbance shall be located in the field and on the storm
drainage plan and shall be retained and protected. Such trees shall
not be removed except as provided on the approved subdivision or land
development plan. The filling of soil over the roots of trees to be
preserved is prohibited. Roots are presumed to extend out from the
tree as far as the tree's branches extend outward.
I.
Landscaping requirements.
(1)
Stormwater management facilities 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.
(2)
Landscaping shall be required in and around all constructed stormwater
management facilities having a surface area of 1,000 square feet or
more for the purposes of:
(3)
A planting plan and planting schedule shall be submitted in accordance
with the following:
(a)
For wet meadows, including floors of stormwater management basins:
[1]
Wet meadows and the floors of stormwater management basins shall
be planted with native wildflowers and native warm season 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 basin or on a berm constructed for impounded water.
[2]
Seeding by drills, corrugated rollers, cyclone or drop seeders,
or hand seeding is preferred for these areas; however, hydroseeding
followed by hydromulching may 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 the nutrient
applied does not exceed plant uptake. Soil for planting of wildflowers
shall contain not less than 3% nor more than 10% organic matter, as
determined by an agricultural chemist, with certification of the test
before planting.
[4]
Seeding shall take place 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, only a single annual mowing when plants are
dormant is required to maintain the landscaping described hereinabove.
Maintenance practices shall include eradication of invasive species
as identified by the Pennsylvania Department of Conservation and Natural
Resources ("DCNR").
(b)
Wet edges that remain wet all or most of the year shall be planted
with wildflowers, grasses, shrubs, or an appropriate combination of
these elements. 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 facilities adjoin wooded areas,
native trees and shrubs shall be selected and planted outside the
facility so as to blend with existing surroundings.
[2]
Plantings in such areas shall be of sufficient density to eliminate
the need for mowing.
[3]
Clusters of trees and shrubs should be planted around stormwater
management facilities but well away from outfalls and constructed
berms to provide for wildlife habitat, wind control, buffering, and
visual 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 may be planted in either spring or
fall; evergreens are best planted in late summer or early fall.
[5]
Maintenance practices shall include eradication of invasive
species as identified by the DCNR.
(d)
Slopes.
[1]
Where slopes are gentle 10H:1V or flatter, a mixture of native
meadow grasses and wildflowers (for wet meadows) shall be planted.
[2]
On steep slopes (as defined by the West Whiteland Township Zoning
Ordinance), native dense spreading 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 berm
acting as an impoundment structure for a stormwater management facility.
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]
Maintenance practices shall include eradication of invasive
species as identified by the DCNR.
(e)
In cases where stormwater management facilities are to be located
in proximity to wetlands or waterways, the applicant shall consider
such conditions and the planting plan and schedule and shall reflect
consideration of existing flora.
(f)
Stormwater management facilities shall be screened in a manner
that complements the existing landscape and provides sufficient access
for maintenance.
