A.
Applicants proposing regulated activities in the municipality which
are not exempt under § 106 shall submit a stormwater management
site plan (SWM site plan) to the municipality for review and approval
in accordance with Articles III and IV. SWM site plans approved by
the municipality shall be on site throughout the duration of the regulated
activity.
B.
The stormwater management and runoff control criteria and standards
in this ordinance shall apply to the total proposed regulated activity,
even if it is to take place in stages. The measurement of impervious
surfaces shall include all of the impervious surfaces in the total
proposed regulated activity even if the development is to take place
in stages.
C.
No regulated activity within the municipality shall commence until:
(1)
The municipality issues approval of a SWM site plan, which demonstrates
compliance with the requirements of this ordinance; and
(2)
The applicant has received a letter of adequacy or approval
for the erosion and sediment control plan review by the municipality
and the Conservation District (if required) and has received all other
local, state and federal permit approvals required for the project
involving the regulated activity.
D.
Neither submission of a SWM site plan under the provisions herein
nor compliance with the provisions of this ordinance 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 § 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 ordinance.
F.
To the maximum extent practicable, postconstruction 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 one-hundred-year storm onto or through adjacent property(ies)
or downgradient property(ies), including diffuse drainage discharge,
shall not be altered in any manner without written permission from,
and, where applicable as determined by the municipality, an easement
and agreement with, the affected landowner(s) for conveyance of discharges
onto or through their property(ies). Such discharge shall be subject
to any applicable discharge criteria specified in this ordinance.
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 §§ 305,
306, 307, and 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 ordinance, the applicant shall propose
alternate methods to mitigate the bypass of the BMPs, subject to the
approval of the municipality. 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 ordinance, to meet the erosion and sediment
control requirements of the municipality, if applicable, and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law.
K.
For all regulated activities, permanent BMPs and conveyances shall
be designed, implemented, operated, and maintained to meet the purposes
and requirements of this ordinance and to meet all requirements under
Title 25 of the Pennsylvania Code, the Clean Streams Law, and Act
167.
L.
The design of all BMPs and conveyances shall incorporate sound engineering
principles and practices in a manner that does not aggravate existing
stormwater problems as identified by the municipality. The municipality
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.
Hotspots runoff controls. Specific structural or pollution prevention
practices may be required, as determined to be necessary by the Municipal
Engineer, to pretreat runoff from hotspots prior to infiltration.
Following is a list of examples of hotspots:
(1)
Vehicle salvage yards and recycling facilities;
(2)
Vehicle fueling stations;
(3)
Vehicle service and maintenance facilities;
(4)
Vehicle and equipment cleaning facilities;
(5)
Fleet storage areas (bus, truck, etc.);
(6)
Industrial sites based on Standard Industrial Classification
Codes;
(7)
Marinas (service and maintenance areas);
(8)
Outdoor liquid container storage;
(9)
Outdoor loading/unloading facilities;
(10)
Public works storage areas;
(11)
Facilities that generate or store hazardous materials;
(12)
Commercial container nursery;
(13)
Contaminated sites/brownfields;
(14)
Other land uses and activities as designated by the municipality.
O.
Contaminated and brownfield sites. Where BMPs may contribute to the
migration of contaminants in groundwater, the water quality and runoff
volume, stream channel protection, and peak rate control standards
shall be met; however, at the Municipal Engineer's discretion, the
minimum infiltration requirement may be reduced or eliminated commensurate
with the contaminated area, and the required water quality and runoff
control measures may be increased to mitigate the reduced infiltration
requirement for the contaminated area.
P.
Additional water quality requirements. The municipality may require
additional stormwater control measures for stormwater discharges to
special management areas including, but not limited to:
(1)
Water bodies listed as "impaired" by PADEP.
(2)
Any water body or watershed with an approved total maximum daily
load (TMDL).
(3)
Areas of known existing flooding problems.
(4)
Critical areas with sensitive resources (e.g., state-designated
special-protection waters, cold-water fisheries, carbonate geology
or other groundwater recharge areas that may be highly vulnerable
to contamination, drainage areas to water supply reservoirs, etc.).
Q.
Applicants shall utilize the Pennsylvania Stormwater Best Management
Practices Manual (PA BMP Manual), as amended, or other sources acceptable
to the Municipal Engineer, for testing and design standards for BMPs,
and where there is a conflict with the provisions of this ordinance,
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 or other design guidance acceptable to the Municipal Engineer.
S.
All regulated activities located within an identified floodplain area shall comply with § 225-24 of the East Caln Township Ordinance, as amended. The natural conveyance characteristics of the site and the receiving floodplain shall be incorporated into the stormwater management practices proposed for the site.
[Amended 7-19-2017 by Ord. No. 2017-05]
T.
Disturbance of existing ground cover during construction of the proposed
regulated activity is prohibited within 50 feet of top-of-bank of
all perennial and intermittent waterways, water bodies (lakes, ponds,
etc.) and wetlands, except for activities otherwise approved by state
or local agencies (e.g., stream restoration projects, road crossings,
subsurface utility projects, etc.). At the Municipal Engineer's discretion,
and with Conservation District and PADEP approval where necessary,
the nondisturbance buffer may be reduced because of setback or other
site constraints, but never be less than 10 feet.
The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the municipality of the SWM site plan
and prior to commencement of any regulated activities, as applicable:
A.
All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at 25 Pa. Code Chapter 102, or erosion
and sediment control requirements of the municipality.
B.
Work within natural drainageways subject to permit by PADEP under
25 Pa. Code Chapter 105.
C.
Any BMP or conveyance that would be located in or adjacent to surface
waters of the commonwealth, including wetlands, subject to permit
by PADEP under 25 Pa. Code Chapter 105.
D.
Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way or require access to or from a state highway
and be subject to approval by PennDOT.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by PADEP under 25 Pa. Code
Chapter 105.
A.
No regulated activity within the municipality shall commence until:
(1)
The municipality receives documentation that the applicant has
received:
(a)
A "letter of adequacy" from the Conservation District or other
approval from PADEP in compliance with 25 Pa. Code Chapter 102 for
an erosion and sediment control plan for construction activities,
if applicable;
(b)
A PADEP NPDES construction activities permit as required under
25 Pa. Code Chapter 92a, if applicable;
(c)
Evidence of any other permit(s) or approvals required for the
regulated activities; and
(2)
An erosion and sediment control plan has been approved by the
municipality, if required.
B.
A copy of the erosion and sediment control plan and any required
permit(s), as required by PADEP regulations, shall be available on
the site at all times.
C.
Additional erosion and sediment control measures shall be applied
where infiltration BMPs are proposed, at a minimum including those
required in § 306M.
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 § 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 ordinance. The site design shall:
A.
First, identify and delineate all existing natural resources and
natural and man-made hydrologic features listed in § 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 § 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 § 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 Subsections B and C. Appendix B presents additional discussion of natural hydrology site design and sources of information for "conservation design," "low-impact design," and "sustainable design."
D.
The procedures set forth above shall be utilized to the maximum extent
practicable for the overall site design and selection, location and
design of features and BMPs to be used to comply with the requirements
of §§ 305, 306, 307 and 308.
To control postconstruction stormwater impacts from regulated
activities and meet state water quality requirements, BMPs shall be
provided in the site design that replicate predevelopment stormwater
infiltration and runoff conditions, such that postconstruction stormwater
discharges do not degrade the physical, chemical, or biological characteristics
of the receiving waters. 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 postconstruction 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 § 309D of this ordinance.
C.
The design of the facility outlet shall provide for protection from
clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E.
Water quality improvement shall be achieved in conjunction with achieving
the infiltration requirements of § 306. The infiltration
volume required under § 306 may be included as a component
of the water quality volume. If the calculated water quality and runoff
volume is greater than the volume infiltrated, then the difference
between the two volumes shall be managed for water quality and runoff
volume control through other techniques or practices but shall not
be discharged from the site.
F.
Runoff from the disturbed area shall be treated for water quality
prior to entering existing waterways or water bodies. If a stormwater
management practice does not provide water quality treatment, then
water quality BMPs shall be utilized to provide pretreatment prior
to the runoff entering the stormwater management practice.
G.
The municipality may require additional water quality and runoff
control measures for stormwater discharging to special management
areas such as those listed in § 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 § 304C that apply to water
quality and volume control.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A.
Wherever possible, infiltration should be designed to accommodate
the entire water quality and runoff volume required in § 305.
B.
For regulated activities involving new development, the volume of
a minimum of one inch of runoff from all proposed impervious surfaces
shall be infiltrated.
D.
If the requirements of Subsection B or C cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Municipal Engineer why this infiltration volume cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc.) and what alternative volume can be infiltrated; however, in all cases at least the first 0.5 inch of runoff volume shall be infiltrated.
E.
Only if a minimum of at least 0.5 inch infiltration requirement cannot
be physically accomplished on the site shall a waiver from § 306
be considered by the municipality. Waivers shall follow the procedures
in § 111.
F.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased an equivalent amount to offset the loss.
G.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
H.
Existing impervious surfaces located in areas outside of the site
(i.e., outside of the regulated activity) may be excluded from the
calculation of the required infiltration volume.
I.
A detailed soils evaluation of the site shall be conducted by a qualified
professional and at a minimum shall address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be conducted by a qualified licensed professional
and shall be consistent with the PA BMP Manual (as amended) (or other
guidance acceptable to the Municipal Engineer) and in general shall:
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(2)
Provide field tests such as double ring infiltrometer or other
hydraulic conductivity tests (at the elevation of the proposed infiltration
surface) to determine the appropriate hydraulic conductivity rate.
Standard septic/sewage percolation tests are not acceptable for design
purposes.
(3)
Design the infiltration facility for the required retention
(infiltration) volume based on field-determined infiltration capacity
(and apply safety factor as per applicable design guidelines) at the
elevation of the proposed infiltration surface.
(4)
On-lot infiltration features are encouraged; however, it shall
be demonstrated to the Municipal Engineer that the soils are conducive
to infiltration on the identified lots.
J.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
(1)
A minimum depth of 24 inches between the bottom of the BMP and
the top of the limiting zone. Additional depth may be required in
areas underlain by karst or carbonate geology (see § 306N).
(2)
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
(3)
The infiltration facility shall completely drain the retention
(infiltration) volume within three days (72 hours) from the end of
the design storm.
K.
All infiltration practices shall:
(1)
Be selected and designed to meet the criteria of § 304C
that are applicable to infiltration;
(2)
Be set back at least 10 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.),
unless otherwise approved by the Municipal Engineer;
(3)
Be set back at least 10 feet from all property lines;
(4)
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.
L.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
(1)
When a hotspot is located in the area draining to a proposed
infiltration facility, an evaluation of the potential of groundwater
contamination from the proposed infiltration facility shall be performed,
including a hydrogeologic investigation (if necessary) by a qualified
licensed professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a "wellhead protection area" of a public
water supply well, as delineated in accordance with the Pennsylvania
Wellhead Protection Program, infiltration practices shall be in conformance
with the applicable approved source water protection assessment or
source water protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
M.
During site construction, all infiltration practice components shall
be protected from compaction due to heavy equipment operation or storage
of fill or construction material. Infiltration areas shall also be
protected from sedimentation. Areas that are accidentally compacted
or graded shall be remediated to restore soil composition and porosity.
Adequate documentation to this effect shall be submitted to the Municipal
Engineer for review. All areas designated for infiltration shall not
receive runoff until the contributory drainage area has achieved final
stabilization.
N.
Consideration of infiltration BMPs for areas underlain by karst or
carbonate geology is encouraged, but only where the design, supporting
calculations, results of soils or other site investigations or other
documentation is provided to the municipality demonstrating that the
potential or likelihood of subsidence or sinkholes is minimal. Evaluation
of site conditions and infiltration design shall rely on guidance
in the PA BMP Manual (as amended) or other guidance acceptable to
the Municipal Engineer.
O.
Groundwater quality of the carbonate aquifer shall be protected from
infiltration of pollutants. At a minimum, stormwater runoff from hotspots
(i.e., sources of significant pollutant runoff) shall first be discharged
through a water quality BMP(s) to remove pollutants prior to infiltration.
Where soil characteristics are insufficient to provide removal of
pollutants from sources other than hotspots, stormwater runoff shall
first be discharged through a water quality BMP(s) to remove pollutants
prior to infiltration.
P.
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
Q.
Where roof drains are designed to discharge to infiltration practices,
they shall have appropriate measures to prevent clogging by unwanted
debris (for example, silt, leaves and vegetation). Such measures shall
include but are not limited to leaf traps, gutter guards and cleanouts.
R.
All infiltration practices shall have appropriate positive overflow
controls.
S.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
T.
The following procedures and materials shall be required during the
construction of all subsurface facilities:
(1)
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
(2)
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
(3)
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
(4)
The tops, bottoms and sides of all seepage beds, trenches, or
like facilities shall be covered with drainage fabric. Fabric shall
be nonwoven fabric acceptable to the Municipal Engineer.
(5)
Stormwater shall be distributed throughout the entire seepage
bed/trench or like facility, and provisions for the collection of
debris shall be provided in all facilities.
For regulated activities involving new development with one
or more acres of earth disturbance, the applicant shall comply with
the following stream channel protection requirements to minimize stream
channel erosion and associated water quality impacts to the receiving
waters:
A.
The peak flow rate of the postconstruction two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour-duration precipitation, using the
SCS Type II distribution.
B.
To the maximum extent practicable, and unless otherwise approved
by the Municipal Engineer, the postconstruction 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 § 309D of this ordinance.
D.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter unless otherwise approved by the Municipal
Engineer, and a trash rack shall be installed to prevent clogging.
For sites with small drainage areas contributing to the BMP that do
not provide enough runoff volume to allow a twenty-four-hour attenuation
with the three-inch orifice, the calculations shall be submitted showing
this condition.
E.
When the calculated orifice size is below three inches, gravel filters
(or other methods) are recommended to discharge low-flow rates subject
to the Municipal Engineer's satisfaction. When filters are utilized,
maintenance provisions shall be provided to ensure filters meet the
design function.
F.
All proposed stormwater 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.
Postconstruction 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 postconstruction design storm shall
be reduced to the peak flow rate of the corresponding predevelopment
design storm shown in the table.)
| |||
|
Predevelopment Design Storm
| |||
|
Postconstruction Design Storm Frequency
(24-Hour Duration)
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| |
|
2-year
|
1-year
|
2-year
| |
|
5-year
|
2-year
|
5-year
| |
|
10-year
|
2-year
|
10-year
| |
|
25-year
|
25-year
|
25-year
| |
|
50-year
|
50-year
|
50-year
| |
|
100-year
|
100-year
|
100-year
| |
B.
For modeling purposes, the predevelopment ground cover conditions
shall be determined using the corresponding ground cover assumptions
presented in § 309D of this ordinance.
C.
For regulated activities involving only redevelopment, no peak flow
rate controls are required when and only if the total proposed 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 § 309D.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this ordinance. 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 309.1 summarizes acceptable computation methods.
The method selected for use shall be based on the individual limitations
and suitability of each method for a particular site. The use of the
Rational Method to estimate peak discharges for drainage areas greater
than five acres shall be permitted only upon approval by the Municipal
Engineer.
|
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
|
U.S. 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 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 ordinance. Rainfall depths used shall be obtained
from NOAA Atlas 14 values consistent with a partial duration series.
When stormwater calculations are performed for routing procedures
or infiltration, water quality and runoff volume functions, the duration
of rainfall shall be 24 hours.
C.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration (duration) and
storm events with rainfall intensities obtained from NOAA Atlas 14
partial duration series estimates or the latest version of the PennDOT
Drainage Manual (PDM Publication 584). Times of concentration shall
be calculated based on the methodology recommended in the respective
model used. Times of concentration for channel and pipe flow shall
be computed using Manning's Equation.
D.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
(1)
For regulated activities involving new development, the following
ground cover assumptions shall be used:
(2)
For regulated activities involving redevelopment, the following
ground cover assumptions shall be used:
(a)
For areas that are woods (as defined in Article II of this ordinance),
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 ordinance.
(b)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this ordinance.
(c)
At the discretion of the Municipal Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this ordinance, 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.
F.
Runoff coefficients (C) for both predevelopment and proposed (postconstruction)
conditions for use in the Rational Method shall be obtained from Table
C-2 in Appendix C of this ordinance.
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
No. 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended). Hydraulic
computations to determine the capacity of open channels shall be consistent
with methods and computations contained in the Federal Highway Administration
Hydraulic Engineering Circular No. 15 (Publication No. FHWA-NHI-05-114
HEC 15, as amended). Values for Manning's roughness coefficient (n)
shall be consistent with Table C-3 in Appendix C of the Ordinance.
A.
Any BMP intended to hold standing water for four days or longer shall
be designed to incorporate biologic controls consistent with the West
Nile Guidance found in Appendix D,[1] PADEP document 363-0300-001, "Design Criteria - Wetlands
Replacement/Monitoring" (as amended), or contact the Pennsylvania
State Cooperative Wetland Center or the Penn State Cooperative Extension
Office for design information.
[1]
Editor's Note: Appendix D is included as an attachment to
this Appendix A185.
B.
Any stormwater basin required or regulated by this ordinance designed
to store runoff and requiring a berm or earthen embankment shall be
designed to provide an emergency spillway to safely convey flow up
to and including the one-hundred-year proposed conditions. The height
of embankment shall provide a minimum one foot of freeboard above
the maximum pool elevation computed when the facility functions for
the one-hundred-year proposed conditions inflow. Should any BMP require
a dam safety permit under 25 Pa. Code Chapter 105 regulations, the
facility shall be designed in accordance with and meet the regulations
of 25 Pa. Code Chapter 105 concerning dam safety. In addition, 25
Pa. Code Chapter 105 may require the safe conveyance of storms larger
than the one-hundred-year event.
C.
Any drainage conveyance facility and/or channel not governed by 25
Pa. Code Chapter 105 regulations shall be designed to convey, without
damage to the drainage facility or roadway, runoff from the twenty-five-year
storm event. Larger storm events (fifty-year and one-hundred-year
storms) shall also be safely conveyed in the direction of natural
flow without creating additional damage to any drainage facilities,
nearby structures, or roadways.
D.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from the facility.
E.
Roadway crossings or structures located within identified floodplain areas shall be able to convey runoff from a one-hundred-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program floodplain management requirements and Chapter 225, Zoning.
[Amended 7-19-2017 by Ord. No. 2017-05]
F.
Any 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 No. 11 (Publication No. FHWA-IP-89-016, as amended)
and the PADEP Erosion and Sediment Pollution Control Program Manual
(Publication No. 363-2134-008, as amended), or other design guidance
acceptable to the Municipal Engineer.
A.
Retention/detention basin standards.
(1)
Retention basins.
(a)
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 § 311B.
(b)
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, the Pennsylvania Fish
and Boat Commission, the Natural Resources 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.
(c)
An outlet structure shall be designed to allow complete drainage
of the pond for maintenance.
(d)
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.
(e)
Sediment storage volume equal to at least 20% of the volume
of the permanent pool shall be provided.
(f)
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.
(g)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the retention basin.
(h)
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).
(i)
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
for unauthorized entry and use of the pond by the general public,
when required by the municipality. 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.
Fences shall be designed as not to restrict the flow of water at an
emergency spillway.
(j)
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.
(k)
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.
(2)
Detention basins.
(a)
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 that the designer
demonstrates to the satisfaction of the municipality that the basin
bottom will be landscaped with appropriate wetland vegetation pursuant
to § 311B.
(b)
Inflow structures. The inflow pipe invert into a basin shall
be a minimum of 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.
(c)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the basin.
(d)
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.
(3)
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)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets in conformance with AASHTO M170, M198 and M207.
(e)
Energy dissipation practices that convert concentrated flow
to uniform shallow sheet flow shall be used where appropriate.
(f)
If required by the municipality, basin outlet structures shall have
childproof nonclogging trash racks over all 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 standpipes.
(4)
Embankment.
(a)
The fill material shall be free of roots, woody vegetation,
oversized stones, rocks, or other objectionable material. Areas on
which fill is to be placed shall be scarified prior to placement of
fill. Fill material will be placed in six-inch to nine-inch layers
and shall be compacted based upon acceptable engineering standards.
(b)
A core trench or key-way. For embankments that exceed three
feet in height, shall include a core trench or key-way along the center
line of the embankment to prevent seepage at the joint between the
existing soil and the fill material. The core trench or key-way shall
be a minimum of three feet below the existing grade and eight feet
wide with a side slope of 1:1 (horizontal:vertical) or flatter.
(5)
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 drainageways discharge to or from
basins. Energy dissipaters shall comply with criteria in the PADEP
Erosion and Sediment Pollution Control Program Manual, Technical Guidance
No. 363-2134-008, March 2012, as amended. Such facilities shall be
both functional and harmonious with the surrounding environment; for
example, native rock shall be used in constructing dissipaters where
practical.
(6)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a one-hundred-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
PADEP Erosion and Sediment Pollution Control Program Manual, Technical
Guidance No. 363-2134-008, March 2012, as amended, 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 monoslab revetments,
grass pavers, and native stone.
(7)
Freeboard. Freeboard is the difference between the elevation
of the design flow in the emergency spillway (usually the one-hundred-year
peak elevation) and the top elevation of the settled basin embankment
(top of berm). The minimum freeboard shall be one foot.
(8)
Discharge points. The minimum distance between a proposed stormwater
management facility (aboveground or subsurface detention/infiltration
facility or retention basin), discharge point (including the energy
dissipater, the emergency spillway, dam breast area, or water storage
area) and a downstream property boundary shall in no case be less
than indicated in the table below. 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
municipality 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.
|
Acres of Drainage Area
|
Distance Between Discharge Point and Downstream Property
Boundary
(feet)
| |
|---|---|---|
|
1 to 2
|
25
| |
|
2 to 4
|
50
| |
|
4 to 10
|
75
| |
|
Over 10
|
100
|
B.
Landscaping of stormwater management practices.
(1)
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.
(2)
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:
(3)
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 facility 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 the
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 weeks 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 steep slopes as defined by the municipality
Code of Ordinances, 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)
All species selection, planting methods, and maintenance shall
be designed and performed in accordance with the landscaping provisions
of the East Caln Township Subdivision and Land Development Ordinance,
where appropriate. The use of native species is encouraged.
(5)
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.
(6)
Stormwater management practices shall be screened in a manner
which complements the existing landscape and provides sufficient access
for maintenance.
C.
Conveyance systems.
(1)
Storm drainage collection systems, inlets and pipes, shall be
designed to convey a twenty-five-year storm event without surcharging
a storm drain structure. Design provisions shall be provided that
allow for the overland conveyance of the one-hundred-year storm flow
to the required stormwater management facilities, if applicable, without
damage to any private or public property.
(2)
Cross-pipes, box culverts and bridges shall be designed to convey
a one-hundred-year storm event.
(3)
Any drainage facility crossing a state highway shall conform
to all applicable PennDOT design standards.
(4)
A five-minute storm duration shall be used if this duration
does not result in a maximum expected discharge that exceeds the capacity
of a thirty-inch pipe.
(5)
If a five-minute storm duration results in a pipe size exceeding
30 inches, the time-of-concentration approach shall be used in determining
storm duration.
(6)
Curb inlets. Curb inlets shall be located at curb tangents on
the uphill side of street intersections and at intervals along the
curbline to control the maximum amount of encroachment of runoff on
the roadway pavement so that it does not exceed a width of four feet
during the design storm event. Design and location of curb inlets
shall be approved by the municipality. The maximum amount of flow
through an intersection may not exceed one inch.
(7)
Inlets and manholes shall be spaced at intervals not exceeding
300 feet and shall be located wherever branches are connected or sizes
are changed and wherever there is a change in alignment or grade.
For drainage lines of at least 36 inches diameter, inlets and manholes
shall be spaced at intervals of a 400 feet maximum.
(8)
Inlets, culverts and basin discharge systems shall be designed
for the worst case condition. Inlet capacity shall be based on design
data provided by the manufacturers and accepted by the Municipal Engineer.
If acceptable information is not available, inlets in nonponding areas
shall be designed for a maximum capacity of four cubic feet per second
(cfs). Where ponding occurs, inlet capacity shall be based on accepted
engineering design practices.
(9)
The capacity of all stormwater pipes shall be calculated utilizing
the Manning's Equation for open channel flow as applied to closed
conduit flow. The Manning's roughness coefficient shall be 0.13 for
all concrete pipe. In cases where pressure flow may occur, the hydraulic
grade line shall be calculated throughout the storm sewer system to
verify that at least one foot of freeboard will be provided in all
inlets and manholes for the design storm event.
(10)
Culverts shall be designed based on procedures contained in
Hydraulic Design of Highway Culverts, HDS No. 5, U.S. Department of
Transportation, Federal Highway Administration.
(11)
Flow velocity. Stormwater pipe systems shall be designed to
produce a minimum velocity of 3.0 feet per second when flowing full.
The maximum permissible velocity shall be 10.0 feet per second. Pipe
slopes shall not be less than 0.5%.
(12)
Minimum pipe size. Minimum pipe size shall be 18 inches.
(13)
Inlet and manhole construction. Inlet and manhole castings and
concrete construction shall be equivalent to PennDOT design standards.
(14)
Open-end pipes must be fitted with concrete endwalls and wing
walls in accordance with PennDOT standards.
(15)
All storm sewer pipes shall be laid to a minimum depth of one
foot from subgrade to crown of pipe.
(16)
Pipe materials. All storm piping shall be in accordance with
PennDOT specifications. The use of corrugated metal pipe (CMP) shall
only be permitted upon approval from the municipality on a case-by-case
basis. Piping shall be saw-cut at ends, as needed, and not hammered
or broken.
(17)
Stormwater pipes must be oriented at right angles to electric,
water, sanitary sewer, and gas utilities when crossing above or beneath
it. Crossing angles of less than 90° will only be permitted at
the discretion of the municipality. When skewed crossings are permitted,
interior angles between alignment of the storm sewer pipe and utility
may not be less than 45°.
(18)
Modified inlet boxes shall be used when required in accordance
with the PennDOT Publication 72 - RC Standards. Pipes shall not be
allowed to enter the corner of boxes. The crown of the pipe shall
be at least four inches below the top of the precast structure box.
(19)
All manholes, endwalls, inlet boxes, inlet grates and inlet
hoods shall be constructed in accordance with PennDOT Publication
408 and Publication 72 - RC standards.
(20)
Permanent open swales shall be designed to convey a one-hundred-year
storm event computed from Manning's Equation.
(21)
Open swales shall be stabilized with vegetation or other materials
in accordance with Title 25 Chapter 102 Rules and Regulations of the
Pennsylvania Department of Environmental Protection. Slopes for swale
banks shall not exceed one foot vertical for every four feet horizontal.
(22)
A minimum of a twenty-foot-wide access and maintenance easement
shall be provided for and centered on all storm sewers and open swales
not located within the public right-of-way.
