[Ord. No. 261, 3/6/2023]
1.
Applicants proposing regulated activities in the municipality which are not exempt under § 23-106 shall submit a stormwater management site plan (SWM site plan) to the municipality for review and approval in accordance with Parts 3 and 4. Regulated activities under Item 1) in § 23-106, Subsection 2, are exempt from a SWM site plan in accordance with Parts 3 and 4 if the regulated activities do not involve §§ 23-310 and/or 23-311. Regulated activities under Item 1) in § 23-106, Subsection 2, may be exempt from the following sections marked with "*." SWM site plans approved by the municipality shall be on-site throughout the duration of the regulated activity.
2.
The intent of these design standards is to encourage environmentally
sound stormwater management practices that provide necessary drainage
facilities while protecting the hydrologic characteristics and water
quality of the site and watershed. Land developments shall be required
to incorporate infiltration and stormwater management controls. Stormwater
management design shall blend into the natural environment and be
aesthetically integrated into the site design. Typical water quality
treatment measures can be found in Appendix E.[1]
[1]
Editor's Note: Appendix E is included as an attachment to this chapter.
3.
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.
4.
No regulated activity within the municipality shall commence until:
A.
The municipality issues approval of a SWM site plan, which demonstrates
compliance with the requirements of this chapter; and
B.
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.
5.
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.
6.
All proposed stormwater management facilities shall be designed to
prevent the pollution of groundwater resources by stormwater, promote
safety, minimize health hazards, preserve natural features and provide
infiltration and groundwater recharge where appropriate.
7.
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 § 23-304 for the overall site design and for selection, location, and design of features and BMPs to be used to comply with the requirements of this chapter.
8.
The stormwater management system shall not create an adverse impact
on stormwater quantity or quality in either upstream or downstream
areas. Off-site areas which discharge to or across a site proposed
for development shall be addressed in the stormwater management plan
prepared for the development. No stormwater management plan shall
be approved until it is demonstrated that the runoff from the project
shall not adversely impact downstream areas.
9.
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.
10.
Where watercourses traverse a development site, drainage easements
shall be provided conforming to the line of such watercourses. The
terms of the easements shall prohibit excavation, the placing of fill
or structures, any alterations that may adversely affect the flow
of stormwater within any portions of the easement and require the
establishment and protection of riparian buffers.
11.
Earth Disturbance.
A.
For regulated activities with equal or more than 1,000 square feet
of proposed earth disturbance and equal or more than 500 square feet
of regulated impervious surfaces, 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 municipality, 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.
B.
For all regulated activities with one acre or less of proposed earth
disturbance, the applicant shall provide proof of written notification
to all adjoining landowner(s) describing the proposed regulated activity
and proposed stormwater control discharge(s). The written notification
must include a request for written comment to be sent to the municipality
within 30 days of the receipt of the notification.
12.
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 §§ 23-305, 23-306, 23-307, and 23-308). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
13.
If site conditions preclude capture of runoff from limited portions
of the disturbed area for achieving water quality volume control standards,
stream channel protection standards, and the two-year, five-year,
and ten-year storm event peak runoff rate reduction standards for
new development required by this chapter, the applicant shall propose
alternate methods to mitigate the bypass of the BMPs, subject to the
approval of the Municipal Engineer. In no case shall resulting peak
rate be greater than the predevelopment peak rate for the equivalent
design storm.
14.
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 municipality, if applicable, and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law.[2]
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
15.
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,[3] and the Storm Water Management Act.[4]
16.
Where deemed necessary by the Municipal Engineer, or as addressed
in an approved Act 167 stormwater management plan, the applicant shall
construct storm drains to handle on-site runoff; to the maximum extent
permitted under the Municipalities Planning Code and Act 167, or any
amendments thereto, provide on-site/off-site drainage easements; and
provide for the conveyance of off-site runoff to an acceptable outlet
in the same watershed.
17.
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).
18.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by the PADEP through the joint permit
application process, or the general permit process, as required by
PADEP regulations. When there is a question whether wetlands may be
involved, it is the responsibility of the applicant or his agent to
demonstrate that the land in question is not classified as wetlands.
Otherwise approval to work in the area shall be obtained from PADEP
as determined through the jurisdictional determination process.
19.
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.
20.
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:
A.
Vehicle salvage yards and recycling facilities;
B.
Vehicle fueling stations;
C.
Vehicle service and maintenance facilities;
D.
Vehicle and equipment cleaning facilities;
E.
Fleet storage areas (bus, truck, etc.);
F.
Industrial sites based on standard industrial classification codes;
G.
Marinas (service and maintenance areas);
H.
Outdoor liquid container storage;
I.
Outdoor loading/unloading facilities;
J.
Public works storage areas;
K.
Facilities that generate or store hazardous materials;
L.
Commercial container nursery;
M.
Contaminated sites/brownfields;
N.
Other land uses and activities as designated by the municipality.
21.
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.
22.
Additional Water Quality Requirements. The municipality may require
additional stormwater control measures for stormwater discharges to
special management areas including, but not limited to:
A.
Water bodies listed as "impaired" by PADEP.
B.
Any water body or watershed with an approved total maximum daily
load (TMDL).
C.
Areas of known existing flooding problems.
D.
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.).
23.
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 chapter,
the most restrictive applies.
24.
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
Municipal Engineer.
25.
All regulated activities located within a special flood hazard area
designated by the Federal Emergency Management Agency (FEMA) shall
comply with the Zoning Ordinance[5] 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.
26.
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.
27.
With conservation district and PADEP approval where necessary, disturbance
of existing ground cover during construction of the proposed regulated
activity is prohibited within 10 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.).
28.
At the time of application for a building permit for any approved
lot created by a subdivision and/or improved as a land development
project, issuance of the permit shall be conditioned upon adherence
to the terms of this chapter.
[Ord. No. 261, 3/6/2023]
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:
1.
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.
2.
All work within natural drainageways that may or may not be subject
to permit by PADEP under 25 Pa. Code Chapter 105.
3.
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.
4.
Any stormwater management facility, BMP, conveyance, or part thereof
regulated by this chapter that would be located on or discharge to
a state highway right-of-way or require access to or from a state
highway or municipal road right-of-way shall be subject to approval
by PennDOT and the municipality.
5.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area.
6.
Any facility which may constitute a dam subject to permit by PADEP
under 25 Pa. Code Chapter 105.
[Ord. No. 261, 3/6/2023]
1.
No regulated activity within the municipality shall commence until:
A.
The municipality receives documentation that the applicant has received:
(1)
A "letter of adequacy" from the conservation district or other
approval from PADEP in compliance with 25 Pa. Code Chapter 102 of
an erosion and sediment control plan for construction activities for
projects where the area of disturbance exceeds 5,000 square feet,
where pond dredging is involved, or when the disturbance is associated
with activities described under 25 Pa. Code Chapter 105 permits;
(2)
A PADEP NPDES permit for stormwater discharges associated with
construction activities as required under 25 Pa. Code Chapter 92a,
if applicable;
(3)
Evidence of any other permit(s) or approvals required for the
regulated activities; and
B.
An erosion and sediment control plan has been approved by the municipality.
Those projects whose earth disturbance will be less than 5,000 square
feet and therefore do not need approval from the Chester County Conservation
District shall submit a plan to the Borough in accordance with Chapter
102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania,
PADEP, Subpart C, Protection of Natural Resources, Part II, Water
Resources, Chapter 102, Erosion Control.
2.
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.
3.
Additional erosion and sediment control measures shall be applied where infiltration BMPs are proposed, at a minimum including those required in § 23-306, Subsection 13.
B.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain their maximum
infiltration capacity.
C.
In order to ensure compliance with Chapter 102, the timing of the
installation and operation of the infiltration BMP shall be at the
discretion of the Borough Engineer.
[Ord. No. 261, 3/6/2023]
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 1,000 or more square feet of proposed earth disturbance or 500 or more square feet of regulated impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in § 23-402, Subsection 2C) 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:
2.
Second, provide a prioritized listing of these resources and features
to identify:
A.
Those to be incorporated into the site design in a manner that provides
protection from any disturbance or impact from the proposed regulated
activity;
B.
Those to be protected from further disturbance or impact but for
which the proposed regulated activity will provide improvement to
existing conditions;
C.
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
D.
Those that may be considered for alteration, disturbance, or removal.
3.
Third, develop the site design to achieve the following:
B.
Minimize earth disturbance (both surface and subsurface);
C.
Maximize protection of or improvement to natural resources and special
management areas;
D.
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;
F.
Maximize infiltration and the use of natural site infiltration features,
patterns and conditions, and evapotranspiration features;
G.
Apply selective grading design methods to provide final grading patterns
or preserve existing topography in order to evenly distribute runoff
and minimize concentrated flows;
H.
Minimize the cumulative area to be covered by impervious surfaces
and:
I.
Minimize the volume and peak discharge rates of stormwater generated;
J.
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
M.
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
landscape-based management in common open space areas, where practicable;
and
N.
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of § 23-304, Subsections 2 and 3. Appendix H[1] presents additional discussion of conservation design
and low-impact development.
[1]
Editor's Note: Appendix H is included as an attachment to this chapter.
[Ord. No. 261, 3/6/2023]
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 H,[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:
1.
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.
3.
The design of the stormwater management facility outlet shall provide
for protection from clogging and unwanted sedimentation.
4.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
5.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 23-306. The infiltration volume required under § 23-306 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.
6.
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. Typical
water quality treatment measures can be found in Appendix E.[2]
[2]
Editor's Note: Appendix E is included as an attachment to this chapter.
8.
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.
9.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
10.
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.
12.
Evapotranspiration may be quantified and credited towards meeting
volume requirements according to the PADEP Post Construction Stormwater
Management (PCSM) Spreadsheet and Instructions (December 2020) or
the most recent guidance from PADEP.
[1]
Editor's Note: Appendix H is included as an attachment to this chapter.
[Ord. No. 261, 3/6/2023]
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:
1.
Wherever possible, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 23-305.
2.
For regulated activities involving both new development and redevelopment (when new additional impervious surface is created), infiltration should be designed to accommodate the entire water quality and runoff volume required in § 23-305. Infiltration BMPs should be consistent with the design and infiltration period guidelines included in the PA BMP Manual or other PADEP design guidance. If the runoff volume required by § 23-305 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) or other PADEP guidance, such as the Managed Release Concept, may be used to manage this volume with approval from the Municipal Engineer.
3.
For regulated activities involving both new development and redevelopment
(when new additional impervious surface is created), the volume of
a minimum of one inch of runoff from all regulated impervious surfaces
shall be infiltrated.
4.
If the requirements of § 23-306, Subsection 2 or 3, 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.
6.
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.
7.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
8.
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.
9.
A detailed soils evaluation of the site shall be conducted by a qualified
professional witnessed by the Municipal Engineer 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:
A.
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.
B.
Provide municipality a minimum of two witnessed field tests such
as double ring infiltrometer or other hydraulic conductivity tests
(at the elevation of the proposed infiltration surface for each infiltration
facility BMP) to determine the appropriate hydraulic conductivity
rate. Standard septic/sewage percolation tests are not acceptable
for design purposes.
C.
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.
D.
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.
10.
Infiltration BMPs shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics:
B.
An infiltration rate sufficient to accept the additional stormwater
volume and drain completely as determined by field tests conducted
by the applicant.
C.
The infiltration facility shall completely drain the retention (infiltration)
volume within three days (72 hours) from the end of the design storm.
11.
All infiltration practices shall:
B.
Be set back at least 15 feet from all buildings and features with
subgrade elements (e.g., basements, foundation walls, etc.), unless
otherwise approved by the Municipal Engineer.
C.
For any infiltration practice that collects runoff from shared or
multiple features and that is located within 25 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.
12.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater:
A.
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.
B.
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.
C.
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
13.
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. Infiltration 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. Remediation must be witnessed
and approved by the municipality. All areas designated for infiltration
shall not receive runoff until the contributory drainage area has
achieved final stabilization.
14.
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 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.
15.
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.
16.
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.
17.
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.
18.
All infiltration practices shall have appropriate positive overflow
controls.
19.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
20.
The following procedures and materials shall be required during the
construction of all subsurface infiltration facilities:
A.
Excavation for the infiltration facility shall be performed with
equipment that will not compact the bottom of the seepage bed/trench
or like facility.
B.
Confirmation infiltration testing per § 23-306, Subsections 9 and 10, shall be conducted to verify the design parameters of the bed and/or trench. Testing yielding different results from the original work will require the construction work be stopped and revised design calculations be submitted for review and approval by the municipality, prior to completion of construction.
C.
The bottom of the bed and/or trench shall be scarified prior to the
placement of aggregate.
D.
Only clean aggregate with documented porosity, free of fines, shall
be allowed.
E.
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.
F.
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.
[Ord. No. 261, 3/6/2023]
For regulated activities involving new development with greater
than 500 square feet 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:
1.
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.
2.
To the maximum extent practicable, and unless otherwise approved
by the Municipal 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).
4.
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.
5.
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.
6.
All proposed stormwater management facilities shall make use of measures
to extend the flow path and increase the travel time of flows in the
facility.
7.
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.
[Ord. No. 261, 3/6/2023]
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities including those
that involve new development and redevelopment:
1.
Post-construction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design storms specified in Table 308.1.
|
Table 308.1
Peak Rate Control Standards
| ||
|---|---|---|
|
(Peak Flow Rate of the Post-Construction Design Storm Shall
Be Reduced to the Peak Flow Rate of the Corresponding Predevelopment
Design Storm Shown in the Table)
| ||
|
Post-Construction Design Storm Frequency (24-Hour Duration)
|
Predevelopment Design Storm
| |
|
New Development Regulated Activities
|
Redevelopment Regulated Activities
| |
|
2-year
|
1-year
|
2-year
|
|
5-year
|
2-year
|
5-year
|
|
10-year
|
2-year
|
10-year
|
|
25-year
|
90% of 25-year
|
25-year
|
|
50-year
|
90% of 50-year
|
50-year
|
|
100-year
|
90% of 100-year
|
100-year
|
3.
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 the same location and equal to 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 § 23-309, Subsection 4.
4.
Only the area of the proposed new additional 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.
5.
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.
6.
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.
7.
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.
[Ord. No. 261, 3/6/2023]
1.
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
|
US Army Corps of Engineers
|
Applicable where use of a full hydrologic computer model is
desirable or necessary
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites up to five acres, or as approved by the municipality
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
|
2.
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.
3.
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.
4.
The applicant shall utilize the following ground cover assumptions
for all predevelopment water quality and runoff volume, infiltration
volume and peak flow rate calculations:
A.
For regulated activities involving new development, the following
ground cover assumptions shall be used:
(1)
For areas that are woods (as defined in Part 2 of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(2)
For all other areas (including all impervious surfaces), predevelopment
calculations shall assume at least 20% of the existing impervious
surface area to be disturbed as "meadow" ground cover.
B.
For regulated activities involving redevelopment (when new additional
impervious surface is created), the following ground cover assumptions
shall be used:
(1)
For areas that are woods (as defined in Part 2 of this chapter), predevelopment calculations shall assume ground cover of "woods in good condition."
(2)
For areas that are not woods or not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(3)
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.
C.
The applicant shall determine which stormwater standards apply to
the proposed regulated activity as follows:
(1)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities
as defined in this chapter.
(2)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only redevelopment activities as
defined in this chapter.
(3)
At the discretion of the Municipal Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this chapter, may either:
(a)
Apply the stormwater standards (redevelopment or new development)
that are associated with the activity that involves the greatest amount
of land area; or
(b)
Apply the redevelopment and new development stormwater standards
to the corresponding redevelopment and new development portions of
the proposed regulated activity.
5.
Runoff curve numbers (CN) for both predevelopment and proposed (post-construction)
conditions to be used in the Soil Cover Complex Method shall be obtained
from Table C-1 in Appendix C[1] of this chapter.
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
6.
Runoff coefficients (C) for both predevelopment and proposed (post-construction)
conditions for use in the Rational Method shall be obtained from Table
C-2 in Appendix C of this chapter.
7.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
8.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5, as amended).
Hydraulic computations to determine the capacity of open channels
shall be consistent with methods and computations contained in the
Federal Highway Administration Hydraulic Engineering Circular Number
15 (Publication No. FHWA-NHI-05-114 HEC 15, as amended). Values for
Manning's roughness coefficient (n) shall be consistent with
Table C-3 in Appendix C of the chapter.
10.
Hydrographs may be obtained from the NRCS method such as TR-55, TR-20,
or from use of the modified or unit hydrograph rational methods. If
modified or unit hydrograph rational methods are used, the ascending
leg of the hydrograph shall have a length equal to three times the
time of concentration (3xTc), and the descending leg shall have a
length equal to seven times the time of concentration (7xTc) to approximate
an SCS Type II hydrograph.
[Ord. No. 261, 3/6/2023]
1.
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 B,[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 B is included as an attachment to this chapter.
2.
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. Code Chapter 105 regulations, the facility shall be designed in
accordance with and meet the regulations of Pa. Code Chapter 105 concerning
dam safety. Pennsylvania Code Chapter 105 may require the safe conveyance
of storms larger than the 100-year event.
3.
All facilities that constitute water obstructions (e.g., culverts,
outfalls, or stream enclosures) and any work involving wetlands governed
by DEP Chapter 105 regulations (as amended or replaced from time to
time by DEP) shall be designed in accordance with Chapter 105 and
will require a permit from DEP.
4.
Any drainage conveyance facility and/or channel not governed by Pa.
Code Chapter 105 regulations shall be designed to convey, without
damage to the drainage facility or roadway, runoff from the twenty-five-year
storm event. Larger storm events (fifty-year and 100-year storms)
shall also be safely conveyed in the direction of natural flow without
creating additional damage to any drainage facilities, nearby structures,
or roadways.
5.
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.
6.
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.
7.
Any stormwater management facility located within a PennDOT right-of-way
shall comply with PennDOT minimum design standards and permit submission
and approval requirements.
8.
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.
9.
Stormwater Management Facilities (BMPs).
A.
Retention Basins. The standards for retention basins listed in this
section are minimum standards, and the professional designer is responsible
for the ultimate final design.
(1)
General Requirements.
(a)
Where retention (permanent pool) basins are utilized, adequate
assurances of access easement, maintenance, indemnification, liability
insurance, and security shall be provided and approved by the Borough.
(b)
Retention facilities shall be designed and located so as not
to present a hazard to the public health or safety. Such facilities
shall be designed so that no adverse effects will result from backwater
flooding. Their design shall be approved by the Borough.
(c)
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. 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 the subdivision regulations.[2] The Chester County Health Department, Pennsylvania Fish
and Boat Commission, the Natural Resource Conservation Service, the
Pennsylvania Extension Service, or other qualified professional consultant
shall be consulted during the design of these facilities in order
to ensure the health of aquatic communities and minimize the risk
of creating mosquito breeding areas.
(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)
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.
(f)
The design of all retention facilities shall incorporate sound
engineering principles and practices. The Borough shall reserve the
right to disapprove any design that would result in the occurrence
or continuation of an adverse hydrologic or hydraulic condition within
the watershed.
(2)
Design Considerations.
(a)
A water surface limit shall be no closer than 100 feet to any
building.
(b)
No retention facility shall be placed within 100 feet of a special
geologic feature. No stormwater subsurface conveyance facility shall
be constructed within 50 feet of a special geologic feature, unless
it is constructed of durable pipe utilizing watertight joints.
(c)
The height of berm shall not exceed 15 feet unless approved
by the Borough and DEP.
(d)
If this retention facility is considered to be a dam as per
DEP Chapter 105, the design of the facility must be consistent with
the Chapter 105 regulations, and may be required to pass a storm greater
than the 100-year event.
(e)
The minimum top of berm width shall be eight feet and the outside
side slopes shall not be steeper than three horizontal to one vertical
(3H:1V).
(f)
The top of berm shall be constructed at least six inches above
the design elevations to allow for settlement of the embankment.
(g)
Any side slopes below the permanent water surface level shall
not be steeper than 5H:1V. Inside slopes above the permanent water
surface level shall not be steeper than three horizontal to one vertical
(3H:1V).
(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 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. 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.
(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)
All retention 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 permanent design water elevation. The
Borough may require an impermeable liner to be installed up to the
100-year design water surface elevation.
(l)
The Borough may require soil samples from the site to be analyzed
to determine if these soils are suitable for berm embankment construction.
If, in the opinion of the Borough, the soils are found to be unsuitable,
the developer shall import suitable soils for constructing the basin.
(m)
Sediment storage volume equal to at least 20% of the volume
of the permanent pool shall be provided.
(n)
A sediment forebay with a hardened bottom shall be provided
at each inlet into the retention basin. The forebay storage capacity
shall at minimum be 10% of the permanent pool storage. The forebay
shall be designed to allow for access by maintenance equipment for
periodic cleaning.
(o)
Inlet pipes and outlet pipes shall be separated to the greatest
extent possible in order to maximize the flow path through the retention
basin.
(p)
Outlets shall meet the following specifications:
1)
The outlet shall be designed to allow complete drainage of the
pond for maintenance.
2)
To minimize clogging and to facilitate cleaning and inspecting,
outlet pipes shall have an internal diameter of at least 18 inches
and a minimum grade of 1%.
3)
Properly spaced antiseep collars as recommended by PADEP shall
be installed on all basin outlet pipes within a constructed berm.
Design calculations shall be provided.
4)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
5)
Outlet pipes shall be constructed of reinforced concrete with
watertight rubber gaskets in conformance with AASHTO M170, M198 and
M207.
6)
Basin outlets shall have childproof, nonclogging, nonrusting
trash racks over all design openings exceeding nine inches in diameter.
Periodic cleaning of debris from trash racks shall be included in
the operation and maintenance plan.
7)
Antivortex devices, consisting of a thin, nonrusting vertical
plate normal to the basin berm, shall be provided at the top of all
circular risers or standpipes.
8)
Discharge Points. The minimum distance between a proposed basin
discharge point (including the energy dissipater, etc.) and a downstream
property boundary shall in no case be less than 15 feet. Where there
is discharge onto or through adjacent properties prior to release
to a stream, designers shall demonstrate how downstream properties
are to be protected. The Municipal Engineer may require that the setback
distance be increased based upon factors such as topography, soil
conditions, the size of structures, the location of pipes, and discharge
rates. A drainage easement may also be required.
9)
Energy dissipaters that convey concentrated flow to uniform
shallow sheet flow shall be used where appropriate. 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.
10)
Energy dissipaters shall comply with criteria in
PADEP Chapter 102. Such facilities shall be both functional and harmonious
with the surrounding environment; for example, native rock shall be
used in constructing dissipaters where practical.
(q)
Basin Emergency Spillways.
1)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a 100-year storm. All stormwater
retention facilities shall provide a minimum of 1.0 foot of freeboard
above the maximum pool elevation associated with the two- through
100-year runoff event. The freeboard shall be measured from the maximum
pool elevation to the invert of the emergency spillway. The two- through
100-year storm events shall be controlled by the primary outlet structure.
An emergency spillway for each basin shall be designed to pass the
100-year return frequency storm peak basin inflow rate with a minimum
of 1.0 foot of freeboard measured from the top of the design flow
to the top of settled basin. The freeboard criteria shall be met considering
any off-site areas tributary to the basin as developed, as applicable.
2)
Emergency spillways shall be constructed with freeboard in undisturbed
earth wherever possible. When constructed in fill, sod, precast concrete
paving blocks, concrete or permanent erosion control matting shall
be used. Lining for emergency spillways shall incorporate native colors
and materials where possible, including mono slab revetments, grass
pavers and native stone. Design calculations shall be submitted indicating
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 existing grade. Emergency spillways shall have side slopes no steeper
than three horizontal to one vertical (3H:1V) and be designed to safely
convey 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.
(r)
Stabilization. Proper stabilization, including stilling basins,
energy dissipaters, and channel lining, shall be constructed at the
outlets of all retention basins and emergency spillways. The stabilization
shall control water to avoid erosion, reduce velocities of released
water, and direct water so that it does not interfere with downstream
activities.
(s)
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 Borough may require jute or erosion control matting to
be installed inside the basin or on the basin embankment.
(t)
A four-foot-high chain-link fence may be required to fully surround
the retention basins. The fence may also be landscaped/screened as
determined by the Borough.
B.
Detention Basins. The standards for detention basins listed in this
section are minimum standards, and the professional designer is responsible
for the ultimate final design.
(1)
General Requirements.
(a)
Where detention (dry bottom) basins are utilized, adequate assurances
of access easement, maintenance, indemnification, liability insurance,
and security shall be provided and approved by the Borough.
(b)
Detention facilities shall be designed and located so as to
not present a hazard to the public health or safety. Such facilities
shall be designed so that no adverse effects will result from backwater
flooding. Their design shall be approved by the Borough.
(c)
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Borough shall reserve
the right to disapprove any design that would result in the occurrence
or continuation of an adverse hydrologic or hydraulic condition within
the watershed.
(d)
Detention basins are generally discouraged as a stormwater management
practice and should only be used as a last resort where no other management
facility is practical. Detention basins typically collect and quickly
release runoff from a site in a manner that is contrary to the principles,
goals and standards presented within this chapter. The landscape standards
of the Subdivision Ordinance[3] shall apply.
(2)
Design Considerations.
(a)
A water surface limit shall be no closer than 50 feet to any
residential building and no closer than 25 feet to any other type
of building.
(b)
No stormwater detention facility shall be placed within 50 feet
of a special geologic feature. No stormwater subsurface conveyance
facility shall be constructed within 50 feet of a special geologic
feature, unless it is constructed of durable pipe utilizing watertight
joints.
(c)
The height of berm shall not exceed 15 feet unless approved
by the Borough and DEP.
(d)
If this detention facility is considered to be a dam as per
DEP Chapter 105, the design of the facility must be consistent with
the Chapter 105 regulations, and may be required to pass a storm greater
than the 100-year event.
(e)
The minimum top of berm width shall be five feet.
(f)
The top of berm shall be constructed at least six inches above
the design elevations to allow for settlement of the embankment.
(g)
The maximum outside and inside side slopes should not be steeper
than three horizontal to one vertical (3H:1V). The minimum required
slope for the basin bottom is 1%. 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 four feet and the minimum top
width of the core shall be two 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 Borough may require an impermeable liner
to be installed up to the 100-year design water surface elevation.
(h)
Where concrete, stone or brick walls are used with side slopes
proposed to be steeper than one foot vertical in three feet horizontal,
the basin shall be fenced by a permanent fence 42 inches in height,
and a ramp of durable, nonslip materials for maintenance vehicles
shall be provided for access into the basin.
(i)
The Borough may require soil samples from the site to be analyzed
to determine if these soils are suitable for berm embankment construction.
If, in the opinion of Borough, the soils are found to be unsuitable,
the developer shall import suitable soils for constructing the basin.
(j)
Inlet pipes and outlet pipes should be separated to the greatest
extent possible in order to maximize the flow path through the retention
basin.
(k)
The inlet pipe invert into a basin shall be six inches above
the basin floor or lining so that the pipe can adequately drain after
rainstorms. Inlets shall discharge into areas of the basin that slope
toward the outlet structure.
(l)
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.
(m)
If infiltration of runoff is considered as a method of runoff
abatement or part of the water quality abatement, infiltration tests
shall be taken at the site and at the lowest depth of proposed detention
facilities and witnessed by the Borough. The results shall be submitted
to the Borough for review in accordance with the provisions of this
chapter.
(n)
Outlets should meet the following specifications:
1)
To minimize clogging and to facilitate cleaning and inspection,
outlet pipes should have an internal diameter of at least 18 inches
and a minimum grade of 1%.
2)
Properly spaced antiseep collars as recommended by PADEP shall
be installed on all basin outlet pipes within a constructed berm.
Design calculations shall be provided.
3)
The minimum circular orifice diameter for controlling discharge
rates from detention facilities shall be three inches. Designs where
a lesser size orifice would be required to fully meet release rates
shall be acceptable, provided that as much of the site runoff as practical
is directed to the detention facilities.
4)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment should be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
5)
Outlet pipes shall be constructed of reinforced concrete with
watertight rubber gaskets in conformance with AASHTO M170, M198 and
M207.
6)
Basin outlet structures shall have childproof, nonclogging,
nonrusting trash racks over all design openings exceeding nine inches
in diameter. Periodic cleaning of debris from trash racks shall be
included in the operation and maintenance plan.
7)
Antivortex devices, consisting of a thin, vertical, nonrusting
plate normal to the basin berm, shall be provided at the top of all
circular risers or stand pipes.
8)
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 PADEP Chapter 102. Such facilities shall be
both functional and attractive; for example, native rock shall be
used in constructing dissipaters where practical.
9)
The minimum distance between a proposed basin discharge point
(including the energy dissipater, etc.) and a downstream property
boundary shall in no case be less than 15 feet. Where there is discharge
onto or through adjacent properties prior to release to a stream,
designers shall demonstrate how downstream properties are to be protected.
The Municipal Engineer may require that the setback distance be increased
based upon factors such as topography, soil conditions, the size of
pipes, the location of structures, and discharge rates. A drainage
easement may also be required.
(o)
Basin Emergency Spillways.
1)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a 100-year storm. The freeboard
shall be measured from the maximum pool elevation to the invert of
the emergency spillway. All stormwater detention facilities shall
provide a minimum of 1.0 foot of freeboard above the maximum pool
elevation associated with the two- through fifty-year runoff events.
Five-tenths foot of freeboard shall be provided above the maximum
pool elevation of the 100-year runoff event. The two- through 100-year
storm events shall be controlled by the primary outlet structure.
An emergency spillway for each basin shall be designed to pass the
100-year return frequency storm peak basin inflow rate with a minimum
of 0.5 foot of freeboard measured from the top of the design flow
to the top of the settled basin. The freeboard criteria shall be met
considering any off-site areas tributary to the basin as developed,
as applicable. If this detention facility is considered to be a dam
as per DEP Chapter 105, the design of the facility must be consistent
with the Chapter 105 regulations, and may be required to pass a storm
greater than the 100-year event.
2)
Emergency spillways shall be constructed in undisturbed earth
with freeboard wherever possible. When constructed in fill, sod, precast
concrete paving blocks, concrete or permanent erosion control matting
shall be used. Lining for emergency spillways shall incorporate native
colors and materials where possible, including mono slab revetments,
grass pavers and native stone. Design calculations shall be submitted
indicating 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 existing grade. Emergency spillways shall have side slopes no steeper
than three horizontal to one vertical (3H:IV) and shall be designed
to safely convey 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.
(p)
Stabilization. Proper stabilization, including stilling basins,
energy dissipaters, and channel lining, shall be constructed at the
outlets of all basins and emergency spillways. The stabilization shall
control water to avoid erosion, reduce velocities of released water,
and direct water so that it does not interfere with downstream activities.
(q)
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 Borough may require jute or erosion control matting to
be installed inside the basin or on the basin embankment.
10.
Other BMP Facilities. Other BMP facilities shall follow the PADEP
BMP Manual for guidelines and sound engineering procedures.
A.
All infiltration practices shall be set back at least 15 feet from
all structures with subgrade elements (e.g., basements, foundation
walls).
B.
The lowest elevation of the infiltration area shall be at least two
feet above the seasonal high-water table and bedrock.
C.
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.
D.
All infiltration practices shall have appropriate positive overflow
controls to prevent storage within one foot of the finished surface
or grade.
[Ord. No. 261, 3/6/2023]
1.
All facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures) and any work involving wetlands
governed by DEP Chapter 105 regulations (as amended or replaced from
time to time by DEP) shall be designed in accordance with Chapter
105 and will require a permit from DEP.
2.
Any drainage conveyance facility and/or channel not governed by Pa.
Code Chapter 105 regulations must be designed to convey, without damage
to the drainage facility or roadway, runoff from the fifty-year storm
event with a minimum of 0.5 foot of freeboard measured below the lowest
point along the top of the roadway. Larger storm events (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.
3.
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.
4.
Storm Sewer Requirements.
A.
Storm sewers, culverts, bridges and other structural installations
shall be provided to safely accommodate stormwater, 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 Borough and its citizens in order to:
(1)
Maintain natural hydrologic regimes of streams and watercourses.
(2)
Promote drainage of all low points along the line of streets
and where necessary to intercept runoff. Overflow swales and channels
shall be designed to contain and 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.
(3)
Intercept stormwater runoff along all points along streets at
reasonable intervals, to provide positive drainage away from buildings
and to prevent substantial flow of water across intersections or flooding
of intersections during the design storm stipulated elsewhere in this
section. Storm drainage facilities shall be designed to convey the
flow of stormwater runoff in a safe and efficient manner.
(4)
Insure adequate and unimpeded flow of stormwater under driveways
in, near, or across natural watercourses or drainage swales and channels.
Suitable pipes or other waterway openings shall be provided as necessary.
(5)
Ensure adequate drainage at intersections of driveways with
streets.
(6)
Provide that where existing storm sewers are reasonably accessible
and of adequate capacity, subdivisions and land developments shall
connect to the existing storm sewers.
(7)
Provide that when storm drainage will be directed into an adjacent
municipality, all provisions for accommodating such storm drainage
shall be submitted to the governing body of that municipality for
review.
B.
Storm sewers, culverts, swales, channels, and related conveyance
facilities shall be designed to accommodate and discharge all runoff
from adjacent upstream drainage areas, assuming the upstream area
is fully developed.
C.
The stormwater conveyance system, inlets, pipes, overland flow and
swales, shall be designed utilizing the Rational Method. Storm drainage
collection systems, inlets and pipes shall be designed to convey a
fifty-year storm event without surcharging an inlet. Adequate controls
to ensure the 100-year storm event is conveyed to the stormwater management
facility must be incorporated into the design.
D.
All drainage facilities shall be designed to adequately handle surface
runoff and carry it to suitable outlets and shall be designed in accordance
with the minimum design standards established by this chapter.
E.
The pavement cross slope on streets to convey stormwater runoff to
drainage facilities shall not be less than 1/8 inch per foot and not
more than 1/2 inch per foot. The slope of the shoulder areas shall
not be less than 3/4 inch per foot and not more than one inch per
foot.
F.
Surface cross drainage at intersections or other roadway sections
will not be permitted.
G.
Applicants are encouraged to design conveyance systems that encourage
infiltration and improve water quality wherever practicable.
H.
Complete stormwater calculations shall be prepared in accordance
with the design standards and appendixes of this chapter. All designs
for drainage facilities shall be submitted to the Borough for review
and approval and shall be designed and installed to meet the following
minimum standards.
I.
The collection system shall be designed by the Rational Method of
Design using the formula Q = CiA, unless otherwise approved by the
Borough.
J.
Rainfall intensity curves and other hydraulic design data provided
by the PennDOT and/or this chapter shall be used for design purposes.
K.
The design of all drainage facilities shall incorporate sound engineering
principles and practices. The Borough shall reserve the right to disapprove
any design that would result in the occurrence or continuation of
an adverse hydrologic or hydraulic condition within the watershed.
5.
Storm Sewer Design.
A.
Inlet efficiency and bypass flow shall be determined for all inlets,
and the gutter flow spread in the road shall not exceed 1/2 the travel
lane width or to a maximum of four feet calculated based on the fifty-year
design storm event. The Borough may require that a hydraulic grade
line analysis be performed on storm sewer systems.
B.
Storm sewer and inlet design shall be based upon PennDOT design methods.
Design shall use inlet efficiency curves included in PennDOT Design
Manual Part 2, latest edition.
C.
Storm sewer sizes shall be determined based upon the following design
storm frequencies:
(1)
Fifty years in all subdivisions or land developments, unless
otherwise specified by the Borough.
(2)
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 Borough, overflow swales or channels may be provided
at low points in streets to safely contain and convey the full 100-year
peak flow to the control facility, in lieu of providing the full capacity
in the storm sewer.
(3)
Cross-pipes, box culverts and bridges shall be designed to convey
a 100-year storm event.
D.
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 for concrete
pipe shall be 0.12. In cases where pressure flow may occur, the hydraulic
grade line shall be calculated throughout the storm sewer system to
verify that at least 1/2 foot of freeboard will be provided in all
inlets and manholes for the design storm event.
E.
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 may only
discharge to the rear yard and must be a minimum of 50 feet beyond
habitable buildings and centered within a twenty-foot-wide easement.
F.
Storm sewers shall have a minimum diameter of 18 inches and only
reinforced cement concrete pipe or smooth bore corrugated polyethylene
pipe up to a maximum diameter of 48 inches shall be used. All pipes
must have a minimum of 12 inches of cover.
G.
Storm drains shall be designed to produce a minimum velocity of 3.0
feet per second when flowing full and shall not be designed or installed
at less than a slope of 0.50%.
H.
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 Borough Code. This should be noted on the plans.
I.
All storm sewer pipes beneath a paved surface shall be bedded and
fully backfilled with PennDOT certified No. 2A stone. This backfill
shall be placed in minimum eight-inch lifts and solidly compacted
to the satisfaction of the Borough.
J.
Curves in pipes or box culverts without an inlet or manhole are prohibited.
Tee joints, elbows and wyes are also prohibited. Subsurface drainage
systems shall have accessible manholes spaced at intervals not exceeding
400 feet and shall be located wherever branches are connected or sizes
are changed and wherever there is a change in alignment or grade.
Inlets or other means of interconnection may be used instead of manholes
when approved by the Borough, or as required for curbed streets.
K.
Storm sewer lines, within street rights-of-way, shall be located
between the center line of the street and the curbline and shall parallel
the center line of the street as far as practical. Locating storm
sewers under curbs in curves or at street intersections will not be
permitted.
L.
Storm facilities not located within a public right-of-way shall be
contained in and centered within a twenty-foot-wide easement. A note
on the plan shall indicate that the easement allows the Borough the
right, though not the responsibility, to perform needed maintenance
and/or repairs and back charge the maintainer or lien the property
of the maintainer.
M.
When there is a change in pipe size through an inlet, the bottom
elevation of the outlet pipe shall be a minimum two inches below the
bottom inside elevations of all incoming pipes.
N.
Open-end pipes must be fitted with end walls in accordance with PennDOT
Roadway Construction Standards, latest edition.
O.
Headwalls shall be used where storm runoff enters the storm sewer
horizontally from a natural or man-made channel. The capacity of such
storm sewers shall be evaluated using both Manning Equation and inlet/outlet
control procedures. The lower flow derived from these two procedures
shall be the design capacity of the storm sewer.
P.
Trash racks should be placed on all stormwater entrance structures.
Q.
On curbed streets, inlets shall be placed at points of abrupt changes in the horizontal or vertical directions of storm sewers, at curb tangents on the uphill side of street intersections, at maximum distances as specified in accordance with the required pipe size, and to limit gutter flow as required. The Manning Equation shall be used to calculate the capacities of gutters. Pennsylvania Department of Transportation inlets should be used in accordance with the design procedures outlined in PennDOT Design Manual, Part 2. Manholes may be substituted for inlets at locations where inlets are not required to handle surface runoff.
R.
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.
S.
Inlets and manholes shall be designed and located to prevent hazards
to vehicles, bicycles, and pedestrians. Inlet and manhole castings
and concrete construction shall be equivalent to PennDOT Roadway Construction
Standards, latest edition.
T.
Inlets shall be located at tangents on the uphill side of street
intersections and at intervals along the gutter line to control the
maximum amount of encroachment of runoff on the roadway pavement so
that same does not exceed a width of four feet during the design storm
event. Design and location of curb inlets shall be approved by the
Borough. The maximum amount of bypass flow through an intersection
may not exceed 1/2 inch.
U.
The maximum flow to inlets located in low points (such as sag vertical
curves) shall include the overland flow directed to the inlet as well
as all bypass runoff from upstream inlets. The bypass flow from upstream
inlets shall be calculated using inlet efficiency curves included
in PennDOT Design Manual Part 2, latest edition or other equivalent
computer program.
V.
Inlets, manholes, covers and frames shall conform to Pennsylvania
Department of Transportation specifications. At street intersections,
every attempt should be made to place inlets in the tangent and not
in the curved portion of the curbing.
W.
When precast concrete inlets or manholes are used within a street,
a maximum of two courses of masonry brick or grade ring may 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 rings are
used, the horizontal joints 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 rings to prevent the washing
of fines into the structure.
X.
Inlets, manholes, headwalls, end walls, 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.
Y.
Manhole covers shall have the word "STORM" cast in two-inch-high
letters on the top of the smooth cover.
Z.
Precast inlet tops and boxes shall meet the requirements of PennDOT
Publication 408 PennDOT Type C precast concrete. Inlet tops are to
be provided with "Dump No Waste - Drains To Waterway" plate with trout
logo to be approved by the Borough. All other PennDOT inlets are to
be provided with either a painted stenciled logo on the roadway or
another acceptable marking approved by the Borough.
AA.
The words "Dump No Waste - Drains To Waterway" in raised letters
of 1 1/4 inches with bass logo shall be cast or stamped into
the storm sewer manhole cover.
BB.
Drainage structures that are to be located within state highway rights-of-way
shall be approved by the Pennsylvania Department of Transportation,
and a letter from that Department indicating such approval shall be
submitted to the Borough.
6.
Open Vegetated Channels (Swales).
A.
Grass swales not specifically designed as BMP devices should have
a minimum bottom slope of 1% and their depth shall be deep enough
to convey the fifty-year storm event with a minimum of 0.5 foot of
freeboard. Swale linings shall be designed based on the ten-year velocity.
Swales shall have sufficient capacity to contain and convey the 100-year
storm discharge within its banks and without creating any safety or
property hazard.
B.
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.
C.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
D.
Freeboard is the difference between the elevation of the design flow
in the channel and the top of berm elevation of the channel.
E.
Trapezoidal channels in cross section should have a minimum bottom
width of two feet.
F.
Channels shall be designed with moderate side slopes of three horizontal
to one vertical. Flatter side slopes may be necessary under certain
circumstances.
G.
The peak velocity of the runoff from the storm shall be nonerosive
for the soil and ground cover provided in the channel.
H.
The bottom of dry swales shall be situated at least two feet above
the seasonal high-water table.
I.
Channels (for example, dry swales) may require an underdrain in order
to function and dewater.
J.
The maximum allowable ponding time in the channel shall be less than
48 hours.
K.
Accumulated sediment within the channel bottom shall be removed when
25% of the original WQ volume has been exceeded.
L.
Check dams along the channel length may be warranted.
M.
Landscape specifications shall address the grass species, wetland
plantings (if applicable), soil amendment and hydric conditions present
along the channel.
N.
Swales, when located outside of the Borough right-of-way, shall be
located within an easement not less than 15 feet wide, but of sufficient
width to allow access for maintenance and to convey all storms up
to and including the 100-year storm. A note on the plan shall indicate
that the easement allows the Borough the right, though not the responsibility,
to perform needed maintenance and/or repairs and back charge the maintainer
or lien the property of the maintainer.
O.
Flood relief channels shall be provided and designed to convey the
runoff from the 100-year, twenty-four-hour storm, such that a positive
discharge of this runoff to an adequate receiving stream or conveyance
system occurs without allowing this runoff to encroach upon other
properties.
P.
Use of grassed swales or open vegetated swales in lieu of curbing
to convey, infiltrate and/or treat stormwater runoff from roadways
is encouraged. Inlets shall be placed at the center of the swale draining
the street and shall be located no closer than four feet from the
edge of the cartway.
Q.
Open channels along existing roadways may be required to be enclosed
by the Borough if PennDOT standards for safety and maintenance cannot
be satisfied. All drainage structures shall conform to the latest
edition of Form 408, PennDOT specifications.
R.
No property owner shall obstruct or alter the flow, location or carrying
capacity of a stream, channel or drainage swale to the detriment of
any other property owner, whether upstream or downstream. All subdivision
and/or land development plans containing streams, channels, drainage
swales, storm sewers or other conveyance systems that cross property
boundaries, existing or proposed, or whose discharge crosses such
boundaries shall contain a note stating the above.
7.
Roof Drains, Sump Pumps, Floor Drains, Building Drains, etc.
A.
Stormwater roof drains, sump pumps, floor drains, building drains,
etc., shall not discharge water directly over a sidewalk or into any
sanitary sewer line or storm sewer line or inlet or manhole.
B.
All proposed roof drains and collector locations shall be shown on
the storm drainage plans. Roof drains and collectors shall meet all
appropriate Borough Code requirements.
8.
Design of Bridges and Culverts.
A.
All bridges, driveway culverts and culverts shall have ample waterway
to carry the design flows, based on a minimum storm frequency of 50
years, unless a larger design flow is required by PADEP or FEMA or
the Borough. One-hundred-year water depths crossing the road or drive
shall not exceed six inches above the roadway or driveway center-line
elevation. Bridge and/or culvert construction shall be in accordance
with the PADOT specifications and shall meet the requirements of the
PADEP or FEMA or the Borough. The appropriate permits and approvals
must be acquired by the applicant prior to final plan approval.
B.
Culverts under roadways and located within the road right-of-way
shall be provided with wing walls and constructed for the full width
of the right-of-way. If the Borough determines that the character
of the road is expected to change in the future, the cartway shall
be made to anticipate this condition. On each side of the cartway,
railings must be set back from the edge of the final cartway, and
this area may be used to place sidewalks, bike trails, etc. Easements
shall be provided for maintenance.
C.
Culverts not in roadways shall be provided with wing walls, and extra
wide easements shall be provided for access and maintenance.
