A.
For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 125-302:
(1)
Preparation and implementation of an approved SWM site plan
is required.
(2)
No regulated activities shall commence until:
(a)
The municipality issues written 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.
B.
Neither submission of an 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.
C.
SWM site plans approved by the municipality, in accordance with § 125-403, shall be on-site throughout the duration of the regulated activity.
D.
The municipality may, after consultation with DEP, approve measures
for meeting the state water quality requirements other than those
in this chapter, provided that such alternative measures meet the
minimum requirements of, and do not conflict with, state law including,
but not limited to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
E.
For all regulated earth disturbance activities, erosion and sediment
control BMPs shall be designed, implemented, operated, and maintained
during the regulated earth disturbance activities (e.g., during construction)
to meet the purposes and requirements of this chapter and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law.[2] Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual), No. 363-2134-008, as amended and updated.[3]
F.
Impervious areas.
(1)
The measurement of impervious areas shall include all of the
impervious areas in the total proposed development even if development
is to take place in stages.
(2)
For development taking place in stages, the entire development
plan shall be used in determining compliance with this chapter.
(3)
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls in § 125-303 and the peak rate controls of § 125-306 do not need to be retrofitted for existing impervious areas that are not being altered by the proposed regulated activity.
G.
Stormwater flows onto adjacent property shall not be created, increased,
relocated, or otherwise altered without written notification to the
adjacent property owner(s). Such stormwater flows shall be subject
to the requirements of this chapter.
H.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this chapter by implementing
measures to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(3)
Incorporate methods described in the PA BMP Manual (NOTE: See § 125-901C.) If methods other than green infrastructure and LID methods are proposed to achieve the volume and rate controls required under this chapter, the SWM site plan must include a detailed justification demonstrating that the use of LID and green infrastructure is not practicable.
I.
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.
J.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
K.
Normally dry, open top storage facilities should completely drain
both the volume control and rate control capacities over a period
of time not less than 24 hours and not more than 72 hours from the
end of the design storm.
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 an existing stormwater problem(s) or continuation
of an existing stormwater problem(s).
M.
The design storm volumes to be used in the analysis of peak rates
of discharge should 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's Atlas 14 (NOTE: See § 125-901E.) can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
N.
For all regulated activities, SWM BMPs 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 Pa.
Code, the Clean Streams Law, and the Storm Water Management Act.[4]
[4]
Editor's Note: See 35 P.S. § 691.1 et seq. and 32
P.S. § 680.1 et seq., respectively.
P.
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 §§ 125-303, 125-304, 125-305, and 125-306). Drainage facilities located on the site shall be designed to safely convey flows from outside of the site through the site.
Q.
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.
R.
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.
S.
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.).
T.
All regulated activities located within a special flood hazard area designated by the Federal Emergency Management Agency (FEMA) shall comply with Article XIV of Chapter 170 of the Londonderry Township Code of Ordinances, more specifically known as the "Londonderry Township Flood Hazard District," 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.
A.
General exemptions. Regulated activities that involve less than or equal to 1,000 square feet of proposed impervious surfaces and less than or equal to 5,000 square feet of earth disturbance or are listed in § 125-302G are exempt from those (and only those) requirements of this chapter that are included in the sections and articles listed in Table 125-302.1. Exemptions are for the items noted in Table 125-302.1 only and shall not relieve the applicant from other applicable requirements of this chapter. Exemption shall not relieve the applicant from implementing such measures as are necessary to protect health, safety, and welfare, property, and water quality.
B.
Agricultural activity is exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.
C.
Forest management and timber operations are exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.
E.
The municipality may deny or revoke any exemption or modified requirements
pursuant to this section at any time for any project that the municipality
believes may pose a threat to public health or safety or the environment.
F.
Requirements for exempt activities.
(1)
An exemption from any requirement of this chapter shall not
relieve the applicant from implementing all other applicable requirements
of this chapter or from implementing such measures as are necessary
to protect public health, safety, and welfare, property and water
quality.
(2)
An exemption shall not relieve the applicant from complying
with the requirements for state-designated special protection waters
designated by PADEP as high-quality (HQ) or exceptional value (EV)
waters, or any other current or future state or municipal water quality
protection requirements.
(3)
An exemption under this chapter shall not relieve the applicant
from complying with all other applicable municipal ordinances or regulations.
(4)
Any applicant desiring exemption from design, plan submission,
and plan processing requirements shall complete an application for
exemption in the form available at the municipality's office
and pay any applicable filing fee.
|
Table 125-302.1
Thresholds for Regulated Activities Exempt From Chapter's Provisions
| |||
|---|---|---|---|
|
Chapter's Article/Section
|
Activities Listed in § 125-302G
|
< or = 1,000 Square Feet of Proposed Impervious Surfaces
AND < or = 5,000 Square Feet of Proposed Earth Disturbance
|
> 1,000 Square Feet of Proposed Impervious Surfaces OR > 5,000
Square Feet of Proposed Earth Disturbance
|
|
Article I
|
Not exempt
|
Not exempt
|
Not exempt
|
|
Article II
|
Not exempt
|
Not exempt
|
Not exempt
|
|
Not exempt
|
Not exempt
|
Not exempt
| |
|
Exempt
|
Exempt
|
Not exempt
| |
|
Article IV
|
Exempt
|
Exempt
|
Not exempt
|
|
Article V
|
Exempt
|
Exempt
|
Not exempt
|
|
Article VI
|
Exempt
|
Exempt
|
Not exempt
|
|
Article VII
|
Not Exempt
|
Not Exempt
|
Not exempt
|
|
Article VIII
|
Not exempt
|
Not exempt
|
Not exempt
|
|
Other erosion, sediment and pollution control requirements
| |||
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
G.
Exemptions for specific activities. The following specific regulated activities are exempt from the requirements of Article III, §§ 125-301, 125-303, 125-304, 125-305, 125-306, 125-307, and 125-308; Article IV; Article V; and Article VI of this chapter (as shown in Table 125-302.1), unless otherwise noted below. All other conveyance and system design standards established by the municipality in other codes or ordinances shall be required, and all other provisions of this chapter shall apply.
(1)
Emergency exemption. Emergency maintenance work performed for
the protection of public health, safety and welfare. This exemption
is limited to repair of an existing stormwater management facility;
upgrades, additions or other improvements are not exempt. A written
description of the scope and extent of any emergency work performed
shall be submitted to the municipality within two business days of
the commencement of the activity. A detailed plan shall be submitted
no later than 30 calendar days following commencement of the activity.
If the municipality finds that the work is not an emergency, then
the work shall cease immediately and the requirements of this chapter
shall be addressed as applicable.
(2)
Maintenance. Any maintenance to an existing Stormwater management
facility, BMP or conveyance made in accordance with plans and specifications
approved by the Municipal Engineer or municipality.
(3)
Existing landscaping. Use of land for maintenance, replacement
or enhancement of existing landscaping.
(4)
Gardening. Use of land for gardening for home consumption.
(5)
Agricultural-related activities.
(b)
High tunnel if:
[1]
The high tunnel or its flooring does not result in an impervious
surface exceeding 25% of all structures located on the landowner's
total contiguous land area under common ownership; and
[2]
The high tunnel meets one of the following:
[a]
The high tunnel is located at least 100 feet from
any perennial stream or watercourse, public road or neighboring property
line; or
[b]
The high tunnel is located at least 35 feet from
any perennial stream or watercourse, public road or neighboring property
line and located on land with a slope not greater than 7%; or
(6)
Forest management. Forest management operations which are consistent with a sound forest management plan as filed with the municipality and which comply with the DEP's management practices contained in its publication "Soil Erosion and Sedimentation Control Guidelines for Forestry" (as amended or replaced by subsequent guidance). Such operations are required to have an erosion and sedimentation control plan which meets the requirements of 25 Pa. Code Chapter 102 and meets the erosion and sediment control standards of § 125-303 of this chapter.
(7)
Maintenance of existing gravel and paved surfaces. Replacement of existing gravel and paved surfaces shall meet the erosion and sediment control requirements of 25 Pa. Code Chapter 102 and § 125-301E of this chapter and is exempt from all other requirements of this chapter listed in § 125-302G above. Resurfacing of existing gravel and paved surfaces and paving of existing gravel surfaces are exempt from the requirements of this chapter listed above. Construction of new or additional impervious surfaces shall comply with all requirements of this chapter as indicated in Table 125-302.1.
(8)
Municipal roadway shoulder improvements. Shoulder improvements
conducted within the existing roadway cross section of municipal-owned
roadways, unless an NPDES permit is required, in which case the proposed
work must comply with all requirements of this chapter.
(9)
In-place replacement of residential dwelling unit. The replacement
in the exact footprint of an existing one- or two-family dwelling
unit.
(10)
In-place replacement, repair, or maintenance of residential
impervious surfaces. The replacement of existing residential patios,
decks, driveways, pools, garages, and/or sidewalks that are accessory
to an existing one- or two-family dwelling unit in the exact footprint
of the existing impervious surface.
H.
Modified requirements for small projects. Regulated activities that
involve both 1,000 square feet to 2,500 square feet of proposed impervious
surfaces and 1,000 square feet to 5,000 square feet of proposed earth
disturbance may apply the modified requirements presented in the "Simplified
Approach to Stormwater Management for Small Projects" (Simplified
Approach) (Appendix A[3]) to comply with the requirements of §§ 125-301 (except § 125-301E), 125-303, 125-304, 125-305, 125-306, 125-307, and 125-308, and Article IV, Article V, Article VI, and Article VII of this chapter (as shown in Table 125-302.2). The applicant shall first contact the Municipal Engineer to confirm that the proposed project is eligible for use of the simplified approach and is not otherwise exempt from this chapter's provisions, determine what components of the proposed project are to be considered as impervious surfaces, and determine if other known site or local conditions exist that may preclude the use of any techniques included in the simplified approach. Appendix A includes instructions and procedures for preparation, submittal, review and approval of documents required when using the simplified approach and shall be adhered to by the applicant. All other provisions of this chapter shall apply.
|
Table 125-302.2
Thresholds for Regulated Activities That Are Eligible for "Modified"
Requirements for the Provisions of This Chapter That Are Listed Below
| |
|---|---|
|
Chapter's Article/Section
|
Activities Listed in § 125-302H
|
|
Article I
|
All provisions apply
|
|
Article II
|
All provisions apply
|
|
All provisions apply
| |
|
Exempt if modified requirements of § 125-302H are applied
| |
|
Article IV
|
Exempt if modified requirements of § 125-302H are applied
|
|
Article V
|
Exempt if Modified Requirements of § 125-302H are Applied
|
|
Article VI
|
Exempt if modified requirements of § 125-302H are applied
|
|
Article VII
|
Exempt if modified requirements of § 125-302H are applied
|
|
Article VIII
|
All provisions apply
|
|
Other erosion, sediment and pollution control requirements
| |
|
Table 125-302.2 Notes:
| |
|
•
|
"Modified requirements" - regulated activities listed within the subsections of this chapter noted in Table 125-302.2 are eligible for exemption only from the indicated sections and subsections of this chapter and only if the modified requirements of § 125-302H are met to the satisfaction of the municipality; all other provisions of this chapter apply.
|
[3]
Editor's Note: Appendix A is included as an attachment to this chapter.
[4]
Editor's Note: See 35 P.S. § 691.1 et seq.
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 LID practices
provided in the PA BMP Manual, as well as the guidance on green infrastructure
LID and CD provided in Appendix B,[1] shall be utilized for all regulated activities wherever
possible. The applicant shall comply with the following water quality
and runoff volume requirements for all regulated activities, including
all new development and redevelopment activities:
A.
The post-construction total runoff volume shall not exceed the predevelopment
total runoff volume for all storms equal to or less than the two-year,
twenty-four-hour duration precipitation (design storm) or a minimum
of 1.5 inches of runoff from all regulated impervious surfaces shall
be managed, whichever volume to be managed is greater. 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 § 125-307D of this chapter.
C.
The design of the stormwater management facility outlet shall provide
for protection from clogging and unwanted sedimentation.
D.
BMPs that moderate the temperature of stormwater shall be used to
protect the temperature of receiving waters.
E.
Water quality improvement shall be achieved in conjunction with achieving the infiltration requirements of § 125-304. The infiltration volume required under § 125-304 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 § 125-301S.
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.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
Providing for infiltration consistent with the natural hydrologic
regime is required to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created or expanded. The applicant shall achieve the following
infiltration requirements:
A.
For regulated activities involving either new development or redevelopment, infiltration should be designed to accommodate the entire water quality and runoff volume required in § 125-303. If the runoff volume required by § 125-303 cannot be infiltrated, then alternative methods consistent with the PA BMP Manual (as amended) may be used to manage this volume with approval from the Municipal Engineer. If the requirements of any subsection of § 125-304 cannot be physically accomplished, then the applicant shall be responsible for demonstrating with data or calculations to the satisfaction of the Municipal Engineer why the required infiltration volume controls 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.
B.
A waiver from § 125-304 shall be considered by the municipality only if a minimum of at least 0.5 inch infiltration requirement cannot be physically accomplished on the site.
C.
If site conditions preclude capture of runoff from portions of the
impervious surfaces, the infiltration volume for the remaining area
shall be increased by an equivalent amount to offset the loss.
D.
When a project contains or is divided by multiple watersheds, the
infiltration volume shall be separately addressed for each watershed.
E.
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.
F.
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.
G.
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.
(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 two days (48 hours) from the end of the
design storm.
H.
All infiltration practices shall:
(1)
Be set back at least 25 feet from all buildings and features
with subgrade elements (e.g., basements, foundation walls, etc.),
unless otherwise approved by the Municipal Engineer.
(2)
For any infiltration practice that collects runoff from shared
or multiple features and that is located within 50 feet of a building
or feature with subgrade elements (e.g., basements, foundation walls,
etc.), the bottom elevation shall be set below the elevation of the
subgrade element.
I.
Infiltration facilities shall, to the maximum extent practicable,
be located to avoid introducing contaminants to groundwater.
(1)
When a hotspot is located in the area draining to a proposed
infiltration facility, an evaluation of the potential of groundwater
contamination from the proposed infiltration facility shall be performed,
including a hydrogeologic investigation (if necessary) by a qualified
licensed professional to determine what, if any, pretreatment or additional
design considerations are needed to protect groundwater quality.
(2)
When located within a wellhead protection area of a public water
supply well, infiltration practices shall be in conformance with the
applicable approved source water protection assessment or source water
protection plan.
(3)
The applicant shall provide appropriate safeguards against groundwater
contamination for land uses that may cause groundwater contamination
should there be a mishap or spill.
J.
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.
K.
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.
L.
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.
M.
All infiltration practices shall have appropriate positive overflow
controls.
N.
No sand, salt or other particulate matter may be applied to a porous
surface material for winter ice conditions.
O.
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 post-construction two-year, twenty-four-hour
design storm shall be reduced to the predevelopment peak flow rate
of the one-year, twenty-four-hour duration precipitation, using the
SCS Type II distribution.
B.
To the maximum extent practicable, and unless otherwise approved
by the 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).
C.
For modeling purposes, the predevelopment ground cover conditions shall be determined using the corresponding ground cover assumptions presented in § 125-307D of this chapter.
D.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter unless otherwise approved by the 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 management facilities shall make use of measures
to extend the flow path and increase the travel time of flows in the
facility.
G.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate control shall be separately addressed for
each drainage area.
The applicant shall comply with the following peak flow rate
control requirements for all regulated activities including those
that involve new development and redevelopment.
A.
Post-construction peak flow rates from any regulated activity shall
not exceed the predevelopment peak flow rates as shown for each of
the design storms specified in Table 125-306.1.
|
Table 125-306.1
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
|
10-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 § 125-307D of this chapter.
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 125-306.1, using the redevelopment ground cover assumptions presented in § 125-307D. This design criterion for redevelopment is only permitted with approval of Municipal Engineer. It shall result in no measurable impact on downstream properties.
D.
Only the area of the proposed regulated activity shall be subject
to the peak flow rate control standards of this chapter. Undisturbed
areas for which the discharge point has not changed are not subject
to the peak flow rate control standards.
E.
Areas located outside of the site (i.e., areas outside of the regulated
activity) that drain through a proposed site are not subject to peak
flow rate control requirements. Drainage facilities located on the
site shall be designed to safely convey flows from outside of the
site through the site.
F.
When a regulated activity contains or is divided by multiple drainage
areas, the peak flow rate controls shall be separately addressed for
each drainage area.
G.
The effect of structural and nonstructural stormwater management
practices implemented as part of the overall site design may be taken
into consideration when calculating total storage volume and peak
flow rates.
A.
Stormwater runoff from all regulated activity sites with a drainage
area of greater than five acres shall be calculated using a generally
accepted calculation technique(s) that is based on the NRCS Soil Cover
Complex Method. Table 125-307.1 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 125-307.1
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 chapter. 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, predevelopment calculations shall
assume ground cover of "woods in good condition."
(b)
For areas that are not woods and not impervious surfaces, predevelopment
calculations shall assume ground cover of "meadow."
(c)
For areas that are impervious surfaces, predevelopment calculations
shall assume at least 20% of the existing impervious surface area
to be disturbed as "meadow" ground cover.
(3)
The applicant shall determine which stormwater standards apply
to the proposed regulated activity as follows:
(a)
Stormwater standards for new development shall apply to all
proposed regulated activities that involve only new development activities,
as defined in this chapter.
(b)
Stormwater standards for redevelopment shall apply to all proposed
regulated activities that involve only Redevelopment activities, as
defined in this chapter.
(c)
At the discretion of the Municipal Engineer, regulated activities
that involve a combination of both new development and redevelopment
activities, as defined in this chapter, may either:
[1]
Apply the redevelopment or new development stormwater standards
that are associated with the activity that involves the greatest amount
of land area; or
[2]
Apply the redevelopment and new development stormwater standards
to the corresponding redevelopment and new development portions of
the proposed regulated activity.
E.
Runoff curve numbers (CN) for both predevelopment and proposed (post-construction)
conditions to be used in the Soil Cover Complex Method shall be obtained
from Table C-1 in Appendix B of this chapter.[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
F.
Runoff coefficients (C) for both predevelopment and proposed (post-construction)
conditions for use in the Rational Method shall be obtained from Table
C-2 in Appendix B of this chapter.
G.
Weighted averaging of runoff coefficients shall not be used for manual
computations or input data for water quality and runoff volume calculations.
H.
Hydraulic computations to determine the capacity of pipes, culverts,
and storm sewers shall be consistent with methods and computations
contained in the Federal Highway Administration Hydraulic Design Series
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 B of this chapter.
A.
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
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. 25 Pa. Code Chapter 105 may require the
safe conveyance of storms larger than 100-year event.
B.
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 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.
C.
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.
D.
Roadway crossings or structures located within designated floodplain
areas shall be able to convey runoff from a 100-year design storm
consistent with Federal Emergency Management Agency National Flood
Insurance Program, Floodplain Management Requirements.
E.
Any stormwater management facility located within a PennDOT right-of-way
shall comply with PennDOT minimum design standards and permit submission
and approval requirements.
F.
Adequate erosion protection and energy dissipation shall be provided
along all open channels and at all points of discharge. Design methods
shall be consistent with the Federal Highway Administration Hydraulic
Engineering Circular 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.
G.
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 chapter.
A.
In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as part of any subdivision
or land development that encompasses a riparian buffer. riparian buffer
easement agreements shall be submitted to the municipality's
Solicitor for review and approval prior to recording.
B.
Except as required by 25 Pa. Code Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100-year floodplain or a minimum of 35 feet from the top of the stream bank (on each side).
C.
Minimum management requirements for riparian buffers.
(1)
Existing native vegetation shall be protected and maintained
within the riparian buffer easement.
(2)
Whenever practicable invasive vegetation shall be actively removed
and the riparian buffer easement shall be planted with native trees,
shrubs and other vegetation to create a diverse native plant community
appropriate to the intended ecological context of the site.
D.
The riparian buffer easement shall be enforceable by the municipality
and shall be recorded in the Chester County Recorder of Deeds Office,
such that it shall run with the land and shall limit the use of the
property located therein. The easement shall allow for continued private
ownership and shall count toward the minimum lot area required by
the Township Zoning Ordinance, unless otherwise specified in the Zoning
Ordinance.[1]
E.
Any permitted use within the riparian buffer easement shall be conducted
in a manner that will maintain the extent of the existing 100-year
floodplain, improve or maintain the stream stability, and preserve
and protect the ecological function of the floodplain.
G.
Septic drain fields and sewage disposal systems shall not be permitted
within the riparian buffer easement and shall comply with setback
requirements established under 25 Pa. Code Chapter 73.
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.
The following minimum performance standards shall apply to all applicable
activities, whether they are new development or redevelopment.
(1)
Water quality management shall be provided through the use of
structural and/or nonstructural BMPs. Water quality stormwater management
practices shall be designed to reduce or eliminate solids, sediment,
nutrients, and other potential pollutants from the site. A stormwater
management practice complies with this requirement if it is:
(2)
Stormwater runoff generated from development shall not be directly
discharged into a jurisdictional wetland or waters of the United States
and their adjacent wetlands without being treated by an approved stormwater
management practice prior to discharge. Natural wetlands shall not
be used to meet the minimum design requirements for stormwater management
or stormwater runoff quality treatment, except when used as part of
a treatment train that incorporates a portion of the outer zone (filter
strip) of the wetland's riparian buffer as a stormwater outfall.
In such instances, the discharge velocity from the terminal end of
a pipe or associated energy dissipation practice shall not exceed
two feet per second for the two-year frequency storm event. Where
such a management strategy is used, all feasible methods shall be
used to convert concentrated flow to uniform, shallow sheet flow before
entering the outer zone of the wetland's riparian buffer. In
addition, it shall be demonstrated that such an approach will not
cause erosion.
(3)
Stormwater shall be infiltrated and/or discharged within the
same drainage area of the stream receiving the runoff from the development
site prior to development.
(4)
Rainfall intensities used for the Modified Rational Method shall
be based on the current PennDOT Storm Intensity-Duration-Frequency
Chart appropriate to the specific site.
(5)
The Rational Method (that is, Q = CIA) shall be used for calculations
of the peak rate of runoff for the design of storm sewers and drainage
swales but not for the design of stormwater management facilities
where a full hydrograph is needed. The equation representing the Rational
Method is comprised of the following (in English units):
|
Q
|
=
|
Peak flow rate, cubic feet per second (CFS)
|
|
C
|
=
|
Runoff coefficient, dependent on land use/cover
|
|
I
|
=
|
Design rainfall intensity, inches per hour
|
|
A
|
=
|
Drainage area, acres
|
(6)
Runoff characteristics of off-site areas that drain through
a proposed development shall be based on actual existing conditions.
(7)
All infiltration practices shall be set back at least 25 feet
from all structures with subgrade elements (e.g., basements, foundation
walls).
(8)
All infiltration practices shall be designed to infiltrate the
stored volume within 48 hours.
(9)
All inflows to infiltration practices shall be treated to prevent
the direct discharge of sediment into the infiltration practice; accumulated
sediment reduces stormwater storage capacity and ultimately clogs
infiltration mechanisms. For storm sewer inflows, "snouts" or other
similar sediment and debris capturing devices shall be provided. No
sand, salt or other particulate matter may be applied to a porous
(pervious) surface for winter ice conditions.
B.
Open vegetated channels.
(1)
Open vegetated channels are conveyance systems that are engineered
to also perform as water quality and infiltration practices. Such
systems can be used for the conveyance, retention, infiltration and
filtration of stormwater runoff.
(2)
Several types of open vegetated channels can provide a water
quality function and augment infiltration. Examples of such systems
include, but are not limited to, swales, grass channels, and biofilters.
Open vegetated channels are primarily applicable for land uses such
as roads, highways, residential developments (dry swales only) and
pervious areas.
(3)
Open vegetated channels shall be designed to meet the following
minimum standards:
(a)
The peak velocity of the runoff from the ten-year storm shall
be nonerosive for the soil and ground cover provided in the channel.
(b)
The longitudinal slope shall be no less than 2%.
(c)
Channels shall be trapezoidal in cross section. The minimum
bottom width shall be two feet. The maximum bottom width shall be
eight feet.
(d)
Channels shall be designed with moderate side slopes of 4H:1V.
Flatter side slopes may be necessary under certain circumstances.
(e)
Channels shall be designed to temporarily store the water quality
volume (WQv) within the system for a maximum period of 48 hours and
a minimum period of one hour. The maximum allowable ponding time in
the channel shall be less than 48 hours.
(f)
At the discretion of the Township Engineer, channels may require
an underdrain in order to function and dewater.
(g)
Accumulated sediment within the channel bottom shall be removed
when 25% of the original WQv has been exceeded.
(h)
The bottom of dry swales shall be situated at least two feet
above the seasonal high water table.
(i)
Check dams along the channel length may be warranted, as determined
by the Municipal Engineer.
C.
Retention basins.
(1)
Retention basins shall be designed to create a healthy ecological community with sufficient circulation of water to prevent the growth of unwanted vegetation and mosquitoes. Care should be taken to landscape retention basins in accordance with § 125-311G.
(2)
The retention basin shall be of sufficient size to allow the
appropriate aquatic community needed to maintain healthy pond ecology
and avoid mosquitoes capable of carrying West Nile virus and other
diseases. The Chester County Health Department, Pennsylvania Fish
and Boat Commission, the Natural Resource Conservation Service, the
Pennsylvania Extension Service, or other qualified professional consultant
shall be consulted during the design of these facilities in order
to ensure the health of aquatic communities and minimize the risk
of creating mosquito breeding areas.
(3)
The outlet structure shall be designed to allow complete drainage
of the pond for maintenance, without the use of pumps.
(4)
The design of a retention basin shall include the determination
of the proposed site's ability to support a viable permanent
pool. The design shall take into account such factors as the available
and required rate and quality of dry weather inflow, the stormwater
inflow, seasonal and longer-term variations in groundwater table,
and impacts of potential pollutant loadings.
(5)
Sediment storage volume equal to at least 20% of the volume
of the permanent pool shall be provided.
(6)
A sediment forebay with a hardened bottom shall be provided
at each inlet into the retention basin. The forebay storage capacity
shall at minimum be 10% of the permanent pool storage. The forebay
shall be designed to allow for access by maintenance equipment for
periodic cleaning.
(7)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a 100-year storm. The maximum
velocities in vegetated spillways excavated in otherwise undisturbed
soil shall be analyzed based upon the velocity of peak flow in the
emergency spillway during an assumed clogged primary outlet condition.
Where maximum velocities exceed design standards contained in the
Engineering Field Manual for Conservation Practices (USDA, SCS, July
1984), suitable lining shall be provided. In general, emergency spillways
should not be located in fill areas; all emergency spillways placed
on fill materials shall be lined. Lining for emergency spillways shall
incorporate native colors and materials where possible including mono
slab revetments, grass pavers and native stone.
(8)
Existing ponds or permanent pool basins can be used for stormwater
management, provided that it can be demonstrated that the ponds are
structurally sound and meet the design requirements herein.
(9)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the retention basin.
(10)
Retention basins shall be designed to provide a length-to-width
ratio of at least 3L:1W as measured in plain view (for example, a
ratio of 4L:1W is too narrow).
(11)
The retention basin depth shall average three feet to six feet
with no area shallower than three feet. In residential areas, ponds
shall be equipped with management practices that reduce the potential
for unauthorized entry and use of the pond by the general public.
Split rail fences equipped with mesh wire or other such practices
that are both functional and attractive shall be provided. 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 Township
regulation.
(12)
An aquatic bench/shelf at least 10 feet wide and with a gentle
slope not exceeding 10H:1V shall be provided along the entire perimeter
of the retention basin. The bench/shelf shall be one foot below the
permanent water surface level.
(13)
The minimum top of berm width shall be 10 feet.
(14)
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.
(15)
Stabilization. Proper stabilization structures, including stilling
basins, energy dissipaters, and channel lining, shall be constructed
at the outlets of all retention basins and emergency spillways. The
stabilization structures shall control water to prevent accelerated
erosion, reduce velocities of released water and direct water so that
it does not interfere with downstream activities.
(16)
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 Hydraulic
Engineering Circular No. 15 - Design for Stable Channels With Flexible
Linings (USDOT, FHWA, 1986) or the Engineering Field Manual for Conservation
Practices (USDA, SCS, July 1984). Such facilities shall be both functional
and harmonious with the surrounding environment; for example, native
rock shall be used in constructing dissipaters where practical.
(17)
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 structures, and discharge rates. A drainage easement, in accordance with § 125-311E(13) shall also be required where discharge enters other property or crosses property lines.
(18)
Soils used for the construction of basins shall have low-erodibility
factors (K factors).
(19)
An impermeable liner may be required to be installed up to the
100-year design water surface elevation.
(20)
The Township may require structures/grates to prevent unauthorized
access to the interior of drainage pipes.
(21)
Outlet pipes shall have a concrete cradle extending up the sides
of the pipe at least 50% of its outside diameter with a minimum thickness
of six inches.
(22)
Properly sized and spaced antiseep collars shall be installed
on all basin outlet pipes. Antiseep collars shall be installed on
the outlet pipe within the phreatic line (saturation zone), which
shall be assumed to start inside slope of the basin at the ten-year
design water surface elevation and slope downwards in the downstream
direction with a slope of four horizontal units to one vertical unit
until intersecting with the outlet pipe invert. Design calculations
shall be provided. Sand filter diaphragms shall be required for all
wet ponds and shall be designed in accordance with PADEP standards.
Antiseep collars shall also be subject to the following criteria:
(a)
Sufficient collars shall be placed to increase the seepage length
along the conduit by a minimum of 15% of the pipe length located within
the saturation zone.
(b)
Maximum collar spacing shall be 14 times the required projection
above the pipe. The minimum collar spacing shall be five times the
minimum required projection.
(c)
Antiseep collars should be placed within the saturated zone.
In cases where the spacing limit will not allow this, at least one
collar will be in the saturated zone.
(d)
All antiseep collars and their connections to the conduit shall
be watertight and made of material compatible with the conduit.
(e)
Collar dimensions shall extend a minimum of two feet in all
directions around the pipe.
(f)
Antiseep collars shall be placed a minimum of two feet from
pipe joints except where flanged joints are used.
(g)
For pipes with concrete cradles, the projection shall be measured
from the cradle.
(23)
For all embankment ponds/basins, a cutoff trench and impervious
core shall be provided. The cutoff trench shall extend at least four
feet into undisturbed subsoil (below topsoil layer) and shall have
a minimum width of eight feet. The impervious core shall be constructed
of compacted relatively impervious (Unified Soil Classification CL-ML
or CL) soils. The side slopes of the cutoff trench and impervious
core shall not exceed 1H:1V, and the top elevation of the impervious
core shall be set at or above the twenty-five-year design water elevation.
(24)
Outlet structures. Outlet structures shall meet the following
specifications:
(a)
To minimize clogging and to facilitate cleaning and inspecting,
outlet pipes shall have an internal diameter of at least 18 inches
and a minimum grade of 1%.
(b)
Antiseep collars shall be provided on all outlet pipes within
a constructed berm.
(c)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(d)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
(e)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets in conformance with AASHTO MI 70, MI 98 and M207.
(f)
Energy dissipation practices that convert concentrated flow
to uniform shallow sheet flow shall be used where appropriate.
(g)
Basin outlet structures shall have childproof nonclogging trash
racks overall design 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 O&M
plan.
(h)
Antivortex devices, consisting of a thin vertical plate normal
to the basin berm, shall be provided at the top of all outlet structure
risers or standpipes.
(i)
The outlet structure shall be analyzed for flotation assuming
all orifices and pipes are plugged. The factor of safety against flotation
shall be 1.2 or greater. Extended concrete base or other means of
ballast shall be provided as necessary to achieve the factor of safety
of 1.2.
D.
Detention basins.
(1)
Detention basins are generally discouraged and should be considered
only where other practices cannot be used effectively. Detention basins
have historically been used in stormwater management systems that
concentrate and pipe flows to the low end of a project site to quickly
release downstream. Such an approach is ineffective in meeting the
stormwater management requirements and the design approach advocated
in this chapter. Stormwater management techniques described herein
strive to disconnect flows (rather than concentrate them), provide
infiltration and water quality treatment, maintain or increase the
predevelopment time of concentration (rather than decrease it), and
manage flows as close to where they are generated as possible. Detention
basins typically do not accomplish these goals.
(2)
The maximum inside side slopes shall not exceed 3H:1V. The minimum required slope for the basin bottom is 2%. A level bottom is acceptable, provided the designer demonstrates to the Township's satisfaction that the basin bottom will be landscaped with appropriate wetland vegetation pursuant to § 125-311G. In addition, detention basins of sufficient size and slope may serve other functions as well, including recreational uses which do not hinder or conflict with the function of the detention basin.
(3)
Inlet structures. The inlet pipe invert into a basin shall be
six inches above the basin floor or lining so that the pipe can adequately
drain after rainstorms. Inlets shall discharge into areas of the basin
that slope toward the outlet structure.
(4)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the detention basin.
(5)
Low-flow channels. Low-flow channels constructed of concrete
or asphalt are not permitted. Where low-flow channels are necessary,
they shall be composed of a natural or bioengineered material. Low-flow
channels shall be designed to promote water quality and slow the rate
of flow through the basin. Low-flow channels may also be designed
to infiltrate where practical.
(6)
Properly sized and spaced antiseep collars shall be installed
on all basin outlet pipes. Antiseep collars shall be installed on
the outlet pipe within the phreatic line (saturation zone), which
shall be assumed to start inside slope of the basin at the ten-year
design water surface elevation and slope downwards in the downstream
direction with a slope of 4H:1V until intersecting with the outlet
pipe invert. Design calculations shall be provided. Sand filter diaphragms
shall be required for all wet ponds and shall be designed in accordance
with PADEP standards. Antiseep collars shall also be subject to the
following criteria:
(a)
Sufficient collars shall be placed to increase the seepage length
along the conduit by a minimum of 15% of the pipe length located within
the saturation zone.
(b)
Maximum collar spacing shall be 14 times the required projection
above the pipe. The minimum collar spacing shall be five times the
minimum required projection.
(c)
Antiseep collars should be placed within the saturated zone.
In cases where the spacing limit will not allow this, at least one
collar will be in the saturated zone.
(d)
All antiseep collars and their connections to the conduit shall
be watertight and made of material compatible with the conduit.
(e)
Collar dimensions shall extend a minimum of two feet in all
directions around the pipe.
(f)
Antiseep collars shall be placed a minimum of two feet from
pipe joints except where flanged joints are used.
(g)
For pipes with concrete cradles, the projection shall be measured
from the cradle.
(7)
For all embankment ponds/basins, a cutoff trench and impervious
core shall be provided. The cutoff trench shall extend at least four
feet into undisturbed subsoil (below topsoil layer) and shall have
a minimum width of eight feet. The impervious core shall be constructed
of compacted relatively impervious (Unified Soil Classification CL-ML
or CL) soils. The side slopes of the cutoff trench and impervious
core shall not exceed 1H:1V, and the top elevation of the impervious
core shall be set at or above the twenty-five-year design water elevation.
(8)
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 basin outlet pipes
within a constructed berm.
(c)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(d)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
(e)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets in conformance with AASHTO M170, M198 and M207.
(f)
Energy dissipation practices that convert concentrated flow
to uniform shallow sheet flow shall be used where appropriate.
(g)
Basin outlet structures shall have childproof nonclogging trash
racks overall design opening exceeding 12 inches in diameter except
those openings designed to carry perennial stream flows. Periodic
cleaning of debris from trash racks shall be included in the O&M
plan.
(h)
Antivortex devices, consisting of a thin vertical plate normal
to the basin berm, shall be provided at the top of all outlet structure
risers or stand pipes.
(i)
The outlet structure shall be analyzed for flotation assuming
all orifices and pipes are plugged. The factor of safety against flotation
shall be 1.2 or greater. Extended concrete base or other means of
ballast shall be provided as necessary to achieve the factor of safety
of 1.2.
(9)
Emergency spillways shall be sized and located to permit the
safe passage of stormwater flows from a 100-year storm. The maximum
velocities in vegetated spillways excavated in otherwise undisturbed
soil shall be analyzed based upon the velocity of peak flow in the
emergency spillway during an assumed clogged primary outlet condition.
Where maximum velocities exceed design standards contained in the
Engineering Field Manual for Conservation Practices (USDA, SCS, July
1984), suitable lining shall be provided. In general, emergency spillways
should not be located in fill areas; all such practices placed on
fill materials shall be lined. Lining for emergency spillways shall
incorporate native colors and materials where possible, including
mono slab revetments, grass pavers and native stone.
(10)
The minimum top of berm width shall be 10 feet.
(11)
The Township may require structures/grates to prevent unauthorized
access to the interior of drainage pipes.
(12)
Freeboard. The minimum freeboard shall be one foot.
(13)
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 Hydraulic
Engineering Circular No. 15 - Design for Stable Channels with Flexible
Linings (USDOT, FHWA, 1986) or the Engineering Field Manual for Conservation
Practices (USDA, SCS, July 1984). Such facilities shall be both functional
and attractive; for example, native rock shall be used in constructing
dissipaters where practical.
(14)
Stabilization. Proper stabilization structures, including stilling
basins, energy dissipaters, and channel lining, shall be constructed
at the outlets of all basins and emergency spillways. The stabilization
structures shall control water to prevent accelerated erosion, reduce
velocities of released water and direct water so that it does not
interfere with downstream activities.
(15)
Discharge points. Where there is discharge onto or through adjacent properties prior to release to a stream, the applicant shall demonstrate how downstream properties are to be protected. 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. The Municipal Engineer may require that the setback distance from the property line be increased based upon factors such as topography, soil conditions, the size of structures, the location of structures, and discharge rates. A drainage easement, in accordance with § 125-311E(13), shall also be required where discharge enters other property or crosses property lines.
(16)
A sediment forebay with a hardened bottom shall be provided
at each inlet into the detention basin. The forebay storage capacity
shall at minimum be 10% of the permanent pool storage. The forebay
shall be designed to allow for access by maintenance equipment for
periodic cleaning.
(17)
Soils used for the construction of basins shall have low-erodibility
factors (K factors).
(18)
An impermeable liner may be required to be installed up to the
100-year design water surface elevation.
E.
Conveyance systems (open channels, drainageways, and storm sewers).
(1)
Applicants are encouraged to design conveyance systems that
encourage infiltration and improve water quality wherever practicable.
(2)
Wherever conveyance channels are necessary, drainage shall be
maintained by an open channel with landscaped banks designed to carry
the twenty-five-year, twenty-four-hour stormwater runoff from upstream
contributory areas. 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. The Municipal Engineer may increase the design storm,
as conditions require. All open channels shall be designed with one
foot of freeboard above the design water surface elevation of the
design runoff condition.
(3)
Flood relief channels shall be provided and designed to convey
the runoff from the 100-year, twenty-four-hour storm, such that a
positive discharge of this runoff to an adequate receiving stream
or conveyance system occurs without allowing this runoff to encroach
upon other properties.
(4)
Open channels along existing roadways that are to be impacted
by new development or redevelopment projects may be required to be
enclosed by the Township if PennDOT standards for safety and maintenance
cannot be satisfied. All drainage structures shall conform to the
latest edition of Form 408, PennDOT specifications.
(5)
Where drainage swales are used in lieu of or in addition to storm sewers, they shall be designed to carry the required runoff without erosion and in a manner not detrimental to the properties they cross. Drainage swales shall provide a minimum grade of 2% but shall not exceed a grade of 9%. Drainage swales used strictly for conveyance are not the same as open vegetated channels. Design standards for open vegetated channels are provided under § 125-311B of this chapter.
(6)
Street curbing for the purpose of stormwater conveyance is discouraged.
On streets that must contain curbing, storm sewers shall be placed
in front of the curbing. To the greatest extent possible, storm sewers
shall not be placed directly under curbing. At curbed street intersections,
storm inlets shall be placed in the tangent section of the road.
(7)
Use of grassed swales or open vegetated swales in lieu of curbing
to convey, infiltrate and/or treat stormwater runoff from roadways
is encouraged. Inlets shall be placed at the center of the shoulder
swale draining the street and shall be located no closer than four
feet from the edge of the cartway.
(8)
Londonderry Township shall be granted a minimum twenty-foot-wide
drainage easement over all storm sewers, drainage swales, channels,
etc., that are a component of the stormwater management system when
located within undedicated land. All permanent detention basins and/or
other stormwater management facilities providing stormwater control
for other than a single residential lot shall be located within a
defined drainage easement that allows proper legal and maintenance
vehicle access by Township personnel if the need arises for such access.
Said easement shall be subject to the approval of the Township Solicitor.
(9)
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.
(10)
Water quality device. Storm drainage systems that collect runoff
from parking areas and/or loading areas exceeding 10,000 square feet
of impervious coverage and discharge to stormwater management systems,
including surface or subsurface infiltration systems, shall have a
minimum of one water quality device, sized appropriately per manufacturer's
recommendations. The purpose of water quality devices is to remove
oil, grease, and heavy particulates or total suspended solids, hydrocarbons
and other floating substances from stormwater runoff. Examples of
water quality devices include, but are not limited to, Vortechs® Stormwater Treatment
System by Contech, and Terre Kleen™ by Terre Hill Stormwater Systems. Methods
other than water quality devices may be permitted if the applicant
demonstrates to the Township's satisfaction that any such alternative
will be as effective and as easily maintained. Periodic cleaning of
these systems shall be addressed in the O&M plan submitted to
the Township.
(11)
Stormwater drainage shall be provided to:
(a)
Permit unimpeded flow of natural watercourses. In general, every
effort shall be made to avoid any disturbance of the existing natural
system of site drainage.
(b)
Ensure adequate drainage of all low points along the line of
streets or any other locations where stormwater concentration would
cause problems.
(c)
Intercept stormwater runoff along streets at intervals related
to the extent and grade of the area drained.
(d)
Provide positive drainage away from on-site sewage disposal
facilities.
(12)
If existing storm sewers are reasonably accessible, proposed
subdivisions may connect therewith, provided that no other alternatives
exist for managing stormwater at the sites themselves. Additionally,
it must be demonstrated that the storm sewer system has adequate capacity
to receive the proposed flows.
(13)
In the design of stormwater facilities and conveyances, special
consideration shall be given to avoidance of situations that may arise
from concentration of stormwater runoff and to the solution of existing
problems. The concentration of stormwater runoff onto adjacent properties
shall not be permitted without a written easement agreement with the
adjacent property(ies) owner(s).
(14)
Storm sewer design requirements.
(a)
Stormwater piping and inlet systems shall be designed for the
twenty-five-year frequency storm, except that storm sewer systems
within the drainage area of stormwater management facilities shall
be analyzed for the 100-year storm. Any storm sewer analysis shall
be a hydraulic grade line analysis that includes the effects of the
downstream tailwater conditions and shall demonstrate that the proposed
storm sewer system can convey the contributory flows without the hydraulic
grade line exceeding the top of any connected storm sewer inlet or
manhole.
(b)
Storm conveyance facilities located at site low points shall
be designed to accommodate the 100-year storm. When approved by the
Township, overflow swales may be provided at low points in streets
to safely convey the full 100-year peak flow to the control facility,
in lieu of providing the full capacity in the storm sewer system.
In no case may the elevation of the topographic overflow from a site
low point be higher than one foot below the finished floor elevation
of any structure that could be impacted by inundation at the low point.
(c)
The site grading shall be designed to direct overland flow resulting
from storm sewer clogs or blockages to the intended stormwater management
facility.
(d)
Storm sewers shall have a minimum diameter of 18 inches, and
only PennDOT Class III or greater reinforced cement concrete pipe
shall be used where the Township will have ultimate maintenance responsibility.
All storm sewers beneath a paved surface shall be bedded and backfilled
with fine-grained or densely graded soils free of stone fragments
greater than six inches in diameter. This backfill shall be placed
in six-inch lifts and solidly compacted to the satisfaction of the
Township. The minimum grade of the pipe shall be 0.50%.
(e)
For storm sewers not to be maintained by the Township, smooth-bore
high-density polyethylene pipe up to a maximum diameter of 36 inches
may be used in lieu of concrete. The minimum diameter shall be 18
inches, and the minimum grade shall be 0.50%, except for stormwater
management facility outlet pipes, which shall have a minimum grade
of 1.0%.
(f)
Headwalls, endwalls or end sections shall be required on all
open pipes, shall be of concrete construction and shall be set on
a minimum of 12 inches of AASHTO No. 57 (PennDOT 2B) coarse aggregate.
Flared end sections, when allowed by the Township, shall be concrete.
(g)
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, as amended, unless
otherwise dictated by Township ordinance.
(h)
Storm sewers shall be straight sections connected by inlets
or manholes, with any changes in alignment occurring only at inlets
or manholes.
(i)
When there is a change in pipe size through an inlet, the top
inside elevation of the outlet pipe shall be at or below the top inside
elevations of all incoming pipes. There shall be a minimum of 0.2
inch difference in inlet and outlet inverts.
(j)
For the twenty-five-year design storm event, inlet efficiency
and bypass flow shall be determined for all inlets, and the gutter
flow spread shall not exceed 1/2 the travel lane width or to a maximum
of eight feet where parking is permitted.
(k)
Rainfall intensity curves and other hydraulic design data provided
by PennDOT shall be used for design purposes.
(l)
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.
(m)
Inlets, manholes, covers and frames shall conform to PennDOT
RC standards and Form 408 specifications, as amended. At street intersections,
every attempt should be made to place inlets at the points of tangency
or curvature sections and not in the curved portion of the curbing.
Intersection grading details at a scale of one inch equals 20 feet
shall be provided for all proposed or modified intersections.
(n)
When precast concrete inlets or manholes are used within a street,
a minimum of two four-inch leveling courses of brick masonry or precast
grade ring shall be placed to bring the grate or cover to proper elevation.
No more than 12 inches of masonry or grade rings shall be permitted.
F.
Londonderry Township recognizes there are several additional BMPs
that are consistent with the objectives of this chapter. For example,
applicants may wish to consider porous paving, water quality inlets,
rain barrels, sand filters, rain gardens and other bioretention facilities,
to name a few. Applicants are encouraged to consider such practices,
provided they document appropriate design manuals and demonstrate
that alternatives will adequately meet the requirements of this chapter.
Also, riparian corridors and other native plant landscaping can provide
a valuable stormwater management benefit; such practices are encouraged
to the extent they can be quantified as providing a useful stormwater
management purpose without compromising their essential function.
