A.
For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 109-15:
B.
All SWM site plans for regulated activities shall include such measures
as necessary to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this chapter, as stated in § 109-3, by including measures that:
(a)
Minimize disturbance to floodplains, wetlands, wooded areas,
and existing vegetation.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of the commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas.
(f)
Minimize soil disturbance and compaction.
(3)
Incorporate the techniques for low-impact development practices described
in the Pennsylvania Stormwater Best Management Practices Manual (BMP
Manual).
C.
Stormwater flows onto adjacent property shall not be created, increased,
or relocated, without the written notification of the adjacent property
owner(s) by the developer. Copies of all such notifications shall
be included in the SWM site plan submission.
D.
For all regulated activities where erosion and sediment control is
required in accordance with Title 25 of the Pennsylvania Code and
the Clean Streams Law,[1] the SWM site plan shall include the required erosion and
sedimentation control measures. Necessary E&S BMPs shall be designed
in accordance with the Erosion and Sediment Pollution Control Program
Manual (E&S Manual) 2, No. 363-2134-008 (April 15, 2000), as amended
and updated. Approval of the SWM site plan by the Township shall be
conditioned on the applicant obtaining erosion and sedimentation control
approval from the appropriate agency(ies), when applicable.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
E.
For all regulated activities where NPDES permitting is required in
accordance with the Clean Water Act [33 U.S.C. § 1251 et
seq. (1972), as amended], the SWM site plan shall include the information
required in the applicant's NPDES permit application. Approval
of the SWM site plan by the Township shall be conditioned on the applicant
obtaining NPDES permit approval from the appropriate agency(ies),
when applicable.
G.
Special management areas. SWM site plans involving regulated activities
within special management areas shall be prepared in a manner consistent
with the guidance provided in Chapter 7 of the BMP Manual. The SWM
site plan submission shall include design details for SWM BMPs within
said special management area, and shall include information on why
the area is deemed to be a "special management area."
H.
A SWM site plan may propose that stormwater related to the proposed
regulated activities be accommodated by existing stormwater management
facilities on adjoining or nearby properties, provided that the SWM
site plan documents the following.
(1)
The use of the stormwater management facilities located on said adjoining
or nearby property is approved in writing by the owner of the property.
(2)
The stormwater management facilities located on said adjoining or
nearby property are designed in a manner that can accommodate the
stormwater management needs of the regulated activity in a manner
consistent with all requirements of this chapter. The SWM site plan
shall include all documentation necessary for the Township to confirm
such compliance.
I.
The design storm volumes to be used in the analysis of peak rates
of discharge shall be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, as amended
and updated, United States Department of Commerce, National Oceanic
and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological
Design Studies Center, Silver Spring, Maryland. NOAA's Atlas
14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
J.
SWM site plans, once approved by the Township, shall remain on site
throughout the duration of the regulated activity and be available
for review as may be necessary by representatives of the Township.
K.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
L.
The Township may, after consultation with DEP, approve measures for
meeting the state water quality requirements other than those in this
chapter, provided that they meet the minimum requirements of, and
do not conflict with, state law including, but not limited to, the
Clean Streams Law. The municipality shall maintain a record of consultations
with DEP pursuant to this subsection.
A.
A property owner or developer of any regulated activity that meets
the following exemption criteria is, upon approval from the Township,
exempt from the formal SWM site plan submission requirements of this
chapter as specified herein. However, the property owner or developer
shall be subject to all other requirements of this chapter other than
the formal SWM site plan submission requirements for which an exemption
or exemptions have been authorized, unless otherwise indicated. The
criteria for exemption in this section apply to the total development
proposed, including instances in which the development is proposed
to take place in phases. The date of enactment of this chapter shall
be the starting point from which future development and the respective
exemption criteria shall be cumulatively considered and regulated.
(1)
Regulated activities that involve equal to or less than 1,000 square
feet of impervious surface may be exempted from the peak rate control,
volume control and the SWM site plan preparation and submission requirements
of this chapter. The applicant shall complete the Municipal Stormwater
Management Worksheet from the Stormwater Management Design Assistance
Manual and file said worksheet with the Township.
(2)
Regulated activities that involve greater than 1,000 square feet
and equal to or less than 10,000 square feet of impervious area, and
where all the proposed impervious area can be entirely disconnected,
may be exempted from the peak rate control, volume control, and the
SWM site plan preparation and submission requirements of this chapter.
The applicant shall complete the Stormwater Management Worksheets
from the Stormwater Management Design Assistance Manual and file said
worksheets with the Township.
(3)
Regulated activities that involve greater than 1,000 square feet and equal to or less than 5,000 square feet of impervious area may be exempted from the peak rate control and volume control preparation and submission requirements of this chapter. A minor stormwater site plan, as detailed in the Stormwater Management Design Assistance Manual, shall be submitted to the Township instead of the submission of a full SWM site plan in accordance with Article IV of this chapter.
(4)
Agricultural activities shall be exempt from the rate control, volume control and SWM site plan preparation and submission requirements of this chapter, provided the agricultural activities are performed in accordance with the requirements of 25 Pa. Code 102. Further, such activities shall not be subject to the exemption approval process of § 109-15B of this chapter.
(5)
Forest management and timber operations are exempted from the rate
control, volume control and SWM site plan preparation and submission
requirements of this chapter, provided the forest management and timber
operations are performed in accordance with the requirements of 25
Pa. Code 102, as amended or repromulgated from time to time.
(6)
Regulated activities involving domestic gardening for single-family consumption shall be exempted from volume control, rate control, and SWM site plan preparation and submission requirements of this chapter, and shall not be subject to the exemption approval process of § 109-15B of this chapter.
(7)
In-kind repair, in-kind replacement, and maintenance of existing surfaces, and structures shall be exempted from volume control, rate control, and SWM site plan preparation and submission requirements of this chapter, and shall not be subject to the exemption approval process of § 109-15B of this chapter.
B.
Authorization of exemptions. The Township shall determine, in accordance
with the following requirements and process, whether a proposed regulated
activity may be exempted from any of the requirements of this chapter.
(1)
The property owner or developer proposing the regulated activity shall submit, in writing on a form supplied by the Township, a request for said proposed regulated activity to be exempted from allowable requirements of this chapter pursuant to Subsection A. The written request shall identify the project and shall indicate the specific exemption criteria, as listed in § 109-15A, that apply to the project and shall be submitted in accordance with the Stormwater Management Worksheets from the Stormwater Management Design Assistance Manual.
(2)
Upon receipt of the exemption request form, the Township or its designee
shall either approve or deny the exemption request. If the exemption
request is denied, the Township or its designee shall direct the property
owner or developer to submit the information required to demonstrate
that the proposed regulated activity complies with the requirements
of this chapter or meets the exemption criteria.
(3)
Exemption request approval shall be at the discretion of the Township,
and shall be subject to the following:
(a)
The Township may deny any exemption request or suspend or revoke
any approved exemption request at any time for any project where the
Township believes that the proposed regulated activity poses a threat
to public health, safety, property, or the environment.
(b)
Approval of an exemption request does not relieve the property
owner or developer from other applicable requirements of this chapter
or of other Township ordinance or regulations.
(c)
The Township reserves the right to deny an exemption request
if a drainage problem is known or identified as existing or is expected
to exist downstream from the proposed regulated activity.
A.
Impervious area.
(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 phases.
(2)
For development taking place in phases, the total proposed impervious
area within the SWM site plan must be used in determining conformance
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 § 109-17 and the peak rate controls of § 109-18 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
B.
Normally, dry, open-top storage facilities, designed as such, shall
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. However, any designed infiltration
volume at such facilities is exempt from the minimum twenty-four-hour
standard, i.e., may infiltrate in a shorter period of time, so long
as none of the stormwater intended for infiltration is discharged
into the surface waters of the commonwealth.
C.
Infiltration BMPs shall 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.
D.
A stormwater management plan shall be submitted for all subdivisions
and/or land developments. The plan shall show all drainage within
the area affecting the subject property, all existing and proposed
drainage facilities and all grading proposed for the subject property,
as well as the additional plan information required in this section.
E.
All land areas shall be graded to secure proper drainage away from
buildings, on-lot sewage disposal facilities and the like and to prevent
the collection of stormwater in pools. Drainage provisions shall be
of such design as to carry surface waters to the nearest practical
natural drainage channel, storm sewer system detention basin or other
drainage facilities. The landowner or developer shall construct and/or
install such drainage structures and/or pipes as are determined necessary
by the Township to prevent soil erosion, damage and siltation and
to satisfactorily carry off surface water. In the design of storm
drainage facilities, special consideration must be given to preventing
excess runoff onto adjacent developed or undeveloped properties. In
no case may any slope exceed the normal angle of slippage of the material
involved. All slopes must be protected against erosion. In no case
may a change be made in the existing topography which would alter
the existing drainage or topography in a way so as to adversely affect
adjoining properties.
F.
Storm sewers, culverts, bridges and related drainage installations
shall be provided to:
(1)
Permit unimpeded flow of natural watercourses. Such flow may be redirected
as required, subject to the approval of the Pennsylvania Department
of Environmental Protection.
(2)
Ensure adequate drainage of all low points as may be related to streets.
(3)
Intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area drained to prevent flow
of stormwater across intersections and to prevent the flooding of
intersections during the design storm.
(4)
Ensure adequate and unimpeded flow of stormwater under driveways
in, near or across natural watercourses or drainage swales. Properly
sized pipes or other conduits shall be provided as necessary.
(5)
Prevent excessive flow on or across streets, sidewalks, drives, parking
areas and any other paved surface or access way.
(6)
Lead stormwater away from springs.
(7)
Provide adequate drainage away from on-site sewage disposal systems.
G.
The stormwater management plan for each subdivision and/or land development
shall take into account and provide for upstream areas within the
entire watershed in computing discharge quantities, sizing of pipes,
inlets and other structures. The runoff from any proposed development
shall be subject to evaluation which includes the anticipated runoff
from other existing or proposed developments within the same watershed.
Stormwater management facilities designed to serve more than one property
or development in the same watershed are encouraged, in which case
consultation with the Township is required prior to design.
H.
All natural streams, channels, swales, drainage systems and/or areas
of concentration of surface water shall be maintained in their existing
condition unless alteration is approved by the Township. In any event,
all encroachment activities shall comply with Chapter 105 of the Commonwealth
of Pennsylvania Department of Environmental Protection, Dam Safety
and Waterway Management Rules and Regulations.
I.
Man-made structures shall be kept to a minimum and bridges, culverts,
or riprap shall be constructed to maintain the natural characteristics
of the stream and shall meet the approval of the Township.
J.
Retention/detention basins shall be designed to utilize the natural
contours of the land. When such design is impracticable, the construction
of the basin shall utilize slopes as shallow as possible to blend
the structures into the existing terrain.
K.
Any subdivision and/or land development within a flood hazard district
shall comply with all of the provisions of the Straban Township Zoning
Ordinance, and the rules and regulations of the Pennsylvania Department
of Environmental Protection.
L.
The Township may require that a landowner or developer provide reasonable
corrective measures to alleviate an existing off-site drainage problem
which may be affected by the proposed subdivision and/or land development.
It shall be the responsibility of the landowner or developer to obtain
all drainage easements in, over or through other properties, and the
Township, its agents, workmen, servants and employees shall be indemnified
and held harmless from any liability.
M.
All lots, tracts or parcels shall be graded to provide proper drainage
away from buildings and dispose of the runoff without ponding, and
all land within a development shall be graded to drain and dispose
of surface water without ponding, except where other arrangements
are approved by the Township. Grading shall not be done in such a
way so as to divert water onto the property of another landowner without
the expressed consent of the Township and the affected landowner.
The low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities. Water volume controls shall be implemented using the Design Storm Method in § 109-17A or the Simplified Method in § 109-17B. For a regulated activity involving less than one acre of impervious coverage that does not require hydrologic routing to design the stormwater facilities, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and/or other factors. The Design Storm Method in § 109-17A shall be used for all regulated activity involving greater than one acre of impervious coverage.
A.
The Design Storm Method (CG-1 in the BMP Manual) may be used for
any size of regulated activity. This method requires detailed modeling
to achieve the following standards.
(1)
The post-development total runoff volume shall not increase for all
storms equal to or less than the two-year twenty-four-hour duration
precipitation.
B.
The Simplified Method (CG-2 in the BMP Manual) is independent of
site conditions and may be used for projects involving regulated activities
proposing equal to or less than one acre of impervious coverage and
that do not require design of stormwater storage facilities. When
the Simplified Method is used to address stormwater management needs
of new impervious surfaces, the following design standards shall be
achieved:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow and shall not be
released into the surface waters of the commonwealth. Removal options
for the first one inch of runoff include, but are not necessarily
limited to, reuse and infiltration.
(3)
Infiltration facilities shall be designed to accommodate infiltration of as much of the permanently removed runoff as site conditions will allow. If the soils within the project area do not allow for infiltration of the entire first one inch of runoff from new impervious surfaces, other forms of runoff volume control shall be used to achieve the required removal volume. Such measures may include, but are not limited to vegetated roofs, biorention, and capture-and-reuse systems. In addition, the infiltration alternative authorized in § 109-17C may be employed.
C.
Infiltration alternative. Where infiltration is not possible due to soil characteristics or is not desirable given other characteristics, water quality control may be proposed as an alternative to strict adherence to the volume control standards of § 109-17 of this chapter. Where water quality control is proposed, the following standards shall be achieved.
(1)
At a minimum, the following documentation shall be provided to justify
the proposal to reduce the infiltration requirements:
(a)
Description of and justification for field infiltration/permeability
testing with respect to the type of test and test locations.
(b)
An interpretive narrative describing existing soils of the site
and their structure as these relate to the interaction between soils
and water characteristics of the site. In addition to providing soil
and soil profile descriptions, this narrative shall identify depth
to seasonal water tables and depth to bedrock and provide a description
of all subsurface elements (restrictive layers, geology, etc.) that
influence the direction and rate of subsurface water movement.
(c)
A qualitative assessment of the site's contribution to
annual aquifer recharge shall be made, along with the identification
of any restrictions or limitations associated with the use of designed
infiltration facilities.
(d)
The provided documentation must be signed and sealed by a professional
engineer or geologist.
(2)
Water quality BMPs shall be implemented on all permanent stormwater
discharges from the proposed project site to achieve pollutant removal
efficiencies in accordance with the Table 109-17.
|
Table 109-17 Required Pollutant Removal Efficiencies for
Infiltration Alternatives
| |||
|---|---|---|---|
|
Pollutant Load
|
Units
|
Required Removal Efficiency
(%)
| |
|
Total suspended solids (TSS)
|
Pounds
|
85%
| |
|
Total phosphorus (TP)
|
Pounds
|
85%
| |
|
Total nitrate (NO3)
|
Pounds
|
50%
| |
(3)
Design guidance from the most current version of the Pennsylvania
Stormwater Best Management Practices Manual, or equivalent resource
as precoordinated with the Township, shall be consulted when choosing
design criteria for water quality BMPs.
A.
Post-development discharge rates shall not exceed the predevelopment
discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-,
and one-hundred-year, twenty-four-hour storms. If it is shown that
the peak rates of discharge indicated by the post-development analysis
are less than or equal to the peak rates of discharge indicated by
the predevelopment analysis for one-, two-, five-, ten-, twenty-five-,
fifty-, and one-hundred-year, twenty-four-hour storms, then the requirements
of this section have been met. Otherwise, the applicant shall provide
additional controls as necessary to satisfy the peak rate of discharge
requirement.
B.
For computation of predevelopment peak discharge rates, 20% of existing
impervious areas, when present, shall be considered meadow.
Where an applicant proposes to utilize riparian buffers as the
means to meet the requirements of this chapter, said riparian buffers
shall be established and/or maintained in accordance with the BMP
Manual or the publication Riparian Forest Buffer Guidance, published
November 2010 by the Pennsylvania Department of Environmental Protection,
and as may be amended or updated.
A.
Any drain or conveyance, whether on the surface or subsurface, that
allows any nonstormwater discharge including sewage, process wastewater,
and wash water to enter the waters of the commonwealth is prohibited.
C.
The following discharges are authorized unless they are determined
to be significant contributors to pollution to the waters of the commonwealth:
(1)
Discharges from fire-fighting activities.
(2)
Potable water sources including waterline flushing.
(3)
Irrigation drainage.
(4)
Air-conditioning condensate.
(5)
Springs.
(6)
Water from crawl space pumps.
(7)
Pavement wash waters where spills or leaks of toxic or hazardous
materials have not occurred (unless all spill material has been removed)
and where detergents are not used.
(8)
Diverted stream flows.
(9)
Flows from riparian habitats and wetlands.
(10)
Uncontaminated water from foundations or from footing drains.
(11)
Lawn watering.
(12)
Dechlorinated swimming pool discharges.
(13)
Uncontaminated groundwater.
(14)
Water from individual residential car washing.
(15)
Routine external building wash-down (which does not use detergents
or other compounds).
(16)
Water discharged in well testing for potable water supplies.
Roof drains, sump pumps, and footer drains should discharge
to infiltration or vegetative BMPs and, to the maximum extent practicable,
satisfy the criteria for DIAs. Discharges of each should be conveyed
in such a manner as to not cause water problems for adjoining property
owners. Roof drains, sump pumps, and footer drains shall not discharge
to roadways, curblines, or storm sewer systems without approval by
the Township.
No person shall modify, remove, fill, landscape, or alter any
SWM BMPs, facilities, areas, or structures in a manner, without the
written approval of the Township, with the exception of necessary
maintenance activities such as mowing.
A.
Standards and criteria.
(1)
Storm drainage system.
(a)
Design flow rate.
[1]
The storm sewer system shall be designed to carry a twenty-five-year
peak flow rate without surcharging inlets. The peak flow rate into
each inlet shall be indicated on the stormwater drainage plan. The
design flow rate shall be determined by the rational formula, Q=CIA:
|
Where:
| ||||
|
Q
|
=
|
Peak runoff rate, cubic feet per second (CFS).
| ||
|
C
|
=
|
Runoff coefficient equal to the ratio of the peak runoff rate
to the average rate of rainfall over a time period equal to the time
of concentration.
| ||
|
I
|
=
|
Average rainfall intensity in inches per hour for a time equal
to the time of concentration.
| ||
|
A
|
=
|
Drainage area in acres.
| ||
[2]
Appropriate values for the runoff coefficient and rainfall intensity
shall be taken from the following source, unless otherwise approved
by the Township:
|
Commonwealth of Pennsylvania
| |
|---|---|
|
Department of Transportation
| |
|
Publication 584, PennDOT Drainage Manual
| |
|
2010 Edition (or the latest revision thereto)
|
(b)
Storm sewer system design.
[1]
The storm sewer system shall be designed to the more restrictive
of the following: to collect stormwater at any point where three to
five cubic feet per second is accumulated during the design storm;
and/or inlets/manholes shall not be spaced more than 300 feet apart
on pipe sizes up to 24 inches in diameter and not more than 400 feet
apart on greater sizes.
[2]
Inlets, manholes, grates, covers, frames and the like shall
conform to the Pennsylvania Department of Transportation Roadway Construction
Standards and Form No. 408 specifications and all amendments, revisions
or updates thereto.
(c)
Bridge/culvert/channel design.
[1]
Bridges and culverts shall have ample waterway to carry expected
flows, based on a minimum storm frequency of 100 years or as required
by the Pennsylvania Department of Environmental Protection. Bridge
and/or culvert design shall be in accordance with the Pennsylvania
Department of Transportation and/or the Pennsylvania Department of
Environmental Protection requirements. All culverts shall be provided
with concrete end walls.
[2]
All drainage channels shall be designed to carry a flow rate
equal to a one-hundred-year, twenty-four-hour storm.
[3]
All drainage channels shall be designed to prevent the erosion
of the stream bed and stream bank areas. The flow velocity in all
vegetated drainage channels shall not exceed the maximum permissible
velocity to prevent soil erosion. Suitable bank stabilization shall
be provided where required to prevent soil erosion of the drainage
channels. Where storm sewers discharge into existing drainage channels
at an angle greater than 30° from parallel with the downstream
channel flow, the far side bank shall be stabilized by the use of
riprap and masonry and/or concrete walls. The stabilization shall
be designed to prevent soil erosion and front heave under and behind
the stabilizing media.
[4]
Any vegetated drainage channel requiring mowing of the vegetation
shall have a maximum slope of four horizontal to one vertical on those
areas to be mowed.
[5]
In all instances where a proposed driveway culvert will cross
a drainage channel/swale, a minimum fifteen-inch diameter driveway
culvert must be provided that will be adequate to convey the design
flows of the drainage channel.
[6]
The design of all channels shall, as a minimum, conform to the
design procedures outlined with the Pennsylvania Department of Transportation
standards, Pennsylvania Stormwater Best Management Practices Manual,
and Pennsylvania Department of Environmental Protection Erosion and
Sediment Pollution Control Program Manual.
(d)
Overflow system. An overflow system shall be provided to carry
flow to the detention basin when the capacity of the storm drain pipe
system is exceeded. The overflow system shall have sufficient capacity
to carry the difference between the one-hundred-year and the twenty-five-year
peak flow rates.
(e)
Inlet capacity.
[1]
All inlets must be designed to accommodate the twenty-five-year
peak flow rate. The capacity of Type C, M, or S inlets shall be determined
from the following source:
|
Commonwealth of Pennsylvania
| |
|---|---|
|
Department of Transportation
| |
|
Publication 584, PennDOT Drainage Manual
| |
|
2010 Edition (or the latest revision thereto)
|
[2]
The capacity of each inlet shall be indicated within the SWM
site plan narrative. All stormwater management plans shall indicate
that inlet grates be installed in such a manner that the roadway stormwater
will be directed into the inlet and away from the roadway. All inlets
shall be designed to create a one-inch sump condition below finished
road surface unless approved otherwise by the Township. At curbed
street/driveway intersections, inlets shall be placed on the tangent
section and not in the curved portion of the curbing.
(f)
Straight pipe sections. All storm sewers shall be designed to
follow straight courses. No angular deflections of storm sewer pipe
sections in excess of 5° shall be permitted. No vertical curves
shall be permitted in the storm sewer system.
(g)
Horizontal pipe deflections. A manhole or inlet shall be provided
at all horizontal deflections in the storm pipe system exceeding 5°.
(h)
Minimum grade and size. All storm drain pipes shall be designed
to maintain a minimum grade that will result in a full flow velocity
of at least two feet per second. All storm sewer pipes shall have
a minimum inside diameter of 15 inches.
(i)
Pipe capacity. The capacity of all pipe culverts shall, as a
minimum, provide the required carrying capacity to meet current Pennsylvania
Department of Transportation standards and criteria.
(j)
Pipe arches. Where headroom is restricted, equivalent pipe arches
may be used in lieu of circular pipes.
(k)
Pipe material and gauge thickness. All storm sewers shall be
either reinforced cement concrete, corrugated aluminum, corrugated
polyethylene pipe, smooth lined corrugated polyethylene pipe, or aluminized
Type II steel pipe. Storm sewers shall be of the proper class and
thickness to support the above fill material. Pipe class and gauge
or thickness shall be noted on the plans. All pipe shall conform to
Pennsylvania Department of Transportation specifications.
(l)
Allowable headwater depth. At all inlets or manholes, the maximum
allowable headwater depth shall be one foot below the top of the inlet
grate or the manhole.
(m)
Minimum and maximum cover. In lawn areas, a minimum of 12 inches
of cover shall be maintained over all storm drain pipes or as specified
by the pipe manufacturer. Under streets, the top of storm drain pipes
shall be a minimum of six inches below sub grade elevation or as specified
by the pipe manufacturer. The maximum cover over storm drain pipes
shall be 10 feet unless otherwise approved by the Township.
(n)
Storm sewer system outlets. Storm sewer system outlet pipes
shall extend to proposed stormwater management facilities, natural
watercourses and the like. A concrete end wall shall be required on
all storm sewer system inlet and outlet pipes. All storm/sewer outlets
24 inches in diameter or greater shall be equipped with a galvanized
child-proof horizontal bar rack, bolted to the end wall.
(o)
Drainage easements.
[1]
All storm sewer easements through undedicated land shall be
a minimum of 20 feet in width.
[2]
Where a site is traversed by a watercourse, a drainage easement
or right-of-way conforming substantially with the line of such watercourse
and of such width as will be adequate to preserve natural drainage
and provide sufficient width for maintenance shall be created, as
determined by the Township.
(p)
Diversion of surface water runoff. All storm sewers and/or drainage
swales shall be designed to carry such runoff into a detention basin
or similar facility utilized to control the rate of runoff, unless
approved otherwise by the Township.
(2)
Detention/retention basins.
(a)
Detention basins shall be designed in accordance with the Soil
Cover Complex Method and the procedures developed by the United States
Department of Agriculture, Soil Conservation Service, as outlined
in their Technical Release No. 55, Urban Hydrology for Small Watersheds,
with specific attention given to antecedent moisture conditions, flood
routing and peak discharge and Hydrology National Engineering Handbook
Section 4, or other method as approved by the Township Engineer.
(b)
Basin design criteria (SCS). Basins shall be designed to safely convey the quantity of water resulting from a one-hundred-year, twenty-four-hour storm under full development conditions. Stormwater management calculations shall ensure that the predevelopment discharge from the site conforms with § 109-18.
(c)
Outlet control structures.
[1]
All outlet control structures shall be constructed of concrete,
properly anchored to prevent flotation and equipped with childproof,
nonclogging removable trash racks over all design openings 12 inches
or greater in diameter, except those openings designed to carry perennial
stream flows.
[2]
Temporary sedimentation controls shall be provided during construction
to prevent the flow of sediment through the basin outlet pipe. Such
measures may include temporary riser pipes, rock-filled gabions, plywood
standboxes, silt fences and the like.
(d)
Emergency spillways. Whenever possible, the emergency spillway
for basins shall be constructed on undisturbed ground. Emergency spillways
shall be constructed of reinforced concrete, concrete moundslabs or
vegetated earth. All emergency spillways shall be constructed so that
the basin berm is protected against soil erosion. The minimum capacity
of the emergency spillway shall be designed to pass the one-hundred-year
post-development flow neglecting the capacity of the principal outlet
structure. Emergency spillways shall extend along the upstream and
downstream berm embankment slopes. The emergency spillway shall not
discharge stormwater over earthen fill and/or easily erodible material
without adequate protection against soil erosion.
(e)
Freeboard. The minimum freeboard shall be one foot. (Freeboard
is the difference between the design flow elevations in the emergency
spillway and the top of the settled basin embankment.)
(f)
Basin outlet pipes. Basin outlet pipes shall be equipped with
watertight joints.
(g)
Antiseep collars. Antiseep collars shall be installed around
the principal pipe barrel within the normal saturation zone of the
basin berms. The antiseep collars and their connections to the pipe
barrel shall be watertight. The antiseep collars shall be designed
in accordance with USDA SCS criteria. Design calculations for antiseep
collars must be submitted with the basin calculations.
(h)
Basin outlets. Energy-dissipating devices (concrete aprons and
the like) shall be placed at all basin outlets. Concrete end walls
shall be placed at all basin outlets. All basin outlet pipes 24 inches
in diameter or greater shall be equipped with childproof devices to
deter entry by pedestrians or animals. Design calculations for proposed
energy dissipaters must be submitted with basin calculations.
(i)
Slope of detention basin embankment.
[1]
The maximum slope of earthen basin embankments shall be four
to one (4:1), unless otherwise approved by the Township. The top or
toe of any slope shall be located a minimum of 15 feet from adjacent
property lines with the exception of the downstream property line
where the toe of the embankment shall be placed a sufficient distance
to allow for energy dissipating devices but in no case less than 40
feet unless approved otherwise by the Township.
[2]
Whenever possible, the side slopes and basin shape shall blend
with the natural topography. Straight side slopes and rectangular
basins shall be avoided whenever possible.
(j)
Width of berm. The minimum top width of detention basin berms
shall be six feet.
(k)
Construction specifications. The plans shall indicate the construction
specifications and compaction requirements for all detention/retention
basins.
(l)
Cutoff trench. A cutoff trench shall be excavated along the
center line of dam on earth fill embankments. The minimum depth shall
be three feet. The minimum bottom width shall be 10 feet or wide enough
to permit operation of compaction equipment. The side slopes shall
be no steeper than 1:1. The trench shall be kept free from standing
water during the backfilling operations.
(m)
Grading and landscaping of basins; cuts and fills. No excavation
or fill shall be made with a cut-and-fill slope steeper than four
feet horizontal to one foot vertical unless otherwise approved by
the Township Engineer. A written statement shall be required from
a civil engineer licensed by the Commonwealth of Pennsylvania having
experience in soils engineering certifying that he has inspected the
site and that any proposed deviation from the slope specified above
should not endanger any property or result in personal injury. Retaining
walls will be required if a stable slope cannot be maintained. Any
retaining wall design must be designed by an experienced structural
engineer licensed by the Commonwealth of Pennsylvania. The toe of
any cut or fill slope must be located a minimum of 15 feet from adjacent
property lines.
(n)
Landscaping.
[1]
A minimum of four inches of topsoil shall be placed on all areas
affected by the basin construction (bottom of basin, side slopes,
top of berm and the like).
[2]
All earthen basins shall be seeded with a standard seed mix
containing temporary and permanent grasses capable of providing a
minimum uniform seventy-percent perennial ground cover, or other approved
ground covers, within seven days after final grading. Application
rate shall be in accordance with the seed supplier's guidelines
and recommendations.
[3]
Fencing may be required around detention/retention basins where
the Township determines that circumstances warrant the fencing.
[4]
All detention/retention basins shall be landscaped.
(o)
Permanent pond.
[1]
A five-foot-wide bench sloping at 4% shall be provided for all
detention/retention basins designed to contain a permanent pond of
water. The toe of the bench shall begin at the permanent water surface
elevation. Alternate designs may be submitted for review to the Township
and its engineer.
[2]
When a permanent pond is proposed, a report prepared by a certified
geotechnical specialist must be provided certifying that the water
will not become stagnant. The basin side slopes below the waterline
must not exceed 4:1.
(p)
Positive drainage. Detention basins must be designed to eliminate standing water or swampy conditions after the basin has drained. This must be accomplished either by the installation of stone-trenched underdrains or by providing a minimum basin bottom slope of 2% to the basin outlet. Other arrangements may be presented for review and approval by the Township, provided the facility is entirely dewatered in accordance with § 109-16B.
(3)
Subsurface infiltration/disposal/retention basin systems. The following
procedures and materials shall be required for all subsurface stormwater
management facilities:
(a)
Prior to starting any excavation for subsurface facilities,
the contractor must notify the Township's Engineer 48 hours in
advance for inspection of said facilities. Inspection is required
for all subsurface stormwater management facilities at the time of
installation and prior to backfilling.
(b)
Excavation for all subsurface facilities shall be performed
in a manner that will minimize compaction of the subsurface facility
floor and surrounding areas as well as minimize smearing of the sidewalls
of the subsurface facility.
(c)
The floor and sidewalls of the subsurface stormwater management
facility shall be roughened prior to placement of the geofabric and
aggregate.
(d)
Only clean, open graded aggregate, free of fines, shall be used
in subsurface stormwater management facilities.
(e)
The top, sides, and floor of all subsurface stormwater management
facilities shall be covered with a drainage filtration fabric which
meets the requirements of the Pennsylvania Department of Transportation,
Publication 408 for Class I Geofabrics.
(f)
All pipes leading into subsurface stormwater management facilities
shall be equipped with screening or water quality devices to prevent
debris from entering the system.
(g)
The floor of all subsurface stormwater management facilities
shall be located a minimum of 12 inches above the seasonal high-water
table or bedrock limiting zone as established by a soil test pit and
site specific soil profile. Depths of less than 12 inches above the
limiting zone will only be allowed where it is certified by a registered
professional engineer, geologist, or hydrogeologist that the proposed
facility will not create an environmental hazard.
(h)
For all subsurface stormwater management facilities that propose to use infiltration as a means to manage stormwater runoff, infiltration testing must be performed at the same elevation of the invert of the proposed facility to determine a design infiltration rate and dewatering time for the proposed facility. The proposed facility dewatering time must be in accordance with § 109-16B.
(i)
Inspection points, cleanouts, and overflow facilities shall
be provided for all subsurface stormwater management facilities. All
inspection points and cleanouts must be located in a configuration
that will allow for cleaning and maintenance of the entire subsurface
facility.
(j)
Detailed maintenance instructions and a proposed maintenance
schedule must be provided on the plan drawings and provided to the
property owner prior to plan approval.
(k)
All subsurface stormwater management facilities must be located
a minimum of 100 feet from any potable water wells.
