A.
For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 136-15:
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 § 136-3, 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,
decreased, relocated, or otherwise altered 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, 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.
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.
An 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 the 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 the 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 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 plan submission requirements for which an exemption or
exemptions have been authorized. 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 Page 1 of the Municipal
Stormwater Management Worksheet from the Stormwater Management Design
Assistance Manual (see Plan Appendix C) 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 the 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 (see Plan
Appendix C) 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 (see Plan Appendix C), 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 that the agricultural activities are performed in accordance with the requirements of 25 Pa. Code Chapter 102. Further, such activities shall not be subject to the exemption approval process of § 136-15B 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 that the forest management
and timber operations are performed in accordance with the requirements
of 25 Pa. Code Chapter 102.
(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 § 136-15B 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 § 136-15B 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 § 136-15A, that apply to the project.
(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 § 136-17 and the peak rate controls of § 136-18 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.
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 § 136-17A or the Simplified Method in § 136-17B. For 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 § 136-17A 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, bioretention, and capture-and-reuse systems. In addition, the infiltration alternative authorized in § 136-17C 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 § 136-17 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)
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 136-17.
|
Table 136-17
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 washwater, 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 washwaters 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 washdown (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.
No person shall modify, remove, fill, landscape, or alter any
SWM BMPs, facilities, areas, or structures in any manner, without
the written approval of the Township, with the exception of necessary
maintenance activities, such as mowing.
A.
General requirements.
(1)
The plan shall show all drainage within the area affecting the subject
property, all existing and proposed drainage facilities, all grading
proposed for the subject property, as well as the additional plan
information required in this section.
(2)
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:
(3)
Storm sewers, culverts, bridges, and related drainage installations
shall be provided:
(a)
To permit unimpeded flow of natural watercourses.
(b)
To ensure adequate drainage of all low points as may be related
to streets.
(c)
To 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.
(d)
To ensure an adequate and impeded 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.
(e)
To prevent excessive flow on or across streets, sidewalks, drives,
parking areas, and any other paved surface or accessway.
(f)
To lead stormwater away from springs.
(g)
To provide adequate drainage away from on-site sewage disposal
systems.
(4)
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.
(5)
All natural streams, channels, swales, drainage systems and/or areas
of concentration of surfaces 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.
(6)
Man-made structures shall be kept to a minimum, and bridges, culverts,
or rip-rap shall be constructed to maintain the natural characteristics
of the stream and shall meet the approval of the Township.
(7)
For the purpose of this subsection, "streams" and "intermittent streams"
are defined as those watercourses on the Franklin Township Zoning
Map, the USGS Quadrangle maps of the area, and/or determined as such
pursuant to an on-site survey by the Township.
(8)
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.
(10)
Any water originating from non-natural, sources such as swimming
pools, air-conditioning units, sump pumps, roof drains or other similar
flow, shall not be discharged directly onto any street or other public
right-of-way used for pedestrian or vehicular access. The Township
may require a plan that provides for these discharges.
(11)
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.
B.
Stormwater management plan requirements. In addition to the drainage
plan requirements in the adopted Franklin Township Subdivision and
Land Development Ordinance,[2] the following requirements apply:
(1)
A plan showing all predevelopment and post-development stormwater
flow to and from basins; a plan showing all post-development flows
to all inlets, headwalls, swales, channels, and the like. The drainage
areas and the design flow to each inlet or structure shall be delineated
on a copy of the stormwater management plan, where applicable.
(2)
A map depicting the total watershed. A USGS Quadrangle Map is suitable
as the source for such a map. However, the watershed area must be
highlighted or otherwise distinguished from other areas outside the
watershed.
(3)
Maps and drawings showing all existing and proposed drainage facilities
affecting the subject property.
(4)
The following stormwater-related items shall be included as part
of the plan submission:
(a)
Definition of existing on- or off-site drainage problems.
(b)
Appropriate stormwater management criteria, such as release
rate percentage, direct discharge and downstream impact elevation.
(c)
Configuration of the storm sewer and sanitary sewer system layouts.
(d)
Location and layout of the stormwater management system, with
a detailed description of its proposed design and operation.
(e)
Profiles of all proposed sewers, including elevation, sizes,
slopes and materials, at a scale of no less than one inch equals 50
feet horizontal and one inch equals five feet vertical. In addition,
a plan (at a smaller scale, where necessary) shall be provided which
shows the overlay development drainage and grading.
(f)
Locations, dimensions and design details required for the construction
of all facilities. All existing and proposed detention/retention basins
shall, at a minimum, be shown in plan view and shall include but not
be limited to the following information:
(g)
Soil percolation test results. Soil testing shall be completed
and reported in accordance with the criteria established by the Township
Engineer. Specific alternate criteria may be submitted for review
and consideration.
(h)
Final provisions for ownership and maintenance of all stormwater
related facilities.
C.
Standards and criteria.
(1)
Stormwater collection system. The design of stormwater collection
and conveyance facilities shall be governed by the following criteria:
(a)
Peak discharge shall be computed using the Rational Formula:
|
Q
|
=
|
C I A
| ||
|
Where:
| ||||
|
Q
|
=
|
Peak discharge in cubic feet per second.
| ||
|
C
|
=
|
Runoff coefficient expressed as the ratio of peak runoff rate
to the average amount of rainfall over a period of time 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.
| ||
|
In general, the procedure from the Pennsylvania Department of
Transportation, Design Manual, Part 2, shall be followed.
| ||||
(b)
Runoff coefficients shall be computed as a weighted average
of conditions which represent maximum development potential on the
property. Soil types, ground slope, and storm frequency shall all
be considered in the selection of runoff coefficients. Reference for
the coefficients applied should be provided in the report.
(c)
Storm intensity (I) shall be computed as a function of the time
of concentration (Tc). A Tc of five minutes shall be used in determining
intensity, unless the drainage area parameters justify the use of
a greater value. Use of a Tc greater than five minutes shall be as
approved by the Township Engineer.
(d)
The minimum full flow velocity of any storm sewer or culvert
shall be 2.5 feet per second (fps).
(e)
Storm frequency.
[1]
The following storm frequencies are to be used for design:
|
Type
|
Frequency
| |
|---|---|---|
|
Local streets
|
25-year
| |
|
Major intersections of local streets
|
25-year
| |
|
Collector streets and arterial streets
|
50-year
|
[2]
All structures shall be designed to convey the one-hundred-year
storm flood without roadway overtopping. The Township may also require
easements to convey the one-hundred-year frequency storm through the
entire project site.
[3]
Alternate criteria may be required following recommendation
of the Township Engineer. In addition, functional classification of
streets, for the purpose of determining storm frequencies, shall be
as interpreted by the Township Engineer.
(f)
For storm sewers that will be dedicated to the Township, all
pipe material shall either be reinforced concrete or smooth lined
corrugated polyethylene pipe, to be as determined by the Township.
Standards as referenced from ASTM or other source acceptable to the
Township Engineer shall be specified.
(g)
The minimum pipe size for storm sewers shall be 15 inches in
diameter, unless otherwise approved by the Township Engineer.
(h)
Installation.
[1]
Storm sewers shall be installed a sufficient time in advance
of final street paving in order to allow for settlement of the trench.
[2]
Installation shall be in accordance with manufacturers' recommendations,
PennDOT Publication 408, and RC standards, or as specified by the
Township Engineer.
[3]
Minimum cover from the top of the pipe to the top of subgrade
shall be 12 inches.
(i)
Safety features shall be incorporated into the storm sewer system
as necessary.
(j)
The minimum thickness of any corrugated steel or metal pipe
shall be 16-gauge or as otherwise required by the Township for anticipated
load conditions.
(k)
Roof and basement drains must not be connected to streets, sanitary
or storm sewers or roadside ditches in order to promote overland flow
and infiltration/percolation of stormwater.
D.
Inlets/manholes/junctions.
(1)
In general, inlets, manholes, grates, covers, frames and the like
shall conform to the Pennsylvania Department of Transportation Standard
Specifications, Publication 408, and RC Standards. Design shall be
performed in accordance with the Pennsylvania Department of Transportation
or Federal Highway Administration Standards. Additional alternate
criteria may be required following review of the Township Engineer.
(a)
Inlets shall be spaced such that they are not subject to flows
higher than five cubic feet per second or at a distance greater than
400 feet along curbed streets and at low points on sag vertical curves
with an inlet on each side of the street. Additional inlets shall
be placed at the upper side of street intersections, to prevent stormwater
from crossing the intersection. Inlets are not allowed on the intersection
radii. In no case shall inlets be placed at a location where they
function at less than 65% efficiency based on criteria in the PennDOT
Design Manual. Design shall be such that the maximum allowable spread
of water on streets shall not exceed 1/2 of the travel lane or three
inches in the gutter line, whichever is more restrictive. At intersections,
depth of flow shall not exceed one inch for the twenty-five-year storm
event.
(b)
Inlets shall have weep holes placed at the appropriate elevations
to drain the bottom of the inlet box and the subgrade prior to placing
the base and surface courses.
(c)
Where structures are subject to traffic loads, the structure
shall be traffic-rated.
(d)
Inlet tops in residential developments shall be bicycle-safe
unless otherwise approved.
(e)
Manholes shall not be placed more than 500 feet apart. Additionally,
manholes shall be placed at points of change in horizontal and/or
vertical direction of storm sewers. Inlets may be substituted for
manholes where they will serve as a means of intercepting runoff.
(f)
If less than a forty-eight-inch diameter, curves in pipes or
box culverts without junction are prohibited. Tee joints, elbows,
and wyes are always prohibited.
(g)
In all cases, inlet and catch basin tops shall be designed and
installed level with the road surface.
(h)
Inlets over four feet in depth shall be provided with steps
for access. Inlets over eight feet in depth may require structural
design, subject to recommendation of the Township Engineer.
(i)
Inlets shall be sumped up to two inches below grade as directed
by the Township Engineer.
E.
Channels and culverts.
(1)
Culvert design. In cases where drainage is collected by means of
a headwall, and inlet or outlet conditions control, the pipe shall
be designed as a culvert.
(a)
The minimum diameter of the culvert shall be 18 inches. Design
shall be in accordance with the United States Federal Highway Administration
design procedure.
(b)
The maximum HW/D ratio for inlet control shall be 1.25 or such
that water surface elevation is 1/2 foot below the edge of street
grade during a twenty-five-year storm event, whichever is more stringent.
Roadway shall not be overtopped for a one-hundred-year storm event.
(c)
Headwalls and endwalls shall be provided for all culverts unless
otherwise approved by the Township Engineer. Material shall be reinforced
concrete unless otherwise approved. Headwalls greater than five feet
in height shall be designed by a professional engineer.
(d)
Culvert pipe and material shall be the same as that required
for storm sewers.
(e)
All applicable modeling results, calculations and supporting
documentation shall be submitted.
(2)
Channel design.
(a)
Manning's equation shall be used for the design of all open
channels. Complete calculations shall be submitted which detail flow,
depth, and velocity. For channels and swales, design for erosion control
must be provided.
(b)
All channels shall be designed to prevent erosion of the channel
bottom and sides. The flow velocity in all vegetated drainage channels
shall not exceed the maximum permissible velocity to prevent soil
erosion. Stabilization techniques such as rip-rap, sodding, geofabrics
and/or premanufactured products shall be utilized where necessary
to minimize erosion potential.
(d)
Where swales are installed and vegetative stabilization has
not or will not occur between November 1 and March 1, other means
of temporary stabilization shall be provided.
(e)
Design criteria for swales and channels shall be the same as
that required for storm sewers.
(f)
Swales must be designed and constructed with a uniform slope
to avoid ponding within the swale. Swales designed or constructed
at less than 1.5% slope may be required to include an underdrain in
the swale bottom.
F.
Stormwater management facilities.
(1)
General design criteria.
(a)
Peak discharge and runoff shall be computed using the Soil-Cover-Complex
Method contained in the Urban Hydrology for Small Watersheds, Technical
Release No. 55, published by the Engineering Division, Soil Conservation
Service, United States Department of Agriculture. Alternate methodology
and/or computing software may be used, subject to approval by the
Township Engineer.
(b)
Stormwater shall not be rerouted or concentrated in a manner
which is inconsistent with downstream conditions or where downstream
properties are likely to be affected. In addition, the proposed stormwater
discharge at the perimeter of the site shall not exceed the capacity
of any existing facility, nor shall it alter the predevelopment flow
characteristics. All new concentrated discharges of stormwater onto
adjacent properties shall be within existing storm sewers or channels.
The Township may require written acknowledgment or easement from adjacent
property owners in the event that these conditions are not met.
(c)
Franklin Township has the authority to require that computed
existing runoff rates be reconciled with field observations and conditions.
If the designer can substantiate, through actual physical calibration,
that more-appropriate runoff and time-of-concentration values should
be utilized at a particular site, then appropriate variations may
be made upon review and recommendations of the Franklin Township Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question.
(d)
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. Franklin Township reserves
the right to disapprove any design that would result in the occupancy
or continuation of an adverse hydrologic or hydraulic condition within
the watershed.
G.
Detention basins.
(1)
Basin design criteria.
(a)
Basins shall provide control of post-development peak runoff rates as specified in § 136-17 of this chapter.
(b)
In locations where known drainage or flooding problems exist
and where stormwater management has not been previously provided,
and it is determined that the proposed development may contribute
to the problem, it may be recommended by the Township Engineer that
the discharge of basins or other stormwater management peak flow rate
controls be such that the post-development release rate from storms
up to the ten-year frequency be limited to the predevelopment flow
rate from a two-year frequency storm.
(c)
The Modified PULS Routing Technique or other method approved
by the Township Engineer shall be used for routing computations.
(2)
Basin construction standards.
(a)
Basins shall not be located over any existing or proposed utility
lines.
(b)
The maximum slope of earthen embankments shall be 4:1, with
3:1 allowed following review and recommendation from the Township
Engineer. The top or toe of any slope shall be located a minimum of
15 feet from adjacent property lines, except for a downstream property
line where there shall be sufficient additional distance for energy
dissipation and for access of maintenance equipment, but in no case
shall be less than 40 feet, unless approved otherwise by the Township.
Greater slopes may be allowed with the provision of a design basis
which considers fill material and stabilization, where approved by
the Township Engineer. In areas which are not easily accessible for
maintenance, side slopes shall not exceed 5:1.
(c)
Where possible, the side slopes and basin shape shall blend
with the natural topography.
(d)
The minimum top width of detention basin berms shall be six
feet.
(e)
All basins shall have provisions for dewatering so as not to
create unmaintainable conditions. The minimum grade of the basin floor
shall be 2% to ensure proper drainage towards the outlet structure.
One percent may be allowed following review and recommendation of
the Township Engineer or if a paved or stabilized low-flow channel
is provided.
(f)
All submitted basin plans shall indicate the construction specifications
and compaction requirements to be used during construction. All earth
fill dams shall be designed and certified by a registered professional
engineer. Construction specifications shall be reviewed and approved
by the Township Engineer.
(g)
A cutoff trench shall be excavated along the center line of
any dam on an earth fill embankment. The minimum depth shall be three
feet. The minimum bottom width shall be 10 feet or wide enough to
permit operation of compaction equipment.
(h)
A minimum of six inches of topsoil shall be placed on all areas
affected by the basin construction (i.e., basin floor, side slopes,
top of berm, and the like) to allow for the establishment of vegetation.
(i)
All basins shall be stabilized using methods acceptable to the
USDA Soil Conservation Service.
(j)
The maximum water depth of a finished detention basin (measured
from the lowest point in the basin floor to the crest of the emergency
spillway) shall not exceed eight feet, unless otherwise approved by
the Township Engineer.
(k)
Fencing. Any stormwater management facility that is designed
so that it detains water on a temporary or permanent basis may be
subject to the following fencing regulations:
[1]
Facilities with water depths exceeding four feet or greater
may require fencing if a public safety hazard is deemed possible by
the Township Supervisors. In determining the need for fencing, the
Township Supervisors shall consider, at a minimum, the following:
[2]
All gates opening through a fence enclosure shall be equipped
with a self-closing and self-locking device for keeping the gate securely
closed at all times.
[3]
The Franklin Township Supervisors may require the installation
of a shrub barrier in lieu of a fence. The Township reserves the right
to review and approve the proposed shrubbery for this application.
[4]
The fencing type, height and style shall be reviewed and approved
at the discretion of the Township.
(l)
A minimum of one foot freeboard shall be provided above the
basin water surface elevation during a one-hundred-year frequency
storm.
(m)
Minimum floor elevations for all structures shall be two feet
(minimum) above the basin water surface elevation during a one-hundred-year
frequency storm. If basements will be provided, detailed calculations
and waterproofing design shall be provided which addresses the effects
of stormwater on the structure.
(n)
The Township may, upon recommendation of the Township Engineer,
impose additional requirements on earth fill dams for the safety and
welfare of the Township.
(o)
For sites of geologic concern, a geotechnical analysis and design
of the site as it relates to the proposed basin shall be provided.
(3)
Emergency spillway standards.
(a)
Minimum freeboard, or the distance between the design flow elevation
and the top of the settled basin embankment, shall be one foot for
a one-hundred-year frequency storm.
(b)
Emergency spillway design should be based on a one-hundred-year
design storm when neglecting the capacity of the outlet structure
and outfall culvert.
(c)
Emergency spillways shall be constructed on undisturbed earth,
where possible. Emergency spillways shall be constructed of vegetated
earth, reinforced concrete or concrete mound slabs. Emergency spillways
shall not discharge stormwater over earthen fill or other easily erodible
material without adequate protection against soil erosion. Detailed
calculations and design shall be submitted. Downstream channels shall
be of adequate design to convey flows from the emergency spillway
to an existing stream, storm sewer or other approved discharge point.
(4)
Outlet pipes and structures. The following measures shall be incorporated
into the design and construction of all outlet structures and pipes.
Supporting calculations and drawings shall be submitted.
(a)
Antiseep collars shall be installed around all outlet pipes
through embankments. The antiseep collars and their connections to
the pipe barrel shall be watertight. Design calculations in accordance
with the USDA Soil Conservation Service shall be submitted.
(b)
Temporary sedimentation controls shall be provided during construction
to prevent the flow of sediment-laden runoff through the basin outlet
pipe. Such measures may include temporary riser pipes, rock-filled
gabions, plywood stand boxes, silt fences, skimmers and the like.
Design of such measures shall comply with the requirements of the
Adams County Conservation District.
(c)
Energy-dissipation shall be provided at the outlet of detention
basins, along outfall channels, and at the discharge end of all conveyance
pipes.
(d)
Outlet control structures shall be constructed to prevent flotation.
(e)
Outlet control structures shall be equipped with a childproof,
nonclogging, removable trash rack for all openings larger than 12
inches in diameter.
(f)
All pipes through earthen embankments shall be of a type for
which watertight joint systems are available. Outfall pipes and culverts
shall be reinforced concrete, unless otherwise approved by the Township
Engineer.
(5)
Where required by the Township, an analysis shall be conducted of
the conditions downstream of the discharge from the property proposed
for development. Such analysis shall consider existing and proposed
flow rates, velocities, potential for erosion, and expected water
surface elevations in relation to existing structures or properties.
Such downstream analysis shall be submitted to the Township Engineer
and, where deemed necessary, to the Adams County Conservation District.
H.
Subsurface disposal/retention basin systems.
(1)
Installation requirements.
(a)
The following procedures and materials shall be required for
all subsurface facilities:
[1]
Excavation for infiltration facility shall be performed with
equipment which will not compact the bottom of the seepage bed, infiltration
trench or like facility.
[2]
The bottom of the bed or trench shall be roughened prior to
placement of aggregate.
[3]
Only clean, open-graded aggregate, free of fines, shall be used
in subsurface systems.
[4]
The top, sides, and bottom of all seepage beds, infiltration
trenches, or like 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.
[5]
All pipes leading into subsurface drainage systems shall be
equipped with screening devices to prevent debris from entering the
system.
[6]
The bottom of all subsurface disposal or retention basin systems
shall be a minimum of 12 inches above the limiting zone as established
by the site-specific soil profile. Depths of less than 12 inches above
the limiting zone will only be allowed where the developer provides
a written report, certified by a registered professional engineer,
geologist, or hydrogeologist, which certifies that the condition will
not create an environmental hazard.
[7]
Inspection points, cleanouts and overflow facilities shall be
provided for subsurface disposal systems.
[8]
All subsurface stormwater disposal systems or retention basins
shall be located a minimum of 100 feet from any potable water wells.
I.
Basins with permanent pools (wet basins).
(1)
Basins designed to have a permanent pool of water stored in the reservoir
shall conform to the design standards of detention or retention basins.
Where deemed to be necessary, after consulting with the Township Engineer,
the Township may impose additional criteria for design and construction
of wet basins. Earthen embankment designs shall be sealed by a registered
professional engineer experienced in such design.
(2)
Embankments shall have a slope not exceeding four horizontal to one
vertical.
(3)
Adequate stabilization shall be provided to control anticipated erosion
due to wave or water level fluctuation.
(4)
Where necessary, wet pond embankments shall be protected from rodent
intrusion.
