[Ord. No. 187, 11/30/2018]
1.
All computations used in conjunction with the analysis and design
of stormwater management facilities shall be based on one or more
of the following methods, or as otherwise approved in advance by the
Township:
3.
The SCS, Type II Rainfall Distribution shall be used for all analyses.
The design storm frequencies for the watersheds are:
|
Design Storm
(year)
|
24 Hour Rainfall Depth
(inches)
|
|---|---|
|
1
|
1.97
|
|
2
|
2.35
|
|
5
|
2.88
|
|
10
|
3.30
|
|
25
|
3.90
|
|
50
|
4.40
|
|
100
|
4.92
|
4.
Predevelopment Conditions. The cover type for all sites will be considered
to be the dominant land cover during the five-year period immediately
preceding a proposed regulated activity:
A.
For the purposes of predevelopment peak flow rate and volume determination,
existing nonforested pervious area conditions shall be considered
as meadow (good condition). Forested land areas shall be considered
in good condition.
B.
For the purposes of predevelopment peak flow rate and volume determination,
20% of existing disturbed impervious area, when present, shall be
considered meadow (good condition).
5.
Postdevelopment Conditions. The hydrologic parameters used to develop
peak flow rates shall be reflective of anticipated soil runoff characteristics
following grading and development of the site.
6.
Time of Concentration. The minimum time of concentration for any
watershed shall be six minutes. A minimum six-minute time of concentration
may be assumed for any postdevelopment watershed. Calculations must
be provided for all predevelopment times of concentrations. A minimum
time of concentration cannot be assumed for predevelopment conditions.
7.
The use of the Basic Rational Method in estimating runoff may be
employed in the design of the storm sewer conveyance system within
the development. The storm sewer system shall be interpreted as the
conduits, culverts, inlets and appurtenant features for the conveying
of stormwater to, through, or from a development site to the point
of final discharge or control facility. The Rational Method shall
not be used in the analysis of stormwater runoff from the development
in its entirety or in conjunction with the design of any retention/detention
facilities or other runoff control measures.
9.
Stormwater runoff shall not be transferred from one watershed to
another unless the watersheds are subareas of a larger watershed that
are tributary to a common point of interest within or near the perimeter
of the property. Transfer of runoff from one watershed to another
under any other circumstances shall only be approved at the discretion
of the Township. Documentation shall be provided that peak flow rates
are not increased following development and there will be no detrimental
impact in downstream areas.
10.
The SWM site plan shall consider all of the stormwater runoff flowing
over the project site. Runoff calculations shall be made to ensure
that runoff from the upstream watershed area (based on the Township
Comprehensive Plan) can be accommodated by the pipes, drainage easements
and watercourses, etc., on the site.
[Ord. No. 187, 11/30/2018]
1.
All stormwater collection and conveyance facilities (pipes, swales,
and structures) shall be designed for a 100-year design storm event,
unless the runoff would naturally drain overland to a stormwater detention
facility, in which case a twenty-five-year design storm event may
be used. The hydraulic gradeline must be a minimum of two feet below
the surface elevation in all structures and within the pipe. Swales
and channels shall provide at least one foot of freeboard above the
energy gradeline. Backwater effects of pipes discharging under surcharge
conditions shall be included.
2.
Runoff calculations shall include complete hydrology and hydraulic
analysis of all downstream swales and pipe facilities to permanent
stream discharge point, to the Township boundary or connection to
a state- or county-owned conveyance system.
3.
Manholes shall not be spaced more than 350 feet apart for pipe sizes
up to 24 inches in diameter and not more than 450 feet apart for larger
pipe sizes.
4.
No public stormwater pipe shall be less than 15 inches in diameter
that conveys surface runoff. The minimum pipe slope shall be 1.0%
grade or maintain velocity of two feet/second. For public storm sewer
systems, only pipes related to construction of stormwater BMPs may
be less than 15 inches in diameter.
5.
All workmanship and materials shall conform to the Township standard
details as contained in Township ordinances, regulations and resolutions.
In addition, all workmanship and materials shall conform to the latest
edition of PennDOT Form 408 and be supplied by manufacturers or suppliers
listed in PennDOT's Bulletin 15, and the Allegheny County Health
Department Plumbing Code for materials installation and inspection
related to roof drains and subsurface drains.
6.
Manhole and inlet castings shall conform to the Pennsylvania Department
of Transportation Form 408 and PennDOT Standards for Roadway Construction.
Inlet grates shall be bicycle safe. Frames and grates shall be cast
iron or structural steel. Concrete frames shall not be permitted.
7.
All connections to existing storm sewer pipes shall be made by construction
of a suitable junction box (inlet or manhole) to provide access for
cleanout. No blind connections shall be permitted.
8.
All storm sewers constructed in road rights-of-way or drainage easements
shall be constructed of reinforced cement concrete pipe (RCP) Class
III or stronger cement or material.
9.
Stormwater collection and conveyance facilities shall not be installed
parallel and close to the top or bottom of major embankments eight
feet or greater in height or 3:1 or steeper in slope in order to avoid
the possibility of failing or causing the embankments to fail.
10.
Low side lots must extend roof and french drains to a sump and then
outlet to a common collector pipe system or natural watercourse in
accordance with the Township-approved SWM site plan for the development
site. No lot shall be permitted to discharge stormwater flow on or
over fill slopes.
11.
The discharge of stormwater runoff shall be to a well-defined drainagecourse,
which has a defined bed and bank. If stormwater runoff cannot be discharged
to a defined drainagecourse, documentation of written permission from
each downstream property owner shall be provided for all properties
between the source of discharge and the defined drainagecourse. The
use of level spreaders or similar BMPs that promote sheet flow may
be permitted if the applicant provides documentation that the peak
postdevelopment 100-year discharge is no greater than predevelopment
two-year discharge and a letter from the Geotechnical Engineer of
Record regarding slope stability of the downstream area is provided.
Use of a level spreader requires a waiver to be granted by the Township.
[Ord. No. 187, 11/30/2018]
1.
All retention/detention facilities shall be equipped with multistage
outlet structures to provide discharge control for each designated
storm frequency. Provisions shall also be made to safely pass the
postdevelopment 100-year storm runoff in the event of an outlet structure
failure without damaging or impairing the continued function of the
facilities. The facility(ies) must have an emergency outlet which
is able to discharge the postdevelopment 100-year peak flow. Should
any stormwater management facilities qualify as a dam under DEP 25
Pa. Code Chapter 105 criteria, the facility shall be designed in accordance
with those regulations and meet the regulations concerning dam safety.
2.
Any stormwater management facility designed to store runoff shall
provide an emergency spillway designed to convey the unattenuated
100-year postdevelopment peak rate flow with a blocked primary outlet
structure and an inundation of stormwater to the invert elevation
of the spillway, with a minimum one-foot freeboard to the crest of
the embankment.
3.
Woody vegetation shall not be permitted on the embankments or within
25 feet of the emergency spillway.
4.
The water depth of a storage facility which is not fenced shall be
limited to two feet unless approved by the Township. Otherwise, appropriate
fencing at least four feet in height shall be required.
5.
Side slopes of storage facilities shall not exceed a ratio of 3:1
horizontal to vertical dimension. The crest of the embankment shall
have a minimum width of not less than 10 feet.
6.
The retention/detention facility shall be equipped with an access
road at least 10 feet wide and with a maximum grade of 15%. Access
roads greater than 12% grade must be paved. Access roads 12% or less
in grade shall have a minimum six-inch-depth noneroding aggregate
surface. A gate with a minimum opening of 10 feet shall be provided
for maintenance access. An access easement with a minimum width of
20 feet to all stormwater detention facilities shall be provided.
The access easement shall include a statement on the recorded plan
from the owner/operator of the facility granting access to the Township.
If there is a sidewalk where the access road intersects a public road,
there must be a portland cement concrete apron between the road and
the sidewalk.
7.
All stormwater facility outlet structures shall have suitable gaskets
to prevent leakage and piping of water through the facility embankment.
All storm pipe installed through the facility embankment must be constructed
of reinforced concrete pipe. Use of high-performance polypropylene
pipe may be permitted with documentation of pipe manufacturer's
backfill requirements.
8.
A geotechnical investigation report for the construction of the stormwater
detention/retention and infiltration facilities must be provided including
design recommendation for embankment construction, interior and exterior
slopes, drainage swales and infiltration areas.
9.
Basin outlet structures shall have nonclogging trash racks over all
design openings. Periodic cleaning of debris from trash racks shall
be included in the operation and maintenance plan.
10.
Inlet structures and outlet structures shall be separated to the
greatest extent possible in order to maximize the flow path through
the basin.
11.
BMPs must be designed to protect and maintain existing uses (e.g.,
drinking water use; cold water fishery use) and maintain the level
of water quality necessary to protect those uses in all streams, and
to protect and maintain water quality in "special protection" streams,
as required by statewide regulations at 25 Pa. Code Chapter 93 (collectively
referred to herein as "State Water Quality Requirements").
