Measures used to collect and carry stormwater
on any site ("stormwater management facilities") shall be designed
to meet the following minimum performance standards:
A.
Prevent erosion damage and satisfactorily carry off
or detain and control the rate of release of surface waters.
B.
When subsurface soil conditions are suitable, require
runoff control measures to percolate the stormwater into the ground
to aid in the recharge of groundwater and the preservation of baseflow.
C.
Carry surface water to the nearest adequate street,
storm drain, detention basin, natural watercourse, or drainage facility.
D.
Take surface water from the bottom of vertical grades,
to lead water away from springs, and to collect water upgrade of all
street intersections at the earliest or most efficient point.
E.
Control/accommodate not only the anticipated peak
discharge from the on-site disturbed area but also the existing runoff
being contributed from all land at a higher elevation in the same
watershed.
F.
Maintain the adequacy of the natural stream channels.
Accelerated bank erosion shall be prevented by controlling the rate
and velocity of runoff discharged to these watercourses so as to avoid
increasing the occurrence of stream bank overflow.
G.
Preserve the adequacy of existing culverts and bridges
by suppressing the new flood peaks created by the new earth disturbances.
H.
If in the course of preparing or reviewing the stormwater
management plan the Township Engineer determines that off-site improvements
are necessary to satisfactorily control the stormwater from the site,
the applicant shall be responsible for such off-site improvements.
I.
All stormwater detention and retention facilities
shall be in place and functioning prior to the creation of any impervious
surface.
J.
Whenever a watercourse, stream or intermittent stream
is located within a grading site, it shall remain open in its natural
state and location and shall not be piped unless permitted by the
Pennsylvania Department of Environmental Protection (DEP) and the
Township.
K.
The existing points of natural drainage discharge
onto adjacent property shall not be altered without written approval
and a drainage easement from the affected landowners.
L.
No stormwater runoff or natural drainage shall be
so diverted as to overload existing drainage systems or create flooding
or the need for additional drainage structures on other private properties
or public lands.
M.
All stormwater management facilities shall be designed
to satisfy the following requirements:
(1)
They shall be capable of withstanding the discharge
associated with the one-hundred-year return rainfall event without
failing or resulting in damage to downstream areas. Some nondetention
facilities may be designed to bypass stormwater discharges which are
in excess of the appropriate design storm. In this case, conveyance
must be provided to transport the one-hundred-year surcharge flow
to downstream facilities, a natural watercourse or storm drainage
system inlet.
(2)
All groundwater recharge devices shall be protected
from sedimentation. Areas designated for recharge shall not receive
runoff until the contributory drainage areas have achieved final stabilization.
A.
Stormwater detention facilities. Stormwater detention
facilities include all structural measures which can reliably and
predictably achieve the peak discharge requirements. Stormwater detention
facilities include, but are not necessarily limited to, detention
basins, retention basins, bioretention areas, open (at-grade) sand
filters, closed (below-grade) sand filters, water quality inlets,
dry wells, below-grade detention chambers, and rooftop detention.
B.
Peak discharge design storms. The design storm criteria
to be used in calculations for the watershed is to limit the post-development
runoff for the two-, ten-, fifty- and one-hundred-year storms to the
predevelopment rates. Any stormwater detention facilities required
by this chapter and subject to the water quality requirements and
stormwater runoff peak rate requirements herein shall meet the applicable
water quality and peak rate requirement for the two-, ten-, fifty-
and one-hundred-year return period runoff events (design storms) consistent
with the standard and accepted calculation methodology and engineering
standards and be satisfactory to the Township Engineer.
C.
Runoff calculation methodology.
(1)
Any stormwater runoff calculation involving drainage
areas greater than 20 acres, including on- and off-site areas, shall
use a generally accepted calculation technique that is based on the
NRCS Soil Cover Complex Method. It is assumed that all methods will
be selected by the design professional based on the individual limitations
and suitability of each method for a particular site.
(2)
All calculations consistent with this chapter using
the Soil Cover Complex Method shall use the appropriate design rainfall
depths for the various return period storms.
(3)
For purposes of predevelopment flow rate determination,
undeveloped land shall be considered as "meadow, in good condition,"
unless the natural ground cover generates a lower curve number or
Rational "C" value.
(4)
All calculations using the Rational Method shall use
rainfall intensities consistent with appropriate times of concentration
for overland flow and return periods from NRCS methodology. Time of
concentration for overland flow (maximum 300 feet) and concentrated
flow shall both be calculated using NRCS methodology. Times of concentration
for channel and pipe flow shall be computed using Manning's Equation
or NRCS methodology.
(5)
The design of any stormwater detention facilities
intended to meet the performance standards of this chapter shall be
verified by routing the design storm hydrograph through these facilities
using accepted methods of practice. The Township Engineer may approve
the use of any generally accepted reservoir routing technique which
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph. The computer routing
program used must take into account the tailwater effect of the discharge
pipe on the orifice design as well as the submergence of the discharge
pipe outlet.
(6)
Outlet structures for stormwater management facilities
shall be designed to meet the performance standards of this chapter
using any generally accepted hydraulic analysis technique or method
approved by the Township Engineer.
D.
Stormwater detention and retention facilities. Stormwater
detention and retention facilities shall meet the following minimum
design/construction standards:
(1)
Detention basin shall be designed to facilitate regular
maintenance, mowing and periodic silt removal and reseeding. Shallow
broad basins are preferred to steep-sided basins.
(2)
The maximum slope of the earth and detention basin
embankment shall be 3:1 with the exception that any slope to be maintained
by the Township shall be 4:1. The top or toe of any slope shall be
located a minimum of five feet from a property line. Whenever possible,
the side slope and basin shape shall conform to the natural topography.
(3)
Unless permitted, detention basins shall not be located
within floodplains nor within areas of floodplain or alluvial soils.
(4)
Detention basins shall be designed so they return
to normal conditions within approximately 24 hours after termination
of the storm, unless the Township Engineer finds that downstream conditions
may warrant other design criteria for stormwater release.
(5)
If retention basins are used, the applicant shall
demonstrate that such ponds are designed to protect public health,
safety and welfare.
(6)
Fences may be required for any detention or retention
basins where there is a permanent water surface or conditions warrant.
(7)
The minimum top width of the detention basin berm
shall be 10 feet. A cutoff trench (keyway) of relative impervious
material shall be provided beneath all embankments requiring fill
material. The keyway shall be a minimum eight feet wide, minimum three
feet deep, and have 1:1 side slopes.
(8)
In order to ensure proper drainage on the basin bottom,
a minimum grade of 2% shall be maintained for sheet flow. Where a
two-percent slope cannot be maintained, low-flow channels at a minimum
grade of 1%, constructed of concrete or other materials approved by
the Township Engineer, shall be constructed between all basin inlets
and the basin outlet.
(9)
All detention and retention basin embankments shall
be placed in eight-inch maximum lifts to a minimum ninety-five-percent
dry density. Prior to proceeding to the next lift, compaction shall
be checked by the Township Engineer or an approved soils engineer,
who shall provide the Township Engineer with a written report. Compaction
tests shall be performed using the Modified Proctor Method in accordance
with ASTM D-1577. Compaction tests shall be run on the leading and
trailing edge as well as the top of the berm.
(10)
Emergency overflow facilities shall be provided
for detention facilities to accommodate runoff in excess of design
flows. Whenever possible, emergency spillway for the detention basins
shall be constructed on undisturbed ground. Emergency spillways shall
be constructed of concrete pavers, gabions or other similar materials
approved by the Township Engineer. All emergency spillways shall be
constructed so that the detention basin berm is protected against
erosion. The minimum capacity of all emergency spillways shall be
the peak flow rate of the one-hundred-year design storm after development.
The construction material of the emergency spillway shall extend along
the upstream and downstream berm embankment slopes. The upstream edge
of the emergency spillway shall be a minimum of three feet below the
spillway crest elevation. The downstream slope of the spillway shall
at a minimum extend to the toe of the berm embankment. The emergency
spillway shall not discharge over earthen fill or easily erodible
material.
(11)
The minimum freeboard shall be one foot.
(12)
Anti-seep collars shall be installed around
the pipe barrel within the normal saturation zone of the detention
basin berms. The anti-seep collars and their connections to the pipe
barrels shall be watertight. The anti-seep collars shall extend a
minimum of two feet beyond the outside of the principal pipe barrel.
The maximum spacing between collars shall be 14 times the minimum
projection of the collar measured perpendicular to the pipe. A minimum
of two anti-seep collars shall be installed on each outlet pipe.
(13)
All outlet pipes through the basin berm shall
be reinforced concrete pipe, designed to withstand the loading caused
by a fully saturated berm, and shall be watertight joints using O-ring
joint pipe. Outlet pipe shall be backfilled with material similar
to the core material (semi-impervious).
(14)
The invert of the inlet pipe(s) into a basin
shall be six inches above the basin floor or lining so that it can
adequately drain after rainstorms. Inlet pipe(s) shall discharge to
areas of the basin that slope toward the outlet structure.
(15)
Energy dissipaters and/or level spreaders shall
be installed at points where pipes or drainageways drain to or from
the basin. Energy dissipaters shall comply with criteria in Hydraulic
Engineering Circular No. 15, Design of Stable Channels with Flexible
Linings, published by the Federal Highway Administration of the United
States Department of Transportation, or the Engineering Field Manual
for Conservation Practices; NRCS energy-dissipating device calculations
shall be submitted for Township review and approval.
(16)
Inlet and outlet structures shall be located
at a maximum distance from one another in order to promote water quality
benefits. The Township Engineer may require a rock filter or rock-filled
gabion for entrapping sediments carried in stormwater if sufficient
separation of inlet and outlet structures cannot be achieved.
(17)
A perforated riser or similar sediment-control
device shall be provided at each outlet of all detention basins during
construction for sediment control. The riser shall be constructed
of metal or concrete. The riser shall extend to a maximum elevation
of two feet below the crest elevation of the emergency spillway. The
perforated riser shall be designed so that the rate of outflow is
controlled by the pipe barrel through the basin berm when the depth
of water within the basin exceeds the height of the riser. Circular
perforations with a maximum diameter of one inch shall be spaced 12
inches vertically. The horizontal spacing shall be in accordance to
DEP Soil Erosion and Sedimentation Control Manual specifications.
The perforations shall be cleanly cut and shall not be susceptible
to enlargement. All metal risers shall be suitably coated to prevent
corrosion. A trash rack or similar appurtenance shall be provided
to prevent debris from entering the pipe. All risers shall have concrete
base attached with a watertight connect. The base shall be of sufficient
weight to prevent flotation of the riser. An anti-vortex device consisting
of a thin vertical plate normal to the base and berm shall be provided
at the top of the riser. Unless this structure is part of the permanent
outlet control, it shall be removed from the site when it has been
adequately stabilized, as determined by the Township Engineer.
(18)
All drainage channels shall be designed to prevent
erosion of the bed and banks. The maximum permissible flow velocity
shall not exceed the design requirements outlined in the current Soil
Erosion and Sedimentation Control Manual published by the Pennsylvania
Department of Environmental Protection. Suitable stabilization shall
be provided where required to prevent erosion of the drainage channels.
(19)
Any vegetated drainage channel requiring mowing
of the vegetation shall have a maximum grade of three horizontal to
one vertical on those areas to be mowed.
(20)
Because of the critical nature of vegetated
drainage channels, the design of all vegetated channels shall at a
minimum conform to the design requirements outlined in the current
Soil Erosion and Sedimentation Control Manual published by the Department
of Environmental Protection.
A.
General.
(1)
Storm sewers, culverts, bridges and related installations
shall be provided:
(a)
To permit unimpeded flow of natural watercourses
and in such a manner as to protect the natural character of the watercourses
and to provide regulated discharge;
(b)
To ensure adequate drainage of all low points
along the line of streets; and
(c)
To intercept stormwater runoff along streets
at intervals reasonably related to the extent and grade of the area
drained and to prevent substantial flow of water across intersections.
(2)
All storm sewer system components shall conform to
current PennDOT standards.
(3)
Drainage structures which drain watershed areas in
excess of 1/2 square mile (320 acres) or which have a span of eight
feet or more shall be designed for a maximum expected runoff as calculated
using the Soil Conservation Service (now Natural Resources Conservation
Service) Technical Release 55, "Urban Hydrology for Small Watersheds
(less than 2000 acres)."
(4)
The design storm for the above structures shall be
a one-hundred-year storm. A water obstruction permit shall be obtained
from the Pennsylvania Department of Environmental Protection for the
waterway opening before final design is undertaken.
(5)
The cartway over the culvert or bridge shall be as
wide as the ultimate width of the roadway approaches. Additional width
may be required to provide sidewalk on one or both sides of the cartway.
B.
Storm sewer design and construction requirements.
(1)
Minimum pipe size is 18 inches.
(2)
Minimum pipe slope shall be 0.005 feet per foot.
(3)
Minimum drop across junctions shall be two inches.
At changes in pipe diameter, pipe crowns shall be matched at junctions
(manhole, inlet or junction box).
(4)
Maximum distance between junctions shall be 300 feet.
(5)
Runoff to proposed storm sewers and inlets shall be
calculated using the Rational Method.
(6)
The time of concentration shall be assumed five minutes
for pipes under 30 inches. For pipes 30 inches or greater, the calculated
time of concentration can be utilized.
(7)
The time of concentration to inlets for grate-capacity
calculations shall be assumed five minutes.
(8)
All storm sewer pipes shall be designed as a minimum
to accommodate a minimum of a ten-year storm. Twenty-five-year storms
shall be used as required by the Township Engineer.
(9)
All storm sewer pipes at inlets in sump condition
shall be designed to accommodate the fifty-year storm.
(10)
All storm sewer pipes and inlets intended to
drain to detention facilities shall be designed to accommodate the
one-hundred-year storm if the bypass or overflow runoff will not reach
the basin by overland flow. In cases where the bypass or overflow
runoff will flow over land, a stable swale shall be constructed to
accommodate the excess runoff.
(11)
All inlets in sump condition shall be six-foot
inlets or dual four-foot inlets, as needed.
(12)
All storm sewer systems shall be analyzed for
both inlet and outlet control (including tailwater effects) by using
the equations and nomographs as shown in the FHA's Hydraulic Design
Services No. 5. In lieu of this, computer programs that calculate
the actual hydraulic grade line for the storm sewer system can be
used, provided all losses (friction, bend, junction, etc.) are taken
into account. Documentation for the program must be submitted for
approval.
(13)
Minimum cover over pipes is 18 inches from finished
grade to outside of pipe bell.
[Amended 2-22-2016 by Ord. No. 2016-07]
(14)
Inlet capacities shall be calculated using PennDOT
or manufacturer's nomographs. Documentation for manufacturer's nomograph
must be provided to the Township Engineer.
C.
Shoulders in cut areas (without swales).
(1)
Water flowing in the shoulder shall not encroach more
than 2/3 the shoulder width during a twenty-five-year frequency storm
of five-minute duration.
(2)
The maximum velocity as determined by Manning's Equation
shall not exceed the allowable velocities for the specific type of
shoulder material.
(3)
Inlets shall be provided to control the shoulder encroachment
and water velocity.
D.
Swales adjacent to shoulders.
(1)
Swales in cut areas shall be designed to prevent the
passage of water on the cartway during a twenty-five-year frequency
storm of five-minute duration.
(2)
The maximum velocity as determined by Manning's equation
shall not exceed the allowable velocities for the specific type of
shoulder material.
F.
Inlets, general.
(1)
At street intersections, inlets shall be placed in
the tangent portion, rather than the curved portion, of the curbing.
(2)
If the capacity of the shoulder, swale, curb section
or depressed median section exceeds the assumed inlet capacities,
the inlet capacities shall govern the spacing of inlets.
(3)
If the capacity of the shoulder, swale, curb section
or depressed median section is less than the inlet capacities, then
the shoulder, swale, curb section or depressed section capacity shall
govern the spacing of inlets.
