Structural post-construction stormwater management best management practices (BMPs) shall be designed and constructed in accordance with the Pennsylvania Stormwater Best Management Practices Manual, latest revision, and Chapter 388, Stormwater Management, of the Township's Code. The intent of these specifications is to supplement the Pennsylvania Stormwater Best Management Practices Manual and Chapter 388, Stormwater Management, of the Township's Code for all structural BMPs designed to manage stormwater runoff from any public improvement; however, if there is a conflict among or between the requirements of the Pennsylvania Stormwater Best Management Practices Manual, Chapter 388, Stormwater Management, of the Township's Code and/or these specifications, the requirements of the Pennsylvania Stormwater Best Management Practices Manual shall prevail, followed by the requirements of the Chapter 388, Stormwater Management, of the Township's Code.
A.
Concrete pipe shall be reinforced concrete, rubber gasketed, and
shall conform to AASHTO M170, M198 and M207.
B.
Smooth bore high-density polyethylene pipe shall be product N-12
as manufactured by ADS, or approved equal.
C.
Cast-in-place outlet structure. Reinforced concrete, minimum 4,000
psi compressive strength in accordance with PADOT Publication 408,
latest edition.
D.
Precast outlet structure. Reinforced concrete, minimum 4,000 psi
compressive strength in 28 days in accordance with PADOT Publication
408, latest edition. Submit design for Township Engineer review and
approval.
E.
Precast reinforced concrete inlet box sections shall be in accordance
with AASHTO M259, PADOT Publication 408, latest edition, and PADOT
Standard Drawings.
F.
Orifice plates. Stainless steel, Type 304, with stainless steel mounting
hardware. Use one-fourth-inch-thick up to twenty-four-inch span and
three-eighths-inch-thick over twenty-four-inch spans.
G.
Headwalls/endwalls. Reinforced concrete, minimum 4,000 psi compressive
strength in accordance with PADOT Publication 408, latest edition.
H.
Anti-seep collars. Reinforced concrete, minimum 4,000 psi compressive
strength in 28 days.
I.
Concrete pipe end sections. Reinforced concrete in accordance with
PADOT Publication 408, latest edition. Concrete pipe end sections
shall be permitted only where approved by the Township Engineer.
J.
Steel grates shall be in accordance with PADOT Publication 408, latest
edition.
A.
Basins shall be installed prior to any earthmoving or land disturbances
that they will service. The phasing of their construction shall be
as noted in the soil erosion and sedimentation pollution control narrative
on the approved soil erosion and sedimentation pollution control plan.
B.
Whenever a basin will be located in an area underlined by limestone,
a geological evaluation of the proposed location will be conducted
to determine susceptibility to sinkhole formations. The design of
all facilities over limestone formations shall include measures to
prevent groundwater contamination and, where necessary, sinkhole formation.
Soils used for the construction of basins shall have low erodibility
factors ("K" factors) with values between 0.17 and 0.49.
C.
The maximum slope of earthen basin embankments shall be four horizontal
to one vertical (4:1). The top and/or toe of any slope shall be located
a minimum of five feet from any property line. Whenever possible,
the side slopes and basin shape shall conform to the natural topography.
D.
The minimum top width of the basin berm shall be 10 feet. A cutoff
trench (keyway) of impervious material shall be provided under all
embankments that require fill material. The cutoff trench shall be
a minimum of eight feet wide, two feet deep and have side slopes of
1:1.
E.
In order to insure proper drainage on the floor of any detention
basin, a minimum grade of 2% shall be maintained for areas of sheet
flow to provide positive drainage in the direction of the outlet structure.
Under certain circumstances, such as continuous seasonal flow, the
Township may require that a low-flow channel be constructed.
F.
All basin embankments shall be placed in maximum of eight-inch lifts,
compacted to a minimum of 95% of the maximum dry density, as established
by ASTM D-1557, PTM No. 112 or PTM No. 402. Prior to proceeding to
the next lift, the compaction shall be checked by a soils engineer.
The improver shall obtain the services of a third-party certified
materials testing firm to perform the tests on the leading and the
trailing edge of the berm along with the top of the berm. All test
results shall be signed and sealed by a professional engineer registered
in the Commonwealth of Pennsylvania and shall be furnished to the
Engineer for review and approval. Any location not meeting the compaction
requirements shall be recompacted and retested until suitable compaction
is achieved.
G.
Whenever possible, the emergency spillway for basins shall be constructed
on undisturbed ground. All emergency spillways shall be constructed
so that the basin berm is protected against erosion. The minimum capacity
of all emergency spillways shall be the peak flow rate from 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, at a minimum,
extend to the toe of the berm embankment. The emergency spillway shall
not discharge over earthen fill and/or easily erodible material.
H.
The minimum freeboard for flow through the emergency spillway shall
be one foot. Freeboard is the difference between the design water
elevation in the spillway at the one-hundred-year design storm peak
flow rate and the top of berm elevation.
I.
Anti-seep collars shall be installed around the outlet pipe barrel
within the normal saturation zone of the basin berms. The anti-seep
collars and their connections to the pipe barrel 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.
