A. 
Retention and detention basins shall be designed to discharge the proposed two-, ten-, twenty-five-, and fifty-year storms through a principal spillway and the proposed one-hundred-year storm through a surface water emergency spillway. The use of inlets and pipes are prohibited for the emergency spillway. Basin berm protection shall be provided for the passage of the one-hundred-year storm through the emergency spillway.
B. 
Basins shall be designed in accordance with soil conservation practices to accommodate sediment during construction.
C. 
Detention basins shall be designed to dewater within 24 hours after a two-year storm event and 96 hours after a fifty-year storm event.
D. 
Detention basins shall be provided with a minimum bottom slope of 1.0%. Paved or concrete low flow channels are recommended in detention basins to completely drain them and to carry low flows from inflow culverts to the principal spillway. With low flow channels, the bottom slope of the basin may be 1/2%.
E. 
Percolation rates may be used in Township-approved areas to offset basin discharge rates as groundwater recharge. Percolation tests shall be performed at the site for each soil type.
F. 
The maximum water depth in basins shall be six feet for a proposed fifty-year storm. Depths of greater than six feet will be permitted only with permission from the Township and may require provisions for fencing around the basin and a structural design of the basin embankment.
G. 
The minimum top width of basin berms shall be five feet.
H. 
Basin berm side slopes shall be a minimum of 3:1.
I. 
Basin berms shall include a cutoff trench from a depth of one foot below the toe of the interior basin berm to the fifty-year stormwater elevation constructed of materials having a coefficient of permeability less than or equal to one times 107 cm/sec. This requirement is waived for berms using undisturbed existing soils. The discharge pipe shall also be backfilled with relatively impervious soil. No stone backfill will be allowed unless anti-seep collars are provided.
J. 
Concrete or a welded galvanized steel anti-seep collar shall be placed around all basin discharge pipes to increase the seepage length along the pipe by 15% within the saturated zone of the pipe based on a 4:1 phreatic line. The connection of the antiseep collar to the discharge pipe shall be completely watertight.
K. 
Basin principal spillway outlet structures shall consist of stainless steel orifice plates and mounting hardware, galvanized corrugated metal or reinforced concrete riser and discharge pipe, and welded structural steel inlet grates with a bituminous coating. The use of PVC or polyethylene pipe is prohibited for basin discharge piping except small single-family on-lot systems. Principal spillways with riser pipes where the designed water depth is deeper than 0.4 times the diameter of the riser, the riser shall have an antivortex device to prevent reduced capacity of the riser. (Example: if the top of an eighteen-inch riser is 100.00, the riser shall have an antivortex device if the water is higher than 100.00 plus [18 inches 0.4/12] equals 100.60). Materials used for design shall be specified on the plans.
L. 
Lots adjacent to basins shall have a finished floor elevation of two feet above the one-hundred-year storm elevation. The plans shall indicate the minimum allowable finished floor elevations.
M. 
Basins located on property of separate ownership and serving another property(ies) shall have a drainage easement placed around them that encloses the one-hundred-year storm elevation.
A. 
Swales shall be designed using Manning's equation with the following "n" values and maximum allowable velocities:
Surface Cover
Manning's "n" Value
Permissible Velocity
(fps)
Concrete
0.014
12
Asphalt
0.016
8
Riprap, 3-inch
0.025
6
Riprap, 6-inch
0.025
9
Riprap, 9-inch
0.030
11
Riprap 12-inch
0.030
13
Riprap 15-inch
0.035
14
Seeded grass swales
0.030
4
Sod grass swales
0.040
5
Existing grass swales
0.050
6
B. 
Swales shall be designed for a twenty-five-year storm and include six inches of freeboard.
C. 
Swales with bare soils are not permissible. All swales shall have ground cover or have a paved or concrete surface up to the twenty-five-year storm depth.
D. 
Swale side slopes shall be 2:1 minimum. Side slopes for grass-covered swales which will be mowed shall be 3:1 minimum.
E. 
Bends in swales shall have adequate protection to avoid erosion and channel meandering.
F. 
Swales shall have a twenty-foot-wide easement minimum. The swale shall be centered in the easement.
G. 
All vegetative swales should have a minimum bottom slope of 1%.
A. 
Storm sewer pipes and culverts shall have a minimum diameter of 15 inches, except single-family on-lot systems shall have a minimum diameter of four inches.
B. 
Storm sewer pipes and culverts shall have sufficient slope to provide a minimum velocity of two feet per second for the ten-year storm.
C. 
Storm sewers and culverts shall be designed using Manning's equation with the following allowable pipe types and "n" values. The pipe types used for design shall be specified on the plans.
Pipe Type
Manning's "n" Value
Smooth lined polyethylene
0.012
Corrugated polyethylene
0.021
Reinforced concrete
0.013
Helical galvanized corrugated metal, 15 to 18 inches
0.016
Helical galvanized corrugated metal, 21 to 30 inches
0.019
Helical galvanized corrugated metal, greater than 30 inches
0.023
Annular galvanized corrugated metal
0.025
D. 
All pipes shall be laid with a minimum cover on top of the crown of the pipe of one foot below the proposed finished grade in unpaved areas or below the stone subgrade for paved areas. All pipes with the cartway, including the shoulder area, shall be bedded and backfilled with 2A stone. In other areas, pipes shall be bedded and backfilled with fine graded, highly compactable soil or stone.
E. 
Storm sewers shall run in a straight line. Bends in the system shall contain a manhole or inlet.
F. 
All pipe ends shall have either head walls or end walls and shall be PennDOT Type "D" or "DW" or flared end sections.
[Amended 8-23-2016 by Ord. No. 2016-6]
G. 
All pipe ends shall have erosion protection which meets the velocities of the end pipe discharge for the design storm.
H. 
Head walls and end walls of pipes shall be protected for child entry by placing removable steel bars and mounting hardware, spaced eight inches apart, across the openings.
I. 
Roof gutters shall not be used to divert water away from a surface drainage area. Roof drains shall not discharge directly into road rights-of-way or storm sewers unless they are being directly conveyed to a detention basin. Pipes from downspouts may be constructed of flexible or rigid plastic pipe of any diameter and shall be installed to prevent erosion.
J. 
Storm sewers and culverts shall have a twenty-foot-wide easement minimum, unless they are within a street or other public right-of-way. The pipe shall be centered in the easement.
K. 
The use of multiple pipe installations will not be permitted, unless a request for such use is made in accordance with Article II and only if approved by the Greene Township Board of Supervisors.
A. 
Inlets shall be placed on both sides of the road at low points, spaced no more than 600 feet apart, at every abrupt change in horizontal or vertical direction of the storm sewer, at points where necessary to prevent gutter flow depths from exceeding three inches for the twenty-five-year storm, and at all curb and road tangent intersections.
B. 
The depths of flow across a street intersection shall not exceed one inch for the twenty-five-year storm.
C. 
Inlets shall be depressed two inches below proposed finished grades.
D. 
Manholes may be substituted for inlets at locations where inlets are not necessary.
E. 
Inlet capacities shall be per the most restrictive prevailing manufacturer design information. The manufacturer used for design shall be specified on the plans.
F. 
Inlets shall be concrete and the inlets and grates shall meet design loads. Steel inlet grates shall be bituminous-coated.
A. 
Sites with streams, intermittent channels, water impounding areas, or low to slow draining areas shall have the one-hundred-year floodplain established.
B. 
The Federal Emergency Management Agency, National Flood Insurance Program, Detailed Flood Areas may serve as the principal means of establishing the one-hundred-year floodplain for an existing site.
C. 
Where detailed flood area information is not available or a more detailed analysis is desired for an existing site, the U.S. Army Corps of Engineers, Hydrology Engineering Center method 2 (HEC2) flood study shall be performed. Other standard step methods for natural channels to determine water surface profiles for flows through a natural river or stream may be used with Township approval.
D. 
Proposed sites, where improvements or changes are made in the existing one-hundred-year floodplain, shall be analyzed for the one-hundred-year floodplain using the HEC2 or other standard step methods with Township approval.
E. 
All flood studies shall be signed and sealed by a Pennsylvania registered professional engineer, a registered landscape architect or a registered professional land surveyor.