A.
General design requirements.
(1)
Stormwater shall not be transferred from one watershed to another,
unless:
(2)
Consideration shall be given to the relationship of the subject property
to the drainage pattern of the watershed. A concentrated discharge
of stormwater to an adjacent property shall be contained within an
existing watercourse or confined in an easement or returned to a predevelopment
flow type condition.
(3)
BMPs shall be designed in accordance with the BMP Manual to meet the standards of Article III of this chapter.
(4)
Alteration of existing watercourses shall be approved by issuance of a permit by PADEP under 25 Pa. Code Chapter 105, as applicable.
(5)
Should any stormwater management facility require a dam safety permit under 25 Pa. Code Chapter 105, the facility shall be designed in accordance with PADEP regulations and obtain the appropriate PADEP permit prior to approval by the municipality.
(6)
Drainageways or site grading shall be adequate to convey the runoff
into the stormwater management facility intended to receive the runoff.
In other words, if a stormwater management facility is intended to
treat the one-hundred-year peak runoff from a drainage area, then
it must be ensured that the one-hundred-year peak runoff from the
drainage area does not bypass that facility.
B.
Stormwater management facility design requirements.
(1)
No outlet structure from a stormwater management facility, or swale,
shall discharge directly onto a municipal or state roadway without
approval from the municipality or PennDOT.
(2)
The top, or toe, of any slope, or perimeter of any stormwater management
facility shall be sufficiently distant from any property line, building,
roadway, or structure to ensure that the adjacent property, building,
roadway, or structure will not be harmed and that adequate access
is provided to the stormwater management facility.
(3)
Where the BMP Manual requires the bottom of infiltration/storage
facilities to be located a minimum of two feet above the seasonal
high groundwater table, the bottom of such facilities may be lower
if adequate subsurface drainage is provided. Flows from subsurface
drainage need not be accounted for in volume control or rate control
calculations.
(4)
Energy dissipaters and/or level spreaders shall be installed at points
where pipes or drainageways discharge to or from basins. Discharges
to drainage swales shall be dissipated, or piped, to an acceptable
point.
(5)
All disturbed areas not otherwise resurfaced shall be stabilized
by a minimum uniform 70% perennial vegetative cover, with a density
capable of resisting accelerated erosion. Specifications shall be
provided for vegetation serving a water quality function as part of
a BMP.
(6)
A safety fence may be required, at the discretion of the municipality,
where the location and layout of a stormwater management facility
is potentially dangerous to the public. The fence shall be a minimum
of four feet high, and of a material acceptable to the municipality.
The fence shall provide for appropriate maintenance access.
(7)
Stormwater management facilities that store runoff shall have appropriately
designed primary outlet structures that direct the facility discharge
downstream to an appropriate location in a manner that protects the
structural integrity of the stormwater management facility. The primary
outlet structure shall be designed to pass all design storms up to
and including the one-hundred-year event. Outlet structures shall
be constructed of structurally stable, corrosion resistant materials
such as reinforced concrete. Trash racks shall be provided where debris
can be reasonably expected to clog an outlet structure, particularly
for small outlet orifices.
(8)
Stormwater management facilities that store runoff shall have an
appropriately designed emergency spillway that protects the structural
integrity of the stormwater management facility. The emergency spillway
shall be designed to convey the one-hundred-year peak flow rate assuming
the primary outlet structure is blocked. Where storage is provided
behind a berm, and the one-hundred-year water surface flowing over
the emergency spillway is over one foot above the lowest elevation
along the toe of the berm, a minimum of one foot of freeboard shall
be provided as measured from the top of the berm to the one-hundred-year
water surface. The spillway shall be armored to prevent erosion during
the one-hundred-year peak flow, assuming a blocked primary outlet
structure, and calculations shall be submitted documenting the adequacy
of the proposed armor.
(9)
Embankments for stormwater management facilities that store runoff
shall be constructed in accordance with Section 206 of PennDOT Publication
408, except in-place density and moisture testing shall not be required
unless the maximum water depth is four feet or greater, or as otherwise
required by the municipality.
C.
Stormwater conveyance facilities.
(1)
All storm sewer pipes, grass waterways, open channels, swales and
other stormwater carrying facilities that service drainage areas within
the site must be able to convey post-development runoff from the ten-year
design storm.
(2)
Stormwater management facilities that convey off-site water through
the site shall be designed to convey the twenty-five-year storm event
(or larger events, as determined by the municipality).
(3)
Storm sewers.
(a)
Storm sewers must be able to convey post-development runoff
without surcharging inlets for the ten-year storm event.
(b)
When connecting to an existing storm sewer system, the applicant
must demonstrate that the proposed system will not exacerbate any
existing stormwater problems and that adequate downstream capacity
exists.
(c)
Inlets, manholes, pipes, and culverts shall be constructed in
accordance with the specifications set forth in PennDOT's Publication
408, and as detailed in the PennDOT's Publication 72M, Standards for
Roadway Construction (RC), or other detail approved by the municipality.
All material and construction details (inlets, manholes, pipe trenches,
etc.), must be shown on the SWM site plan, and a note added that all
construction must be in accordance with PennDOT's Publication 408
and PennDOT's Publication 72M, latest edition.
(d)
A minimum pipe size of 18 inches nominal diameter shall be used
for all public or private roadway systems for new installations and
developments. Minimum pipe slope shall be that necessary to provide
a minimum velocity of two feet per second when flowing full, but no
less then 0.5%. Arch pipe of equivalent cross-sectional area may be
substituted in lieu of circular pipe. The minimum pipe size for replacement
of existing roadway pipes shall be at the discretion of the Township.
(e)
All storm sewer pipes shall be laid to a minimum depth of one
foot from roadway subgrade to the crown of pipe.
(f)
In curbed roadway sections, the maximum encroachment of water
on the roadway pavement shall not exceed half of a through travel
lane or one inch less than the depth of curb during the ten-year design
storm of five-minute duration. Gutter depth shall be verified by inlet
capture/capacity calculations that account for road slope and opening
area.
(g)
Inlets shall be placed at a maximum of 400 feet apart. Inlets
shall be placed so drainage cannot cross intersections or street centerlines.
All inlets in paved areas shall have heavy-duty bicycle safe grating.
PennDOT Standard Type C inlets with eight-inch hoods shall be used
along curbed roadway networks.
(h)
Inlets must be sized to accept the specified pipe sizes without
knocking out any of the inlet corners. All pipes entering or exiting
inlets shall be cut flush with the inside wall of the inlet. All inlets
shall provide a minimum one-inch drop between the lowest inlet pipe
invert elevation and the outlet pipe invert elevation. Inlets shall
not have a sump condition in the bottom (unless designed as a water
quality BMP or specifically approved by the municipality). Pipe shall
be flush with the bottom of the box or concrete channels shall be
poured. Inlets, junction boxes, or manholes greater then five feet
in depth shall be equipped with ladder rungs.
(i)
Inlets or manholes shall be located at all vertical dislocations,
at all locations where a transition in storm sewer pipe sizing is
required, at all vertical and horizontal angle points exceeding 5°,
and at all points of convergence of two or more storm sewer pipes.
(j)
Outlet protection and energy dissipaters shall be provided at
all surface discharge points in order to minimize erosion consistent
with the E&S Manual.
(k)
Stormwater roof drains and pipes shall not be connected to storm
sewers or discharge onto impervious areas without approval by the
municipality.
(4)
Swales.
(a)
Swales must be able to convey post-development runoff from a
ten-year design storm with six inches of freeboard to top of the swale.
(b)
Swales shall have side slopes no steeper than 3:1.
(c)
All swales shall be designed, labeled on the SWM site plan,
and details provided to adequately construct and maintain the design
dimension of the swales.
(d)
Swales shall be designed for stability using velocity or shear
criteria. Velocity criteria may be used for channels with less than
ten-percent slope. Shear criteria may be used for all swales. Documentation
must be provided to support velocity and/or shear limitations used
in calculations. Where swale bends occur, the computed velocities
or shear stresses shall be multiplied by the following factor for
the purpose of designing swale erosion protection:
A.
General. All calculations shall be consistent with the stormwater
calculations and methodology described herein, as further set forth
in the BMP Manual.
B.
Rainfall values. Rainfall precipitation-frequency data shall be obtained
from NOAA Atlas 14. The following twenty-four-hour rainfall totals
for Erie County may be used:
|
Return Interval
(years)
|
24-hour Rainfall Total
(inches)
| |
|---|---|---|
|
1
|
2.13
| |
|
2
|
2.56
| |
|
10
|
3.71
| |
|
25
|
4.46
| |
|
50
|
5.09
| |
|
100
|
5.76
|
C.
Runoff volumes. Runoff volumes shall be calculated using the NRCS
Runoff Curve Number Method. Combining Curve Numbers for land areas
proposed for development with Curve Numbers for areas unaffected by
the proposed development into a single weighted curve number is not
acceptable.
D.
Runoff hydrographs. Runoff hydrographs shall be calculated by the
NRCS Unit Hydrograph Method. Computer models utilizing this method
may be used where appropriate for the application. Curve Numbers shall
be based an appropriate engineering reference, such as NRCS Publication
TR-55, "Urban Hydrology for Small Watersheds." Average antecedent
runoff conditions shall be used. Computations shall be that produce
a full hydrograph shall be used. Time of concentration shall be calculated
using the NRCS lag equation or NRCS segmental method, as appropriate.
E.
Peak runoff rates. Peak runoff rates shall be calculated using the
NRCS Unit Hydrograph Method per the previous subsection. The Rational
Method may be used to calculate peak runoff only for the design of
conveyance structures. The Rational Method may not be used to document
compliance with the volume control and rate control requirements of
this chapter, or for developing stormwater hydrographs.
F.
Hydrograph routing. The routing of hydrographs through stormwater
management facilities shall be calculated using the storage indication
method or modified puls method.
G.
Pipe and channel capacity. Manning's equation shall be used for hydraulic
computations and to determine the capacity of open channels, pipes,
and storm sewers. Manning's "n" values shall be obtained from an appropriate
engineering reference, such as PennDOT's Drainage Manual, Publication
584. Culvert design shall consider inlet and outlet control in accordance
with the procedure of Federal Highway Administration Publication HDS
No. 5, "Hydraulic Design of Highway Culverts."
