A.
General design guidelines.
(1)
Stormwater shall not be transferred from one watershed to another,
unless the watersheds are subwatersheds of a common watershed which
join together within the perimeter of the property; the effect of
the transfer does not alter the peak rate discharge onto adjacent
lands; or easements from the affected landowner(s) are provided.
(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 within an existing
watercourse or confined in an easement or returned to a predevelopment
flow type condition.
(3)
Stormwater BMPs and recharge facilities are encouraged (e.g., rooftop
storage, dry wells, cisterns, recreation area ponding, diversion structures,
porous pavements, holding tanks, infiltration systems, stream channel
storage, in-line storage in storm sewers, and grading patterns). They
shall be located, designed, and constructed in accordance with the
latest technical guidance published by PADEP, provided they are accompanied
by detailed engineering plans and performance capabilities and supporting
site specific soils, geology, runoff and groundwater and infiltration
rate data to verify proposed designs. Additional guidance from other
sources may be accepted at the discretion of the Borough Engineer
(a preapplication meeting is suggested).
(4)
All existing and natural watercourses, channels, drainage systems
and areas of surface water concentration shall be maintained in their
existing condition unless an alteration is approved by the appropriate
regulatory agency.
(5)
No outlet structure from a stormwater management facility, or swale,
shall discharge directly onto a Borough or state roadway.
(6)
The invert of all stormwater management facilities and underground
infiltration/storage facilities shall be located a minimum of two
feet above the seasonal high groundwater table or other soil limiting
zone. The invert of stormwater facilities may be lowered if adequate
subsurface drainage, which does not alter the existing water table
level, is provided.
(7)
Any stormwater management facility may be required to be fenced with
a minimum four-foot high fence of material acceptable to the Borough.
Gates with a minimum opening of 10 feet shall be provided for access.
(8)
Stormwater management facilities excavated to carbonate rock must
either be fitted with an impervious clay liner, or over-excavated
four feet and refilled with a suitable material mix. Suitable backfill
material is subject to the approval of the Borough Engineer.
(9)
The type, location, and number of landscaping and planting specification
shall be provided for all stormwater management facilities and be
specific for each type of facility.
B.
Stormwater management facilities (with a depth of water equal to
or greater than three feet measured from the lowest point inside a
facility to the crest of the emergency spillway).
(1)
Any stormwater management facility designed to store runoff and requiring
a berm or earthen embankment, shall be designed to provide an emergency
spillway to handle peak rate of stormwater runoff up to and including
the one-hundred-year post-development flow, with a blocked primary
outlet structure. The height of embankment must be set as to provide
a minimum one foot of freeboard through the spillway, above the maximum
water surface elevation, computed when the spillway functions for
the one-hundred-year post-development inflow, with a blocked outlet
structure. The primary outflow structure must be designed to pass
all design storms (up to and including the one-hundred-year event)
without discharging through the emergency spillway. The maximum water
depth within any stormwater management facility shall be no greater
than eight feet when functioning through the primary outlet structure.
(2)
Emergency spillways shall be armored to prevent erosion during the
one-hundred-year post-development flow, with blocked primary outlet
structure. Synthetic liners or riprap may be used, and calculations
sufficient to support proposed armor must be provided. An earthen
plug must be used to accurately control the spillway invert if riprap
is the proposed armoring material. Emergency spillway armor must extend
up the sides of the spillway, and continue at full width to a minimum
of 10 feet past the toe of slope.
(3)
A stormwater management facility berm cross sections must be at least
five feet wide at the top, and eight feet wide through the emergency
spillway. For fill embankments, the side slopes shall be no steeper
than 3:1 on the inside of the facility and 2:1 on the outside of the
facility. For cut slopes, the side slopes shall be no steeper than
2:1.
(4)
A cutoff and key trench of impervious material shall be provided
under all embankments four feet or greater in height.
(5)
Soils used for the construction of stormwater management facilities
shall have low erodibility factors ("K" factors) (refer to E&S
Manual) and be identified on the SWM site plan.
(6)
Trash racks must be provided to prevent clogging of primary outflow
structure stages for all orifices equivalent to 12 inches or smaller
in diameter.
(7)
Anti-seep collars must be provided on all outflow culverts in accordance
with the methodology contained in the latest edition of the E&S
Manual. An increase in seepage length of 15% must be used in accordance
with the requirements for permanent anti-seep collars.
(8)
Conventional, non-BMP stormwater management facilities (i.e., dry
detention basins) must empty over a period of time not less than 24
hours and not more than 72 hours from the end of the facility's
inflow hydrograph. Infiltration tests performed at the facility locations
and proposed basin bottom depths, in accordance with the BMP Manual,
must support time-to-empty calculations if infiltration is a factor
is the sizing of the stormwater management facility.
(9)
Impervious low flow channels are not permitted within stormwater
management facilities to promote water quality and groundwater recharge
for frequent storm events. Facilities designed as water quality/infiltration
BMPs may have a bottom slope of zero. Minimal maintenance, saturation
tolerant vegetation must be provided in basins designed as water quality/infiltration
BMPs. Conventional, non-BMP stormwater management facilities must
have a minimum slope of 1% extending radially out from the primary
outlet structure. Water storage below the lowest outlet structure
stage (i.e., dead storage) is permitted in stormwater management facilities
designed as water quality/infiltration BMPs.
(10)
Stormwater management facilities bottom elevations must be greater
than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If
no floodplain is defined, bottom elevations must be higher than existing
ground elevations 50 feet from top of stream bank in the facilities
vicinity.
(11)
Basin outflow culverts discharging into floodplains must account
for tailwater. Tailwater corresponding to the one-hundred-year floodplain
elevation may be used for all design storms, or the applicant may
elect to determine flood elevations of the adjacent watercourse for
each design storm. The floodplain is assumed to be 50 feet from top
of stream bank in areas where a floodplain is not designated, or where
no other evidence is provided.
(12)
Exceptions to these requirements may be made at the discretion
of the Borough for BMPs that retain or detain water, but are of a
much smaller scale than traditional stormwater management facilities.
C.
Storm sewer facilities.
(1)
Storm sewers must be able to convey post-development runoff from
the design storm without surcharging inlets where appropriate. 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.
(2)
A minimum pipe size of 15 inches in diameter shall be used in all
roadway systems (public or private) proposed for construction. Pipes
shall be designed to provide a minimum velocity of 2 1/2 feet
per second when flowing full, but in all cases, the slope shall be
no less than 0.5%. Arch pipe of equivalent cross-sectional area may
be substituted in lieu of circular pipe where cover or utility conflict
conditions exist.
(3)
In proposed 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. The maximum distance between inlets in curbed roadway sections
shall be no more than 600 feet; however access to underground pipes
shall be provided every 300 feet.
(4)
Standard Type "C" inlets with eight-inch hoods shall be used along
vertical concrete curbs roadway networks. Type "C" inlets with ten-inch
hoods that provide a two-inch sump condition may be used with approval
of the Borough Engineer when roadway longitudinal slopes are 1.0%
or less.
(5)
For inlets containing a change in pipe size, the elevation for the
crown of the pipes shall be the same or the smaller pipe's crown
shall be at a higher elevation.
(6)
All inlets shall provide a minimum two-inch drop between the lowest
inlet pipe invert elevation and the outlet pipe invert elevation.
(7)
On curbed sections, a double inlet shall be placed at the low point
of sag vertical curves, or an inlet shall be placed at the low point
and on each side of the low point at a distance not to exceed 100
feet, or at an elevation not to exceed 0.2 feet above the low point.
(8)
At all roadway low points, swales and easements shall be provided
behind the curb or swale and through adjacent properties to channelize
and direct any overflow of stormwater runoff away from dwellings and
structures.
(9)
Inlets shall be placed so drainage cannot cross intersections or
street center lines.
(10)
All inlets in paved areas shall have heavy-duty bicycle-safe
grating consistent with PennDOT Publication 72M. A note to this effect
shall be added to the SWM site plan or inlet details therein.
(11)
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 inlet wall. A note to this effect
shall be added to the SWM site plan or inlet details therein.
(12)
Inlets shall have weep holes covered with geotextile fabric
placed at appropriate elevations to completely drain the sub grade
prior to placing the base and surface course on roadways.
(13)
Inlets, junction boxes, or manholes greater than five feet in
depth shall be equipped with ladder rungs and shall be detailed on
the SWM site plan.
(14)
Inlets shall not have a sump condition in the bottom (unless
designed as a water quality BMP). Pipes shall be flush with the bottom
of the box or concrete channels shall be poured.
(15)
Inlets, manholes, pipes, and culverts shall be constructed in
accordance with the specifications set forth in PennDOT's Publication
408, latest edition, and as detailed in the PennDOT's Publication
72M, Standards for Roadway Construction (RC), latest edition, or as
approved by the Borough Engineer. 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. A note shall be added to the plan stating that
all frames, concrete top units, and grade adjustment rings shall be
set in a bed of full mortar according to Publication 408.
(16)
Accessible drainage structures shall be located on continuous
storm sewer system 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.
(17)
All storm drainage piping (equal to or greater than 12 inches)
discharging to the ground surface shall be provided with either reinforced
concrete headwalls and end sections or plastic and metal pipe end
sections compatible with the pipe size involved in accordance with
PennDOT Publication 408 and Publication 72M.
(18)
Outlet protection shall be provided at all surface discharge
points with storm drainage piping (equal to or greater than 12 inches)
in order to minimize erosion consistent with the E&S Manual.
(19)
Pavement base drain shall be provided at all low points in cut
areas, toe of slope areas, and other areas as dictated by proven engineering
principles and design judgment. All base drain shall be in accordance
with PennDOT Publication 408.
D.
Swale conveyance facilities.
(1)
Swales must be able to convey post-development runoff from the design
storm with six inches of freeboard to top of the swale.
(2)
Swales shall have side slopes no steeper than 3:1.
(3)
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.
(4)
Swales shall be designed for stability using velocity or shear criteria.
Velocity criteria may be used for channels with less than 10% slope.
Shear criteria may be used for all swales. Documentation must be provided
to support velocity and/or shear limitations used in calculations.
(6)
Swales must be designed for both temporary and permanent conditions
in accordance with the latest E&S Manual.
A.
All calculations shall be consistent with the guidelines set forth
in the BMP Manual.
B.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil Cover Complex methodology.
Methods shall be selected by the qualified professional based on the
individual limitations and suitability of each method for a particular
site.
C.
Rainfall values.
(1)
Rational Method. The Pennsylvania Department of Transportation Drainage
Manual, Intensity-Duration-Frequency Curves, Publication 584, Chapter
7A, latest edition, shall be used in conjunction with the appropriate
time of concentration and return period.
(2)
Soil Cover Complex Method. The Soil Conservation Service Type II,
twenty-four-hour rainfall distribution shall be used in conjunction
with rainfall depths from NOAA Atlas 14 or consistent with the following
table.
|
Return Interval
(year)
|
24-hour Rainfall Total
(inches)
|
|---|---|
|
1
|
2.40
|
|
2
|
2.90
|
|
10
|
4.36
|
|
25
|
5.43
|
|
50
|
6.38
|
|
100
|
7.48
|
D.
Peak flow rates.
(1)
Rational Method. May be used for drainage areas up to 200 acres for
stormwater conveyance and up to 20 acres for stormwater management.
Extreme caution should be used by the qualified professional if the
watershed has more than one main drainage channel, if the watershed
is divided so that hydrologic properties are significantly different
in one verses the other, if the time of concentration exceeds 60 minutes,
or if stormwater runoff volume is an important factor. The combination
of Rational Method hydrographs based on timing shall be prohibited.
(a)
The use of the Modified Rational Method to design stormwater
management facilities must be approved by the Borough Engineer.
(2)
Soil Cover Complex Method. May be used for drainage areas greater
than 20 acres. This method is recommended for design of stormwater
management facilities and where stormwater runoff volume must be taken
into consideration.
(3)
For comparison of peak flow rates, flows shall be rounded to a tenth
of a cubic foot per second (cfs).
E.
Runoff coefficients.
(3)
For the purposes of predevelopment peak flow rate and volume determination,
existing nonforested pervious areas conditions shall be considered
as meadow (good condition).
(4)
For the purposes of predevelopment volume determination, 20% of existing
impervious area, when present on the project site, and contained within
the new proposed limit of disturbance, shall be considered meadow
(good condition) for predevelopment hydrologic calculations for redevelopment.
F.
Design storm.
(1)
All drainage facilities (inlets, pipes, and swales) shall be designed
to safely convey the ten-year storm.
(2)
All stormwater management facilities shall be verified by routing
the proposed one-year, two-year, ten-year, twenty-five-year, fifty-year,
and one-hundred-year hydrographs through the facility using the Storage
Indication (Modified Puls) Method. The design storm hydrograph shall
be computed using a calculation method that produces a full hydrograph.
(3)
The stormwater management and drainage system shall be designed to
safely convey the post-development one-hundred-year storm event to
stormwater detention facilities, for the purpose of meeting peak rate
control.
(4)
All structures (culvert or bridges) proposed to convey runoff under
a Borough road shall be designed to pass the fifty-year design storm
with a minimum one foot of freeboard measured below the lowest point
along the top of the roadway.
(5)
All design within state or federal rights-of-way or that falls under
the design criteria of any higher authority must meet the requirements
of that agency in addition to meeting the minimum requirements of
this chapter.
G.
Time of concentration.
(1)
Time of concentration shall be computed using the NRCS Segmental
Method as described in TR-55 (SCS 1986 or most current update). The
length of sheet flow shall be limited to 100 feet. The Manning's
"n" roughness coefficient for TR-55 sheet flow can be found in Table
C-4 (Appendix C[3]). Time of concentration for channel and pipe flow shall
be computed using Manning's Equation.
[3]
Editor's Note: Said appendix is included as an attachment
to this chapter.
(2)
For sites with insignificant channelized flow and less than 20% imperviousness
coverage, the time of concentration may be computed using the NRCS
equation for lag time:
|
Time of concentration = Tc = [(Tlag/.6) * 60] (minutes)
|
 |
|
Where:
| |||
|
Tlag
|
=
|
Lag time (hours)
| |
|
L
|
=
|
Hydraulic length of watershed (feet)
| |
|
Y
|
=
|
Average overland slope of watershed (percent)
| |
|
S
|
=
|
Maximum retention in watershed as defined by: S = [(1,000/CN)
- 10]
| |
|
CN
|
=
|
NRCS curve number for watershed as defined by the NRCS Loss
Method
| |
(3)
Additionally, the following provisions shall apply to calculations
for time of concentration:
(a)
The post-development time of concentration shall never be greater
that the predevelopment time of concentration for any watershed or
subwatershed.
(b)
The minimum time of concentration for any watershed shall be
five minutes.
(c)
The designer may choose to assume a five-minute time of concentration
for any post-development watershed or subwatershed without providing
any computations.
(d)
The designer must provide computations for all predevelopment
time of concentration paths. A five-minute time of concentration can
not be assumed for predevelopment.
(e)
Undetained fringe areas (areas that are not tributary to a stormwater
facility but where a reasonable effort has been made to convey runoff
from all new impervious coverage to best management practices) may
be assumed to represent the predevelopment conditions for purpose
of time of concentration calculations.
H.
Drainage areas tributary to sinkholes or closed depressions in areas
underlain by limestone or carbonate geologic features shall be excluded
from the modeled point of analysis defining predevelopment flows.
If left undisturbed during construction activities, areas draining
to closed depressions may also be removed from peak runoff rates in
the post-development analysis. New, additional contributing runoff
shall not be directed to existing sinkholes or closed depressions.
I.
Where uniform flow is anticipated, the Manning's Equation shall
be used for hydraulic computations and to determine the capacity of
open channels, pipes, and storm sewers. The Manning's Equation
should not be used for analysis of pipes under pressure flow or for
analysis of culverts. Manning's "n" values shall be obtained
from Table C-3 (Appendix C[4]). Inlet control shall be checked at all inlet boxes to
ensure the headwater depth during the ten-year design event is contained
below the top of grate for each inlet box.
[4]
Editor's Note: Said appendix is included as an attachment
to this chapter.
J.
The Borough may approve the use of any generally accepted full hydrograph
approximation technique that shall use a total runoff volume that
is consistent with the volume from a method that produces a full hydrograph.
K.
The Borough has the authority to require that computed existing runoff
rates be reconciled with field observations, conditions and site history.
If the designer can substantiate, through actual physical calibration,
that more appropriate runoff and time of concentration values should
be utilized at a particular site, then appropriate variations may
be made upon review and approval of the Borough.
