A.
General design guidelines.
(1)
Stormwater shall not be transferred from one watershed to another,
unless (1) the watersheds are subwatersheds of a common watershed
which join together within the perimeter of the property; (2) the
effect of the transfer does not alter the peak rate discharge onto
adjacent lands; or (3) 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, drywells, cisterns, recreation area ponding, diversion structures,
porous pavements, holding tanks, infiltration systems, 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 municipal engineer (a pre-application 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)
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The municipality shall
reserve the right to disapprove any design that would result in the
continuation or exacerbation of a documented adverse hydrologic or
hydraulic condition within the watershed, as identified in the plan.
(6)
The design and construction of multiple use stormwater detention
facilities are strongly encouraged. In addition to stormwater management,
facilities should, where appropriate, allow for recreational uses
including ball fields, play areas, picnic grounds, etc. Consultation
with the municipality, and prior approval are required before design.
Provision for permanent wet ponds with stormwater management capabilities
may also be appropriate.
(a)
Multiple use basins should be constructed so that potentially
dangerous conditions are not created.
(b)
Water quality basins or recharge basins that are designed for
a slow release of water or other extended detention ponds are not
permitted for recreational uses, unless the ponded areas are clearly
separated and secure.
(7)
Should any stormwater management facility require a dam safety permit
under PADEP Chapter 105, the facility shall be designed in accordance
with Chapter 105 and meet the regulations of Chapter 105 concerning
dam safety.
B.
Stormwater management facility design considerations. All stormwater
management facilities shall meet the following design requirements:
(1)
No outlet structure from a stormwater management facility, or swale,
shall discharge directly onto a municipal or state roadway.
(2)
The top, or toe, of any slope shall be located a minimum of 10 feet
from any property line.
(3)
The minimum horizontal distance between any structure and any stormwater
facility shall be 25 feet. The lowest floor elevation of any structure
constructed immediately adjacent to a detention basin or other stormwater
facility shall be a minimum of two feet above the 100-year water surface
elevation.
(4)
Stormwater management facility bottom (or surface of permanent pool)
elevations must be greater than adjacent floodplain elevations (FEMA
or HEC-RAS analysis). If no floodplain is defined, bottom elevations
must be greater than existing ground elevations 50 feet from top of
stream bank in the facilities' vicinity.
(5)
Basin outflow culverts discharging into floodplains must account
for tailwater. Tailwater corresponding to the 100-year floodplain
elevation must 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 no
other evidence is provided.
(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. The invert of stormwater
facilities may be lowered if adequate subsurface drainage is provided.
(7)
Whenever possible the side slopes and basin shape shall be amenable
to the natural topography. Vertical side slopes and rectangular basins
shall be avoided whenever possible.
(8)
Exterior slopes of compacted soil shall not exceed 3:1, and may be
further reduced if the soil has unstable characteristics.
(9)
Interior slopes of the basin shall not exceed 3:1.
(10)
Unless specifically designed as a volume control facility, all
stormwater management facilities shall have a minimum slope of 1%
extending radially out from the principal outlet structure. Facilities
designed as water quality/infiltration BMPs may have a bottom slope
of zero.
(11)
Impervious low-flow channels are not permitted within stormwater
management facilities.
(12)
Unless specifically designed as a volume control or water quality
facility, all stormwater management facilities 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.
(13)
Energy dissipators 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.
(14)
Landscaping and planting specifications must be provided for
all stormwater management basins and be specific for each type of
basin.
(a)
Minimal maintenance, saturation tolerant vegetation must be
provided in basins designed as water quality/infiltration BMPs.
(15)
A safety fence may be required, at the discretion of the municipality,
for any stormwater management facility. The fence shall be a minimum
of four feet high, and of a material acceptable to the municipality.
A gate with a minimum opening of 10 feet shall be provided for maintenance
access.
(16)
Principal outlet structures. The primary outlet structure shall
be designed to pass all design storms (up to and including the 100-year
event) without discharging through the emergency spillway. All principal
outlet structures shall:
(a)
Be constructed of reinforced concrete or an alternative material
approved by the municipal engineer. When approved for use, all metal
risers shall:
[1]
Be suitably coated to prevent corrosion.
[2]
Have a concrete base attached with a watertight connection.
The base shall be sufficient weight to prevent flotation of the riser.
[3]
Provide a trash rack or similar appurtenance to prevent debris
from entering the riser.
[4]
Provide an antivortex device, consisting of a thin vertical
plate normal to the basin berm.
(b)
Provide trash racks to prevent clogging of primary outflow structure
stages for all orifices equivalent to 12 inches or smaller in diameter.
(c)
Provide outlet aprons and shall extend to the toe of the basin
slope at a minimum.
(d)
Where spillways will be used to control peak discharges in excess
of the ten-year storm, the control weirs shall be constructed to withstand
the pressures of impounded waters and convey flows at computed outlet
velocities without erosion.
(17)
Emergency spillways. Any stormwater management facility designed
to store runoff shall provide an emergency spillway designed to convey
the 100-year post-development peak rate flow with a blocked primary
outlet structure. The emergency spillway shall be designed per the
following requirements:
(a)
The top of embankment elevation shall provide a minimum one
foot of freeboard above the maximum water surface elevation. This
is to be calculated when the spillway functions for the 100-year post-development
inflow, with a blocked outlet structure.
(b)
Avoid locating on fill areas, whenever possible.
(c)
The spillway shall be armored to prevent erosion during the
100-year post- development flow, with a blocked primary outlet structure.
[1]
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.
(d)
Municipal engineer may require the use of additional protection
when slopes exceed 4:1 and spillway velocities might exceed NRCS standards
for the particular soils involved.
(e)
Any underground stormwater management facility (pipe storage
systems) must have a method to bypass flows higher than the required
design (up to a 100-year post-development inflow) without structural
failure, or causing downstream harm or safety risks.
(18)
Stormwater management basins. Design of stormwater management
facilities having three feet or more of water depth (measured vertically
from the lowest elevation in the facility to the crest of the emergency
spillway) shall meet the following additional requirements:
(a)
The maximum water depth within any stormwater management facility
shall be no greater than eight feet when functioning through the primary
outlet structure.
(b)
The top of embankment width shall be at least 10 feet.
(c)
A ten-foot-wide access to the basin bottom must be provided
with a maximum longitudinal slope of 10%.
(d)
Berms shall be constructed using soils that conform to the unified
soil classification of CH, MH, CL or ML. The embankments will be constructed
in a maximum of six-inch lifts. The lifts will each be compacted to
a density of 98% of a standard proctor analysis as per each layer
of compacted fill shall be tested to determine its density analysis
per ASTM 698. Each layer of compacted fill shall be tested to determine
its density per ASTM 2922 or ASTM 3017.
(e)
A cutoff and key trench of impervious material shall be provided
under all embankments four feet or greater in height. The cutoff trench
shall run the entire length of the embankment and tie into undisturbed
natural ground.
(f)
Design details for placement of all outflow pipes and culverts
must include measures to prevent seepage (piping) through the fill.
(19)
Construction of stormwater management facilities.
(a)
Basins used for rate control only shall be installed prior to
or concurrent with any earth moving or land disturbances, which they
will serve. The phasing of their construction shall be noted in the
narrative and on the plan.
(b)
Basins that include water quality or recharge components shall
have those components installed in such a manner as to not disturb
or diminish their effectiveness.
(c)
Compaction test reports shall be kept on file at the site and
be subject to review at all times with copies being forwarded to the
municipal engineer upon request.
(d)
Temporary and permanent grasses or stabilization measures shall
be established on the sides and base of all earthen basins within
15 days of construction.
(20)
Exceptions to these requirements may be made at the discretion
of the municipality for BMPs that retain or detain water, but are
of a much smaller scale than traditional stormwater management facilities.
C.
Stormwater carrying 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 municipal engineer).
(3)
All developments shall include provisions that allow for the overland
conveyance and flow of the post-development 100-year storm event without
damage to public or private property.
(4)
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 municipal
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.
(d)
A minimum pipe size of 18 inches in diameter shall be used in
all roadway systems (public or private) proposed for construction
in the municipality. 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 then 0.5%. Arch pipe of equivalent
cross-sectional area may be substituted in lieu of circular pipe where
cover or utility conflict conditions exist.
(e)
All storm sewer pipes shall be laid to a minimum depth of one
foot from 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)
Standard type "C" inlets with eight-inch hoods shall be used
along curbed roadway networks. Type "C" inlets with 10 inch hoods
that provide a two-inch sump condition may be used with approval of
the municipal engineer when roadway longitudinal slopes are 1% or
less.
(h)
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.
(i)
All inlets shall provide a minimum two-inch drop between the
lowest inlet pipe invert elevation and the outlet pipe invert elevation.
(j)
On curbed sections, a double inlet shall be placed at the low
point of sag vertical curves, or an inlet shall be placed on each
side of the low point at a distance not to exceed 100 feet, or at
an elevation not to exceed 0.2 foot above the low point.
(k)
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.
(l)
All inlets in paved areas shall have heavy duty bicycle safe
grating. A note to this effect shall be added to the SWM site plan
or inlet details therein.
(m)
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. A note
to this effect shall be added to the SWM site plan or inlet details
therein.
(n)
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.
(o)
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.
(p)
Inlets shall not have a sump condition in the bottom (unless
designed as a water quality BMP). Pipe shall be flush with the bottom
of the box or concrete channels shall be poured.
(q)
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 five degrees, and at
all points of convergence of two or more storm sewer pipes.
(r)
All storm drainage piping shall be provided with either reinforced
concrete headwalls or end sections compatible with the pipe size involved
at its entrance and discharge.
(s)
Outlet protection and energy dissipaters shall be provided at
all surface discharge points in order to minimize erosion consistent
with the PennDOT Publication 13M, or with FHWA Publication HEC 14.
[1]
Flow velocities and volumes from any storm sewer shall not result
in a degradation of the receiving channel.
(t)
Stormwater roof drains and pipes shall not be connected to storm
sewers or discharge onto impervious areas without approval by the
municipal engineer.
(5)
Swale conveyance facilities.
(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
10% slope. Shear criteria may be used for all swales. Documentation
must be provided to support velocity and/or shear limitations used
in calculations.
(e)
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:
(f)
Manning's "n" values used for swale capacity design must
reflect the permanent condition.
A.
All calculations shall be consistent with the guidelines set forth
in the BMP Manual, as amended herein.
B.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or the NRCS Rainfall-Runoff Methodology.
Methods shall be selected by the design 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)
NRCS Rainfall-Runoff 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 be consistent with the
following table:
|
Return interval
(Year)
|
24-hour rainfall total
(inches)
|
|---|---|
|
1
|
2.06
|
|
2
|
2.46
|
|
10
|
3.46
|
|
25
|
4.11
|
|
50
|
4.64
|
|
100
|
5.21
|
D.
Runoff volume.
(1)
Rational Method. Not to be used to calculate runoff volume.
(2)
NRCS Rainfall-Runoff Method. This method shall be used to estimate
the change in volume due to regulated activities. 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.
E.
Peak flow rates.
(1)
Rational Method. This method may be used for design of conveyance
facilities only. Extreme caution should be used by the design professional
if the watershed has more then one main drainage channel, if the watershed
is divided so that hydrologic properties are significantly different
in one versus 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.
(2)
NRCS Rainfall-Runoff Method. This method is recommended for design
of stormwater management facilities and where stormwater runoff volume
must be taken into consideration. The following provides guidance
on the model applicability:
(a)
NRCS's TR-55 - limited to 100 acres in size.
(b)
NRCS's TR-20 or HEC-HMS - no size limitations.
(c)
Other models as preapproved by the municipal engineer.
(d)
The NRCS antecedent runoff condition II (ARC II, previously
AMC II) must be used for all simulations. The use of continuous simulation
models that vary the ARC are not permitted for stormwater management
purposes.
(3)
For comparison of peak flow rates, flows shall be rounded to a tenth
of a cubic foot per second (cfs).
F.
Runoff coefficients.
(1)
Rational Method. Use Table C-1 (Appendix C).
(2)
NRCS Rainfall-Runoff Method. Use Table C-2 (Appendix C). Curve numbers
(CN) should be rounded to tenths for use in hydrologic models as they
are a design tool with statistical variability. For large sites, CN's
should realistically be rounded to the nearest whole number.
(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 peak flow rate and volume determination,
20% of existing impervious area, when present, shall be considered
meadow (good condition).
G.
Design storm.
(1)
All stormwater management facilities shall be verified by routing
the proposed one-year, two-year, ten-year, twenty-five-year, fifty-year,
and 100-year hydrographs through the facility using the storage indication
method or modified puls method. The design storm hydrograph shall
be computed using a calculation method that produces a full hydrograph.
(2)
The stormwater management and drainage system shall be designed to
safely convey the post-development 100-year storm event to stormwater
detention facilities, for the purpose of meeting peak rate control.
(3)
All structures (culvert or bridges) proposed to convey runoff under
a municipal 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.
H.
Time of concentration.
(1)
The time of concentration is to represent the average condition that
best reflects the hydrologic response of the area. The following time
of concentration (Tc) computational methodologies
shall be used unless another method is preapproved by the municipal
engineer:
(a)
Predevelopment - NRCS's Log Equation:
|
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 = [(1000/CN)
- 10]
|
|
CN
|
=
|
NRCS curve number for watershed
|
(b)
Post-development; commercial, industrial, or other areas with
large impervious areas (>20% impervious area) - NRCS Segmental Method.
The length of sheet flow shall be limited to 100 feet. Tc for channel and pipe flow shall be computed using Manning's
equation.
(c)
Post-development; residential, cluster, or other low impact
designs less than or equal to 20% impervious area - NRCS Lag Equation
or NRCS Segmental Method.
(2)
Additionally, the following provisions shall apply to calculations
for time of concentration:
(a)
The post-development Tc shall never be
greater that the predevelopment Tc for any
watershed or subwatershed. This includes when the designer has specifically
used swales to reduce flow velocities. In the event that the designer
believes that the post-development Tc is greater,
it will still be set by default equal to the predevelopment Tc for modeling purposes.
(b)
The minimum Tc for any watershed shall
be five minutes.
(c)
The designer may choose to assume a five-minute Tc for any post-development watershed or subwatershed
without providing any computations.
(d)
The designer must provide computations for all predevelopment
Tc paths. A five-minute Tc 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 Tc calculation.
I.
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 used to reduce peak runoff rates
in the post-development analysis. New, additional contributing runoff
should not be directed to existing sinkholes or closed depressions.
J.
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 PENNDOT's Drainage Manual, Publication 584. 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.
K.
The municipality 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.
L.
The municipality 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 recommendation of the municipality.
