A.
General design guidelines.
(1)
Stormwater shall not be transferred from one watershed to another,
unless: a) the watersheds are subwatersheds of a common watershed
which join together within the perimeter of the property; b) the effect
of the transfer does not alter the peak rate discharge onto adjacent
lands; or c) 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)
Innovative 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, 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 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)
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.
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The following stormwater management design criteria provide
a compilation of components recommended for effective stormwater management.
The criteria reflect a moderate level of restrictions in its standards
and are based on technically sound principles. The design criteria
are provided to assist both municipalities and designers to implement
good stormwater management systems.
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B.
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)
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)
All storm sewer pipes shall be laid to a minimum depth of one
foot from subgrade to the crown of pipe.
(e)
Outlet protection and energy dissipaters shall be provided at
all surface discharge points in order to minimize erosion consistent
with the E&S Manual.
[1]
Flow velocities and volumes from any storm sewer shall not result
in a degradation of the receiving channel.
(f)
Stormwater roof drains and pipes shall not be connected to storm
sewers or discharge onto impervious areas without approval by the
Municipal Engineer.
(4)
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.
A.
All calculations shall be consistent with the guidelines set forth
in the BMP Manual, with the following amendments.
B.
Stormwater runoff from all development sites shall be calculated
using the NRCS Rainfall-Runoff Methodology. Other methods shall be
selected by the design professional based on the individual limitations
and suitability of each method for a particular site and approved
by the Municipal Engineer.
C.
Rainfall values.
(1)
NRCS Rainfall-Runoff Method. The Natural Resources 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
(years)
|
24-Hour Rainfall Total
(inches)
| |
|---|---|---|
|
1
|
2.09
| |
|
2
|
2.49
| |
|
10
|
3.50
| |
|
25
|
4.14
| |
|
50
|
4.67
| |
|
100
|
5.22
|
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)
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.
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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.
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(2)
For comparison of peak flow rates, flows shall be rounded to a tenth
of a cubic foot per second (cfs).
F.
Runoff coefficients.
(1)
NRCS Rainfall-Runoff Method. 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, CNs should realistically
be rounded to the nearest whole number.
(2)
For the purposes of predevelopment peak flow rate and volume determination,
existing nonforested pervious areas conditions shall be considered
as meadow (good condition).
(3)
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-, two-, ten-, twenty-five-, fifty-, and one-hundred-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 one-hundred-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.
Where uniform flow is anticipated, 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.
I.
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.
