A.
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.
B.
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.
C.
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.)
D.
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.
E.
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.
F.
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.
(1)
Multiple-use basins should be constructed so that potentially dangerous
conditions are not created.
(2)
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.
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
|
1.99
| |
|
2
|
2.38
| |
|
10
|
3.35
| |
|
25
|
3.96
| |
|
50
|
4.46
| |
|
100
|
4.99
|
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 than 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.
(a)
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:
(b)
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).[1]
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
(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, CNs
should realistically be rounded to the nearest whole number.
(3)
For the purposes of predevelopment peak flow rate and volume determination,
existing nonforested pervious area 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.
Twenty-four-hour 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 twenty-four-hour 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.
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 Lag Equation:
|
Time of concentration = Tc = [(Tlag/0.6) * 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
| ||
(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 cannot 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.
