[Ord. No. 383, 8/27/2019]
1.
General Design Guidelines:
A.
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.
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 pre-development flow type condition. See § 26-301 of this chapter for BMP location restrictions.
C.
Innovative stormwater BMPs and recharge facilities are encouraged.
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
Township (a pre-application 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 Township 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 Township 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.
G.
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.
2.
Stormwater Management Facility Design Considerations: All stormwater
management facilities shall meet the following design requirements:
A.
All areas of a subdivision, land development plan or mobile home
park to be developed with streets, residential lots or sites, recreation
areas or other uses shall be graded to assure positive drainage.
B.
Storm sewers, culverts and related installations shall be provided
to ensure the controlled flow of natural watercourses and to guarantee
the drainage of all low points along the curb or gutter lines, as
well as at intervals related to slope, of all streets in or adjacent
to the plan. Where driveways cross gutter, line or passage via a pipe
below the driveway entrance of the type and size required by the North
Strabane Township.
C.
In designing the storm drainage system, the developer shall use as
his guide the publication "Urban Hydrology for Small Watersheds,"
Technical Release No. 55, U.S. Department of Agriculture, Soil Conservation
Service, June 1986, and as amended.
D.
The predevelopment condition of the parcel to be developed, as determined
by the Township, shall be considered the state of the land surface
on the date that the developer entered into an agreement to purchase
the property. If the property has been partially or completely developed
at the time of purchase, the Township shall require the new owner,
if he proposes to redevelop or further develop the property, to take
remedial action for compliance with the requirements of this subsection.
This shall include situations where removal of existing impervious
areas and replacing them with new impervious areas is proposed, where
no or inadequate existing stormwater management is present. If remedial
action is required, regardless of the condition of the surface, the
design runoff shall not exceed curve number 75 applied to class C
"good meadow" soils (see Tables 2-1 and 2-2 of T.R. 55). Stormwater
runoff originating upstream of the property that is bypassed through
or around the site may be removed from the hydrographical analysis
for on-site stormwater management facilities. Conveyance structures
designed to bypass flow must be designed for the 100-year twenty-four-hour
peak flow event. If a property is to be laid out in lots, no reduction
in lot size shall be permitted to accommodate storm drainage detention
structures. Adequate space shall be reserved for all proposed structures
and access roads.
E.
The stormwater drainage plan of a subdivision, land development plan
or mobile home park shall be designed to assure that, after development,
stormwater does not leave the property at a greater velocity or volume
per second than before development commenced.
(1)
The difference between the pre-development and post-development
condition for all storms up to the 100-year storm shall be stored
on the property for gradual release at pre-development rate.
(2)
Outlet works shall be designed so as to store water and control
runoff for all storms of frequency up to and including the one-hundred-year
storm.
(3)
Vertical riser pipes with trash racks and anti-vortex devices
shall be provided in detention basins, unless an alternate design
is approved by the Township.
(4)
Detention basins shall be designed to drain completely in a
period between 24 and 72 hours.
(5)
No outlet structure from a stormwater management facility, or
swale, shall discharge directly onto a Municipal or state roadway
or right-of-way without approval from the Township or PennDOT.
F.
Detention areas, where required to impede runoff, shall be designed
to meet or exceed the following standards:
(1)
Except where a permanent pond is approved by the Township Commissioners,
detention areas shall be designed to drain completely. All stormwater
management BMPs shall be equipped with a drain line no less than four
inches in diameter and a valve that is normally closed, preferably
on the outside of the facility where it is easily accessed, so that
the facility may be completely dewatered for maintenance and emergencies.
(2)
The height of the impoundment dam shall be not more than 10
feet measured between the top of the dam and the toe of the slope
on the downstream side of the dam. The level of water within the impoundment
shall not exceed 10 feet measured between the lowest point in the
impoundment area behind the dam and the top of the emergency spillway.
(3)
The area to be occupied by the dam shall be cleared of all topsoil
and organic materials prior to construction. The dam shall be built
up in layers not to exceed six inches in depth with equipment providing
95% compaction at optimum moisture conditions. The top of the dam
shall be constructed 10% higher than its designed height to allow
for settlement.
(4)
The core trench stabilizing the dam shall extend the full length
under the dam and full height each side of the dam and shall be designed
after a soil investigation at the dam site carried to at least 10
feet below the level of the toe of the dam or to bedrock. The core
trench shall be not less than three feet in depth, not less than three
feet in width at its bottom, and its sides shall slope not less than
one-foot vertical rise to each one foot of horizontal run (one to
one).
(5)
The settled elevation of the top of the dam at its lowest point
shall be not less than one foot above the maximum water level to be
impounded behind the dam and not less than two feet above the elevation
of the emergency spillway.
(6)
The surfaces of the dam shall be planted in a seed mixture as
specified by PaDEP requirements.
(7)
The horizontal drain pipe passing through the dam shall be reinforced
concrete pipe. Anti-seepage collars shall be placed not less than
28 feet apart along the length of the pipe. Each collar shall be fabricated
of not less than one-fourth-inch steel plate, shall extend not less
than two feet in all directions at right angles from the outer face
of the pipe, shall be welded in place all around and shall be completely
enclosed within the dam fill.
(8)
The top of the riser pipe to drain the pond shall be not less
than three feet in height above the invert elevation of the horizontal
pipe, shall be not less than four inches in diameter and shall be
fitted with an anti-vortex device and a trash rack.
(9)
The emergency spillway shall be capable of passing the flow
created by the 100-year, twenty-four-hour storm event with riser inoperable,
demonstrating one foot of freeboard or greater.
(10)
Stone low-flow channels with underdrains shall be installed
in all aboveground earthen detention facilities, and channel slopes
within the basin shall be 2% minimum.
(11)
Notwithstanding the above requirements, dams as classified under
Pennsylvania Code 25 Chapter 105, Pennsylvania Dam Safety, shall meet
all requirements of said act.
G.
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.
Flows from underdrains shall not be counted in volume or rate control
calculations.
H.
The outfall pipe passing through the embankment shall be reinforced
concrete pipe. Antiseepage collars shall be placed not less than 28
feet apart along the length of the pipe and shall be constructed in
accordance with construction detail.
I.
Exterior slopes of the basin embankment shall not exceed 3:1, and
may be further reduced if the soil has unstable characteristics.
J.
Interior slopes of the basin shall not exceed 3:1.
K.
Unless specifically designed as a volume control facility, all stormwater
management facilities shall have a minimum slope of 2% extending radially
out from the principal outlet structure. A concrete-lined low-flow
channel shall be provided in accordance with the construction detail.
Facilities designed as water quality/infiltration BMPs may have a
bottom slope of zero.
L.
Landscaping and planting specifications must be provided for all
stormwater management basins and be specific for each type of basin.
(1)
Minimal maintenance, saturation-tolerant vegetation must be
provided in basins designed as water quality/infiltration BMPs.
M.
The fence shall be nonprivacy to allow for sight into the basin.
A fence may be required for each aboveground stormwater management
facility as determined at the time of dedication of the facility to
the homeowner's association or other responsible party as named
and required by the Township. All required fencing shall be a minimum
four feet high and require access points for maintenance, as required
by the Township.
N.
An access road shall be provided to stormwater management facilities
that are not directly adjacent to a public right-of-way, in accordance
with the construction detail. Two points of access to the facility
must be provided, with a gated access point in the fence location.
Adequate turning locations adjacent to the stormwater management facility
shall be provided for maintenance vehicles. Easement dedicate to the
township will be shown on subdivision or site plans.
O.
Written design reports and calculations shall be submitted for review
and approval by the Township. Routing calculations using modified
PULS methodology shall be included to assure outflow rates are in
conformance with the requirements of this chapter. Calculations for
storm pipe sizing shall also be included using the rational method
and Manning's formula for a twenty-five-year storm using PennDOT
intensity duration frequency curves from the Engineering Design Manual
(latest edition).
P.
The owner or developer shall enter into a legal agreement approved
by the Township Solicitor which shall hold the Township harmless from
any and all liability relating to storm drainage collection and its
discharge during construction of the system and thereafter. The owner
or developer shall secure, where necessary, off-site easements for
storm drainage.
Q.
The costs of review and inspection by the Township of the stormwater
management system as proposed by the developer shall be borne by the
developer, whether or not the plan is ultimately approved. The Township
shall review such changes as the developer may make in his plan for
compliance with the Township's recommendations and shall advise
the Board of Supervisors whether or not the revised plan is in compliance.
Inspection reports prepared by the contractor and/or geotechnical
engineer shall be provided to the Township.
R.
Catch basins shall meet the Township standards and shall have inverts
poured to allow self-cleaning. Catch basins shall be located no further
apart then 300 feet, measured between catch basins on the same side
of the street.
S.
Graded areas shall be stabilized with erosion-resisting plantings
placed immediately after the completion of grading. Graded slopes
produced by placing fill earth over the preexisting surface shall
be keyed in accordance with sound geotechnical practices.
T.
All persons, partnerships or corporations intending to excavate,
fill or grade land in the Township shall be required to apply for
a grading permit under the terms of the SALDO,[1] as amended, and shall have a valid grading permit in their
possession prior to the start of any grading work. Work shall be undertaken
in conformance with the requirements of this chapter and applicable
regulations of Washington County Conservation District.
[Ord. No. 383, 8/27/2019]
1.
All calculations shall be consistent with the guidelines set forth
in the BMP Manual, as amended herein.
2.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or 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 Township.
3.
Rainfall Values:
A.
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.
B.
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)
|
Twenty-Four-Hour Rainfall Total
(inches)
|
|---|---|
|
1
|
1.99
|
|
2
|
2.38
|
|
10
|
3.35
|
|
25
|
3.96
|
|
50
|
4.46
|
|
100
|
4.99
|
4.
Runoff Volume:
A.
Rational Method. Not to be used to calculate runoff volume.
B.
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.
5.
Peak Flow Rates:
A.
Rational Method. This method may be used for design of conveyance
facilities only. 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.
B.
NRCS Rainfall-Runoff Method. This method is recommended for design
of stormwater management facilities and where stormwater runoff volume
must be taken into consideration.
C.
For comparison of peak flow rates, flows shall be rounded to a tenth
of a cubic foot per second (cfs).
6.
Runoff Coefficients:
B.
NRCS Rainfall-Runoff Method. Use Table C-2 (Appendix C).[2] 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]
Editor's Note: Said appendix is included as an attachment
to this chapter.
C.
For the purposes of pre-development peak flow rate and volume determination,
existing nonforested pervious areas conditions shall be considered
as meadow (good condition).
D.
For the purposes of pre-development peak flow rate and volume determination,
20% of existing impervious area, when present, shall be considered
meadow (good condition).
7.
Design Storm:
A.
All stormwater management facilities shall be verified by routing
the proposed one-, two-, ten-, twenty-five-, fifty-, and 100-year
hydrographs through the facility using the storage indication method
or modified plus method. The design storm hydrograph shall be computed
using a calculation method that produces a full hydrograph.
B.
The stormwater drainage system (minor calculations) shall be designed
to safely convey the post-development 100-year storm event to stable
discharge locations, demonstrating that the pipe capacity exceeds
the total flow through the pipe, and that the hydraulic grade line
does not surcharge connecting structures.
C.
Permanent channels shall be designed to convey the 100-year storm
event to stable discharge locations, demonstrating a minimum of six
inches of freeboard and that channel lining material sheer stress
does not exceed the maximum allowable sheer stress.
D.
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, using FHWA HY-8 or HEC-RAS methodology,
as applicable.
8.
Time of Concentration:
A.
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 Township:
(1)
Pre-development: NRCS's Lag 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 = [(1,000/CN)
- 10]
|
|
CN
|
=
|
NRCS Curve Number for watershed
|
(2)
Post-development: 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.
B.
Additionally, the following provisions shall apply to calculations
for time of concentration:
(1)
The post-development Tc shall never be greater that the pre-development
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 pre-development Tc for
modeling purposes.
(2)
The minimum Tc for any watershed shall be five minutes.
(3)
The designer may choose to assume a five-minute Tc for any post-development
watershed or subwatershed without providing any computations.
(4)
The designer must provide computations for all pre-development
Tc paths. A five-minute Tc cannot be assumed for pre-development.
(5)
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 pre-development conditions for purpose
of Tc calculation.
9.
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.
10.
The Township 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 Township.
