A.
For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 252-17:
B.
SWM site plans approved by the municipality, in accordance with § 252-30, shall be on site throughout the duration of the regulated activity.
C.
The municipality may, after consultation with the DEP, approve measures
for meeting the state water quality requirements other than those
in this chapter, provided that they meet the minimum requirements
of, and do not conflict with, state law, including, but not limited
to, the Clean Streams Law.
D.
For all regulated earth disturbance activities, erosion and sediment
control BMPs shall be designed, implemented, operated, and maintained
during the regulated earth disturbance activities (e.g., during construction)
to meet the purposes and requirements of this chapter and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law. Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual),[1] Technical Guidance No. 363-2134-008, as amended and updated.
E.
Impervious areas.
(1)
The measurement of impervious areas shall include all of the impervious
areas in the total proposed development even if development is to
take place in stages.
(2)
For development taking place in stages, the entire development plan
must be used in determining conformance with this chapter.
(3)
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls in § 252-18 and the peak rate controls of § 252-19 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
F.
If diffused flow is proposed to be concentrated and discharged onto
adjacent property, the applicant must document that adequate downstream
conveyance facilities exist to safely transport the concentrated discharge
or otherwise prove that no erosion, sedimentation, flooding, or other
harm will result from the concentrated discharge.
G.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this chapter by implementing measures
to:
(a)
Minimize disturbance to floodplains, wetlands, natural slopes,
existing native vegetation and woodlands.
(b)
Encourage the creation, maintenance and extension of riparian
buffers and the protection of existing forested buffers.
(c)
Provide trees and woodlands adjacent to impervious areas whenever
feasible.
(d)
Minimize the creation of impervious surfaces and the degradation
of waters of the Commonwealth and promote groundwater recharge.
(e)
Protect natural systems and processes (drainageways, vegetation,
soils, and sensitive areas) and maintain, as much as possible, the
natural hydrologic regime.
(f)
Incorporate natural site elements (wetlands, stream corridors,
mature forests) as design elements.
(g)
Avoid erosive flow conditions in natural flow pathways.
(h)
Minimize soil disturbance and soil compaction.
(i)
Minimize thermal impacts to waters of the commonwealth.
(j)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible, and decentralize and manage stormwater at
its source.
H.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
I.
Infiltration BMPs should be spread out, made as shallow as practicable,
located at least a minimum distance of 10 feet away from basements
and foundations, and located to maximize use of natural on-site infiltration
features while still meeting the other requirements of this chapter.
J.
Normally dry, open-top, storage facilities should completely drain
both the volume control and rate control capacities over a period
of time not less than 24 and not more than 72 hours from the end of
the design storm.
K.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the latest version of the Precipitation-Frequency
Atlas of the United States, National Oceanic and Atmospheric Administration
(NOAA), National Weather Service, Hydrometeorological Design Studies
Center, Silver Spring, Maryland.[3]
L.
NOAA's Atlas 14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this chapter and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law, and the Stormwater Management Act.
O.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
P.
Where watercourses traverse a development site, drainage easements
with a minimum width of 20 feet and which include the one-hundred-year
water surface shall be provided conforming to the center line of such
watercourses. Excavation, the placing of fill or structures, and any
alterations that may adversely affect the flow of stormwater within
any portion of the drainage area shall be prohibited. Also, maintenance,
including mowing of vegetation within the drainage area, may be required,
except as approved by the appropriate governing authority.
Q.
When it can be shown that, due to topographic conditions, natural
drainageways on the site cannot adequately provide for drainage, open
channels may be constructed conforming substantially to the line and
grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by the PA DEP under regulations at 25
Pa. Code Chapter 105 through the joint permit application process
or, where deemed appropriate by the PA DEP, through the general permit
process.
R.
Any stormwater management facilities or any facilities that constitute
water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures,
etc.) that are regulated by this chapter, that will be located in
or adjacent to waters of the commonwealth (including wetlands), shall
be subject to approval by the PA DEP under regulations at 25 Pa. Code
Chapter 105 through the joint permit application process or, where
deemed appropriate by the PA DEP, the general permit process. When
there is a question whether wetlands may be involved, it is the responsibility
of the applicant or his agent to show that the land in question cannot
be classified as wetlands; otherwise, approval to work in the area
must be obtained from the PA DEP.
S.
Should any stormwater management facility require a dam safety permit
under Pa. Code Chapter 105, the facility shall be designed in accordance
with Chapter 105 and meet the regulations of Chapter 105 concerning
dam safety.
T.
Any stormwater management facilities regulated by this chapter that
will be located on, or discharged onto, state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
U.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., are encouraged, where soil
conditions and geology permit, to reduce the size of or eliminate
the need for detention facilities.
V.
Roof drains shall not be connected to streets, sanitary or storm
sewers, or roadside ditches in order to promote overland flow and
infiltration/percolation of stormwater where it is advantageous to
do so. When it is more advantageous to connect directly to streets
or storm sewers, then the municipality shall permit it on a case-by-case
basis.
W.
Applicants are encouraged to use low-impact development practices
to reduce the costs of complying with the requirements of this chapter
and the state water quality requirements.
X.
When stormwater management facilities are proposed within 1,000 feet
of a downstream municipality, the developer shall notify the downstream
municipality and provide a copy of the SWM site plan, if requested,
for review and comment.
B.
The applicant must demonstrate that the following BMPs are being
utilized to the maximum extent practicable to receive consideration
for an exemption:
(1)
Design around and limit disturbance of floodplains, wetlands, natural
slopes over 15%, existing native vegetation, and other sensitive and
special value features.
(2)
Maintain riparian and forested buffers.
(3)
Limit grading and maintain nonerosive flow conditions in natural
flow paths.
(4)
Maintain existing tree canopies near impervious areas.
(5)
Minimize soil disturbance and reclaim disturbed areas with topsoil
and vegetation.
(6)
Direct runoff to pervious areas.
C.
The applicant must demonstrate that the proposed development/additional
impervious area will not adversely impact the following:
D.
An applicant proposing regulated activities may be eligible for exemption
from rate control, volume control, or SWM site plan requirements in
this chapter according to the following table:
|
Table 17.1
| |
|---|---|
|
Exemptions and Submission Requirements
| |
|
New Impervious Area
(square feet)
|
Applicant Must Provide
|
|
0 to <5,000
|
Documentation of new impervious surfaces
|
|
5,000 and greater
|
Rate controls, volume controls and SWM site plan
|
|
NOTES:
| ||
|---|---|---|
|
1.
|
New impervious area since the date of adoption of this chapter.
| |
|
2.
|
Gravel in existing condition shall be considered pervious and
gravel in proposed condition shall be considered impervious.
| |
E.
Single-family residential activities are exempt from these requirements,
provided that the construction:
(2)
Driveways.
(a)
Runoff must discharge onto pervious surface with a gravel strip
or other spreading device.
(b)
No more than 1,000 square feet of paved surface may discharge
to any one point as a concentrated discharge.
(c)
The length of flow on the pervious surface must exceed the length
of flow on the paved surface.
(3)
The municipality can require more information or require mitigation
of certain impacts through installation of stormwater management BMPs
if there is a threat to property, health, or safety.
F.
An applicant proposing regulated activities, after demonstrating compliance with Subsections A, B and C of this section, may be exempted from various requirements of this chapter if documentation can be provided that a downstream man-made water body (i.e., reservoir, lake, or man-made wetlands) has been designed or modified to address the potential stormwater flooding impacts of the proposed development.
G.
The purpose of this section is to ensure consistency of stormwater
management planning between local ordinances and NPDES permitting
(when required) and to ensure that the applicant has a single and
clear set of stormwater management standards to which the applicant
is subject. The municipality may accept alternative stormwater management
controls under this section, provided that:
(1)
The municipality, in consultation with the PA DEP (or delegated authority), determines that meeting the volume control requirements (see § 252-18) is not possible or places an undue hardship on the applicant.
(2)
The alternative controls are documented to be acceptable to the PA
DEP (or delegated authority) for NPDES requirements pertaining to
post-construction stormwater management requirements.
H.
Agricultural activities are exempt from the rate control and SWM
site plan preparation requirements of this chapter, provided that
the activities are performed according to the requirements of 25 Pa.
Code Chapter 102.
I.
Forest management and timber operations are exempt from the rate
and volume control requirement and SWM site plan preparation requirement
of this chapter, provided that the activities are performed according
to the requirements of 25 Pa. Code Chapter 102.
J.
The municipality may deny or revoke any exemption pursuant to this
section at any time for any project that the municipality believes
may pose a threat to public health, safety, property or the environment.
A.
The low-impact development practices provided in the Pennsylvania
SWM BMP Manual shall be utilized for all regulated activities to the
maximum extent practicable.
B.
Stormwater runoff volume controls shall be implemented using the
Design Storm Method or the Simplified Method, as defined below. For
regulated activity areas equal or less than one acre that do not require
hydrologic routing to design the stormwater facilities, this chapter
establishes no preference for either method; therefore, the applicant
may select either method on the basis of economic considerations,
the intrinsic limitations on applicability of the analytical procedures
associated with each methodology, and other factors.
(1)
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size regulated activity. This method requires detailed modeling
based on site conditions.
(a)
Do not increase the post-development total runoff volume when
compared to the predevelopment total runoff volume for the two-year/twenty-four-hour
storm event.
(b)
For hydrologic modeling purposes:
[1]
Existing nonforested pervious areas must be considered meadow
(good condition) for predevelopment hydrologic calculations.
[2]
Twenty percent of existing impervious area, when present within
the proposed project site, shall be considered meadow (good condition)
for predevelopment hydrologic calculations for redevelopment.
(2)
The Simplified Method (CG-2 in the BMP Manual) is independent of
site conditions and should be used if the Design Storm Method is not
followed. This method is not applicable to regulated activities greater
than one acre or for projects that require detailed design of stormwater
storage facilities. For new impervious surfaces:
(a)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(b)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into surface waters of the commonwealth. Removal options
include reuse, evaporation, transpiration, and infiltration.
(c)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases, at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
(d)
Actual field infiltration tests at the location of the proposed
elevation of the stormwater BMPs are required. Infiltration tests
shall be conducted in accordance with the BMP Manual. Notification
of the municipality shall be provided to allow witnessing of the testing.
C.
The applicable worksheets from the BMP Manual must be used in calculations
to establish volume control.
A.
For areas not covered by a release rate map from an approved Act
167 Stormwater Management Plan:
(1)
Post-development discharge rates shall not exceed predevelopment
discharge rates for the two-, ten-, twenty-five-, fifty-, and one-hundred-year,
twenty-four-hour storm events. If it is shown that the peak rates
of discharge indicated by the post-development analysis are less than
or equal to the peak rates of discharge indicated by the predevelopment
analysis for two-, ten-, twenty-five-, fifty-, and one-hundred-year,
twenty-four-hour storms, then the requirements of this section have
been met. Otherwise, the applicant shall provide additional controls
as necessary to satisfy the peak rate of discharge requirement.
B.
For areas covered by a release rate map from an approved Act 167
Stormwater Management Plan:
(1)
For the two-, ten-, twenty-five-, fifty-, and one-hundred-year, twenty-four-hour
storm events, the post-development peak discharge rates will follow
the applicable approved release rate maps. For any areas not shown
on the release rate maps, the post-development discharge rates shall
not exceed the predevelopment discharge rates.
A.
Sensitive areas and water-quality-sensitive developments, as defined
below, which require special consideration with regard to stormwater
management.
(1)
"Sensitive areas" are defined as those areas that, if developed,
have the potential to endanger a water supply. These areas consist
of the delineated one-year zone of contribution and direct upslope
areas tributary to the water supply wells. The City of Lower Burrell
may update the sensitive area boundaries based on new research or
studies as required.
(2)
"Stormwater hotspots" are defined as a land development project that
has a high potential to endanger local water quality and could potentially
threaten groundwater reservoirs. The Municipal Engineer will determine
what constitutes these classifications on a case-by-case basis. The
PA DEP wellhead protection contaminant source list shall be used as
a guide in these determinations. Industrial manufacturing site and
hazardous material storage areas must provide NPDES SIC codes.
B.
Performance standards.
(1)
The location of the boundaries of sensitive areas is set by drainage
areas tributary to any public water supply. The exact location of
these boundaries, as they apply to a given development site, shall
be determined using mapping at a scale which accurately defines the
limits of the sensitive area. If the project site is within the sensitive
area (in whole or in part), two-foot contour interval mapping shall
be provided to define the limits of the sensitive area. If the project
site is adjacent to but within 500 linear feet of a defined sensitive
area, a five-foot contour interval map defining the limits of the
sensitive area shall be included in the stormwater management plan
to document the site's location relative to the sensitive area.
(2)
Stormwater hotspots may be required to prepare and implement a stormwater
pollution prevention plan and file notice of intent as required under
the provision of the EPA industrial stormwater NPDES permit requirements.
(3)
Stormwater hotspots must use an acceptable pretreatment BMP prior
to volume control and/or rate control BMPs. Acceptable pretreatment
BMPs for these developments include those based on filtering, settling,
or chemical reaction processes such as coagulation.
For any regulated activity within a protected watershed (high-quality
or exceptional-value), the applicant shall meet requirements as contained
in 25 Pa. Code Chapter 93 as required and applicable.
A.
General design guidelines.
(1)
Stormwater shall not be transferred from one watershed to another,
unless:
(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 the PA DEP, provided that 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 Pa. Code 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)
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.
(2)
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 the slope of all streets in or adjacent
to the plan. Where driveways cross curb- or gutter lines, a pipe below
the driveway entrance of the type and size required by the City of
Lower Burrell Engineer shall be required.
(3)
In designing the stormwater management facility, the developer shall
use as his guide the publication "Urban Hydrology for Small Watersheds,"
Technical Release No. 55, United States Department of Agriculture,
Soil Conservation Service, June 1986, and as amended.[1]
(a)
The predevelopment condition shall be as defined in § 252-15, Definitions, regarding "existing condition." If the property has been partially or completely developed at the time of purchase, the City Council, at its option, may 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. 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 TR-55.) The developer may pass through stormwater originating upstream of his property, but if he elects to control it, bypass devices he installs shall be capable of handling the one-hundred-year, twenty-four-hour peak flow. If a property is to be laid out in lots, no reduction in lot size shall be permitted to accommodate storm drainage detention structures.
(b)
The developer shall consult with the Westmoreland County Conservation
District before developing his/her stormwater management plans. The
municipality and others reviewing a stormwater control plan will use
discretion in considering unusual conditions such as drainage from
several basins on the same property under development.
(4)
The stormwater management 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.
(a)
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.
(b)
Vertical riser pipes with trash racks and anti-vortex devices
shall be provided in detention basins, unless an alternate design
is approved by the municipality.
(c)
No outlet structure from a stormwater management facility, or
swale, shall discharge directly onto a municipal or state roadway
without approval from the municipality or PennDOT.
(5)
Detention areas, where required to impede runoff, shall be designed
to meet or exceed the following standards:
(a)
Except where a permanent pond is approved by the municipality,
detention areas shall be designed to drain completely.
(b)
The height of the embankment shall be not more than 10 feet
measured between the top of the embankment and the toe of the slope
on the downstream side of the embankment. The level of water within
the impoundment shall not exceed nine feet measured between the lowest
point in the impoundment area behind the embankment and the top of
the emergency spillway.
(c)
The area to be occupied by the embankment shall be cleared of
all topsoil and organic materials prior to construction. The embankment
shall be built up in layers not to exceed six inches in depth with
equipment providing 95% compaction at optimum moisture conditions.
(d)
The settled elevation of the top of the embankment at its lowest
point shall be not less than one foot above the maximum water level
to be impounded behind the embankment and not less than two feet above
the elevation of the emergency spillway.
(e)
The surfaces of the embankment shall be planted in a mixture
of perennial quick-catching grasses.
(f)
The interior and exterior side slopes of the embankment shall
not slope on either side less than one foot vertical rise for each
two feet of horizontal run and not more than a total on both sides
of one foot vertical rise for each five feet of horizontal run.
(g)
The horizontal drain pipe passing through the embankment shall
be reinforced concrete pipe or equivalent. Anti-seepage collars shall
be placed in accordance with proper design procedures. The design
and spacing of the collars shall be submitted for review.
(h)
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 six inches in diameter and shall be fitted
with an anti-vortex device and a trash rack.
(i)
The emergency spillway shall be capable of passing the flow
created by the unattenuated one-hundred-year storm.
(j)
The low edge of a parking area, curbed or not, may serve the
purpose of an emergency spillway to pass the overflow from a stormwater
detention area, provided that the embankment below the edge is well
stabilized with planting materials and the angle of the slope will
not encourage erosion, in the opinion of the municipality and other
reviewing agencies. Parking areas shall not be used to store stormwater.
(k)
Unless specifically designed as a volume control facility, stone
low-flow channels with underdrains shall be installed in all aboveground
earthen detention facilities. Side slopes and channel slopes within
the basin shall be 2% minimum.
(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.
Flows from underdrains need not be accounted for in volume or rate
control calculations.
(7)
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. Facilities designed as water
quality/infiltration BMPs may have a bottom slope of zero.
(8)
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.
(9)
Written design reports and calculations shall be submitted for review
and approval by the municipality. 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 ten-year storm unless conditions warrant
otherwise using PennDOT intensity-duration-frequency curves from the
Engineering Design Manual (latest edition).
(10)
The owner or developer shall enter into a legal agreement approved
by the Municipal Solicitor which shall hold the municipality 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.
(11)
The costs of review and inspection by the Municipal Engineer
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 Engineer shall review any changes as the developer may make in
his plan for compliance with the Engineer's recommendations and shall
advise the City Council whether the revised plan is in compliance
or not. The Council shall direct the Engineer to inspect an approved
embankment or any other proposed design element during its various
stages of construction and to point out to the contractor, the developer
and Council any deviations from the design as approved.
(12)
Catch basins shall meet the municipal standards and shall have
inverts poured to allow self-cleaning. Catch basins shall be located
no further apart than 300 feet, measured between catch basins on the
same side of the street.
(13)
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.
(14)
Grades for embankments shall not exceed one foot vertical rise
for every two feet of horizontal run for fill slopes or one foot vertical
rise for every 1 1/2 feet horizontal run for slopes created by
excavating in areas that have lain dormant for at least two years.
Steeper slopes may be permitted in areas where, in the opinion of
a professional engineer, as evidenced in a written report, concurred
with by the Municipal Engineer, conditions are such as to allow slopes
up to a maximum grade determined by the engineers.
(15)
No grading shall occur within five feet of any subdivision or
development plan boundary, except as is needed for the entrance of
streets or to grade off land immediately adjacent to a street to the
street's elevation.
(16)
All persons, partnerships or corporations intending to excavate,
fill or grade land in the municipality shall be required to undertake
all work in conformance with the requirements of this chapter and
applicable regulations of Westmoreland County.
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.
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)
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 Natural Resources Conservation Service
Type II, twenty-four-hour rainfall distribution shall be used in conjunction
with rainfall depths from NOAA Atlas 14. A copy of the rainfall data
from NOAA Atlas 14 shall accompany the SWM report for review.
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. 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.
(c)
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 B-1 (Appendix B).[1]
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
(2)
NRCS Rainfall-Runoff Method. Use Table B-2 (Appendix B). 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.
Design storm.
(1)
All stormwater management facilities shall be verified by routing
the proposed 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 (culverts or bridges) proposed to convey runoff under
a municipal road shall be designed to pass the twenty-five-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
| |
|---|---|
|
Tc = (T\0.6) * 60 (minutes)
| |
|
T = l0.8 (S + 1)0.7\1900Y5
|
|
Where:
| ||||
|
T
|
=
|
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 than 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.
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 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.
J.
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.
