[Ord. 2010-10, 10/4/2010, § 301]
1.
Applicants proposing regulated activities in the Township which do not fall under the exemption criteria in § 23-402 shall submit a drainage plan consistent with the applicable Act 167 Plan and this chapter to the Township for review and approval. These exemption criteria shall apply to the total proposed development even if development is to take place in stages.
2.
The applicant is required to perform and submit an alternatives analysis
to find practicable alternatives to the surface discharge of stormwater,
the creation of impervious surfaces and the degradation of waters
of the commonwealth, and must maintain as much as possible the natural
hydrologic regime.
3.
The applicant shall design the drainage plan through an alternatives analysis consistent with the sequencing provisions of § 23-302 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan design professional must proceed sequentially in accordance with Part 3 of this chapter and present evidence demonstrating that he has followed the sequence with the drainage plan.
4.
The applicant shall provide Stormwater drainage systems in order
to permit unimpeded flow along natural watercourses, except as modified
by stormwater management facilities or open channels consistent with
this chapter.
5.
No person shall alter existing points of concentrated drainage that
discharge onto adjacent property in any manner which could cause property
damage without permission of the affected property owner. All alteration
of existing points of discharge shall be subject to any applicable
discharge criteria specified in this chapter.
6.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If diffused
drainage discharge is proposed to be concentrated and discharged onto
adjacent property, the applicant shall document that adequate downstream
conveyance facilities exist to safely transport the concentrated discharge,
or otherwise prove that no erosion, sedimentation, flooding or other
impacts will result from the concentrated discharge.
7.
Where a development site is traversed by existing watercourses, drainage easements (for natural and artificial watercourse) shall be provided conforming to the line of such watercourses. The terms of the easement shall conform to the stream buffer requirements (for natural watercourses) contained in § 23-303, Subsection I(3), of this chapter.
8.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to Waters of the commonwealth or wetlands
shall be subject to approval by DEP through the joint permit application
process, or, where deemed appropriate by DEP, the general permit process.
When there is a question whether wetlands may be involved, it is the
responsibility of the applicant to show that the land in question
cannot be classified as wetlands, otherwise approval to work in the
area must be obtained from DEP.
9.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way shall be subject to
approval by PennDOT.
10.
Infiltration of runoff through seepage beds, infiltration trenches, etc., where soil conditions permit, and minimization of impervious surfaces to the extent permitted by the Zoning Ordinance [Chapter 27] are encouraged to reduce the size or eliminate the need for, detention facilities or other structural BMPs.
11.
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 advantageous to do so. Considering potential pollutant
loading, roof drain runoff in most cases will not require pretreatment.
[Ord. 2010-10, 10/4/2010, § 302]
1.
The design of all regulated activities shall include the following
steps in sequence to minimize stormwater impacts:
A.
The applicant is required to find practicable alternatives to the
surface discharge of stormwater and the creation of impervious surfaces
and the degradation of waters of the commonwealth, and the applicant
shall maintain as much as possible the natural hydrologic regime of
the development site.
B.
An alternative is practicable if it is available and capable of being
done after taking into consideration cost, existing technology and
logistics in light of overall project purposes.
C.
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
2.
The applicant shall demonstrate that it designed the regulated activities
in the following sequence to minimize the increases in stormwater
runoff and impacts to water quality:
A.
Prepare an existing resource and site analysis map (ERSAM), showing
environmentally sensitive areas including, but not limited to, steep
slopes, ponds, lakes, streams, wetlands, hydric soils, vernal ponds,
floodplains, buffer areas, Hydrologic Soil Groups A and B (areas conducive
to infiltration), any existing recharge areas and any other requirements
outlined in the SALDO.
C.
Prepare a draft project layout avoiding earth disturbance in sensitive
areas identified in Subsection 2A and minimizing total site earth
disturbance as much as possible. The ratio of the disturbed area to
the entire property area and measures taken to minimize earth disturbance
shall be included on the ERSAM.
D.
Identify development site specific predevelopment drainage areas,
discharge points, recharge areas to be preserved and Hydrologic Soil
Groups A and B to be utilized for recharge.
G.
Satisfy groundwater recharge (infiltration) objective (§ 23-304) and provide for stormwater treatment prior to infiltration.
I.
Prepare final project design to maintain predevelopment drainage
areas and discharge points, to minimize earth disturbance and impervious
surfaces, and to reduce runoff to the maximum extent possible.
J.
Conduct a post development runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 23-305).
K.
Manage any remaining stormwater runoff through treatment prior to
discharge, as part of detention, bioretention, direct discharge or
other structural control.
[Ord. 2010-10, 10/4/2010, § 303]
1.
In addition to the performance standards and design criteria requirements
of this chapter, the applicant shall comply with the following water
quality requirements of this Part:
A.
For water quality and streambank erosion, the applicant shall design
a water quality BMP to detain the proposed conditions two-year, twenty-four-hour
design storm to the existing conditions one-year flow using the SCS
Type II distribution. The applicant shall demonstrate that the applicant
has made provisions (such as adding a small orifice at the bottom
of the outlet structure) so that the one-year storm takes a minimum
of 24 hours to drain from the facility from a point where the maximum
volume of water from the one-year storm is captured. (i.e., the maximum
water surface elevation is achieved in the facility). At the same
time, the applicant shall not attenuate the larger storms in "no detention"
areas (District C) only. This can be accomplished by configuration
of the outlet structure not to control the larger storms, or by a
bypass or channel to divert only the two-year flood into the basin
or divert flows in excess of the two-year storm away from the basin.
Where practicable, wet basins shall be utilized for water quality
control and shall meet the requirements found in the Pennsylvania
Stormwater BMP manual as revised. Release of water can begin at the
start of the storm (i.e., the invert of the water quality orifice
is at the invert of the facility). The design of the facility shall
consider and minimize the chances of clogging and sedimentation. Orifices
smaller than three inches diameter are not recommended. However, if
the design professional can provide proof that the smaller orifices
are protected from clogging by use of trash racks, etc., smaller orifices
may be permitted.
B.
In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
(1)
Total contributing area.
(2)
Permeability and infiltration rate of the development site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high water table.
(5)
Proximity to building foundations and well heads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream bank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
C.
The applicant shall maintain or improve the temperature and quality
of water, wetlands and streams through the use of temperature sensitive
BMPs and stormwater conveyance systems.
F.
Streambank restoration projects shall include the following:
(1)
No restoration or stabilization projects may be undertaken without
examining the fluvial geomorphology of stable reaches above and below
the unstable reach.
(2)
Restoration project design must then consider maintenance of
stability in the adjacent stable reaches of the stream channel.
(3)
An erosion and sediment control plan approved by the Conservation
District must be provided by the applicant.
(4)
All applicable state and federal permits must be obtained.
H.
To accomplish the above, the applicant shall submit original and
innovative designs to the Township Engineer for review and approval.
Such designs may achieve the water quality objectives through a combination
of BMPs (best management practices).
I.
Buffers.
(1)
Wetlands and Vernal Ponds.
(a)
Wetland Identification. Wetlands shall be identified in accord
with the 1987 U.S. Army Corps of Engineers Manual for Identifying
and Delineating Wetlands, as may be amended, and properly flagged
and surveyed on Development Site to ensure they are protected.
(b)
Wetlands in an Artificial Watercourse. Wetlands contained within
the banks of an artificial watercourse shall not be considered for
buffer delineation purposes.
(c)
Wetlands in a Natural Watercourse. Wetlands contained within
the banks of a natural watercourse, only the stream buffer shall apply.
(d)
Wetland and Vernal Pond Buffer Delineation. A one-hundred-foot
inner buffer and fifty-foot outer buffer, measured perpendicular to
and horizontally from the edge of the delineated wetland or vernal
pond for a total distance of 150 feet, shall be maintained for all
wetlands and vernal ponds.
1)
Inner Buffer. Measured perpendicular to and horizontally from
the edge of the delineated wetland or vernal pond, for a distance
of 100 feet.
a)
Permitted Activities/Development. Stormwater conveyance
approved by the Township, buffer maintenance and restoration, the
correction of hazardous conditions, disturbance activities requiring
a DEP permit in conjunction with a Township-approved project, and
passive unpaved stable trails shall be permitted. No other earth disturbance,
grading, filling, buildings, structures, new construction, or development
shall be permitted.
b)
The area of the inner buffer altered by activities
permitted in accord with item a) shall be minimized to the greatest
extent practicable.
2)
Outer Buffer. Measured perpendicular to and horizontally from
the outer edge of the inner buffer for a distance of 50 feet resulting
in a total buffer of 150 feet.
a)
Permitted Activities/Development. Stormwater conveyance and best management practices (BMPs) approved by the Township, buffer maintenance and restoration, the correction of hazardous conditions, disturbance activities requiring a DEP permit in conjunction with a Township-approved project, roads constructed within two feet of existing grade, unpaved trails, and limited forestry activities that do not clear cut the buffer (e.g., selective regeneration harvest) in accord with a forestry management plan shall be permitted provided no buildings are involved, and those activities permitted under Subsections 1I(5) and (6).
b)
No more than 20% of the cumulative outer buffer
on the subject parcel shall be altered by the activities permitted
in accord with item a).
(2)
Lakes and Ponds.
(a)
There is no outer buffer around lakes and ponds.
(b)
Lake and Pond Buffer Delineation. A one-hundred-foot buffer
measured perpendicular to and horizontally from the edge of any water
body, shall be maintained around any water body.
1)
Permitted Activities/Development. Stormwater conveyance approved
by the Township, buffer maintenance and restoration, the correction
of hazardous conditions, lake front views, boat docks and unpaved
trails shall be permitted, provided no buildings are involved.
2)
The area of the buffer impacted by activities permitted in Subsection
1I(2)(b)1) shall not exceed 35% of the cumulative buffer on the property.
(3)
Streams.
(a)
Stream Buffer Delineation. A one-hundred-foot inner buffer and
fifty-foot outer buffer, measured perpendicular to and horizontally
from the top-of-bank on both sides of any stream, for a total distance
of 150 feet, shall be maintained on both sides of any stream. See
Figure 303.1 of Brodhead-McMichaels Creek Plan.
1)
Inner Buffer. Measured perpendicular to and horizontally from
the top-of-bank of the stream for a distance of 100 feet.
a)
Permitted Activities/Development. Stormwater conveyance
approved by the Township, buffer maintenance and restoration, the
correction of hazardous conditions, disturbance activities requiring
a DEP permit in conjunction with a Township-approved project, fish
hatcheries, wildlife sanctuaries and boat launch sites constructed
so as not to alter the floodplain cross section, and unpaved trails
shall be permitted, provided no buildings are involved. No other earth
disturbance, grading, filling, buildings, structures, new construction,
or development shall be permitted.
b)
The area of the inner buffer altered by activities
permitted in accord with item a) shall be minimized to the greatest
extent practicable.
2)
Outer Buffer. Measured perpendicular to and horizontally from
the outer edge of the inner buffer for a distance of 50 feet resulting
in a total buffer of 150 feet.
a)
Permitted Activities/Development. Stormwater conveyance
and best management practices (BMPs) approved by the Township, buffer
maintenance and restoration, the correction of hazardous conditions,
agricultural activities, plant nurseries, parking lots constructed
to existing grade, temporary fairs and carnivals, accessory uses for
residential purposes, private sportsmen's club activities, athletic
facilities, orchards, wildlife sanctuaries, boat launch sites, roads
constructed within two feet of existing grade, road and utility maintenance
and restoration, disturbance activities requiring a DEP permit in
conjunction with a Township-approved project, and unpaved trails and
limited forestry activities that do not clear cut the buffer (i.e.,
select cut timber harvest) in accord with a forestry management plan
shall be permitted, provided no buildings are involved.
b)
In areas of the outer buffer which are not wetlands,
vernal ponds or slopes of 30% or greater, stormwater management facilities
which improve water quality of stormwater discharge shall be permitted
unless prohibited by other Township or state requirements. No other
earth disturbance, grading, filling buildings, structures, new construction,
or development shall be permitted.
c)
No more than 20% of the cumulative outer buffer
on the property shall be altered by the activities permitted in accord
with item a).
(4)
Where resource buffers overlap, the more restrictive requirements
shall apply.
(5)
Preexisting Lots or Parcels/Development in Outer Buffers. In the case of legally preexisting lots (approved prior to the effective date of this chapter) where the useable area of a lot lies within an outer buffer area, the applicant may apply for a waiver from the Board of Supervisors in accordance with Part 8. In considering a waiver application, the Board of Supervisors shall determine whether the applicant has presented evidence of hardship in addition to the standards for waivers set forth in Part 8.
(6)
Improvements to Existing Structures in Outer Buffers. The provisions of this paragraph do not require any changes or improvements to be made to lawfully existing structures in buffers. However, when any improvement to an existing structure is proposed which results in a 10% or greater horizontal expansion of that structure, the improvement may only be permitted by the granting of a waiver from the requirements of this chapter. In considering a waiver application, the Board of Supervisors shall determine whether the applicant has presented evidence of hardship in addition to the standards for waivers set forth in Part 8. Improvements to an existing structure resulting in less than 10% horizontal expansion of that structure are permitted in outer buffers and do not require a waiver from the Board of Supervisors provided all other provisions of this chapter are met.
[Ord. 2010-10, 10/4/2010, § 304]
1.
Maximizing the ground water recharge capacity of the development site being developed is required. The applicant shall demonstrate that the design of the infiltration/recharge stormwater management facilities has provided for, to the maximum extent feasible, ground water recharge to compensate for the reduction in the percolation that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 23-102, and take advantage of utilizing any existing recharge areas.
A.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Maximum Infiltration Requirements. The applicant shall demonstrate that all regulated activities shall recharge (infiltrate), where practicable, a portion of the runoff created by the development as part of an overall drainage plan. The volume of runoff to be recharged shall be determined from Subsection 1A(2)(a) or (b), depending upon demonstrated development site conditions.
(2)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
(b)
An infiltration and/or percolation rate sufficient to accept
the additional stormwater load and drain completely as determined
by field tests conducted by the applicant's design professional.
(c)
The recharge facility shall be capable of completely infiltrating
the recharge volume within four days.
(3)
The size of the recharge facility shall be based upon the following
volume criteria:
(a)
NRCS Curve Number Equation. The NRCS runoff shall be utilized
to calculate infiltration requirements (P) in inches.
For zero runoff: P = I (Infiltration) (in.) = (200/CN) - 2 Eqn:
304.1
Where: CN=SCS (NRCS) curve number of existing conditions contributing
to the recharge facility.
This equation is displayed graphically in, and the infiltration
requirement can be determined from, Figure 23-304.1.
The recharge volume required would therefore be computed as:
Rev (cubic feet) = I x impervious area
(square feet)/12 Eqn: 304.2
Where: I = infiltration requirements (in.)
 Figure 23-304.1 Infiltration Requirement Based upon NRCS
Curve Number
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(b)
Annual Recharge — Water Budget Approach. It has been determined that infiltrating 0.6 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of § 23-304, Subsection 1A(2)(a), cannot be achieved, then 0.6 inches of rainfall shall be infiltrated from all impervious areas, up to an existing site condition curve number of 77. Above a curve number of 77, Equation 304.1 or the curve in Figure 23-304.1 should be used to determine the infiltration requirement.
The recharge volume (Rev) required would
therefore be computed as: Rev = 0.6 or I, whichever
is less, x impervious area.
B.
Soils. The applicant shall submit a detailed soils evaluation of
the development site to determine the suitability of recharge facilities.
The evaluation shall be performed by a design professional, and at
a minimum, address soil permeability, depth to bedrock and subgrade
stability. The general process for designing the infiltration BMP
shall be:
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the watershed to determine general areas of suitability
for infiltration practices.
(2)
Provide site-specific infiltration test results (at the level
of the proposed infiltration surface) in accord with ASTM Guide No.
D 5126, or other method approved by the Township Engineer, to determine
the appropriate hydraulic conductivity rate.
(3)
Design the infiltration structure for the required storm volume
based on field determined capacity at the level of the proposed infiltration
surface.
(4)
If on-lot infiltration structures are proposed by the design
professional, the applicant shall demonstrate to the Township that
the soils are conducive to infiltrate on the lots identified.
C.
Stormwater Hotspots. A stormwater hotspot is defined as a land use
activity that generates higher concentrations of hydrocarbons, trace
metals or toxicants than are found in typical stormwater runoff, based
on monitoring studies. Table 23-304.1 provides samples of designated
hotspots. If a development site is designated as a hotspot, it has
important implications for how stormwater is managed. First and foremost,
untreated stormwater runoff from hotspots shall not be allowed to
infiltrate into groundwater where it may contaminate water supplies.
Therefore, the Rev requirement is NOT applied
to development sites that fit into the hotspot category (the entire
WQv must still be treated). Second, a greater
level of stormwater treatment may be needed at hotspot sites to prevent
pollutant washoff after construction. EPA's NPDES stormwater
program requires some industrial sites to prepare and implement a
stormwater pollution prevention plan.
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Table 23-304.1-Classification of Stormwater Hotspots
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The following land uses and activities are samples of stormwater
hotspots:
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•
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Vehicle salvage yards and recycling facilities
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•
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Fleet storage areas (bus, truck, etc.)
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•
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Public works storage areas
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•
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Facilities that generate or store hazardous materials
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The applicant shall exercise extreme caution where salt or chloride
would be a pollutant since soils do little to filter this pollutant
and it may contaminate the groundwater. The design professional shall
evaluate the possibility of groundwater contamination from the proposed
infiltration/recharge facility and perform a hydrogeologic justification
study if necessary. The infiltration requirement in high quality/exceptional
value waters shall be subject to DEP 25 Pa. Code, Chapter 93, Antidegradation
Regulations. The Township may require the installation of an impermeable
liner in detention basins where the possibility of groundwater contamination
exists. The Township may require a detailed hydrogeologic investigation.
The applicant shall provide safeguards against groundwater contamination
for uses which may cause groundwater contamination, should there be
a mishap or spill. (See Appendix 23-E.)[1]
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[1]
Editor's Note: Appendix E is included as an attachment to
this chapter.
D.
The applicant shall demonstrate that extreme caution has been exercised
where infiltration is proposed in source water protection areas or
that may affect a wellhead or surface water intake.
E.
Recharge/infiltration facilities shall be used in conjunction with
other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
[Ord. 2010-10, 10/4/2010, § 305]
1.
The Brodhead-McMichaels Watershed has been divided into stormwater
management districts as shown on the Watershed Map in Appendix 23-D.[1]
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Standards for managing runoff from each subarea in the Brodhead-McMichaels
Watershed for design storms are shown in Table 23-305.1. Development
sites located in each of the A, B, or C Districts must control proposed
conditions runoff rates to existing conditions runoff rates for the
design storms in accord with Table 23-305.1.
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Areas located outside of the Brodhead-McMichaels Creek Watershed
are subject to the same requirements as if they were located in Stormwater
Management District A of Appendix 23-D, Stormwater Management District
Watershed Map.[2]
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Table 23-305.1-Water Quantity Requirements
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District
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Proposed conditions
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(reduce to)
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Existing conditions
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A
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2-year
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1-year
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5-year
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5-year
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10-year
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10-year
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25-year
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25-year
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50-year
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50-year
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100-year
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100-year
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B-1
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2-year
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1-year
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5-year
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2-year
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10-year
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5-year
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25-year
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10-year
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50-year
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25-year
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100-year
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100-year
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B-2
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2-year
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1-year
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5-year
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2-year
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25-year
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5-year
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50-year
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10-year
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100-year
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50-year
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B-3
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50-year
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10-year
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100-year
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50-year
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Due to the effect of the Leavitt Branch Dam, detention of storms
less than the 50-year are not required in this district.
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C
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Provisional Direct Discharge District. Development sites which can discharge directly to the main channel or major tributaries or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) which meets the "Downstream Hydraulic Capacity Analysis" in Subsection 8 and is shown by the design professional to not cause a downstream problem, may allow an increase in flow as long as no downstream harm is demonstrated. However, sites in District C shall comply with the criteria for water quality and streambank erosion (§ 23-303); and groundwater recharge (§ 23-304). If the proposed conditions runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the increased peak flows or will be provided with improvements to furnish the required capacity. When adequate capacity of the downstream system does not exist and will not be provided through improvements, the proposed conditions peak rate of runoff must be controlled to the existing conditions peak rate as required in District A provisions (i.e., 10-year proposed conditions flows to 10-year existing conditions flows) for the specified design storms.
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2.
General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff prior to development
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix 23-D)[3] and § 23-302 of this chapter.
[3]
Editor's Note: Appendix D is included as an attachment to
this chapter.
3.
District Boundaries. The boundaries of the Stormwater Management
Districts are shown on an Official Map that is available for inspection
at the Township office. A copy of the Official Map at a reduced scale
is included in Appendix 23-D. The exact location of the stormwater
management district boundaries as they apply to a given development
site shall be determined by mapping the boundaries using the two-foot
topographic contours (or most accurate data required) provided as
part of the drainage plan.
4.
Sites Located in More than One District. If a development site is located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall be the existing conditions peak discharge for that subarea as indicated in § 23-302. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. The Board of Supervisors may grant a waiver if discharges from multiple subareas recombine in proximity to the development site. In this case, peak discharge in any direction may be a 100% release rate provided that the overall site discharge meets the weighted average release rate.
5.
Off-Site Areas. Off-site areas that drain through a development site
are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
6.
Site Areas. Where the development site area differs significantly
from the total property area, only the proposed impact area utilizing
stormwater management measures shall be subject to the management
district criteria. In other words, unimpacted areas bypassing the
stormwater management facilities would not be subject to the management
district criteria.
7.
"No Harm" Option. For any proposed development site not located in a provisional direct discharge district, the applicant has the option of using a less restrictive runoff control (including no detention) if the applicant can prove that "no harm" would be caused by discharging at a higher runoff rate than that specified by the applicable Act 167 Plan. The "no harm" option is used when an applicant can prove that the proposed hydrographs can match existing hydrographs, or if it can be proved that the proposed conditions will not cause increases in peaks at all points downstream. Proof of "no harm" must be shown based upon the following "Downstream Impact Evaluation" which shall include a "downstream hydraulic capacity analysis" consistent with Subsection 8 to determine if adequate hydraulic capacity exists. The applicant shall submit this evaluation of the impacts due to increased downstream stormwater flows in the watershed to the Township.
A.
The hydrologic regime of the site must be maintained.
B.
The "downstream impact evaluation" shall include hydrologic and hydraulic
calculations necessary to determine the impact of hydrograph timing
modifications due to the proposed development upon a dam, highway,
structure, natural point of restricted streamflow or any stream channel
section, established with the concurrence of the Township.
C.
The evaluation shall continue downstream until the increase in flow
diminishes due to additional flow from tributaries and/or stream attenuation.
D.
The peak flow values to be used for downstream areas for the design
return period storms (two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year) shall be the values from the calibrated model for
the applicable Act 167 Plan.
E.
Applicant-proposed conditions runoff controls which would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove "no-harm," except in conjunction with proposed capacity improvements for the problem areas consistent with Subsection 8.
F.
A financial distress shall not constitute grounds for the Township
to approve the use of the "no-harm" option.
G.
Downstream capacity improvements may be provided as necessary to
achieve the "no harm" option.
H.
Any "no harm" justifications shall be submitted by the applicant
as part of the drainage plan submission per Part 4.
8.
Downstream Hydraulic Capacity Analysis. Any downstream hydraulic
capacity analysis conducted in accordance with this chapter shall
use the following criteria for determining adequacy for accepting
increased peak flow rates:
A.
Existing natural or man-made channels or swales must be able to convey
the increased runoff associated with a two-year return period event
within their banks at velocities consistent with protection of the
channels from erosion. Acceptable velocities shall be based upon criteria
included in the DEP Erosion and Sediment Pollution Control Program
Manual.
B.
Existing natural or man-made channels or swales must be able to convey
increased twenty-five-year return period runoff without creating any
hazard to persons or property.
C.
Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area must be designed in accordance
with 25 Pa. Code, Chapter 105 (if applicable) and, at minimum, pass
the increased twenty-five-year return period runoff.
9.
Hardship Option. The applicable Act 167 Plan and its standards and
criteria are designed to maintain existing conditions peak flows and
volumes throughout the watershed as the watershed becomes developed.
There may be certain instances, however, where the standards and criteria
established are too restrictive for a particular applicant. The existing
drainage network in some areas may be capable of safely transporting
slight increases in flows without causing a problem or increasing
flows elsewhere. If an applicant cannot meet the stormwater standards
due to lot conditions or if conformance would become a hardship to
an applicant, the hardship option may be applied. A financial distress
shall not constitute grounds for the Township to approve the use of
the hardship option. The applicant would have to plead his/her case
to the Board of Supervisors with the final determination made by the
Township. Any applicant's pleading the "hardship option" will
assume all liabilities that may arise due to exercising this option.
A financial distress shall not constitute grounds for the Township
to approve the use of the "no-harm" option.
[Ord. 2010-10, 10/4/2010, § 306]
1.
Stormwater runoff from all development sites shall be calculated
using a generally accepted calculation technique that is based on
the NRCS soil cover complex method. Table 23-306-1 summarizes acceptable
computation methods and the method selected by the design professional
shall be based on the individual limitations and suitability of each
method for a particular development site. The Township may allow the
use of the rational method after consultation with the Township Engineer
to estimate peak discharges from drainage areas on a case by case
basis.
|
Table 23-306-1 Acceptable Computation Methodologies For
Stormwater Management Plans
| ||
|---|---|---|
|
Method
|
Method Developed By
|
Applicability
|
|
TR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55
|
USDA NRCS
|
Applicable for land development plans with limitations described
in TR55
|
|
HEC-1/HEC-HMS
|
US Army Corps of Engineer
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
|
Rational Method
|
Emil Kuichling
|
As approved by the Township and/or Township Engineer
|
|
Other Methods Varies
|
Other computation methodologies approved by the Township and/or
Township Engineer
| |
2.
All calculations consistent with this chapter using the soil cover
complex method shall use the appropriate design rainfall depths for
the various return period storms according to the region in which
they are located as presented in Table B-1 in Appendix 23-B.[1] If a hydrologic computer model such as PSRM or HEC-1 is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The SCS 'S' curve shown in Figure B-1
of Appendix 23-B shall be used for the rainfall distribution.
[1]
Editor's Note: Appendix B is included as an attachment to
this chapter.
3.
For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
Rational 'C' value, as listed in Table B-2 or B-3 in Appendix
B.
4.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration for overland flow
and return periods from the Design Storm Curves from the PennDOT Design
Rainfall Curves (1986) (Figures B-2 to B-4). Times-of-concentration
for overland flow shall be calculated using the methodology presented
in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55
(as amended or replaced from time to time by NRCS). Times-of-concentration
for channel and pipe flow shall be computed using Manning's equation.
5.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the soil cover complex method shall be obtained from
Table B-2 in Appendix 23-B.
6.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table B-3 in
Appendix 23-B.
7.
The design professional shall consider that the runoff from the development
sites graded to the subsoil will not have the same runoff conditions
as the site under existing conditions, even after topsoiling or seeding.
The design professional may adjust his proposed condition "CN" or
"c" to better reflect proposed soil conditions.
8.
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's
roughness coefficient (n) shall be consistent with Table B-4 in Appendix
23-B.
9.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
10.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. The design storm hydrograph shall be computed using a generally
accepted calculation technique that is based on the NRCS Soil Cover
Complex Method. The Township 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.
[Ord. 2010-10, 10/4/2010, § 307]
1.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by PennDOT.
4.
Any stormwater management facility (i.e., BMP, detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year proposed
conditions. The height of embankment must provide a minimum 1.0 foot
of freeboard above the maximum pool elevation computed when the facility
functions for the one-hundred-year proposed conditions inflow. Should
any stormwater management facility require a dam safety permit under
25 Pa. Code, Chapter 105, the facility shall be designed in accordance
with 25 Pa. Code, Chapter 105, and meet the regulations of 25 Pa.
Code, Chapter 105, concerning dam safety which may be required to
pass storms larger than the one-hundred-year event.
5.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), and any work involving wetlands
governed by 25 Pa. Code, Chapter 105, shall be designed in accordance
with 25 Pa. Code, Chapter 105, and will require a permit from DEP.
6.
Any other drainage conveyance facility that does not fall under 25
Pa. Code, Chapter 105, must be able to convey at a minimum, without
damage to the drainage structure or roadway, runoff from the twenty-five-year
design storm with a minimum 1.0 foot of freeboard measured below the
lowest point along the top of the roadway. Drainage conveyance systems
may be required to convey larger design storms based on individual
drainage/sub-drainage area characteristics as recommended by the Township
Engineer and approved at the discretion of the Board of Supervisors.
Any facility that constitutes a dam as defined in 25 Pa. Code, Chapter
105, may require a permit under dam safety regulations. Any facility
located within a PennDOT right-of-way must meet PennDOT minimum design
standards and permit submission requirements.
7.
Any drainage conveyance facility and/or channel not governed by 25
Pa. Code, Chapter 105, must be able to convey at a minimum, without
damage to the drainage structure, adjacent properties or roadway,
runoff from the twenty-five-year design storm. Drainage conveyance
systems may be required to convey larger design storms based on individual
drainage/sub-drainage area characteristics as recommended by the Township
Engineer and approved at the discretion of the Board of Supervisors.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from that structure. Roadway crossings located within designated
floodplain areas must be able to convey runoff from a one-hundred-year
design storm. Any facility located within a PennDOT right-of-way must
meet PennDOT minimum design standards and permit submission requirements.
8.
At a minimum, storm sewers must be able to convey the proposed conditions
runoff from a twenty-five-year design storm without surcharging inlets,
where appropriate. Storm sewers and other drainage conveyance systems
may be required to convey larger design storms based on individual
drainage/sub-drainage area characteristics as recommended by the Township
Engineer and approved at the discretion of the Board of Supervisors.
9.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
10.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Township reserves
the right to disapprove any design that would result in the construction
of or continuation of a stormwater problem area.
[Ord. 2010-10, 10/4/2010, § 308]
1.
Any earth disturbance must be conducted in conformance with 25 Pa.
Code, Chapter 102.
2.
Additional erosion and sediment control design standards and criteria
that must be or are recommended to be applied where infiltration BMPs
are proposed shall include the following:
A.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity.
B.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
achieved final stabilization.
