In addition to the infiltration and water quality requirements
of this chapter, peak flow from those activities resulting in increases
in impervious surface and/or regrading and compaction shall be attenuated
consistent with the following stormwater calculation methods:
A. The following design storms shall be used for analysis of the predevelopment
and post-development conditions. These values are applicable to the
Soil-Cover-Complex Method:
Return Period
(years)
|
24-Hour Storm
(inches)
|
---|
1
|
2.00
|
2
|
2.35
|
5
|
2.88
|
10
|
3.30
|
25
|
3.91
|
50
|
4.40
|
100
|
4.92
|
NOTES: The precipitation values for each frequency storm listed
above were abstracted from the precipitation frequency estimates developed
by the National Oceanic and Atmospheric Administration as set forth
in NOAA Atlas 14, Volume 2 (NOAA, June 2004). The NOAA data is available
from the Hydrometeorological Design Studies Center of the National
Weather Service.
|
B. The following assumptions shall be used for runoff calculations:
(1) For new development sites, the ground cover used as the predevelopment
assumption for runoff calculations shall be as follows:
(a)
Wooded sites shall use a ground cover of woodland in good condition.
Portions of a site having more than one viable tree of a DBH (diameter
at breast height; DBH is the diameter of the tree stem 4 1/2
feet above the ground) of six inches or greater per 1,500 square feet
shall be considered wooded where such trees existed within 10 years
of application. If there is evidence of logging within the ten-year
period, the logged area shall be considered as woodland in good condition.
(b)
Agricultural sites shall use a ground cover of pasture in good
condition.
(c)
All other portions of a site shall use a ground cover of meadow
in good condition.
(d)
All watershed areas contributing to the point of interest, including
off-site area, shall be considered.
(2) The runoff curve numbers listed in the table below shall be used in developing the runoff calculations for the ground covers noted in §
190-15B(1). These values are referenced from the Urban Hydrology for Small Watersheds Technical Release No. 55 (USDA, 1986). Coefficients for equivalent ground cover conditions shall be used if a runoff method other than the Soil-Cover-Complex Method is used.
|
Hydrologic Soil Group Curve Numbers
|
---|
Ground Cover
|
A
|
B
|
C
|
D
|
---|
Woodland
|
30
|
55
|
70
|
77
|
Meadow
|
30
|
58
|
71
|
78
|
Grass
|
39
|
61
|
74
|
80
|
(3) Impervious cover shall have a curve number of 98.
(4) Gravel pavement shall have a curve number of 89.
(5) Average antecedent moisture conditions, or AMC II, shall be used
(for the Soil-Cover-Complex Method only, for example, TR-55, TR-20).
(6) A Type II distribution storm (for the Soil-Cover-Complex Method only,
for example, TR-55, TR-20).
(7) For time of concentration calculations, sheet flow lengths shall
not exceed 100 feet and shallow concentrated flow lengths shall not
exceed 1,000 feet.
(8) The kinematic "n" value in the sheet flow equation shall be applied
as per the following table. (Values taken from TR-55.)
Surface
|
Value
|
---|
Impervious surfaces
|
0.011
|
Agricultural lands
|
0.17
|
Grass, lawn, or open space
|
0.24
|
Wooded areas
|
0.40
|
C. In all plans and designs for stormwater management systems and facilities
submitted to the Municipal Engineer for approval, stormwater peak
discharge and runoff shall be determined through the use of the NRCS
Soil-Cover-Complex Method, as set forth in Urban Hydrology for Small
Watersheds, Technical Release No. 55 (USDA, 1986), with specific attention
given to antecedent moisture conditions, flood routing, time of concentration,
and peak discharge specifications included therein and in Hydrology
National Engineering Handbook, Section 4 (USDA, 1985), both by the
United States Department of Agriculture, Natural Resources Conservation
Service. Note that when TR-55 is used for natural system-based approaches
and practices encouraged herein, calculations must be performed on
a detailed small subarea basis. Use of Technical Release No. 20 and
other methods listed in Table 1 is also acceptable. The design professional's
selection of a specific method shall be based on the suitability of
the method for the given project site conditions with due consideration
to the limitations of the method chosen. Table 1 herein summarizes
the computational methods available.
Table 1
|
---|
Acceptable Computation Methodologies for Stormwater Management
Plan
|
---|
Method
|
Source
|
Applicability
|
---|
TR-20 or commercial package based on TR-20
|
USDA NRCS
|
When use of full model is desirable or necessary
|
TR-55 or commercial package based on TR-55
|
USDA NRCS
|
Applicable for plans within the model's limitations
|
HEC - HMS
|
U.S. Army Corps of Engineers
|
When use of full model is desirable or necessary
|
PSRM
|
Penn State University
|
When use of full model is desirable or necessary
|
VT/PSUHM
|
Virginia Polytechnic Institute and Penn State University
|
When use of full model is desirable or necessary
|
Modified Rational Method or commercial package based on this
method
|
Emil Kuiching (1889)
|
For sites less than 20 acres
|
SWMM or commercial package based on SWMM
|
U.S. EPA
|
Most applicable in urban areas
|
Small-storm hydrology method (as included in SLAMM)
|
PV & Associates or the website www.winslamm.com
|
Calculation of runoff volume from urban and suburban areas
|
D. A modified Rational Method analysis may be used for drainage areas
smaller than two acres when permitted by the Municipal Engineer. The
term "modified Rational Method" used herein refers to a procedure
for manipulation of the basic Rational Method techniques to reflect
the fact that storms with a duration greater than the normal time
of concentration for a basin will result in a larger volume of runoff
even though the peak discharge is reduced. The methodology and model
chosen for use shall be well documented as being appropriate for use
in this region, and all relevant assumptions, methodologies, calculations
and data used shall be provided to the Municipal Engineer for review.
Information on the modified Rational Method is presented in the Recommended
Hydrologic Procedures for Computing Urban Runoff from Small Watersheds
in Pennsylvania (PADEP, 1982).
E. Rainfall intensities used for the modified Rational Method shall
be based on the precipitation frequency estimates developed by the
National Oceanic and Atmospheric Administration as set forth in NOAA
Atlas 14.
F. The Rational Method (that is, Q = CIA) shall be used for calculations
of the peak rate of runoff for the design of storm sewers and drainage
swales but not for the design of stormwater management facilities
where a full hydrograph is needed. The equation representing the Rational
Method is comprised of the following (in English units):
Q
|
=
|
Peak flow rate, cubic feet per second (cfs).
|
C
|
=
|
Runoff coefficient, dependent on land use/cover.
|
I
|
=
|
Design rainfall intensity, inches per hour.
|
A
|
=
|
Drainage area, acres.
|
G. Runoff characteristics of off-site areas that drain through a proposed
development shall be considered and be based on the existing conditions
in the off-site area.
The Pennsylvania Stormwater Best Management Practices Manual
shall serve as a guide for the design of stormwater management practices.
Additional design guidance may also be obtained from other related
sources, including the 2000 Maryland Stormwater Design Manual, Volumes
I and II (MDE, 2000), Design of Stormwater Filtering Systems (CWP,
1996), and the American Society of Civil Engineers Manual and Report
on Engineering Practice, No. 87, Urban Runoff Quality Management (ASCE,
1998), for the design of stormwater runoff quality control features
for site development. The water quality volume design measures used
herein are based partially on the methodology expressed in the Maryland
manual referenced above. Pursuant to the design options recommended
in the above documents, the following standards shall be adhered to:
A. Extended detention, water quality volume, infiltration and nonstructural
BMP credits criteria. The following sizing criteria shall be followed
at all sites required to meet the standards of this chapter:
(1) Extended detention.
(a)
The one-year twenty-four-hour design storm shall be detained
using the SCS Type II distribution. Provisions shall be made 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). 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 potential.
(b)
Detention ponds shall detain the one-year storm event and allow
it to naturally infiltrate and recharge the groundwater table. All
subsequent orifices for the two-, ten-, twenty-five- and one-hundred-year
storm events shall be placed above the maximum water surface elevation
of the one-year storm.
(c)
Flow from off-site areas must be considered as pass-through
flow if it is conveyed through the BMP and shall be modeled as "present
condition" for the one-year storm event.
(d)
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.
(e)
The length of overland flow used in time of concentration (tc)
calculations is limited to no more than 100 feet for post-development
conditions.
(f)
The models TR-55 and TR-20 (or approved equivalent) may be used
for determining peak discharge rates.
(2) Water quality volume.
(a)
Treatment of the water quality volume (WQv) of stormwater prior
to its release to receiving waters or water bodies shall be provided
at all developments where stormwater management is required. The WQv
equals the storage volume needed to capture and treat the runoff from
storms of one inch or less. Runoff from the first one inch of rainfall
transports most of the total pollutant load. The WQv is based on the
following equation:
WQv = [(P)(Rv)(A)]/12 (acre-feet)
|
Where
|
|
|
P
|
=
|
Rainfall depth in inches (set to 1 inch).
|
Rv
|
=
|
Volumetric runoff coefficient, 0.05 + 0.009(I), where I is percent
impervious cover.
|
A
|
=
|
Site area (acres).
|
(b)
The formula assumes approximately 5% runoff from pervious surfaces
and 90% runoff from impervious surfaces. A minimum of 0.2 inch per
acre of runoff volume shall be met at sites or in drainage areas that
have less than 15% impervious cover.
(c)
Drainage areas having no impervious cover and no proposed disturbance
during development may be excluded from the WQv calculations. However,
designers are encouraged to incorporate water quality treatment practices
for these areas.
(d)
Stormwater quality treatment. The final WQv shall be treated
by an acceptable stormwater management practice from those described
in this section or as approved by the Township.
(e)
For new developments and redevelopments, infiltration is considered
an acceptable method of satisfying part or all of the water quality
volume.
(f)
For new developments, the WQv requirements of this section shall be sized and designed in conjunction with the standards under §
190-16A(1).
(g)
As a basis for design, the following assumptions may be made:
[1]
Multiple drainage areas. When a project contains or is divided
by multiple drainage areas, the WQv volume shall be addressed for
each drainage area.
[2]
Off-site drainage areas. The WQv shall be based on the impervious
cover of the proposed site. Off-site existing impervious areas may
be excluded from the calculation of the water quality volume requirements.
(3) Infiltration volume. Where possible, all of the water quality volume
shall be treated using infiltration BMPs. The following calculation
shall be used to determine the minimum recharge goal for the site:
Recharge volume (Rev), (acre-feet)
|
Fraction of WQv, depending on soil hydrologic group
|
Rev
|
=
|
(S)(A1)
|
Where
|
|
|
S
|
=
|
Soil-specific recharge factor in inches.
|
A1
|
=
|
The measured impervious cover.
|
Hydrologic Soil Group
|
Soil-Specific Recharge Factor (S)
(inches of runoff)
|
---|
A
|
0.40
|
B
|
0.25
|
C
|
0.10
|
D
|
0.05
|
(a)
Infiltrated volume may be subtracted from the total site WQv.
(b)
Infiltration shall not be considered for sites or areas of sites
that have activities that may allow pollution to be infiltrated. For
example, the use of infiltration for the runoff of a service station's
paved lot would not be appropriate, although roof water from the service
station may be infiltrated.
(c)
Infiltration shall only be used when, in the opinion of a professional
engineer, it will not contribute to slope instability or cause seepage
problems into basements or developed downgradient areas.
(d)
If more than one hydrologic soil group is present at a site,
a composite recharge volume shall be computed based upon the proportion
of total site area within each hydrologic soil group.
(e)
All infiltration facilities shall be set back at least 15 feet
from all structures with subgrade elements (e.g., basements, foundation
walls).
(4) Credits for use of nonstructural BMPs. The developer may obtain credits
for the use of nonstructural BMPs using the procedures outlined below.
Examples of nonstructural credit calculations are provided in Appendix
E.
(a)
Volume reduction method No. 1: natural area conservation. A
water quality volume reduction may be taken when undisturbed natural
areas are conserved on a site, thereby retaining their predevelopment
hydrologic and water quality characteristics. Under this method, a
designer would be able to subtract the conservation areas from the
total site area when computing the water quality protection volume.
An added benefit is that the post-development peak discharges will
be smaller, and hence, water quantity control volumes will be reduced
due to lower post-development curve numbers or rational formula "C"
values.
[1]
Rule: Subtract conservation areas from total site area when
computing water quality protection volume requirements.
[2]
Criteria.
[a] The conservation area cannot be disturbed during
project construction and must be protected from sediment deposition.
The conservation area shall be protected with a safety fence until
construction has been completed. After construction, the area shall
be posted with signage indicating that it is a conservation area.
[b] The conservation area shall be protected by limits
of disturbance clearly shown on all construction drawings.
[c] The conservation area shall be located within an
acceptable conservation easement instrument that ensures perpetual
protection of the proposed area. The easement must clearly specify
how the natural area vegetation shall be managed and boundaries will
be marked. [Note: Managed turf (e.g., playgrounds, regularly maintained
open areas) is not an acceptable form of vegetation management.]
[d] The conservation area shall have a minimum contiguous
area requirement of 10,000 square feet.
[e] Rv is kept constant when calculating WQv.
[f] The conservation area must be forested or have
a stable, natural ground cover.
(b)
Volume reduction method No. 2: stream buffers. This reduction
may be taken when a stream buffer effectively treats stormwater runoff.
Effective treatment constitutes treating runoff through overland flow
in a naturally vegetated or forested buffer. Under the proposed method,
a designer would be able to subtract areas draining via overland flow
to the buffer from total site area when computing water quality protection
volume requirements. The design of the stream buffer treatment system
shall use appropriate methods for conveying flows above the annual
recurrence (one-year storm) event.
[1]
Rule: Subtract areas draining via overland flow to the buffer
from total site area when computing water quality protection volume
requirements.
[2]
Criteria.
[a] The minimum undisturbed buffer width shall be 50
feet from top of bank.
[b] The maximum contributing length shall be 150 feet
for pervious surfaces and 75 feet for impervious surfaces.
[c] The average contributing slope shall be 3% maximum
unless a flow spreader is used. In no case shall the average contributing
slope be greater than 10%.
[d] Runoff shall enter the buffer as overland sheet
flow. A flow spreader may be installed to ensure this.
[e] Buffers shall remain as naturally vegetated or
forested areas and shall require only routine debris removal or erosion
repairs.
[f] Rv is kept constant when calculating WQv.
[g] Not applicable if overland flow filtration/groundwater
recharge reduction is already being taken.
(c)
Volume reduction method No. 3: enhanced swales. This reduction
may be taken when enhanced swales are used for water quality protection.
Under the proposed method, a designer would be able to subtract the
areas draining to an enhanced swale from total site area when computing
water quality protection volume requirements. An enhanced swale can
fully meet the water quality protection volume requirements for certain
kinds of low-density residential development (see volume reduction
method No. 5). An added benefit is the post-development peak discharges
will likely be lower due to a longer time of concentration for the
site.
[1]
Rule: Subtract the areas draining to an enhanced swale from
total site area when computing water quality protection volume requirements.
[2]
Criteria.
[a] This method is typically only applicable to moderate-
or low-density residential land uses (three dwelling units per acre
maximum).
[b] The maximum flow velocity for water quality design
storm shall be less than or equal to 1.0 foot per second.
[c] The minimum residence time for the water quality
storm shall be five minutes.
[d] The bottom width shall be a maximum of six feet.
If a larger channel is needed, use of a compound cross section is
required.
[e] The side slopes shall be 3:1 (horizontal:vertical)
or flatter.
[f] The channel slope shall be 3% or less.
[g] Rv is kept constant when calculating WQv.
(d)
Volume reduction method No. 4: overland flow filtration/groundwater
recharge. This reduction may be taken when overland flow filtration/infiltration
zones are incorporated into the site design to receive runoff from
rooftops or other small impervious areas (e.g., driveways, small parking
lots, etc.). This can be achieved by grading the site to promote overland
vegetative filtering or by providing infiltration or rain garden areas.
If impervious areas are adequately disconnected, they can be deducted
from total site area when computing the water quality protection volume
requirements. An added benefit will be that the post-development peak
discharges will likely be lower due to a longer time of concentration
for the site.
[1]
Rule: If impervious areas are adequately disconnected, they
may be deducted from total site area when computing the water quality
protection volume requirements.
[2]
Criteria.
[a] Relatively permeable soils (hydrologic soil groups
A and B) shall be present.
[b] Runoff shall not come from a designated hot spot.
[c] The maximum contributing impervious flow path length
shall be 75 feet.
[d] Downspouts shall be at least 10 feet away from
the nearest impervious surface to discourage reconnections.
[e] The disconnection shall drain continuously through
a vegetated channel, swale or filter strip to the property line or
structural stormwater control.
[f] The length of the disconnection shall be equal
to or greater than the contributing length.
[g] The entire vegetative disconnection shall be on
a slope less than or equal to 3%.
[h] The surface impervious area tributary to any one
discharge location shall not exceed 5,000 square feet.
[i] For those areas draining directly to a buffer,
reduction can be obtained from either overland flow filtration or
stream buffers (see method No. 2).
[j] Rv is kept constant when calculating WQv.
(e)
Volume reduction method No. 5: environmentally sensitive large-lot
subdivisions. This reduction may be taken when a group of environmental
site design techniques are applied to low- and very-low-density residential
development [e.g., one dwelling unit per two acres (du/ac) or lower].
The use of this method can eliminate the need for structural stormwater
controls to treat water quality protection volume requirements. This
method is targeted towards large lot subdivisions and will likely
have limited application.
[1]
Rule: targeted towards large-lot subdivisions (e.g., two-acre
lots and greater). The requirement for structural facilities to treat
the water quality protection volume may be waived.
[2]
Criteria.
[a] For single-lot development:
[i] Total site impervious cover is less than 15%.
[ii] Lot size shall be at least two acres.
[iii] Rooftop runoff is disconnected in accordance
with the criteria in method No. 4.
[iv] Grass channels are used to convey runoff versus
curb and gutter.
[b] For multiple lots:
[i] Total impervious cover footprint shall be less
than 15% of the area.
[ii] Lot areas shall be at least two acres, unless
clustering is implemented. Open space developments shall have a minimum
of 25% of the site protected as natural conservation areas and shall
be at least a half-acre average individual lot size.
[iii] Grass channels shall be used to convey runoff
versus curb and gutter (see method No. 3).
[iv] Overland flow filtration/infiltration zones shall
be established (see method No. 4).
B. Stormwater infiltration/practices.
(1) In selecting the appropriate infiltration BMPs, the applicant shall
consider the following:
(a)
Permeability and infiltration rate of the site soils.
(b)
Slope and depth to bedrock.
(c)
Seasonal high-water table.
(d)
Proximity to building foundations and wellheads.
(f)
Land availability and topography.
(h)
Effects on nearby properties and structures.
(2) A detailed soils evaluation of the project site shall be performed
to determine the suitability of infiltration BMPs. The evaluation
shall be performed by a qualified professional and, at a minimum,
address soil permeability, depth to bedrock and slope stability. The
general process for designing the infiltration BMP shall:
(a)
Analyze hydrologic soil groups as well as natural and man-made
features within the watershed to determine general areas of suitability
for infiltration BMPs.
(b)
Provide field testing data to determine appropriate percolation
rate and/or hydraulic connectivity.
(c)
Design infiltration BMPs for required stormwater volume based
on field-determined capacity at the level of the proposed infiltration
surface.
(3) Soil characteristics are subject to the following specific considerations:
(a)
Infiltration BMPs are particularly appropriate in hydrologic
soil groups A and B, as described in the Natural Resources Conservation
Manual TR-55.
(b)
Low-erodibility factors ("K" factors) are preferred for the
construction of basins.
(c)
There shall be a minimum depth of 48 inches between the bottom
of any facility and the seasonal high-water table and/or bedrock (limiting
zones), except for infiltration BMPs receiving only roof runoff, which
shall be placed in soils having a minimum depth of 24 inches between
the bottom of the facility and the limiting zone.
(d)
There shall be an infiltration and/or percolation rate sufficient
to accept the additional stormwater load and to drain completely as
determined by field tests.
(e)
The infiltration system shall have positive overflow controls
to prevent storage within one foot of the finished surface or grade.
(f)
Infiltration rates shall not be used in computing the storage
volume of the infiltration system.
(g)
Surface inflows shall be designed to prevent direct discharge
of sediment into the infiltration system.
(4) The recharge volume provided at the site shall be directed to the
most permeable hydrologic soil group available, except where other
considerations apply, such as in limestone geology.
(5) Any infiltration BMP shall be capable of completely infiltrating
the impounded water within 48 hours. The forty-eight-hour period is
to be measured from the end of the twenty-four-hour design storm.
(6) The Township may require additional analyses for stormwater management
facilities proposed for susceptible areas, such as:
(b)
Storage areas for salt, chloride, other materials for winter
deicing.
(7) During the period of land disturbance, runoff shall be controlled
prior to entering any proposed infiltration area. Areas proposed for
infiltration BMPs shall be protected from sedimentation and compaction
during the construction phase, so as to maintain their maximum infiltration
capacity.
(8) Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
received final stabilization.
(9) Infiltration facilities shall be selected based on suitability of
soils and site conditions. Acceptable infiltration facilities include,
but are not limited to, filter strips or stormwater filtering systems
(for example bioretention facilities, sand filters); open vegetated
channels (that is, dry swales and wet swales); infiltration trenches;
dry wells; infiltration basins; porous paving systems; retention basins;
wet extended detention ponds; riparian corridor management; riparian
forested buffers; rooftop runoff management systems; and sand filters
(closed or open).
(10)
Where sediment transport in the stormwater runoff is anticipated
to reach the infiltration system, appropriate permanent measures to
prevent or collect sediment shall be installed prior to discharge
to the infiltration system.
(11)
All infiltration facilities shall be set back at least 15 feet
from all structures with subgrade elements (e.g., basements, foundation
walls).
(12)
All infiltration facilities that serve more than one lot and
are considered a common facility shall have a drainage easement. The
easement shall provide to the Township the right of access and shall
be recorded in the Department of Real Estate of Allegheny County,
Pennsylvania.
(13)
If a detailed infiltration study is required, the following
rules shall be followed:
(a)
Soil evaluations shall be performed to determine the feasibility
and extent to which infiltration systems can be used. The evaluation
shall be performed by a qualified, licensed geologist, geotechnical/civil
engineer or soil scientist and, at a minimum, address soil types,
soil permeability, depth to bedrock, limitations of soils, presence/absence
of carbonate geology susceptibility to subsidence and/or sinkhole
formation and subgrade stability. The testing and evaluation shall
be completed at the preliminary design stage.
(b)
Infiltration requirements shall be based on the portions of
the site that are permeable prior to disturbance and the degree to
which development will reduce the permeability of the site. Permeability
of the site shall be determined based on the detailed evaluations
described herein. Use of stormwater management facilities to retain
stormwater for infiltration shall be applied to all areas where the
soils evaluation indicates favorable conditions. Areas generally not
favorable for infiltration shall still be provided with an appropriate
water quality practice.
(c)
Soil infiltration tests shall be performed to an equivalent
depth or elevation of the bottom of the proposed infiltration areas.
These tests shall follow the procedures of percolation test holes
as established by the Allegheny County Health Department (ACHD) for
on-lot septic systems. The testing shall include a test pit and percolation
test holes. The test hole shall be excavated to a depth so that the
presence or absence of bedrock and/or seasonal high-water table can
be determined. A soil log describing the soils present in each test
pit shall be performed. All test holes used for evaluating the percolation
rate shall be presoaked in accordance with the procedures established
by the ACHD. The location and number of test pits and percolation
holes shall be determined based on the type(s) of stormwater management
facilities being designed. Acceptability of infiltration rates shall
be based on sound engineering judgment and recommended design considerations
described in the design manuals listed in the references or other
source material acceptable to the Municipal Engineer.
(14)
The following design and construction standards shall be followed
when planning and constructing infiltration BMPs:
(a)
The lowest elevation of the infiltration area shall be at least
two feet above the seasonal high-water table and bedrock.
(b)
Where roof drains are designed to discharge to infiltration
facilities, they shall have appropriate measures to prevent clogging
by unwanted debris (for example, silt, leaves and vegetation). Such
measures shall include, but are not limited to, leaf traps, gutter
guards and cleanouts.
(c)
All infiltration facilities shall have appropriate positive
overflow controls to prevent storage within one foot of the finished
surface or grade, unless a specific amount of surface storage away
from pedestrian and vehicular traffic is provided and such areas infiltrate
the stored volume within 48 hours after the end of the twenty-four-hour
design storm.
(d)
All infiltration facilities shall be designed to infiltrate
the stored volume within 48 hours after the end of the twenty-four-hour
design storm.
(e)
All surface inflows shall be treated to prevent the direct discharge
of sediment into the infiltration practice; accumulated sediment reduces
stormwater storage capacity and ultimately clogs the infiltration
mechanism. No sand, salt or other particulate matter may be applied
to a porous (pervious) surface for winter ice conditions.
(f)
During site construction, all infiltration practice components
shall be protected from compaction due to heavy equipment operation
or storage of fill or construction material. Infiltration areas shall
also be protected from sedimentation. Areas that are accidentally
compacted or graded shall be remediated to restore soil composition
and porosity. Adequate documentation to this effect shall be submitted
for review by the Municipal Engineer. All areas designated for infiltration
shall not receive runoff until the contributory drainage area has
achieved final stabilization.
(g)
The following procedures and materials shall be required during
the construction of all subsurface facilities:
[1]
Excavation for the infiltration facility shall be performed
with equipment that will not compact the bottom of the seepage bed/trench
or like facility.
[2]
The bottom of the bed and/or trench shall be scarified prior
to the placement of aggregate.
[3]
Only clean aggregate with documented porosity, free of fines,
shall be allowed.
[4]
The tops and sides of all seepage beds, trenches or like facilities
shall be covered with drainage fabric. Fabric shall meet the specifications
of PennDOT Publication 408, Section 735, Construction Class 1.
[5]
Perforated distribution pipes connected to centralized catch
basins and/or manholes with the provision for the collection of debris
shall be provided in all facilities. Where perforated pipes are used
to distribute stormwater to the infiltration practice, stormwater
shall be distributed uniformly throughout the entire seepage bed/trench
or like facility.
C. Open vegetated channels.
(1) Open vegetated channels are conveyance systems that are engineered
to also perform as water quality and infiltration facilities. Such
systems can be used for the conveyance, retention, infiltration and
filtration of stormwater runoff.
(2) Open vegetated channels primarily serve a water quality function
(WQv). They also have the potential to augment infiltration. Examples
of such systems include, but are not limited to, dry swales, wet swales,
grass channels and biofilters. Open vegetated channels are primarily
applicable for land uses such as roads, highways, residential developments
(dry swales only) and pervious areas.
(3) Open vegetated channels shall be designed to meet the following minimum
standards:
(a)
The channel shall be designed to safely convey the ten-year-frequency
storm event with a freeboard of at least 12 inches. "Freeboard" is
the difference between the elevation of the design flow in the channel
and the top elevation of the channel.
(b)
The peak velocity of the runoff from the ten-year storm shall
be nonerosive for the soil and ground cover provided in the channel.
(c)
The longitudinal slope shall be no greater than 4%.
(d)
Channels shall be trapezoidal in cross section.
(e)
Channels shall be designed with moderate side slopes of four
horizontal to one vertical. Flatter side slopes may be necessary under
certain circumstances.
(f)
The maximum allowable ponding time in the channel shall be less
than 48 hours.
(g)
Channels (for example, dry swales) may require an underdrain
in order to function and dewater.
(h)
Channels shall be designed to temporarily store the WQv within
the system for a maximum period of 48 hours and a minimum period of
one hour.
(i)
Landscape specifications shall address the grass species, wetland
plantings (if applicable), soil amendment and hydric conditions present
along the channel.
(j)
Accumulated sediment within the channel bottom shall be removed
when 25% of the original WQv volume has been exceeded. The channel
shall be provided with a permanent concrete cleanout marker that indicates
the 25% loss level.
(k)
Check dams along the channel length may be warranted.
(l)
The bottom of dry swales shall be situated at least two feet
above the seasonal high-water table.
(4) Additional design information for open vegetated channels is available
in Design of Roadside Channels with Flexible Linings, HEC 15, FHWA,
September 2005.
D. Retention basins.
(1) Retention basins shall be designed to create a healthy ecological community with sufficient circulation of water to prevent the growth of unwanted vegetation and mosquitoes or other vectors. If circulation cannot be provided via natural means, then artificial aeration and circulation shall be provided. Care shall be taken to landscape retention basins in accordance with §
190-17.
(2) The retention basin shall be of sufficient size to allow the appropriate
aquatic community needed to maintain healthy pond ecology and avoid
mosquitoes capable of carrying West Nile Virus and other diseases.
The Allegheny County Health Department, Pennsylvania Fish and Boat
Commission, the Natural Resources Conservation Service, the Pennsylvania
Extension Service, or other qualified professional consultant shall
be consulted during the design of these facilities in order to ensure
the health of aquatic communities and minimize the risk of creating
mosquito breeding areas.
(3) An outlet structure shall be designed to allow complete drainage
of the pond for maintenance.
(4) The design of a retention basin shall include the determination of
the proposed site's ability to support a viable permanent pool.
The design shall take into account such factors as the available and
required rate and quality of dry weather inflow, the stormwater inflow,
seasonal and longer-term variations in groundwater table, and impacts
of potential pollutant loadings.
(5) Sediment storage volume equal to at least 20% of the volume of the
permanent pool shall be provided.
(6) A sediment forebay with a hardened bottom shall be provided at each
inlet into the retention basin. The forebay storage capacity shall
at minimum be 10% of the permanent pool storage. The forebay shall
be designed to allow for access by maintenance equipment for periodic
cleaning. A permanent concrete cleanout marker shall be installed
in the forebay to indicate the level where 25% capacity for the forebay
storage has been used.
(7) Emergency spillways shall be sized and located to permit the safe
passage of stormwater flows from an unattenuated one-hundred- year
post-development storm with one foot of freeboard. The maximum velocities
in vegetated spillways excavated in otherwise undisturbed soil shall
be analyzed based upon the velocity of peak flow in the emergency
spillway during an assumed clogged primary outlet condition. Where
maximum velocities exceed design standards contained in the Engineering
Field Manual for Conservation Practices (USDA SCS, July 1984), suitable
lining shall be provided. All emergency spillways placed on fill materials
shall be lined. Lining for emergency spillways shall incorporate native
colors and materials where possible, including monoslab revetments,
grass pavers, riprap and native stone.
(8) Basin and pond embankments shall be designed by a professional engineer
registered in the Commonwealth of Pennsylvania. The design shall include
an investigation of the subsurface conditions at the proposed embankment
location to evaluate settlement potential, groundwater impacts, and
the need for seepage controls. The submittal of a geotechnical report
from a geotechnical engineer for any embankment over 10 feet in effective
height or posing a significant hazard to downstream property or life
is required. The selection of fill materials shall be subject to approval
of the design engineer. Fill shall be free of frozen soil, rocks over
six inches, sod, brush, stumps, tree roots, wood or other perishable
materials. Embankment fills less than 10 feet in fill height shall
be compacted using compaction methods that would reasonably guarantee
that the fill density is at least 90% of the maximum density as determined
by standard proctor (ASTM-698). All embankment fills more than 10
feet in fill height shall be compacted to at least 90% of the maximum
density as determined by standard proctor (ASTM-698) and shall have
their density verified by field density testing. A PADEP dam permit
is required for embankments having a maximum depth of water, measured
from the upstream toe of the dam to the top of the dam at maximum
storage elevation, of greater than 15 feet; and/or for ponds having
a contributory drainage area of greater than 100 acres; and/or for
impoundments of greater than 50 acre-feet.
(9) The embankment's interior slope shall not be steeper than 3:1
(three horizontal to one vertical). The exterior slope of the embankment
may not exceed 2:1 (two horizontal to one vertical).
(10)
The minimum embankment width shall be four feet for embankments
less than six feet in height, six feet if the embankment is between
6.1 feet and 9.9 feet in height and eight feet if the embankment is
between 10 feet and 15 feet in height.
(11)
Existing ponds or permanent pool basins may be used for stormwater
management, provided that it can be demonstrated that the ponds are
structurally sound and meet the design requirements herein.
(12)
Inlet structures and outlet structures shall be separated to
the greatest extent possible in order to maximize the flow path through
the retention basin.
(13)
Retention basins shall be designed to provide a length-to-width
ratio of at least 3L:1W as measured in plan view (for example, a ratio
of 4L:1W is too narrow).
(14)
The retention basin depth shall average three feet to six feet.
(15)
Fencing of the facility is not required if the interior slope
of the pond is 4H:1V or flatter and the design also includes a five-foot-wide
bench around the pond perimeter at an elevation one foot below the
permanent water surface elevation.
(16)
Any side slopes below the permanent water surface level shall
not exceed 3H:1V. Interior side slopes above the permanent water surface
level shall not exceed 3H:1V.
(17)
Stabilization. Proper stabilization structures, including stilling
basins, energy dissipators and channel lining, shall be constructed
at the outlets of all retention basins and emergency spillways. The
stabilization structures shall control water to avoid erosion, reduce
velocities of released water, and direct water so that it does not
interfere with downstream activities.
(18)
Energy dissipators and/or level spreaders shall be installed
to prevent erosion and/or initiate sheet flow at points where pipes
or drainageways discharge to or from basins. Level spreaders shall
be used only where the maximum slope between the discharge point and
the waterway does not exceed 5%. Energy dissipators shall comply with
criteria in Hydraulic Design of Energy Dissipators for Culverts and
Channels, HEC 14, FHWA, July 2006. Such facilities shall be both functional
and harmonious with the surrounding environment, for example, native
rock shall be used in constructing dissipators where practical.
(19)
Discharge points. The minimum distance between a proposed basin
discharge point (including the energy dissipator, etc.) and a downstream
property boundary shall in no case be less than 15 feet. Where there
is discharge onto or through adjacent properties prior to release
to a stream, designers shall demonstrate how downstream properties
are to be protected. The Municipal Engineer may require that the setback
distance be increased based upon factors such as topography, soil
conditions, the size of structures, the location of structures, and
discharge rates. A drainage easement may also be required.
(20)
Outlet structures. Outlet structures shall meet the following
specifications:
(a)
To minimize clogging and to facilitate cleaning and inspecting,
outlet pipes shall have an internal diameter of at least 15 inches
and a minimum grade of 1%.
(b)
Bentonite plugs shall be provided on all outlet pipes within
a constructed berm.
(c)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(d)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
(e)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets, in conformance with AASHTO M170, M198 and M207, or
smooth interior HDPE pipe in conformance with AASHTO M252 or M294.
(f)
Basin outlet structures shall have childproof, nonclogging trash
racks over all design openings exceeding 12 inches in diameter, except
those openings designed to carry perennial stream flows. Periodic
cleaning of debris from trash racks shall be included in the operation
and maintenance plan.
(g)
Antivortex devices, consisting of a thin vertical plate normal
to the basin berm, shall be provided at the top of all circular risers
or standpipes.
E. Detention basins.
(1) The landscape standards of §
190-17 shall apply.
(2) The maximum inside side slopes shall not exceed three horizontal to one vertical (3H:1V). The minimum required slope for the basin bottom is 2%. A level bottom is acceptable, provided that the designer demonstrates to the Township's satisfaction that the basin bottom shall be landscaped with appropriate wetland vegetation pursuant to §
190-17. In addition, detention basins of sufficient size and slope may serve other functions as well, including recreational uses which do not hinder or conflict with the function of the detention basin.
(3) Inlet structures. The inlet pipe invert into a basin shall be six
inches above the basin floor or lining so that the pipe can adequately
drain after rainstorms. Inlets shall discharge into areas of the basin
that slope toward the outlet structure.
(4) Inlet structures and outlet structures shall be separated to the
greatest extent possible in order to maximize the flow path through
the retention basin.
(5) Low-flow channels. Low-flow channels constructed of concrete or asphalt
are not permitted. Where low-flow channels are necessary, they shall
be composed of a natural or bioengineered material. Low-flow channels
shall be designed to promote water quality and slow the rate of flow
through the basin. Low-flow channels may also be designed to infiltrate
where practical.
(6) Outlet structures. Outlet structures shall meet the following specifications:
(a)
To minimize clogging and to facilitate cleaning and inspection,
outlet pipes shall have an internal diameter of at least 15 inches
and a minimum grade of 1%.
(b)
Bentonite plugs shall be provided on all outlet pipes within
a constructed berm.
(c)
All principal outlet structures shall be built using reinforced
concrete with watertight construction joints.
(d)
The use of architecturally treated concrete, stucco, painted
surface or stone facade treatment shall be considered for enhancing
the outlet structure. Such facilities shall be both functional and
harmonious in design with the surrounding environment.
(e)
Outlet pipes shall be constructed of reinforced concrete with
rubber gaskets, in conformance with AASHTO M170, M198 and M207, or
smooth-interior HDPE pipe in conformance with AASHTO M252 or M294.
(f)
Energy-dissipation facilities that convert concentrated flow
to uniform shallow sheet flow shall be used where appropriate.
(g)
Basin outlet structures shall have childproof, nonclogging trash
racks over all design openings exceeding 12 inches in diameter, except
those openings designed to carry perennial stream flows.
(h)
Antivortex devices, consisting of a thin vertical plate normal
to the basin berm, shall be provided at the top of all circular risers
or standpipes.
(7) Emergency spillways shall be sized and located to permit the safe
passage of stormwater flows from an unattenuated one-hundred- year
post-development storm with one foot of freeboard. The maximum velocities
in vegetated spillways excavated in otherwise undisturbed soil shall
be analyzed based upon the velocity of peak flow in the emergency
spillway during an assumed clogged primary outlet condition. Where
maximum velocities exceed design standards contained in the Engineering
Field Manual for Conservation Practices (USDA SCS, July 1984), suitable
lining shall be provided. In general, emergency spillways shall not
be located in fill areas; all such facilities placed on fill materials
shall be lined. Lining for emergency spillways shall incorporate native
colors and materials where possible, including monoslab revetments,
grass pavers, riprap and native stone.
(8) Basin and pond embankments shall be designed by a professional engineer
registered in the Commonwealth of Pennsylvania. The design shall include
an investigation of the subsurface conditions at the proposed embankment
location to evaluate settlement potential, groundwater impacts and
the need for seepage controls. The submittal of a geotechnical report
from a geotechnical engineer for any embankment over 10 feet in effective
height or posing a significant hazard to downstream property or life
is required. The selection of fill materials shall be subject to approval
of the design engineer. Fill shall be free of frozen soil, rocks over
six inches, sod, brush, stumps, tree roots, wood or other perishable
materials. Embankment fills less than 10 feet in fill height shall
be compacted using compaction methods that would reasonably guarantee
that the fill density is at least 90% of the maximum density as determined
by standard proctor (ASTM-698). All embankment fills more than 10
feet in fill height shall be compacted to at least 90% of the maximum
density as determined by standard proctor (ASTM-698) and shall have
their density verified by field density testing. A PADEP dam permit
is required for embankments having a maximum depth of water, measured
from the upstream toe of the dam to the top of the dam at maximum
storage elevation, of greater than 15 feet; and/or for ponds having
contributory drainage area of greater than 100 acres; and/or for impoundments
of greater than 50 acre-feet.
(9) The embankment's interior slope shall not be steeper than 3:1
(three horizontal to one vertical). The exterior slope of the embankment
shall not exceed 2:1 (two horizontal to one vertical).
(10)
The minimum embankment width shall be four feet for embankments
less than six feet in height, six feet if the embankment is between
6.1 feet and 9.9 feet in height, and eight feet if the embankment
is between 10 feet and 15 feet in height.
(11)
Fencing of the facility is not required if the interior slope
of the pond is 4:1 or flatter.
(12)
Freeboard. Freeboard is the difference between the elevation
of the design flow in the emergency spillway (usually the one-hundred-year
peak elevation) and the top elevation of the settled basin embankment
(that is, top of berm). The minimum freeboard shall be one foot.
(13)
Energy dissipators and/or level spreaders shall be installed
to prevent erosion and/or initiate sheet flow at points where pipes
or drainageways discharge to or from basins. Level spreaders shall
be used only where the maximum slope between the discharge point and
the waterway does not exceed 5%. Energy dissipators shall comply with
criteria in Hydraulic Design of Energy Dissipators for Culverts and
Channels, HEC 14, FHWA, July 2006. Such facilities shall be both functional
and attractive; for example, native rock shall be used in constructing
dissipators where practical.
(14)
Stabilization. Proper stabilization structures, including stilling
basins, energy dissipators and channel lining, shall be constructed
at the outlets of all basins and emergency spillways. The stabilization
structures shall control water to avoid erosion, reduce velocities
of released water, and direct water so that it does not interfere
with downstream activities.
(15)
Discharge points. The minimum distance between a proposed basin
discharge point (including the energy dissipator, etc.) and a downstream
property boundary shall in no case be less than 15 feet. Where there
is discharge onto or through adjacent properties prior to release
to a stream, designers shall demonstrate how downstream properties
are to be protected. The Municipal Engineer may require that the setback
distance be increased based upon factors such as topography, soil
conditions, the size of structures, the location of structures and
discharge rates. A drainage easement may also be required.
(16)
A sediment forebay with a hardened bottom shall be provided
at each inlet into the detention basin. The forebay storage capacity
shall at minimum be 10% of the permanent pool storage. The forebay
shall be designed to allow for access by maintenance equipment for
periodic cleaning.
F. Conveyance systems (open channels, drainageways and storm sewers).
(1) Applicants are encouraged to design conveyance systems that encourage
infiltration and improve water quality wherever practicable.
(2) Wherever conveyance channels are necessary, drainage shall be maintained
by an open channel with landscaped banks designed to carry the ten-year,
twenty-four-hour stormwater runoff from upstream contributory areas.
The Municipal Engineer may increase the design storm as conditions
require. All open channels shall be designed with one foot of freeboard
above the design water surface elevation of the design runoff condition.
(3) Flood relief channels shall be provided and designed to convey the
runoff from the one-hundred-year, twenty-four-hour storm such that
a positive discharge of this runoff to an adequate receiving stream
or conveyance system occurs without allowing this runoff to encroach
upon other properties.
(4) Manholes and/or inlets shall not be spaced more than 300 feet apart
for pipe sizes up to 24 inches in diameter and not more than 450 feet
apart for larger pipe sizes.
(5) Where drainage swales are used in lieu of or in addition to storm
sewers, they shall be designed to carry the required runoff without
erosion and in a manner not detrimental to the properties they cross.
Drainage swales shall provide a minimum grade of 2% but shall not
exceed a grade of 9%. Drainage swales used strictly for conveyance
are not the same as open vegetated channels. Design standards for
open vegetated channels are provided under of this chapter.
(6) Street curbing for the purpose of stormwater conveyance is discouraged.
On streets that must contain curbing, storm sewers shall be placed
in front of the curbing. To the greatest extent possible, storm sewers
shall not be placed directly under curbing. At curbed street intersections,
storm inlets shall be placed in the tangent section of the road.
(7) Use of grassed swales or open vegetated swales, in lieu of curbing,
to convey, infiltrate and/or treat stormwater runoff from roadways
is encouraged. Inlets shall be placed at the center of the shoulder
swale draining the street and shall be located no closer than four
feet from the edge of the cartway.
(8) When requested by the Township, the applicant shall obtain or grant
a minimum twenty-foot-wide drainage easement over all storm sewers,
drainage swales, channels, etc., that are a component of the stormwater
management system when located within undedicated land. All permanent
detention basins and/or other stormwater management facilities providing
stormwater control for other than a single residential lot shall be
located within a defined drainage easement that allows proper legal
access and maintenance vehicle access.
(9) No property owner or applicant shall obstruct or alter the flow,
location or carrying capacity of a stream, channel or drainage swale
to the detriment of any other property owner, whether upstream or
downstream. All subdivision and/or land development plans containing
streams, channels, drainage swales, storm sewers or other conveyance
systems that cross property boundaries, existing or proposed, or whose
discharge crosses such boundaries shall contain a note stating the
above.
(10)
Water quality inlets. Storm drainage systems that collect runoff
from parking areas and/or loading areas exceeding 10,000 square feet
of impervious coverage and discharge to stormwater management systems,
including surface or subsurface infiltration systems, shall have a
minimum of one water quality inlet per each acre of drainage area.
The purpose of water quality inlets is to remove oil, grease and heavy
particulates or total suspended solids, hydrocarbons and other floating
substances from stormwater runoff. Methods other than water quality
inlets may be permitted if the applicant demonstrates to the Township's
satisfaction that any such alternative will be as effective and as
easily maintained. Periodic cleaning of these systems shall be addressed
in the operation and maintenance plan submitted to the Township.
Stormwater management facilities shall be landscaped in accordance
with the following standards:
A. Landscaping shall be required in and around all constructed stormwater
management facilities with a minimum surface area of 1,000 square
feet for the purposes of:
(1) Assisting in the management of stormwater;
(2) Stabilizing the soil within such facilities to minimize and control
erosion;
(3) Enhancing the visual appearance of such facilities; and
(4) Mitigating maintenance problems commonly associated with the creation
of such facilities.
B. A planting plan and planting schedule shall be submitted in accordance
with the following:
(1) Wet meadows, including floors of stormwater management facilities.
(a)
Wet meadows and floors of stormwater management facilities shall
be planted with noninvasive plants native to western Pennsylvania,
such as wildflowers and noninvasive grasses, the intent being to create
a mixed meadow of such plantings, where appropriate. Selection of
plantings shall be based on whether the area in question is usually
well drained or permanently wet and whether the area will be used
for recreation purposes. No woody plants shall be planted within the
saturated zone (phreatic line) of a stormwater management practice
or on a berm constructed for impounded water.
(b)
Seeding by drills, corrugated rollers, cyclone or drop seeders
or hand seeding of such areas is preferred; however, hydroseeding
followed by hydromulching can be used on wet ground and steep slopes.
(c)
Fertilizers, as a nutrient supplement, shall not be used unless
it is documented that soil conditions warrant such use and nutrient
applied does not exceed plant uptake. Soil for planting of wildflowers
shall contain not less than 3% or more than 10% organic matter, as
determined by an agricultural chemist, with certification of the test
before planting.
(d)
Seeding shall take place either between April 1 and May 15 or
between September 1 and October 15. Planting areas shall be soaked
to maintain a consistent level of moisture for at least four to six
weeks after planting. For seeding recommendations, reference the DEP's
E&S Pollution Control Program Manual.
(e)
Once established, a single annual mowing when plants are dormant
should be sufficient to maintain a wet meadow and/or floor of a stormwater
management practice.
(2) Wet edges that remain wet all or most of the year shall be planted
with wildflowers, grasses and shrubs. Plants to be located on rims
or banks which remain dry most of the year shall be planted with species
tolerant of dry soil conditions.
(3) Wooded areas.
(a)
Where stormwater management facilities adjoin wooded areas,
trees and shrubs shall be selected and planted outside the practice
so as to blend with existing surroundings.
(b)
Plantings in such areas shall be of sufficient density to eliminate
the need for mowing.
(c)
It is recommended that clusters of trees and shrubs be planted
around stormwater management facilities but well away from outfalls
and any constructed berms, where applicable, to provide for wildlife
habitat, wind control and buffering and screening.
(d)
Vegetation shall be planted during appropriate times of the
year, predominantly between late March and mid-May or from early October
until evidence of ground freezing, depending upon the species selected.
Most deciduous trees and shrubs can be planted in either spring or
fall. Evergreens are best planted in late summer or early fall.
(4) Slopes.
(a)
Where slopes are gentle, a mixture of meadow grasses and wildflowers
(for wet meadows) shall be planted.
(b)
On steep slopes, as defined by the Township's Code, dense
spreading shrubs (shrubs tolerant of dry soils) shall be planted.
Heavy mat mulch shall be used during the period of establishment.
(c)
No woody plant materials or trees shall be located on a constructed
or natural berm acting as the impoundment structure of a stormwater
management practice. Trees shall be located downstream of an impoundment
berm a sufficient distance from the toe of the constructed slope to
assure that the toe of the slope is outside the dripline of the species
planted at maturity but in no case less than 15 feet.
(5) In cases where stormwater management facilities are to be located
in proximity to wetlands or waterways, the applicant's planting
plan and schedule shall consider the sensitive conditions existing
therein and be modified accordingly to reflect existing flora.
(6) Stormwater management facilities shall be screened in a manner which
complements the existing landscape and provides sufficient access
for maintenance.
Stream buffers shall be provided for new development sites as
per the following requirements:
A. In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as part of any subdivision
or land development that encompasses a riparian buffer. The intent
of this chapter in establishing a riparian buffer is to protect and
improve stream water quality. The riparian buffer is intended to slow
overland flow to the stream through the presence of native grasses,
trees and shrubs, allowing infiltration/groundwater recharge; causing
deposition of sediment, nutrients, pesticides, and other pollutants
in the buffer rather than in the stream; and reducing erosion by providing
stream bank stabilization. The trees provide shade for streams, keeping
waters cooler and reducing evaporation.
B. Except as required by 25 Pa. Code Chapter 102, the riparian buffer easement shall be required for all streams (as defined in Article
II) with a contributing watershed area of greater than 10 acres. The riparian buffer easement shall be measured to be a minimum of 35 feet from the top of the stream bank (on each side).
C. Minimum management requirements for riparian buffers.
(1) No use or construction within the riparian buffer shall be permitted that is inconsistent with the intent of the riparian buffer as described in §
190-18A.
(2) Existing native vegetation shall be protected and maintained within
the riparian buffer easement.
(3) Whenever practicable, invasive vegetation shall be actively removed
and the riparian buffer easement shall be planted with native trees,
shrubs and other vegetation to create a diverse native plant community
appropriate to the intended ecological context of the site.
D. The riparian buffer easement shall be enforceable by the Township
and shall be recorded in the appropriate County Recorder of Deeds
office, so that it shall run with the land and shall limit the use
of the property located therein. The easement shall allow for the
continued private ownership and shall count toward the minimum lot
area required by zoning, unless otherwise specified in the Township
Zoning Ordinance.
E. Any permitted use within the riparian buffer easement shall be conducted
in a manner that will maintain the extent of the existing one-hundred-year
floodplain, improve or maintain the stream stability, and preserve
and protect the ecological function of the floodplain.
F. Stormwater drainage pipes shall be permitted within the riparian
buffer easement, but they shall cross the easement in the shortest
practical distance. Other structural stormwater management facilities
are not permitted within the riparian buffer easement.
G. The following conditions shall apply when public and/or private recreation
trails are permitted by the Township within riparian buffers:
(1) It is preferred that trails be designed to be permeable and for nonmotorized
use only; however, impermeable trails are permitted, provided that
they have adequate drainage.
(2) Trails shall be designed to have the least impact on native plant
species and other sensitive environmental features.
H. Septic drainfields and sewage disposal systems shall not be permitted
within the riparian buffer easement and shall comply with setback
requirements established under 25 Pa. Code Chapter 73.
I. Underground utilities shall be permitted within the riparian buffer
easement; however, work shall be performed to minimize disturbance
area and removal of trees. Restoration within the riparian buffer
easement shall be with native species of trees, grasses, and other
plantings. One tree shall be planted for each tree removed, and the
restoration shall be designed by a registered professional with the
requisite experience. Aboveground utilities shall only be permitted
to cross the easement perpendicular to the easement or in the shortest
practical distance. Existing utilities may remain and be maintained
as required.