A.
The management of stormwater on site,
both during and upon completion of the disturbances associated with
activities permitted under § 178-104, shall be accomplished in
accordance with the standards and criteria of this chapter. The design
of any temporary or permanent facilities and structures and the utilization
of any natural drainage systems shall be in full compliance with this
chapter. The intent of these design standards is to encourage environmentally
sound stormwater management practices that provide necessary drainage
facilities while protecting the hydrologic characteristics and water
quality of the site and watershed. Developments shall be required
to incorporate stormwater management controls. Stormwater management
design shall blend into the natural environment and be aesthetically
integrated into the site design.
B.
For all regulated activities, unless
preparation of an SWM site plan is specifically exempted in §
178-302:
(1)
Preparation and implementation of
an approved SWM site plan is required.
(2)
No regulated activities shall commence
until the Borough issues written approval of an SWM site plan, which
demonstrates compliance with the requirements of this chapter.
(3)
All SWM design work must be sealed
by a qualified design professional. All designs proposing the use
of a SWM retention or detention facility, with or without outlet structure(s),
shall be completed by a design engineer licensed in the Commonwealth
of Pennsylvania.
(4)
SWM site plans approved by the Borough,
in accordance with § 178-406, shall be on site throughout the
duration of the regulated activity.
C.
These standards apply to the landowner
and any person engaged in regulated activities.
D.
For all regulated earth disturbance
activities, erosion and sediment control BMPs shall be designed, implemented,
operated, and maintained during the regulated earth disturbance activities
(e.g., during construction) to meet the purposes and requirements
of this chapter and to meet all requirements under Title 25 of the
Pennsylvania Code, the Clean Streams Law,[1] and the Storm Water Management Act.[2] Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual), No. 363-2134-008, as amended and updated. In addition, the
following standards apply:
(1)
No regulated earth disturbance activities
within the municipality shall commence until the municipality receives
a copy of any required approvals from the Conservation District or
DEP for an erosion and sediment control plan.
(2)
DEP has regulations that require
an erosion and sediment control plan for any earth disturbance activity
of 5,000 square feet or more, under 25 Pa. Code § 102.4(b).
(3)
In addition, under 25 Pa. Code Chapter
92a, a DEP NPDES construction activities permit is required for regulated
earth disturbance activities.
(4)
Evidence of any necessary permit(s)
for regulated earth disturbance activities from the appropriate DEP
regional office or County Conservation District must be provided to
the municipality.
(5)
A copy of the erosion and sediment
control plan and any required permits, as required by DEP regulations,
shall be available at the project site at all times.
(6)
Additional erosion and sediment control
design standards and criteria are recommended to be applied where
infiltration BMPs are proposed and 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.
(c)
Infiltration BMPs should
be dispersed throughout the project site at strategic locations, made
as shallow as practicable, and located to maximize use of natural
on-site infiltration features while still meeting the other requirements
of this chapter.
(7)
The municipality shall be authorized
to enforce regulations specified under 25 Pa. Code Chapters 92a, 102
and 105 for regulated earth disturbance activities authorized by the
Allegheny County Conservation District and/or the DEP.
E.
All development activity within a
special flood hazard area designated by the Federal Emergency Management
Agency (FEMA) shall comply with § 212-1503, Floodplain management,
of the Zoning Ordinance, and this subsection. All development shall
be designated to maintain the flood-carrying capacity of the floodway
such that the base flood elevations are not increased, either upstream
or downstream, unless an approval is received from the PADEP. The
natural conveyance characteristics of the site and the receiving floodplain
shall be incorporated into the stormwater management practices proposed
for the site.
F.
The stormwater management system
shall not create an adverse impact on stormwater quantity or quality
in either upstream or downstream areas. Off-site areas which discharge
to or across a site proposed for development shall be addressed in
the stormwater management plan prepared for the development. No stormwater
management plan shall be approved until it is demonstrated that the
runoff from the project shall not adversely impact downstream areas.
G.
Wetlands shall not be used to meet
the minimum design requirements for stormwater management or stormwater
runoff quality treatment, except when used as part of a treatment
train that incorporates a portion of the outer zone (filter strip)
of the wetland's riparian buffer as a stormwater outfall.
H.
Stormwater management designs shall
not alter the alignment of any natural watercourse or adversely impact
its flow.
I.
All proposed stormwater management
systems shall be designed to prevent the pollution of groundwater
resources by stormwater, promote safety, minimize health hazards,
preserve natural features and provide infiltration and groundwater
recharge where appropriate. Neither submission of a plan under the
provision herein nor compliance with the provisions of these regulations
shall relieve any person from responsibility for damage to any person
or property otherwise imposed by law.
J.
Where deemed necessary by the Borough
Engineer, or as addressed in an approved Act 167 stormwater management
plan, the applicant shall construct storm drains to handle on-site
runoff. To the maximum extent permitted under the Municipalities Planning
Code, 53 P.S. § 10101, et seq., and Act 167, or any amendments
thereto, the applicant shall provide on-site/off-site drainage easements
and provide for the conveyance of off-site runoff to an acceptable
outlet in the same watershed.
K.
Where watercourses traverse a development
site, drainage easements shall be provided conforming to the line
of such watercourses. The terms of the easements shall prohibit excavation,
the placing of fill or structures, except as needed for roadways,
driveways and walkways, or any alterations that may adversely affect
the flow of stormwater within any portions of the easement, and require
the establishment and protection of riparian buffers.
L.
Any regulated activity that may affect
the stormwater flows toward or onto a state or county highway right-of-way
or facility shall be designed, implemented, operated and maintained
in accordance with the regulations of the Pennsylvania Department
of Transportation (PennDOT) or Allegheny County, as the case may be.
M.
At the time of the application for
a residential or nonresidential building permit or authorization for
improvements as a land development project, issuance of the permit
shall be conditioned upon adherence to the terms and requirements
of this chapter.
N.
Stormwater discharges to critical
areas with sensitive resources (e.g., special protection waters, cold-water
fisheries, recharge areas, water supply reservoirs, etc.) may be subject
to additional performance criteria or may need to utilize or restrict
certain stormwater management practices.
O.
The applicant must document that
downstream (off-site) conveyance systems exist to safely transport
the discharge and that no erosion, sedimentation, flooding or other
adverse impacts will result from said discharge from each subarea.
P.
All stormwater runoff from disturbed
areas shall be pretreated prior to discharge to surfaces or groundwater.
Q.
Post-construction water quality protection
shall be addressed as required by the stormwater management requirements
contained in this chapter.
R.
All BMPs used to meet the requirements
of this chapter shall conform to the state water quality requirements
and any more stringent requirements as required by the municipality.
S.
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 site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations
and wellheads.
(6)
Erodability 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.
T.
Transference of runoff from one DEP-designated
Act 167 watershed to another shall be prohibited unless approved by
the municipality.
U.
Impervious areas:
(1)
The measurement of impervious areas
shall include all of the impervious areas in the total proposed development
even if development is to take place in stages.
(2)
For development taking place in stages,
the entire development plan must be used in determining conformance
with this chapter.
(3)
For projects that add impervious
area to a parcel, the total impervious area on the parcel is subject
to the requirements of this chapter; except that the volume controls
in § 178-303 and the peak rate controls of § 178-304 do
not need to be retrofitted to existing impervious areas that are not
being altered by the proposed regulated activity.
V.
Stormwater flows onto adjacent or
downstream property shall not be created, increased, decreased, relocated,
impeded, or otherwise altered without written notification of the
affected property owner(s). Notification shall include a description
of the proposed development and the stormwater flows that are being
created, increased, decreased, relocated, impeded, or otherwise altered.
Adjacent property shall at a minimum include any property having a
shared boundary with the subject property of the SWM site plan, however,
if in the judgement of the designated plan reviewer additional properties
are being affected, additional notifications may be required. Proof
of notification (signed postal receipt for example) shall be included
as part of the SWM plan submission to the Borough. Such stormwater
flows shall be subject to the requirements of this chapter.
W.
All regulated activities shall include
such measures as necessary to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this
chapter by implementing measures to:
(a)
Minimize disturbance
to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian
buffers.
(c)
Avoid erosive flow conditions
in natural flow pathways.
(d)
Minimize thermal impacts
to waters of this commonwealth.
(e)
Disconnect impervious
surfaces by directing runoff to pervious areas, wherever possible.
(3)
Incorporate methods described in
the Pennsylvania Stormwater Best Management Practices Manual (BMP
Manual), Pennsylvania Handbook of Best Management Practices for Developing
Areas, the Maryland Stormwater Design Manual, or other appropriate
references for guidance in the design of SWM facilities most appropriate
to individual site conditions. Objectives for design are to reduce
the volume of stormwater generated, infiltrate runoff at its source
to the maximum extent possible, achieve water quality improvement
at the source or during conveyance, and provide for peak flow attenuation.
Applicants shall examine design alternatives by viewing them in a
series. In addition, applicants are strongly encouraged to use structural
and nonstructural stormwater management practices that reduce or eliminate
the need for detention basins. If methods other than green infrastructure
and LID methods are proposed to achieve the volume and rate controls
required under this chapter, the SWM site plan must include a detailed
justification, acceptable to the designated plan reviewer, demonstrating
that the use of LID and green infrastructure is not practicable.
X.
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.
Y.
The design storm-precipitation depths
to be used in the analysis of peak rates of discharge shall be as
obtained in PennDOT's Drainage Manual, Publication 584, Appendix 7A;
or obtained from the latest version of the Precipitation-Frequency
Atlas of the United States, National Oceanic and Atmospheric Administration
(NOAA), National Weather Service, Hydrometeorological Design Studies
Center, Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at:
http://hdsc.nws.noaa.gov/hdsc/pfds/.
Z.
The Borough may, after consultation
with DEP, approve measures for meeting the state water quality requirements
other than those in this chapter, provided that they meet the minimum
requirements of, and do not conflict with, state law including, but
not limited to, the Clean Streams Law.[3]
[3]
Editor's Note: See 35 P.S. § 691.1 et
seq.
AA.
Appendix F contains Worksheets 11,
12, and 13 from Chapter 8 of the Pennsylvania Stormwater Best Management
Practices Manual (DEP, 2006) and shall be used for computing pollutant
loads from common land uses and the resultant loads after application
of the most common stormwater management BMPs. Requirements shall
be met per the worksheets.
A.
Regulated activities that result in cumulative earth disturbances
less than one acre are exempt from the requirements in § 178-401
of this chapter except as provided below. The Project Standardized
SWM Guidance document provided in Appendix E was prepared to assist
applicants in meeting this requirement for individual lots only.
B.
Earth disturbances between 0.25 acre (10,890 square feet) and
one acre of earth disturbance must submit a SWM site plan to the Borough
which shall consist of the following items and related supportive
material needed to determine compliance with §§ 178-303
through 178-305. The applicant can use the protocols in the Small
Project Stormwater Management Site Plan pursuant to Appendix C.[1]
(1)
General description of proposed stormwater management techniques,
including construction specifications of the materials to be used
for stormwater management facilities.
(2)
An erosion and sediment control plan, including all reviews
and letters of adequacy from the Conservation District where appropriate.
(3)
Limits of earth disturbance, including the type and amount of
impervious area that is proposed; proposed structures, roads, paved
areas, and buildings; and a statement, signed by the applicant, acknowledging
that any revision to the approved drainage plan must be approved by
the Borough, and that a revised erosion and sediment control plan
must be submitted to the Borough or Conservation District for approval.
(4)
All stormwater management facilities must be located on a plan
and described in detail; and all calculations, assumptions, and criteria
used in the design of the stormwater management facilities must be
shown. Plans must be designed by a design professional.
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
C.
Earth disturbances under 0.25 acre (10,890 square feet) require
plans in accordance with the project standardized SWM guidance document
provided in Appendix E and the site plan provisions in the Small Project
stormwater management site plan pursuant to Appendix C. These appendices
were prepared to assist applicants in meeting this requirement for
individual lots only.[2]
[2]
Editor's Note: Said appendices are included as attachments to this chapter.
D.
Agricultural activity is exempt from the SWM site plan preparation
requirements of this chapter provided the activities are performed
according to the requirements of 25 Pa. Code Chapter 102.
E.
Forest management and timber operations are exempt from the
SWM site plan preparation requirements of this chapter provided the
activities are performed according to the requirements of 25 Pa. Code
Chapter 102.
F.
Roadway resurfacing and maintenance projects, which do not increase
impervious area, and underground infrastructure projects are exempt
from the provisions of this chapter, provided the activities meet
the requirements of all other Municipal, State and Federal requirements.
G.
Gravel driveways: An existing gravel driveway is considered
semipervious, and the conversion of an existing gravel driveway to
a conventional paved driveway that is less than 1,600 square feet
shall be exempt from the requirements of this chapter to implement
SWM BMPs. The conversion of an existing gravel driveway to a conventional
paved driveway that is at least 1,600 square feet but not more than
5,000 square feet may be accomplished using standardized BMPs. The
Stormwater Management Planning Guidelines for Existing Gravel Driveways
document provided in Appendix D contains two tables labeled "Gravel
Driveways" and has been prepared to assist applicants in meeting this
requirement for individual lots only.[3]
[3]
Editor's Note: Appendix D is included as an attachment to this chapter.
H.
Exemptions from any provisions of this chapter shall not relieve
the applicant from the requirements in § 178-301D through
J.
I.
The Borough may deny or revoke any exemption pursuant to this
section at any time for any project that the Borough believes may
pose a threat to public health and safety or the environment.
J.
Voluntary Green Stormwater Infrastructure (GSI) retrofit projects
that are solely intended to better manage runoff from existing development
and are not part of new development or redevelopment, are exempt from
the stormwater management provisions of this chapter. This does not
exempt such projects from any other municipal, state, or federal regulation.
K.
A stormwater management plan will
not be required for regulated activities previously approved by the
Borough after January 1, 2008.
[Added 12-18-2019 by Ord. No. 655-2019]
The green infrastructure and low-impact development practices
provided in the BMP Manual shall be utilized for all regulated activities
wherever possible. Water volume controls shall be implemented using
the Design Storm Method in Subsection A or the Simplified Method in
Subsection B below. Water volume controls shall be implemented using
the Design Storm Method in Subsection A or the Simplified Method in
Subsection B below, or alternative design criteria as allowed by PA
Code Title 25, Chapter 102.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable
as a method to any size of regulated activity. This method requires
detailed modeling based on site conditions. The following shall be
incorporated into the CG-1 method:
(1)
Do not increase the post-development total runoff volume for
all storms equal to or less than the two-year twenty-four-hour duration
precipitation.
(2)
At least the first one inch of runoff from the net increase
in impervious surfaces shall be permanently removed from the runoff
flow, i.e., it shall not be released into the surface waters of this
commonwealth. Removal options include reuse, evaporation, transpiration,
and infiltration. If the developer provides justification that the
listed removal options are not feasible, and the designated plan reviewer
agrees, runoff shall be detained in a facility designed for a twenty-four-to-seventy-two-hour
dewatering time in an area with a dedicated stormwater system (not
contributory to a combined sewer system) and shall be detained in
a facility designed for a seventy-two-hour dewatering time in an area
contributory to a combined sewer system before discharge to local
stormwater systems or the environment.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below
is independent of site conditions and should be used if the Design
Storm Method is not followed. This method is not applicable to regulated
activities greater than one acre or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from the net increase in impervious surfaces.
(2)
At least the first one inch of runoff from the net increase
in impervious surfaces shall be permanently removed from the runoff
flow, i.e., it shall not be released into the surface waters of this
commonwealth. Removal options include reuse, evaporation, transpiration,
and infiltration. If the developer provides justification that the
listed removal options are not feasible, and the designated plan reviewer
agrees, runoff shall be detained in a facility designed for a twenty-four-hour
dewatering time in an area with a dedicated stormwater system (not
contributory to a combined sewer system) and shall be detained in
a facility designed for a seventy-two-hour dewatering time in an area
contributory to a combined sewer system before discharge to local
stormwater systems or the environment.
(3)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
(4)
This method is exempt from the requirements of § 178-304,
Rate controls.
A.
For areas not covered by a release rate map from an approved
Act 167 Stormwater Management Plan: Post-development discharge rates
shall not exceed the predevelopment discharge rates for the one-,
two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour
storm events. This is the equivalent to a 100% release rate area when
compared to those rates shown in the maps contained in Appendix A
(New and Existing Release Rate Management Districts).[1] This predevelopment to post-development control is not
to be misconstrued as the same as the "conditional direct discharge"
areas on the release rate maps. If it is shown that the peak rates
of discharge indicated by the post-development analysis are less than
or equal to the peak rates of discharge indicated by the predevelopment
analysis for one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year,
twenty-four-hour storms, then the requirements of this section have
been met. Otherwise, the applicant shall provide additional controls
as necessary to satisfy the peak rate of discharge requirement. Peak
flows should be computed using the methods included in the chapter
titled "Stormwater Calculations and Methodology" of the PADEP Stormwater
Management BMP Manual.
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
B.
The post-development peak discharge
rate shall not exceed the predevelopment peak discharge rate multiplied
by the subbasin release rate percentage for the one-year, two-year,
five-year, ten-year, twenty-five-year, fifty-year, and 100-year twenty-four-hour
storm events pursuant to the predevelopment cover assumption described
in § 178-306.
[Amended 12-18-2019 by Ord. No. 655-2019]
A.
In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as an article 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 PA Code Title 25 Chapter 102, the riparian
buffer easement shall be required for all streams (as defined in Article
200) 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 streambank (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 § 178-305A.
(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 Borough and shall be recorded with the Allegheny County Department of Real Estate, 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 Chapter 212 of the Borough's Code of Ordinances, Zoning.
E.
Any permitted use within the riparian buffer easement shall
be conducted in a manner that will maintain the extent of the existing
100-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 Borough within riparian buffers:
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.
[Amended 12-18-2019 by Ord. No. 655-2019]
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:
|
Twenty-Four-Hour Peak Flows for Watersheds
| |
|---|---|
|
Return Period
(years)
|
24-Hour Storm
(inches)
|
|
1
|
1.97
|
|
2
|
2.35
|
|
5
|
2.87
|
|
10
|
3.29
|
|
25
|
3.89
|
|
50
|
4.39
|
|
100
|
4.90
|
(1)
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 are 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), 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, logged area shall be considered
as woodland in good condition. The intent of this section is to recognize
the existing woodland conditions and not inadvertently encourage tree
harvesting.
(b)
Agricultural sites shall
use a ground cover of pasture in good condition.
(c)
All other portions of
a site shall use the appropriately designated ground cover in good
condition.
(d)
All watershed area(s)
contributing to the point of interest, including off-site area, shall
be considered.
(e)
For redevelopment sites,
see § 178-307.
(2)
The runoff curve numbers listed in
the table below shall be used in developing the runoff calculations
for the ground covers noted in § 178-307B. 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.
|
Runoff Curve Numbers for Ground Covers
| ||||
|---|---|---|---|---|
|
Ground Cover
|
Hydrologic Soil Group Curve Numbers
| |||
|
A
|
B
|
C
|
D
| |
|
Woodland
|
30
|
55
|
70
|
77
|
|
Meadow (unmowed)
|
30
|
58
|
71
|
78
|
|
Grass (lawn)
|
39
|
61
|
74
|
80
|
|
Pasture (grazed)
|
39
|
61
|
74
|
80
|
|
Brush
|
30
|
48
|
65
|
73
|
(3)
Impervious cover for the predevelopment
condition shall have a runoff curve number of 95, and the impervious
cover for the post-development condition shall have a runoff 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.
(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 should be applied as per the following table. (Values
taken from TR-55.)
|
Runoff Curve Numbers for Ground Covers
| |
|---|---|
|
Ground Cover
|
Kinematic "n" 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 runoff 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 are 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.
|
Acceptable Computation Methodologies
for Stormwater Management Plans
| ||
|---|---|---|
|
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 full model is desirable or necessary
|
|
PSRM
|
Penn State University
|
When full model is desirable or necessary
|
|
VT/PSUHM
|
Virginia Polytechnic Institute and
Penn State University
|
When full model is desirable or necessary
|
|
Modified Rational Method or commercial
package based on this method
|
Emil Kuiching (1889)
|
For drainage areas less than or equal
to 2 acres
|
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 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):
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.
[Added 12-18-2019 by Ord. No. 655-2019]
A.
Design goals, principles, and standards.
(1)
Design goals. Applicants shall adhere
to a holistic design process incorporating the goals listed below.
The objective is to achieve post-development hydrologic conditions
that are consistent with the predevelopment ground cover assumption
for new development and improve the runoff conditions for redevelopment.
The design goals are:
(a)
Minimize the volume
of runoff that must be collected, conveyed, treated and released by
stormwater management facilities. Site design should implement runoff
reduction techniques such as those described in Chapters 4 and 5 of
the Pennsylvania Stormwater BMP Manual, latest revision.
(b)
Water quality requirements
can be met using BMPs to control stormwater impacts from regulated
areas. Replication of predevelopment volume and peak runoff levels
shall be met to avoid degradation of the physical, chemical and/or
biological characteristics of the receiving waters. This will be achieved
by:
(c)
Maintain the natural
infiltration process and rate, and infiltrate runoff at its source
when appropriate. Avoid development and/or compaction of soils where
infiltration rates are above two inches per hour.
(d)
Remove and/or treat
pollutants at the source or during conveyance.
(f)
Attenuate peak volume
runoff to protect the in-stream channel of the receiving stream.
(g)
For these goals to be
met, the applicant shall submit approved, original and/or innovative
designs. Original/innovative designs will require supporting documentation
to verify that their discharge will meet the Clean Water Act.[2] Pennsylvania Stormwater BMP Manual, and this chapter.
[2]
Editor's Note: See 35 P.S. § 691.1 et
seq.
(2)
General principles. The following
general principles apply to all applicable activities under this chapter:
(a)
Incorporate conservation
design practices to minimize the amount of stormwater generated on
a site, encourage the disconnection of impervious land cover, and
maximize the use of pervious areas for stormwater treatment and on-site
rainfall infiltration.
(b)
Infiltration of surface
water runoff at its source is to be a mechanism for stormwater management
based on hydrologic soil group (or infiltration testing). Selection
of infiltration practices can be influenced by those factors referenced
in § 178-307B(2)(a) and as outlined in the publications listed
in § 178-307B. Infiltration practices shall adhere to the following
criteria:
[2]
Applicants shall first
consider minimum disturbance/minimum maintenance techniques combined
with site grading that distributes runoff to reduce concentration.
Next, applicants shall consider depression areas combined with subsurface
infiltration practices followed by other subsurface measures, including,
but not limited to, porous paving and perforated pipe storage.
[4]
The temperature and
water quality from subwatersheds within or discharging into receiving
waters classified as EV or HQ (PADEP Chapter 93) shall be less than
or equal to predevelopment levels.
[5]
Where high-water tables,
subsurface contamination, slope stability concerns, or other site
constraints preclude achieving the required infiltration volume, additional
conservation design practices and alternative stormwater management
practices should be implemented to reduce to the maximum extent practicable
the total volume of stormwater released to streams. The applicant
shall follow the stormwater runoff hierarchy of § 178-307B(1)(a).
[6]
Infiltration areas should
be designed to maintain any broad and even infiltration pattern which
existed prior to development. Such facilities should use the natural
topography and vegetation in order to blend in with the site. Infiltration
designs which do not provide this may be used if the applicant demonstrates
to the municipality's satisfaction that alternative approaches would
be more effective, more harmonious with their existing environment
and as easily maintained.
[7]
Aboveground stormwater
infiltration facilities should be as shallow as possible while still
achieving the requirements of this chapter.
(c)
Water quality improvement
shall be achieved in conjunction with or as part of infiltration practices.
Water quality improvements shall also be provided for drainage areas
not otherwise addressed by infiltration practices either at the source
of runoff and/or during conveyance away from the source of runoff.
(d)
To reduce the need for
large retention and/or detention basins designed to satisfy the peak
flow attenuation and extended detention requirements, other innovative
stormwater management practices located close to the source of runoff
generation shall be considered, including a combination of practices
(e.g., rooftop storage, open vegetated channels, bioretention, pervious
pavement systems and infiltration trenches).
(e)
When designing stormwater
management facilities to satisfy the peak flow attenuation and extended
detention requirements, the effect of structural and nonstructural
stormwater management practices implemented as part of the overall
site design may be taken into consideration when calculating total
storage volume and release rates.
(f)
Site hydrology and natural
infiltration patterns shall guide site design, construction and vegetation
decisions. All channels, drainageways, swales, natural streams and
other surface water concentrations shall be considered and where possible
incorporated into design decisions.
(3)
Minimum performance criteria.
(a)
The following minimum
performance standards shall apply to all applicable activities, whether
they are new development or redevelopment, under this chapter:
[1]
Water quality treatment
of stormwater runoff shall be provided for all discharges prior to
release to a receiving water body. If a stormwater management practice
does not provide water quality treatment, then water quality best
management practices shall be utilized prior to the runoff entering
the stormwater management practice.
[2]
Water quality management
shall be provided through the use of structural and/or nonstructural
stormwater management practices. Water quality stormwater management
practices shall be designed to reduce or eliminate solids, sediment,
nutrients and other potential pollutants from the site. It is presumed
that a stormwater management practice complies with this requirement
if it is:
[a]
Designed according to
the specific performance criteria outlined in the Pennsylvania Stormwater
Best Management Practices Manual;
[b]
Constructed in accordance
with all permits and approved plans and specifications; and
[c]
Maintained per an approved
operation and maintenance plan or agreement or, in lieu of that, in
accordance with customary practices.
[3]
Stormwater discharges
from land uses or activities with higher potential for pollutant loadings
(hot spots) may require the use of specific structural stormwater
management practices and pollution prevention practices. In addition,
stormwater from a hot spot land use shall be provided with proper
pretreatment prior to infiltration. For the purpose of this chapter,
the sites/facilities listed in § 178-307A(3)(a)[4] below are
considered hot spots.
[4]
Certain industrial sites
may be required to prepare and implement a stormwater pollution prevention
plan and file notice of intent as required under the provision of
the EPA industrial stormwater NPDES permit requirements. Other industrial
sites storing significant quantities of chemicals/wastes should also
prepare a prevention plan. Owners of properties keeping or storing
significant quantities of hazardous materials shall follow the requirements
as specified in the current requirements of the Pennsylvania Construction
Code Act[3] and its regulations. Sites that are required by EPA to
prepare a plan include, but are not limited to:
[a]
Vehicle salvage yards
and recycling facilities;
[b]
Vehicle and equipment
cleaning facilities;
[c]
Fleet storage areas
for buses, trucks, etc.;
[d]
Marinas (service and
maintenance);
[e]
Facilities that generate
or store hazardous materials.
[3]
Editor's Note: See 35 P.S. § 7210.101
et seq.
[5]
Conveyance structures/channels
shall be designed and adequately sized so as to protect the properties
receiving runoff from impacts of flooding and erosion. Where necessary,
and to the maximum extent permitted under the Municipalities Planning
Code[4] and Act 167, or any amendments thereto, drainage easements
from adjoining properties shall be obtained to ensure the drainageway
and the property and shall also establish the operation and maintenance
requirements for the drainageway. Storm drainage systems shall be
designed to convey the 100-year storm event to the stormwater management
facility or BMP to which they discharge.
[4]
Editor's Note: See 53 P.S. § 10101 et
seq.
[6]
All stormwater management
practices shall have an operation and maintenance plan, and if to
be privately owned, an enforceable operations and maintenance agreement
per § 178-502 of this chapter to ensure the system functions
as designed and to provide remedies for system failure.
[7]
Stormwater runoff generated
from development and discharged directly into a jurisdictional wetland
or waters of the United States and their adjacent wetlands shall be
treated by an approved stormwater management practice prior to release
into a natural wetlands and shall not be used to meet the minimum
design requirements for stormwater management or stormwater runoff
quality treatment, except when used as part of a treatment train that
incorporates a portion of the outer zone (filter strip) of the wetland's
riparian buffer as a stormwater outfall. In such instances, the discharge
velocity from the terminal end of a pipe or associated energy-dissipation
practice shall not exceed two feet per second for the two-year frequency
storm event. Where such a management strategy is used, all feasible
methods shall be used to convert concentrated flow to uniform, shallow
sheet flow before entering the outer zone of the wetland's riparian
buffer. In addition, it shall be demonstrated that such an approach
will not cause erosion.
(b)
The following minimum
performance standards shall apply to all applicable new development
activities under this chapter:
[1]
Water quality improvement
shall be achieved in conjunction with or as part of infiltration practices
(if used). Water quality improvements shall also be provided for drainage
areas not otherwise addressed by infiltration practices either at
the source of runoff and/or during conveyance away from the source
of runoff. Stormwater quality management practices shall be designed
to capture and treat stormwater runoff generated by the one-inch rainfall
event. Refer to § 178-307B(1)(b) for water quality volume design
standards and assumptions. Stormwater quality management practice
selection, design and implementation shall be based upon appropriate
reference materials.
[2]
The post-development
peak discharge rate shall not exceed the predevelopment peak discharge
rate multiplied by the subbasin release rate percentage for the two-year,
ten-year, twenty-five-year, and 100-year twenty-four-hour storm events
pursuant to the predevelopment cover assumption.
[3]
Facilities capable of
attenuating rainfall runoff shall be provided and be designed to attenuate
the runoff volume from the one-year, twenty-four-hour storm event
for at least 24 hours.
[4]
Stormwater shall be
infiltrated and/or discharged within the same drainage area of the
stream receiving the runoff from the development site prior to development.
[5]
Structural and nonstructural
stormwater management practices that make best possible use of infiltration
on site shall be considered in all site designs, when appropriate.
(c)
The following minimum
performance standards shall apply to all applicable redevelopment
activities:
[1]
One of the following
minimum performance standards shall be accomplished. Selection of
the performance standard shall be whichever is most appropriate for
the given site conditions:
[a]
Reduce the total impervious
cover on the site (e.g., by using pervious pavement, replacement of
pavement with pervious planting areas or green roof systems) by at
least 25%, based on a comparison of existing impervious cover to proposed
impervious cover; or
[b]
Provide facilities designed
to attenuate the runoff volume from the one-year, twenty-four-hour
post-development storm event for at least 24 hours; or
[c]
Provide facilities to
ensure that the post-development peak discharge rate shall not exceed
the predevelopment peak discharge rate multiplied by the subbasin
release rate percentage for the two-year and ten-year, twenty-four-hour
storm events. A predevelopment cover CN of 71 shall be assumed.
[2]
In addition to the minimum
performance standards for redevelopment areas, water quality improvements
shall be provided for drainage areas not otherwise addressed by infiltration
practices either at the source of runoff and/or during conveyance
away from the source of runoff. Stormwater quality management facilities
shall be designed to capture and treat 1/4 inch of runoff from all
pavement areas (i.e., parking lots, pavements and noncovered sidewalks).
Roof area may be excluded from this calculation.
B.
Standards for stormwater management
practices. 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. A list of references is provided with this chapter.
The water quality volume design measures used herein are partially
based 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:
(1)
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.
(a)
Extended detention.
[1]
Detain the one-year,
twenty-four-hour design storm 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.
[2]
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 100-year storm events shall be placed
above the maximum water surface elevation of the one-year storm.
[3]
Flow from off-site areas
must be considered as pass-through flow if it is conveyed through
the BMP and should be modeled as "present condition" for the one-year
storm event.
[4]
The length of overland
flow used in time-of-concentration (tc) calculations
is limited to no more than 100 feet for post-development conditions.
[5]
The models TR-55 and
TR-20 (or approved equivalent) can be used for determining peak discharge
rates.
(b)
Water quality volume.
[1]
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 (Impervious Area/Total Area x 100%)
A = site area (acres)
[2]
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.
[3]
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.
[4]
Stormwater quality treatment.
The final WQv shall be treated by an acceptable stormwater management
practice(s) from those described in this section or as approved by
the municipality.
[5]
For new developments
and redevelopments, infiltration is considered an acceptable method
of satisfying part or all of the water quality volume.
[6]
For new developments,
the WQv requirements of this section shall be sized and designed in
conjunction with the standards under § 178-307B(1)(a).
[7]
As a basis for design,
the following assumptions may be made:
[a]
Multiple drainage areas.
When a project contains or is divided by multiple drainage areas,
the WQv volume shall be addressed for each drainage area.
[b]
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.
[c]
Infiltration volume.
Where possible, all of the water quality volume should 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)(Ai); where S = soil specific recharge factor in inches
Ai = the measured impervious cover
|
Soil Recharge Factors(S) for Hydrologic
Soil Groups
| |
|---|---|
|
Hydrologic Soil Group
|
Soil-Specific Recharge Factor (S)
|
|
A
|
0.40 inch
|
|
B
|
0.25 inch of runoff
|
|
C
|
0.10 inch of runoff
|
|
D
|
0.05 inch of runoff
|
[i]
Infiltrated volume may
be subtracted from the total site WQv.
[ii]
Infiltration should
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.
[iii]
Infiltration should
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.
[iv]
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.
[v]
All infiltration facilities
shall be set back at least 15 feet from all structures with subgrade
elements (e.g., basements, foundation walls).
(c)
Credits for use of nonstructural
BMPs. The developer may obtain credits for the use of nonstructural
BMPs, as well as within the Pennsylvania Best Management Practice
Manual.
(2)
Stormwater infiltration practices.
(a)
In selecting the appropriate
infiltration BMPs, the applicant shall consider the following:
[1]
Permeability and infiltration
rate of the site soils.
[2]
Slope and depth to bedrock.
[3]
Seasonal high-water
table.
[4]
Proximity to building
foundations and wellheads.
[5]
Erodibility of soils.
[6]
Land availability and
topography.
[7]
Slope stability.
[8]
Effects on nearby properties
and structures.
(b)
A detailed soils evaluation
of the project site shall be performed to determine the suitability
of infiltration BMPs and the extent to which infiltration systems
can be used. The evaluation shall be performed by a qualified professional
and, at a minimum, address soil types, soil permeability, depth to
bedrock, limitations of soil, presence/absence of carbonate geology,
susceptibility to subsidence and/or sinkhole formation, and stability
of subgrade and slope. The testing and evaluation shall be completed
at the preliminary design stage. 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 BMPs.
[2]
Provide field-testing
data to determine appropriate percolation rate and/or hydraulic connectivity.
[3]
Design infiltration
BMPs for required stormwater volume based on field-determined capacity
at the level of the proposed infiltration surface.
[4]
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 should
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.
[5]
Soil infiltration tests
shall be performed to a minimum depth or elevation of four feet below
the bottom of the proposed infiltration areas. These tests shall follow
the procedures of percolation test holes as established by Appendix
C of the Pennsylvania Stormwater BMP Manual 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 Appendix C of the
Pennsylvania Stormwater BMP Manual. 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.
(c)
Soil characteristics,
as subject to the specific considerations below:
[1]
Infiltration BMPs are
particularly appropriate in hydrologic soil groups A and B, as described
in the Natural Resources Conservation Manual TR-55.
[2]
Low-erodibility factors
("K" factors) are preferred for the construction of basins.
[3]
There must 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.
[4]
There must be an infiltration
and/or percolation rate sufficient to accept the additional stormwater
load and to drain completely as determined by field tests.
[5]
The infiltration system
shall have positive overflow controls to prevent storage within one
foot of the finished surface or grade.
[6]
Infiltration rates shall
not be used in computing the storage volume of the infiltration system.
[7]
Surface inflows shall
be designed to prevent direct discharge of sediment into the infiltration
system.
(d)
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.
(e)
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.
(g)
In susceptible areas,
the applicant is required to provide:
[1]
Design(s) addressing
these special issues;
[2]
A verification of the
presence or nonpresence of limestone;
[3]
A detailed geologic
report describing the findings and recommendations; and
[4]
A statement signed and
sealed by a licensed geologist affirming: "I certify that the proposed
facility is/is not underlain by limestone."
(h)
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.
(i)
Infiltration BMPs shall
not be constructed nor receive runoff until the entire contributory
drainage area to the infiltration BMP has received final stabilization.
(j)
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).
(k)
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.
(l)
All infiltration facilities
shall be set back at least 15 feet from all structures with subgrade
elements (e.g., basements, foundation walls).
(m)
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
municipality the right of access.
(n)
The following design
and construction standards shall be followed when planning and constructing
infiltration BMPs:
[1]
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, cleanouts and pretreatment
chambers.
[2]
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.
[3]
All infiltration facilities
shall be designed to infiltrate the stored volume within 48 hours
after the end of the twenty-four-hour design storm.
[4]
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.
[5]
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.
[6]
The following procedures
and materials shall be required during the construction of all subsurface
facilities:
[a]
All excavation should
occur from outside of the filtration facility. If not possible to
construct from the outside, excavation shall be performed by a light,
rubber-tired piece of equipment to avoid compacting the bottom or
sides of the seepage bed/trench or like facility. All construction
equipment traffic must also be kept to an absolute minimum.
[b]
The bottom of the bed
and/or trench shall be scarified prior to the placement of aggregate.
[c]
Only clean-washed aggregate
with documented porosity, free of fines, shall be allowed.
[d]
The bottom, top 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.
[e]
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. An observation well(s) and cleanout(s) shall be
provided.
[f]
Pretreatment shall be
provided upstream of each unit. Vegetation shall not be the only measure
provided.
(3)
Open vegetated channels.
(a)
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.
(b)
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.
(c)
Open vegetated channels
shall be designed to meet the following minimum standards:
[1]
The channel shall be
designed to safely convey the ten-year (or design storm) 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.
[2]
The peak velocity of
the runoff from the ten-year storm (or design storm) shall be nonerosive
for the soil and ground cover provided in the channel.
[3]
The longitudinal slope
shall be no greater than 4%.
[4]
Channels shall be trapezoidal
in cross section.
[5]
Channels shall be designed
with moderate side slopes of four horizontal to one vertical. Flatter
side slopes may be necessary under certain circumstances.
[6]
The maximum allowable
ponding time in the channel shall be less than 48 hours.
[7]
Channels (for example,
dry swales) may require an underdrain in order to function and dewater.
[8]
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.
[9]
Landscape specifications
shall address the grass species, wetland plantings (if applicable),
soil amendment and hydric conditions present along the channel.
[10]
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.
[11]
Check dams along the
channel length may be warranted.
[12]
The bottom of dry swales
shall be situated at least two feet above the seasonal high-water
table.
(d)
Additional design information
for open vegetated channels is available in Design of Roadside Channels
with Flexible Linings, HEC 15, FHWA, September 2005.
(4)
Retention basins.
(a)
Retention basins (retention
ponds) 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.
The minimum contributing drainage area to a retention basin without
a groundwater source shall be four acres. The area of the basin may
be limited by the drainage area size. If the groundwater source and/or
drainage area cannot maintain the water surface elevation necessary
to sustain the ecological community, clean water from an outside source
shall be provided by the responsible organization.
(b)
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, Penn State Cooperative Extension,
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.
(c)
The inflow from the
groundwater source shall be estimated by a qualified professional
and included in the geologic report.
(d)
An outlet structure
shall be designed to allow complete drainage of the pond for maintenance.
(e)
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.
(f)
Sediment storage volume
equal to at least 20% of the volume of the permanent pool shall be
provided.
(g)
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% of the forebay storage has been used.
(h)
Emergency spillways
shall be sized and located to permit the safe passage of stormwater
flows from an unattenuated 100-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.
(i)
Basin and pond embankments
must be designed by a professional engineer registered in the Commonwealth
of Pennsylvania. The design must 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 must be subject to approval of the design engineer.
Fill must 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 must 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
must be compacted to at least 90% of the maximum density as determined
by standard proctor (ASTM-698) and must 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.
(j)
The embankment's interior
slope may 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).
(k)
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.
(l)
Existing ponds or permanent
pool basins can be used for stormwater management, provided that it
can be demonstrated that the ponds are structurally sound and meet
the design requirements herein.
(m)
Inlet structures and
outlet structures shall be separated to the greatest extent possible
in order to maximize the flow path through the retention basin.
(n)
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).
(o)
The retention basin
depth shall average three feet to six feet.
(p)
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.
(q)
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.
(r)
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.
(s)
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.
(t)
Discharge points. The
minimum distance between a proposed basin discharge point (including
the energy dissipater, 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.
(u)
Outlet structures. Outlet
structures shall meet the following specifications:
[1]
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%.
[2]
Bentonite plugs shall
be provided on all outlet pipes within a constructed berm.
[3]
All principal outlet
structures shall be built using reinforced concrete with watertight
construction joints.
[4]
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.
[5]
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.
[6]
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.
[7]
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.
(5)
Detention basins.
(a)
The landscape standards
of § 178-307C shall apply.
(b)
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 the designer demonstrates to the municipality's
satisfaction that the basin bottom will be landscaped with appropriate
wetland vegetation. 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.
(c)
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.
(d)
Inlet structures and
outlet structures shall be separated to the greatest extent possible
in order to maximize the flow path through the detention basin.
(e)
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.
(f)
Outlet structures. Outlet
structures shall meet the following specifications:
[1]
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%.
[2]
Bentonite plugs shall
be provided on all outlet pipes within a constructed berm.
[3]
All principal outlet
structures shall be built using reinforced concrete with watertight
construction joints.
[4]
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.
[5]
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.
[6]
Energy-dissipation facilities
that convert concentrated flow to uniform shallow sheet flow shall
be used where appropriate.
[7]
Basin outlet structures
shall have childproof nonclogging trash racks overall design openings
exceeding 12 inches in diameter except those openings designed to
carry perennial stream flows.
[8]
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.
(g)
Emergency spillways
shall be sized and located to permit the safe passage of stormwater
flows from an unattenuated 100-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 should 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.
(h)
Basin and pond embankments
must be designed by a professional engineer registered in the State
of Pennsylvania. The design must 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 must be subject to the approval of the design engineer.
Fill must 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 must 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
must be compacted to at least 90% of the maximum density as determined
by standard proctor (ASTM-698) and must 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.
(i)
The embankment's interior
slope may 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).
(j)
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.
(k)
Fencing of the facility
is not required if the interior slope of the pond is 4:1 or flatter.
(l)
Freeboard. Freeboard
is the difference between the elevation of the design flow in the
emergency spillway (usually the 100-year peak elevation) and the top
elevation of the settled basin embankment (that is, top of berm).
The minimum freeboard shall be one foot.
(m)
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.
(n)
Stabilization. Proper
stabilization structures, including stilling basins, energy dissipators
and channel linings, 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.
(o)
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.
(p)
A sediment forebay with
a hardened bottom shall be provided at each inlet into the detention
basin. The forebay storage capacity shall, at a minimum, be 10% of
the permanent pool storage. The forebay shall be designed to allow
for access by maintenance equipment for periodic cleaning.
(q)
All detention facilities
shall be provided with an access road for maintenance purposes. Such
roads shall be a minimum of 10 feet in width and have a maximum grade
of 15%.
(6)
Conveyance systems (open channels,
drainageways, and storm sewers).
(a)
Applicants are encouraged
to design conveyance systems that encourage infiltration and improve
water quality wherever practicable.
(b)
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.
(c)
Flood relief channels
shall be provided and designed to convey the runoff from the 100-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.
(d)
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.
(e)
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 § 178-307C(3) of this chapter.
(f)
Street curbing for the
purpose of stormwater conveyance in rural areas 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.
(g)
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.
(h)
The developers 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.
(i)
No property owner 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.
(j)
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 municipality'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 municipality.[5]
[5]
Editor's Note: Former Subsection B(6)(k), regarding other
best management practices, which immediately followed, was repealed 10-18-2023 by Ord. No.
682-2023.
(7)
Underground detention facilities.
[Added 10-18-2023 by Ord. No. 682-2023]
(a)
Underground detention tanks shall utilize the largest practicable
pipe diameter in order to provide ease of access and maintenance.
(b)
Design measures must be implemented to prevent pipe flotation and
allow for visual inspection capabilities, adequate pipe ventilation,
and access maintenance.
(c)
Underground facilities shall be designed parallel to existing and/or
proposed contours.
(d)
The underground facilities shall be designed to include overflow
controls able to pass the unattenuated 100-year-peak storm without
overtopping the access point.
(e)
A minimum forty-eight-inch diameter manhole with steps must be provided
at either end of the facility and at all terminal ends and junctions
for maintenance purposes. A minimum of two accesses must be provided.
(f)
Maximum life expectancy of the underground system shall also be a
design consideration for pipe material specifications. The use of
corrugated metal pipe (CMP) for underground facilities proposed for
ownership and maintenance by the Borough is prohibited.
(g)
For underground rock sumps, refer to Appendix E of this chapter.[6]
[6]
Editor's Note: Appendix E is included as an attachment to this chapter.
(8)
Other best management practices that appear within the Pennsylvania
Best Management Practice Manual may be utilized within the Borough.
However, use of BMPs not described within this chapter will be subject
to a thorough review by the Borough Engineer and approval from the
Borough prior to implementation. This would include, but is not limited
to, such practices as bioretention, rain barrels, sand filters, etc.
[Added 10-18-2023 by Ord. No. 682-2023]
C.
Landscaping of stormwater management
facilities. Stormwater management facilities shall be landscaped in
accordance with the following standards:
(1)
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:
(2)
A planting plan and planting schedule
shall be submitted in accordance with the following:
(a)
Wet meadows, including
floors of stormwater management facilities.
[1]
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.
[2]
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.
[3]
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.
[4]
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.
[5]
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.
(b)
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.
(c)
Wooded areas.
[1]
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.
[2]
Plantings in such areas
shall be of sufficient density to eliminate the need for mowing.
[3]
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.
[4]
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.
(d)
Slopes.
[1]
Where slopes are gentle,
a mixture of meadow grasses and wildflowers (for wet meadows) shall
be planted.
[2]
On steep slopes as defined
by the municipality's Code of Ordinances, dense spreading shrubs (shrubs
tolerant of dry soils) shall be planted. Heavy mat mulch shall be
used during the period of establishment.
[3]
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.
(e)
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.
(f)
Stormwater management
facilities shall be screened in a manner which complements the existing
landscape and provides sufficient access for maintenance.
(g)
All landscaping plans
must be sealed by a professional landscape architect (PLA) registered
in the Commonwealth of Pennsylvania.
D.
Stream buffer requirements. Stream
buffers shall be provided for new development sites as per the following
requirements:
(1)
A minimum stream buffer width of
50 feet landward in each direction from the top of stream banks is
required for all waterways having both a defined bank and a contributing
watershed area of greater than 100 acres.
(2)
A minimum stream buffer width of
15 feet landward in each direction from the center line of the waterway
is required for smaller waterways having a contributing watershed
area of less than 100 acres and greater than 10 acres.
(3)
The stream buffer area should be
maintained in a natural state.
(4)
When wetland(s) extend beyond the
edge of the required buffer width, the buffer shall be adjusted so
that the buffer consists of the extent of the wetland plus a twenty-five-foot
zone extending beyond the wetland edge.
(5)
Stream buffer averaging may be applied
to account for encroachments such as road crossings. The following
criteria must be met in order to utilize buffer averaging on a development
site:
(a)
Buffer averaging is
required for water quality buffers that have stream crossings.
(b)
An overall average buffer
width of at least 50 feet must be achieved within the boundaries of
the property to be developed. Stream buffer corridors on adjoining
properties cannot be included with buffer averaging on a separate
property, even if owned by the same property owner.
(c)
The average width must
be calculated based upon the entire length of stream bank that is
located within the boundaries of the property to be developed. When
calculating the buffer length, the natural stream channel should be
followed.
(d)
Stream buffer averaging
shall be applied to each side of a stream independently. If the property
being developed encompasses both sides of a stream, buffer averaging
can be applied to both sides of the stream but must be applied to
both sides of the stream independently.
(e)
On each stream bank,
the total width of the buffer shall not be less than 25 feet at any
location, except at approved stream crossings. Those areas of the
buffer having a minimum width of 25 feet (or less at approved stream
crossings) can comprise no more than 50% of the buffer length.
(6)
Stream buffer locations and widths
should be illustrated on all subdivision plans with notations requiring
that they be maintained in a natural state.
(7)
Stream buffers should be illustrated
on all grading and erosion and sedimentation control plans. The defined
stream buffer location should be properly recorded. The recording
should provide a plan illustrating the stream buffer location, width
and the requirement that it be maintained in a natural state.
