This article is adopted and implemented to achieve the following
general purposes and objectives:
B.
To utilize and preserve the desirable existing natural drainage systems
and to preserve the flood-carrying capacity of streams.
C.
To encourage natural infiltration of rainfall to preserve groundwater
supplies and stream flows.
D.
To provide for adequate maintenance of all permanent stormwater management
facilities in the municipality.
The provisions of this article shall apply to all subdivisions
and land developments, unless specifically exempted or otherwise modified
herein.
A.
General requirements. No final subdivision or land development plan
shall be approved, no permit authorizing construction issued, nor
any earthmoving or land disturbance activity initiated until the final
stormwater management plan for the development site is approved in
accordance with the provisions of this article.
B.
Exemptions for small developments.
(1)
At the time of application, the Township shall determine if the subdivision
or land development qualifies as a small development and, therefore,
is eligible for a simplified stormwater plan submission. For the purposes
of this article, a "small development" is any subdivision or land
development which results (or will result when fully constructed)
in the creation of 3,000 or less square feet of impervious surface
area.
(2)
Small developments shall be exempt from the preparation of a stormwater management plan as specified by Subsection C and D of this section. However, such developments must still provide safe management of stormwater runoff in accordance with the performance standards of § 350-48B of this article and as approved by the Township Engineer. Any stormwater management facilities constructed as part of the development shall be designed to control the peak stormwater runoff in accordance with §§ 350-48 and 350-49 of this article.
(3)
Applications for small developments shall include a plan which describes,
narratively and graphically, the type and location of proposed on-site
stormwater management techniques or the proposed connection to an
existing storm sewer system. The plan should show accurately site
boundaries; contours at two-foot intervals for all sites, with spot
elevation for relatively level tracts; location of watershed and/or
subarea boundaries on the site (if applicable); and any watercourses,
floodplains or existing drainage facilities or structures located
on the site.
(4)
Stormwater management plans for small developments do not have to be prepared by a registered professional engineer or land surveyor. Whenever the submission of runoff calculations is required by the Township Engineer, they shall be prepared in accordance with § 350-49 of this article.
(5)
The Township Engineer shall review and approve the proposed provisions
for stormwater management for a small development. Where the applicant
is proposing to connect to an existing storm sewer, the applicant
shall demonstrate that sufficient capacity exists in the storm sewer
from the point of connection to the point of outlet in the natural
drainage system. The Township Engineer shall determine if the proposed
development site is part of a larger parcel or tract for which a stormwater
management plan was approved previously and, therefore, subject to
any specific stormwater management controls contained in the prior
plan.
(6)
For a parcel or tract of land held under single ownership, only one
application for a small development as defined above shall be permitted
before requiring a stormwater management plan for the entire parcel.
C.
Stormwater plan contents: preliminary S/LD plan submission.
(1)
General format. The stormwater plan shall be prepared using the general
requirements for plan format contained in this section with the following
additions:
(a)
Watershed location. Provide a locator map showing the development
site location within the watershed and watershed sub-basin(s) as identified
by the Pennsylvania Department of Environmental Protection. On all
site drawings, show the boundaries of the watershed(s) and (where
applicable) sub-basin(s) as they are located on the development site
and identify watershed name(s) and sub-basin number(s).
(b)
Floodplain boundaries. Identify one-hundred-year floodplain
boundary on the development site (as applicable), as identified on
the Township's Flood Insurance Rate Map (FIRM).
(c)
Natural features. Show all bodies of water (natural and artificial),
watercourses (permanent and intermittent), swales, wetlands and other
natural drainage features on the development site, or which will be
affected by runoff from the development.
(d)
Soils. Provide an overlay showing soil types and boundaries
within the development site (consult county, NRCS, U.S. Geologic Survey
for information).
(e)
Contours. Show existing and final contours at intervals of two
feet; in areas with relatively level slope, spot elevation may be
used.
(f)
Stormwater management controls. Show any existing stormwater
management or drainage controls and/or structures, such as storm sewers,
swales, culverts, etc., which are located on the development site
or which are located off site but will be affected by runoff from
the development.
(2)
Professional certification. The stormwater management plan (including
all calculations) must be prepared and sealed by a registered professional
engineer, or such other professional persons who are qualified to
make certification under the laws of Pennsylvania.
(3)
Runoff calculations. Calculations for estimating predevelopment and post-development discharge rates and for designing proposed stormwater control facilities must be submitted with the stormwater management plan. All calculations shall be prepared using the methods and criteria prescribed by §§ 350-48 and 350-49 of this article.
(4)
Stormwater controls. All proposed stormwater runoff control measures
must be shown on the plan, including methods for collecting, conveying
and storing stormwater runoff on site which are to be used both during
and after construction. Erosion and sedimentation controls should
be shown. The preliminary plan should provide information on the general
type, location, sizing, etc., of all proposed facilities and their
relationship to the existing watershed drainage system.
(5)
Easements, rights-of-way, deed restrictions. All existing and proposed
easements and rights-of-way for drainage and/or access to stormwater
control facilities shall be shown, and the proposed owner identified.
Any areas subject to special deed restrictions relative to or affecting
stormwater management on the development site shall be shown.
(6)
Other permits/approvals. A list of any approvals/permits relative
to stormwater management that will be required from other governmental
agencies (e.g., an obstruction permit from PADEP) and anticipated
dates of submission/receipt should be included with the preliminary
plan submission. Copies of applications may be requested by the Township
where they may be helpful for the stormwater plan review.
(7)
Maintenance program. The preliminary application shall contain a
proposed maintenance plan for all stormwater control facilities, in
accordance with the following:
(a)
Identify the proposed ownership entity (e.g., Township, property
owner, homeowners' association, other management entity).
(b)
A maintenance program for all facilities, outlining the type
of maintenance activities, probable frequencies, personnel and equipment
requirements, and estimated annual maintenance costs.
(c)
Identify the method of financing continuing operation and maintenance
if the facility is to be owned by other than the Township or other
governmental agency.
D.
Stormwater plan contents: final S/LD plan submission.
(1)
All information pertaining to stormwater management from the preliminary
plan, along with any changes.
(2)
Final maps showing the exact nature and location of all temporary
and permanent stormwater management controls, along with design and
construction specifications. Details for the construction of all facilities
shall be included as part of the construction drawings.
(3)
A schedule for the installation of all temporary and permanent stormwater
control measures and devices.
(4)
An accurate survey showing all current and proposed easements and
rights-of-way and copies of all proposed deed restrictions.
(5)
A maintenance program establishing ownership and maintenance responsibilities for all stormwater control facilities (identify specific person or entity) and detailing financial requirements and sources of funding. Submit any legal agreements required to implement the maintenance program and copies of the maintenance agreement as specified by § 350-51B of this article.
A.
Preapplication phase.
(1)
During the preapplication phase (see § 350-13 of this chapter), applicants are urged to consult with the Township, the Allegheny County Planning Department, and the County Conservation District on the requirements for safely managing stormwater runoff from the development site in a manner consistent with the Township ordinances and the watershed stormwater management plan; these agencies may be helpful in providing the data that is necessary for preparing the stormwater management plan for the development site.
(2)
Applicants are encouraged to submit a sketch plan with a narrative
description of the proposed stormwater management controls for general
guidance and discussion with Township and county agencies.
(3)
The preapplication phase is not mandatory, and any review comments
provided by Township or county agencies are advisory only and do not
constitute any legally binding action on the part of the Township
or any county agency.
B.
Preliminary and final stormwater plan reviews.
(1)
Requirements. Two sets of the stormwater management plan, in accordance with the requirements of § 350-44, will be submitted with the preliminary and final application.
(2)
Review by the Township Engineer and the County Conservation District.
Preliminary and final plans will be reviewed by the Township Engineer
and the County Conservation District.
(3)
Review by County Planning Department. A copy of the preliminary plan,
along with all runoff calculations, will be forwarded to the Allegheny
County Planning Department. A report of the findings will be returned
to the Township within 30 days. No plan will be approved which receives
a negative watershed impact review by the county.
(4)
Notification of affected municipalities. When a plan is submitted
for review, municipalities downstream of the proposed project will
be notified by the Township in writing. As part of the notification,
a date will be established by which any comments from the affected
municipalities must be received. Copies of the plans will be made
available to the municipalities upon request. Comments received will
be submitted to the Planning Commission and the Township Commissioners.
(5)
Township Engineer review. The Township Engineer shall approve or
disapprove the preliminary and final stormwater management plan based
on the requirements of the Township ordinances, the standards and
criteria of the watershed stormwater management plan, and good engineering
practice. The Engineer shall submit a written report, along with supporting
documentation, to the Township Planning Commission for consideration
as part of the overall plan review.
(6)
The Township Engineer's decision to approve or disapprove a
stormwater management plan shall be final.
(7)
Approval. No preliminary or final approval for any application involving
land disturbance will be granted until a stormwater management plan
for the site has been approved.
(8)
Permits. Where the application requires an obstruction or erosion/sedimentation
control permit from PADEP, then final plan approval shall be contingent
upon receipt of such permits. However, no building permit shall be
issued, or construction started, until the permits are received and
copies filed with the Township.
Upon recording of the final plat, the applicant may commence to install or implement the approved stormwater management controls, subject to the provisions of § 350-45B above. If site development or building construction does not begin within 12 months of the date of the final approval of the plan, then before doing so, the applicant shall resubmit the stormwater management plans to verify that no condition has changed within the watershed that would affect the feasibility or effectiveness of the previously approved stormwater management controls. Furthermore, if for any reason development activities are suspended for 12 months or more, then the same requirement for resubmission of the stormwater management plan shall apply. The terms of these subsequent reviews shall be subject to the provisions of Article III of this chapter.
Requests for modifications of the final approved stormwater
management controls shall be submitted to the Township Engineer as
follows:
A.
If the request is initiated before construction begins, the stormwater plan must be resubmitted and reviewed according to the procedures in § 350-45 of this article.
B.
If the request is initiated after construction is underway, the Township Engineer shall have the authority to approve or disapprove the modification, based on field inspection, provided the requested changes in stormwater controls do not result in any modifications to other approved Township land use/development requirements (such as required building setbacks, yards, etc.). A plan modification, in accordance with applicable Township procedures, shall be necessary if any such requirements are affected. The Township Engineer shall submit a record of all approved changes for the stormwater management controls to the Township prior to the acceptance of any improvements by the Township. Modifications must not affect the compliance of the plan with the design standards as set forth in Article IV of this chapter and Article III and § 400-31 of Chapter 400, Zoning.
A.
General performance standards. The following provisions shall be
considered the overriding performance standards against which all
proposed stormwater control measures shall be evaluated, and they
shall apply in all watersheds in the Township.
(1)
Any landowner and any person engaged in the alteration or development
of land which may affect stormwater runoff characteristics shall implement
such measures as are reasonably necessary to prevent injury to health,
safety or other property. Such measures shall include such actions
as are required:
(a)
To assure that the maximum rate of stormwater runoff is no greater
after development than prior to development activities; or
(b)
To manage the quantity, velocity and direction of resulting
stormwater runoff in a manner which otherwise adequately protects
health and property from possible injury.
(2)
The stormwater management plan for the development site must consider
all the stormwater runoff flowing over the site.
(3)
Where the existing storm sewers are reasonably accessible, proposed
developments may be required to connect with the storm sewer system
unless insufficient capacity or other reasons can be demonstrated
to prevent the connection.
B.
Stormwater performance standards. The stormwater performance standards
contained in this section are intended to implement the standards
contained in the Pennsylvania Storm Water Management Act (Act 167
of 1978, as amended).[1] If there is any discrepancy between the provisions of
this chapter and the standards of the Act, or subsequent amendments,
then the standards of the current Act shall govern.
[1]
Editor's Note: See 32 P.S. § 680.1 et seq.
A.
Design storms.
(1)
The two-, ten- and one-hundred-year design storm frequencies shall
be used for analyzing stormwater runoff for predevelopment and post-development
conditions as well as for designing runoff control facilities in all
Township watersheds. The NRCS twenty-four-hour, Type II Rainfall Distribution
shall be used for all analyses. The twenty-four-hour rainfall volumes
for each of these design storms is shown below:
|
Design Storm Frequency
|
Rainfall Depth in Inches
| |
|---|---|---|
|
2-year
|
2.14
| |
|
10-year
|
3.24
| |
|
100-year
|
4.59
|
(2)
For additional information or data on other return periods, consult
"Rainfall Duration Frequency Tables for Pennsylvania," published by
PADEP Office of Resource Management, Bureau of Dams and Waterways
Management, Division of Stormwater Management, Harrisburg, Pennsylvania,
February 1983.
B.
Calculation methods.
(1)
Development sites. For the purposes of computing peak flow rates,
runoff hydrographs, and storage requirements for development sites,
the NRCS Soil Cover Complex Method as presented in the most recent
version of Technical Release 55 (TR-55), or equal, shall be used.
(2)
Stormwater collection/conveyance systems. For the purposes of designing
storm sewers, open swales and other stormwater runoff collection and
conveyance facilities, the Rational Method shall be applied. Rainfall
intensities for design shall be obtained from the Pennsylvania Department
of Transportation Rainfall Charts, unless otherwise specified by the
Township Engineer.
(3)
Predevelopment conditions. For the purposes of calculating predevelopment
peak flow rates, all sites will be considered to be in a heavily forested
cover type at the time of development.
C.
Release rate percentage.
(1)
Application. All subdivisions and land development activities which
result in an increase in the post-development peak rate of stormwater
runoff from any outfall on the development site shall be assumed to
have a release rate percentage of 100%, unless otherwise specified
by the Township Engineer.
(2)
Definition. The release rate percentage defines the percentage of
the predevelopment peak rate of runoff that can be discharged from
the site after development. It applies uniformly to all land developments
or alterations within a sub-basin, and the post-development rate of
runoff discharged from the development site cannot exceed the release
rate percentage for the sub-basin in which it is located.
(3)
Procedure for use. The steps that must be followed to use the release
rate percentage for a particular development site are:
(a)
Compute the predevelopment and post-development runoff hydrographs for the site using the prescribed calculation method for the two-, ten- and one-hundred-year design storms, applying no on-site detention for stormwater management. If the post-development peak runoff rate is less than or equal to the predevelopment peak runoff rate, then additional stormwater control shall not be required. If the post-development peak runoff rate is greater than the predevelopment value, then stormwater detention will be required and proceed to § 350-54B.
(b)
Multiply the release rate percentage(s) by the predevelopment
rate of runoff from the development site to find the allowable release
rate from the development site for the two-, ten- and one-hundred-year
storm events.
D.
No harm evaluation.
(1)
An applicant may seek to exceed the otherwise applicable sub-basin
release rate percentage by performing the no harm evaluation, which
requires an independent engineering analysis to demonstrate that other
reasonable options exist to protect downstream areas from harmful
storm runoff impacts.
(2)
The no harm evaluation shall be prepared by a registered engineer, or land surveyor who is experienced in hydrology and hydraulics, in accordance with the procedures contained in § 350-54 of this article.
(3)
The analysis for the no harm evaluation shall be submitted to the
Township Engineer for review and approval.
A.
No earthmoving activity, including cuts and fills, excavation, removal
of topsoil, trees, or vegetative cover of the land, shall commence
until the plans for minimizing soil erosion and sedimentation, both
during and after construction, have been approved.
B.
The erosion and sedimentation plan shall be prepared in accordance
with the Pennsylvania erosion and sedimentation regulations (25 Pa.
Code Chapter 102) and the standards and specifications of the Allegheny
County Conservation District.
C.
The erosion and sedimentation control plan shall specifically indicate
all erosion and sedimentation control measures to be utilized on the
site. The plan shall meet the following requirements:
(1)
The erosion and sedimentation control plan shall be prepared by persons
trained and experienced in erosion and sedimentation control methods
and techniques.
(2)
The erosion and sedimentation control plan shall be designed to prevent
accelerated erosion and sedimentation and shall consider all factors
which contribute to erosion and sedimentation, including, but not
limited to, the following:
(a)
The topographic features of the project area;
(b)
The types, depth, slope and areal extent of the soils;
(c)
The proposed alteration to the area;
(d)
The amount of runoff from the project area and the upstream
watershed area;
(e)
The staging of earthmoving activities;
(f)
Temporary control measures and facilities for use during earthmoving;
(g)
Permanent control measures and facilities for long-term protection;
and
(h)
A maintenance program for the control facilities, including
disposal of materials removed from the control facilities or project
area.
D.
Proposed erosion/sedimentation control measures shall be submitted
with the stormwater management plan as part of the applicant's
preliminary and final plans.
E.
The plan should show the type, location, and application of the proposed
erosion/sedimentation control measures and include the calculations
and criteria used in designing them.
F.
Some of the guides to be followed in designing the erosion/sedimentation
control measures include:
(1)
The smallest practicable area of soil should be exposed at any one
time during construction, and exposure should be limited to the shortest
practicable period of time.
(2)
Temporary ditches, dikes, vegetation, and/or mulching shall be used
to protect critical areas exposed during development.
(3)
Sediment basins (debris basins, desilting basins, or silt traps)
shall be installed and maintained to removed sediment from runoff
waters from land undergoing development.
(4)
Wherever possible, temporary erosion/sedimentation control measures
should be designed to serve as part of a site's permanent stormwater
management control system.
(5)
Permanent vegetation and erosion control structures should be installed
as soon as practicable during construction activities.
(6)
Wherever feasible, natural vegetation should be retained and protected,
and natural grade alterations kept to a minimum.
A.
Maintenance responsibilities.
(1)
The stormwater management plan for the development site shall establish
responsibilities for the continuing operation and maintenance of all
proposed stormwater control facilities.
(2)
The Township Commissioners shall make the final determination of
the continuing maintenance responsibilities as part of the final application
review based on the recommendation of the Planning Commission. The
Township Commissioners reserve the right to accept or reject the ownership
and operating responsibility of any or all of the stormwater management
controls.
B.
Maintenance agreement.
(1)
Prior to final approval of the stormwater management plan for the
site, the property owner/applicant shall sign and record a maintenance
agreement covering all stormwater control facilities which are to
be privately owned. The agreement shall have the following stipulations:
(a)
The owner/applicant shall maintain all facilities in accordance
with the approved maintenance schedule and shall keep all facilities
maintained in a safe and attractive manner.
(b)
The owner/applicant shall convey to the Township easements and/or
rights-of-way to assure access for periodic inspections by the Township
and maintenance, if required.
(c)
The owner/applicant shall keep on file with the Township the
name, address and telephone number of the person or company responsible
for maintenance activities. In the event of a change, new information
will be submitted to the Township within 10 days of the change.
(d)
The owner/applicant shall establish any special maintenance
funds or other financing sources, in accordance with the approved
maintenance plan.
(e)
If the owner/applicant fails to maintain the stormwater control
facilities, following due notice by the Township to correct the problems,
the Township shall perform the necessary maintenance or corrective
work. The owner/applicant shall reimburse the Township for all costs.
(2)
Other items may be included in the agreement where determined necessary
to guarantee the satisfactory maintenance of all facilities. The maintenance
agreement shall be subject to the review and approval of the Township
Solicitor.
A.
The Township Engineer or a designated representative shall inspect
the construction of the temporary and permanent stormwater management
controls for the development site. The permittee shall notify the
Township at least 48 hours in advance of the completion of the following
key development phases:
(1)
At the completion of preliminary site preparation, including stripping
of vegetation, stockpiling of topsoil, and construction of temporary
stormwater management and erosion control facilities.
(2)
At the completion of rough grading, but prior to placing topsoil,
permanent drainage, or other site development improvements and ground
covers.
(3)
During construction of the permanent stormwater facilities at such
times as specified by the Township Engineer.
(4)
Completion of permanent stormwater management facilities, including
established ground covers and plantings.
(5)
Completion of any final grading, vegetative control measures, or
other site restoration work done in accordance with the approved plan
and permit.
B.
No work shall commence on any subsequent phase until the preceding
one has been inspected and approved. If there are deficiencies in
any phase, the Township Engineer shall issue a written description
of the required corrections and stipulate the time by which they must
be made.
C.
If, during construction, the contractor or permittee identifies any
site conditions, such as subsurface soil conditions, alterations in
surface or subsurface drainage, which could affect the feasibility
of the stormwater management design, the Township Engineer shall be
notified within 24 hours of the discovery of such condition and request
a field inspection. The Township Engineer shall determine if the condition
requires a stormwater plan modification.
D.
In cases where stormwater facilities are to be installed in areas
of landslide-prone soils, or where other special site conditions exist,
the Township may require special precautions such as soil tests and
core borings, full-time resident inspectors, and/or similar measures.
All costs of any such measures shall be borne by the developer/subdivider.
A.
Neither the granting of any approval under the stormwater management
provisions of this section, nor the compliance with the provisions
of this section, or with any condition imposed by a Township official
hereunder, shall relieve any person from any responsibility imposed
by law, nor impose any liability upon the Township for damages to
persons or property.
B.
The granting of a permit which includes any stormwater management
facilities shall not constitute a representation, guarantee or warranty
of any kind by the Township, or by an official, employee or consultant
thereof, of the practicability of safety of any structure, use or
other plan proposed and shall create no liability upon or cause of
action against such public body, official, employee or consultant
for any damage that may result pursuant thereto.
Design criteria for stormwater management facilities:
A.
General design guidelines.
(1)
Applicants may select runoff control techniques, or combinations
of techniques, which are most suitable to control stormwater runoff
from the development site. All controls are subject to the approval
of the Township Engineer. The Engineer may request specific information
on design and/or operating features of the proposed stormwater controls
in order to determine their suitability and adequacy in terms of the
standards of this chapter.
(2)
In selecting and designing stormwater management systems and controls,
applicants may be guided by the following references:
(a)
"Urban Hydrology for Small Watersheds," Technical Release No.
55, USDA, Natural Resources Conservation Service, 1975 (or most recent
edition).
(b)
"Soil Erosion and Sedimentation Control Manual," Pennsylvania
Department of Environmental Protection, March 1982.
(c)
"Standards and Specifications for Soil Erosion and Sediment
Control," Maryland Water Resources Administration, 1983.
(d)
"Urban Stormwater Management," Special Report No. 49, American
Public Works Administration, 1981.
(e)
"Water Resources Protection Measures in Land Development, A
Handbook," University of Delaware Water Resources Center, April 1974.
(f)
"Design and Construction of Sanitary and Storm Sewers," WPCF
Manual of Practice No. 9, Water Pollution Control Federation, 1970.
(3)
Methods of stormwater runoff detention and control.
(a)
The following is a listing of detention and control methods
which may be used in stormwater management systems, if appropriate.
The choice of control techniques is not limited to those appearing
on this list.
[2]
Rooftop storage.
[3]
Parking lot and street storage.
[4]
Seepage pits, seepage trenches, level spreaders, or other infiltration
structures.
[5]
Porous pavement and concrete lattice block surfaces.
[6]
Grassed channels and vegetated strips.
[7]
Routed flow over grass.
[8]
Decreased impervious area coverage.
(b)
The use of other control methods which meet the criteria in
this section will be permitted when approved by the Township Engineer.
Various combinations of methods should be designed to suit the particular
requirements of the type of development and the topographic features
of the project area.
(4)
Maintenance of natural drainageways. All natural streams, channels,
swales, drainage systems and/or areas of surface water concentration
shall be maintained in their existing condition unless an alteration
is approved by the Township. All encroachment activities shall comply
with the requirements of Chapter 105 (Water Obstructions and Encroachments)
of Title 25, Rules and Regulations of the Pennsylvania Department
of Environmental Protection.
(5)
The applicant should consider the effect of the proposed stormwater
management techniques on any special soils conditions or geological
hazards which may exist on the development site. In the event such
conditions are identified on the development site, the Township may
require in-depth studies by a competent geotechnical engineer.
B.
Criteria for stormwater detention facilities.
(1)
If detention facilities are utilized for the development site, the
facility(ies) shall be designed such that the post-development peak
runoff rates from the developed site are controlled to those rates
defined by the sub-basin release rate percentage or no harm evaluation
for the two-, ten- and one-hundred-year design storms.
(2)
All detention facilities shall be equipped with outlet structures
to provide discharge control for the two-, ten- and one-hundred-year
storm frequencies. Provisions shall also be made for auxiliary structures
that are capable of passing the post-development one-hundred-year
storm runoff flows without damaging the facilities.
(3)
Shared storage facilities, which provide stormwater detention for
more than the development site, may be considered within a single
sub-basin. Such facilities shall meet the design criteria contained
in this section. In addition, runoff from the development sites involved
shall be conveyed to the facility in a manner so as to avoid adverse
impacts, such as flooding or erosion, to channels and properties located
between the development site and the shared storage facilities.
(4)
Where detention facilities are used, the design of multiple-use facilities,
such as ball fields or similar recreational uses, is encouraged wherever
feasible.
(5)
As a general rule, detention facilities will be designed as dry basins,
although wet facilities will be considered in specific situations
where they can be shown to represent a significant amenity to the
development and/or the Township.
(6)
Except in approved wet basins, stormwater detention basins will be
designed to drain completely. All interior portions of the basin will
slope toward the outlet, or low-flow sluice, at a minimum slope of
2%.
(7)
In general, facilities should be designed to have design water depths
as shallow as possible.
(8)
All detention facilities involving an earth embankment shall be designed
with a minimum freeboard of one foot between the peak emergency spillway
design flow elevation and the top of the embankment.
(9)
All embankments will be designed according to sound engineering practice
for such structures and must meet the approval of the Township. Facilities
with a design water depth in excess of 10 feet shall require a supporting
report from an engineer experienced in the design of earth embankments
and shall be constructed under the supervision of the engineer.
(10)
The outside slopes of the embankment shall not exceed two horizontal
to one vertical. The interior slopes of the structure within the pool
area should not exceed a slope of three horizontal to one vertical.
(11)
Except where special erosion protection measures are provided,
all disturbed areas will be graded evenly, topped with four inches
of topsoil, fertilized, seeded, and mulched by methods approved by
the Township.
(12)
Each inlet and outlet to the facility shall be provided with
erosion control measures approved by the Township.
(13)
Outlet control structures shall be constructed of reinforced
concrete (cast-in-place, precast, or block) and provided with debris
grates approved by the Township Engineer.
(14)
All impoundment areas shall be adequately underdrained to prevent
long-term ponding of water.
(15)
All detention facilities shall be provided with an access road
(with a legal easement) for maintenance purposes. Such roads shall
be a minimum of 10 feet wide and have a maximum grade of 15%.
(16)
Control and removal of debris both in the storage facility and
in all inlet or outlet devices shall be a design consideration.
(17)
Inflow and outflow structures, pumping stations, and other structures
shall be protected and designed to minimize safety hazards.
(18)
Landscaping shall be provided for the facility which harmonizes
with the surrounding area.
(19)
An as-built drawing shall be required for each stormwater detention
facility constructed. The drawing shall represent an engineering certification
of the volume of the facility and the depth vs. storage relationship.
This relationship shall be shown on the drawing in table form. The
drawing shall be stamped by a registered professional engineer and
submitted to the Township within 60 days of the completion of the
facility. No facility will be accepted until this requirement has
been fulfilled.
C.
Criteria for collection/conveyance facilities.
(1)
As a general rule, no stormwater may be discharged to unprotected
areas such as hillsides without special erosion and/or energy dissipation
controls being installed. Stormwater shall either be conveyed to the
nearest established stream channel as approved by the Township Engineer
or be provided with an approved energy dissipation device. Conveyance
shall be by pipe or erosion-protected ditch.
(2)
The design for culverts, pipes and other stormwater conveyance structures
shall be consistent with the design of the other stormwater management
facilities.
(3)
All sites shall be graded to provide drainage away from and around
structures to prevent potential flooding damage.
(4)
Lots located on the high side of streets shall extend roof and french
drains to a properly sized storm sewer located within the street right-of-way.
Low-side lots may extend roof and french drains to a stormwater collection/conveyance
system or natural watercourse in accordance with the approved stormwater
management plan for the development site.
(5)
Collection/conveyance facilities should not be installed parallel
and close to the top or bottom of major embankments to avoid the possibility
of failing or causing the embankments to fail.
D.
Criteria for dry sumps.
(2)
Each sump shall be designed to store a minimum water volume equivalent
to one inch of water covering the tributary area. For example, a sump
for a one-thousand-two-hundred-square-foot roof area would be designed
to store a water volume of 100 cubic feet. The total storage area,
assuming the sump is 50% rock and 50% voids, would be 200 square feet.
(3)
Dry sumps should be elongated in a three-to-one length to width ratio
and be oriented with the long dimension parallel to the contour.
(4)
Dry sumps must be constructed in undisturbed ground only. No dry
sumps shall be permitted in fill material.
(5)
The minimum distance between a dry sump and the property line shall
be 20 feet.
(6)
Dry sumps shall not be permitted in or upslope of areas determined
by the county or other competent person to be susceptible to landslides.
E.
Disposal of stormwater from roof and driveway drains.
(1)
Unless otherwise approved by the Township, no stormwater from roofs
or driveway drains shall be discharged to the street surface or curb
underdrain.
(2)
Acceptable methods of disposal include properly designed dry sumps,
grassed or other ground surfaces with adequate consideration being
given to erosion protection, storm sewers, or any other method approved
by the Township.
