This article is adopted and implemented to achieve the following
general purposes and objectives:
A. To manage stormwater runoff resulting from land alteration and disturbance
activities as required by the Pennsylvania Storm Water Management
Act (Act 167 of 1978, as amended).
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.
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.
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.
[1]
Detention basins. See standards in Subsection
B of this section.
[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.
[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.
(1) All dry sumps designed for accepting surface water from roof or driveway
areas shall be designed according to Exhibit No. 4, shown in Appendix
No. 1, unless otherwise approved by the Township Engineer.
(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.