A.
Applicants may select runoff control techniques, or
a combination of techniques, which are most suitable to control stormwater
runoff from the development site. All controls shall be subject to
approval of the Municipal Engineer. The Municipal 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.
B.
The applicant should consider the effect of the proposed
stormwater management techniques on any special soil conditions or
geological hazards which may exist on the development site. In the
event such conditions are identified on the site, the Municipal Engineer
may require in-depth studies by a competent geotechnical engineer.
Not all stormwater control methods may be advisable or allowable at
a particular development site.
C.
In developing a stormwater management plan for a particular
site, stormwater controls shall be selected according to the following
order of preference:
D.
Infiltration practices shall be used to the extent
practicable to reduce volume increases and promote groundwater recharge.
A combination of successive practices may be used to achieve the applicable
minimum control requirements. Justification shall be provided by the
applicant for rejecting each of the preferred practices based on actual
site conditions.
A.
Infiltration systems shall be sized and designed based
upon local soil and groundwater conditions.
B.
Infiltration systems greater than three feet deep
shall be located at least 10 feet from basement walls.
C.
Infiltration systems shall not be used to handle runoff
from commercial or industrial working or parking areas. This prohibition
does not extend to roof areas which are demonstrated to be suitably
protected from the effects of the commercial/industrial activities.
D.
Infiltration systems may not receive runoff until
the entire drainage area to the system has received final stabilization.
E.
The stormwater infiltration facility design shall
provide an overflow system with measures to provide a nonerosive velocity
of flow along its length and at the outfall.
A.
If flow attenuation facilities are employed to assist
in the control of peak rates of discharge, their effects must be quantified
using the SCS Technical Release (TR) 55 Urban Hydrology for Small
Watersheds or other approved method. The effects of the flow attenuation
facilities on travel time should be reflected in the calculations.
B.
Flow attenuation facilities such as swales and natural
depressions should be properly graded to ensure positive drainage
and avoid prolonged ponding of water.
C.
Swales shall be properly vegetatively stabilized or
otherwise lined to prevent erosion.
D.
Swales shall be designed according to the recommendations
contained in the Commonwealth of Pennsylvania Erosion and Sediment
Pollution Control Program Manual.
A.
If detention facilities are utilized for the development
site, the facility(ies) shall be designed such that post-development
peak runoff rates from the developed site are controlled to those
rates defined by the subarea release rate percentage for the two-,
five-, ten-, twenty-five-, fifty-, and one-hundred-year storm frequencies.
B.
All detention facilities shall be equipped with outlet
structures to provide discharge control for the six designated storm
frequencies. Provisions shall also be made to safely pass the post-development
one-hundred-year storm runoff without damaging or impairing the continued
function of the facilities. Should any stormwater management facilities
be regulated by PA DEP Chapter 105 regulations, the facility shall
be designed in accordance with those regulations and meet the regulations
concerning dam safety.
C.
Shared-storage facilities which provide detention
of runoff for more than one development site within a single subarea
are encouraged wherever feasible and provided such facilities meet
the criteria contained in this section. In addition, runoff from the
development sites involved shall be conveyed to the facility in a
manner that avoids adverse impacts (such as flooding or erosion) to
channels and properties located between the development site and the
shared-storage facilities.
D.
Where detention facilities will be utilized, multiple
use facilities, such as wetlands, lakes, ballfields or similar recreational/open
space uses are encouraged wherever feasible, subject to the approval
of the municipality and compliance with the Pennsylvania Department
of Environmental Protection's Chapter 105 regulations.
E.
Other considerations which should be incorporated
into the design of the detention facilities include:
(1)
Inflow and outflow structures shall be designed and
installed to prevent erosion and bottoms of impoundment type structures
should be protected from soil erosion.
(2)
Control and removal of debris both in the storage
structure and in all inlet or outlet devices shall be a design consideration.
(3)
Inflow and outflow structures, pumping stations, and
other structures shall be designed and protected to minimize safety
hazards.
(4)
The water depth at the perimeter of a storage pond
should be limited to that which is safe for children. Restriction
of access (fence, walls, etc.) may be necessary depending on the location
of the facility and the maximum depths of water.
(5)
Inside slope of storage ponds shall not exceed a ratio
of 3:1 horizontal to vertical dimension. Outside slope shall not exceed
a ratio of 2:1.
(6)
Landscaping shall be provided for the facility which
harmonizes with the surrounding area.
(7)
Facilities shall be located to facilitate maintenance,
considering the frequency and type of equipment that will be required.
(8)
Bottoms of detention basins should be graded with
sufficient slope to provide positive surface drainage. A subdrainage
system may be required depending on the location of the pond bottom
relative to groundwater levels.
(9)
Fencing shall be provided if required by the municipality.
A.
All stormwater runoff collection or conveyance facilities,
whether storm sewers or other open or closed channels, shall be designed
in accordance with the following basic standards:
(1)
All sites shall be graded to provide drainage away
from and around the structure in order to prevent any potential flooding
damage.
(2)
Lots located on the high side of streets shall extend
roof and French drains to the curbline storm sewer (if applicable).
Low side lots shall extend roof and french drains to a stormwater
collection/conveyance/control system or natural watercourse in accordance
with the approved stormwater management plan for the development site.
(3)
Collection/conveyance facilities should not be installed
parallel and close to the top or bottom of a major embankment to avoid
the possibility of failing or causing the embankment to fail.
(4)
All collection/conveyance facilities shall be designed
to convey the twenty-five-year storm peak flow rate from the contributing
drainage area and to carry it to the nearest suitable outlet such
as a stormwater control facility, curbed street, storm sewer or natural
watercourse.
(5)
Where drainage swales or open channels are used, they
shall be suitably lined to prevent erosion and designed to avoid excessive
velocities.
B.
In the Ohio River bluff areas (as defined by the USGS
contours on USGS maps), special care should be taken to properly direct,
collect, and discharge stormwater runoff so as to protect against
bluff recession and erosion. The developer's activities shall be coordinated
with the municipality to effect such protections. The Pennsylvania
Department of Environmental Protection and the U.S. Corps of Engineers
may be able to provide guidance in the form of technical assistance.
C.
Wherever storm sewers are proposed to be utilized,
they shall comply with the following criteria:
(1)
Where practical, designed to traverse under seeded
and planted areas. If constructed within 10 feet of road paving, walks
or other surfaced areas, drains shall have a narrow trench and maximum
compaction of backfill to prevent settlement of the superimposed surface
or development.
(2)
Preferably installed after excavating and filling
in the area to be traversed is completed, unless the drain is installed
in the original ground with a minimum of three feet of cover and/or
adequate protection during the fill construction.
(3)
Designed with cradle when traversing fill areas of
indeterminate stability, with anchors when gradient exceeds 20%, and
with encasement or special backfill requirements when traversing under
a paved area.
(4)
Designed to adequately handle the anticipated stormwater
flow and be economical to construct and maintain. The minimum pipe
size shall be 15 inches in diameter.
(5)
Drain pipe, trenching, bedding and backfilling requirements
shall conform to the requirements of the municipality and/or applicable
PennDOT Specifications, Form 408.
(6)
All pipe material shall be high-density polyethylene;
PVC; 10-gauge (minimum) galvanized corrugated metal pipe or as approved
by the Board of Supervisors.
(7)
Storm inlets and structures shall be designed to be
adequate, safe, self-cleaning and unobtrusive and consistent with
municipal standards.
(8)
Approved grates shall be designed for all catch basins,
stormwater inlets and other entrance appurtenances.
(9)
Manholes shall be designed so that the top shall be
at finished grade and sloped to conform to the slope of the finished
grade. Top castings of structures located in roads or parking areas
shall be machined or installed to preclude "rattling."
(10)
Where a proposed storm sewer connects with an
existing storm sewer system, the applicant shall demonstrate that
sufficient capacity exists in the downstream system to handle the
additional flow.
(11)
Storm sewer outfalls shall be equipped with
energy dissipation devices to prevent erosion and conform with applicable
requirements of the Pennsylvania DEP for stream encroachments (Chapter
105 of Pennsylvania DEP Rules and Regulations).
(12)
The developer shall install all catch basins
such that they are flush with the pavement. Where the wearing course
will not be placed immediately following the BCBC, the contractor
shall leave the top of the grate flush with the top of the BCBC and
raise the grate and frame when the wearing course is applied. The
contractor shall saw-cut any pavement needed to be removed in order
to raise the frame and grate. All exposed saw-cut edges shall be tack-coated
in accordance with the latest PennDOT specifications.
D.
Parallel drain system.
(1)
The discharge of sumps, footer drains and water softeners,
etc., through the development of lots or parcels onto any roadways
(cartway or right-of-way area) shall not be permitted.
(2)
Access to the storm sewer shall be provided for all
discharges from sumps, footer drains, roof drains and water softeners.
This access may be provided to the street catch basin, provided the
catch basin is on the same side of the street. A six-inch pipe running
parallel to the street may be provided, within the right-of-way, that
drains to a street catch basin, provided that the six-inch pipe does
not cross the street. The pipe shall be Schedule 40 pipe or an approved
equal. A saddle "T" and cleanout shall be provided at property line
for all pipes carrying sumps, footer drain, roof drain and water softener
discharges. The parallel storm system is not to be used for street
or yard runoff or runoff from other developed or undeveloped areas.
