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 Township Engineer or other engineer designated by the Board
of Supervisors. 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.
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 Township 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.
Stormwater shall not be transferred from one watershed to another,
unless:
(1)
The
watersheds are subwatersheds of a common watershed which join together
within the perimeter of the property;
(2)
The
effect of the transfer does not alter the peak rate discharge onto
adjacent lands; or
(3)
Permanent
easements from the affected landowner(s) are provided and shall be
recorded.
B.
Consideration shall be given to the relationship of the subject property
to the drainage pattern of the watershed. A concentrated discharge
of stormwater to an adjacent property shall be within an existing
watercourse or confined in an easement or returned to a predevelopment
flow-type condition.
C.
Innovative stormwater BMPs and recharge facilities are encouraged
(e.g., rooftop storage, dry wells, cisterns, recreation area ponding,
diversion structures, porous pavements, holding tanks, infiltration
systems, in-line storage in storm sewers, and grading patterns). They
shall be located, designed, and constructed in accordance with the
latest technical guidance published by PADEP, provided they are accompanied
by detailed engineering plans and performance capabilities and supporting
site-specific soils, geology, runoff and groundwater and infiltration
rate data to verify proposed designs. Additional guidance from other
sources may be accepted at the discretion of the Township (a preapplication
meeting is suggested).
D.
All existing and natural watercourses, channels, drainage systems
and areas of surface water concentration shall be maintained in their
existing condition unless an alteration is approved by the appropriate
regulatory agency.
E.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Township shall reserve
the right to disapprove any design that would result in the continuation
or exacerbation of a documented adverse hydrologic or hydraulic condition
within the watershed, as identified in the plan.
F.
The design and construction of multiple use stormwater detention
facilities are strongly encouraged. In addition to stormwater management,
facilities should, where appropriate, allow for recreational uses
including ballfields, play areas, picnic grounds, etc. Consultation
with the Township and prior approval are required before design. Provision
for permanent wet ponds with stormwater management capabilities may
also be appropriate.
(1)
Multiple use basins should be constructed so that potentially dangerous
conditions are not created.
(2)
Water quality basins or recharge basins that are designed for a slow
release of water or other extended detention ponds are not permitted
for recreational uses, unless the ponded areas are clearly separated
and secure.
A.
Infiltration systems shall be sized and designed based upon local
soil and groundwater conditions.
B.
The use of practical structural and nonstructural BMPs is encouraged
in the design of site drainage and infiltration systems and development
of land. Design of such measures shall consider the limitations of
the site's topography and soils, and shall identify the benefits
of proposed measures and maintenance requirements. Technical requirements
and limitations of BMPs are provided in the BMP Manual.
C.
Infiltration systems greater than three feet deep shall be located
at least 10 feet from basement walls.
D.
Surface drainage inlets shall be fitted with sediment control devices.
E.
Infiltration systems may not receive runoff until the entire drainage
area to the system has received final stabilization.
F.
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 method approved by the Township. 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.
All stormwater management facilities shall meet the following
general design requirements:
A.
If detention facilities are utilized for the development site, the
facility(ies) shall be designed such that postdevelopment peak runoff
rates from the developed site are controlled to those rates defined
by the subarea release rate percentage for the one-, two-, ten-, twenty-five-,
fifty- and one-hundred-year storm frequencies/rainfall values.
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 postdevelopment one-hundred-year
storm runoff without damaging or impairing the continued function
of the facilities. Any stormwater management facilities regulated
by PaDEP Chapter 105 regulations shall be designed in accordance with
those regulations.
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.
Discharging stormwater off the property to an existing facility may
require supporting calculations, including hydraulic grade line calculations,
as deemed necessary by the Engineer, to prove the adequacy of the
downstream facilities. The developer, subject to Township approval,
may elect to upgrade downstream facilities so as to accommodate the
generated runoff.
E.
A multistage outlet may be used for detention basins in order to
provide discharge control for different storm frequency events, provided
that the siltation facility precedes stormwater entry into the detention
basin or outlet structure.
G.
Setback regulations. Aboveground detention facilities shall constitute structures for purposes of application of yard setback regulations established in Chapter 145, Zoning, of the Code of the Township of Millcreek, as amended. Said facilities shall comply with those setback regulations from structures, adjacent parcels, streets, etc. established in Chapter 145, Zoning, generally and as specifically provided in its § 145-41. Subject to compliance with these setback regulations, a detention facility may be located within a required buffer area.
H.
No outlet structure from a stormwater management facility or swale
shall discharge directly onto a Township street or a state highway
without approval from the Township or PennDOT, as applicable.
I.
The top, or toe, of any slope of stormwater management facilities
shall be located a minimum of 20 feet from any property line.
J.
Interior slopes of the basin shall not exceed 3:1.
K.
Exterior slopes of compacted soil shall not exceed 3:1, and may be
further reduced if the soil has unstable characteristics.
L.
Bottoms of detention basins should be graded with sufficient slope
(minimum of 2%) to provide positive surface drainage. A subdrainage
system may be required depending on the location of the pond bottom
relative to groundwater levels. Slopes of less than 2% may be approved
by the Township if warranted by site specifics, provided that in such
event, a subdrainage system must then be installed. Facilities designed
as water quality/infiltration BMPs may have a bottom slope of zero.
M.
The minimum horizontal distance between any stormwater holding facility
and a building or structure shall be 20 feet. The lowest floor elevation
of any structure constructed immediately adjacent to a detention basin
or other stormwater management facility shall be a minimum of two
feet above the one-hundred-year water surface elevation.
N.
Stormwater management facility bottom (or surface of permanent pool)
elevations must be greater than adjacent floodplain elevations (FEMA
or HEC-RAS analysis). If no floodplain is defined, bottom elevations
must be greater than existing ground elevations 50 feet from top of
stream bank in the facilities' vicinity.
O.
Basin outflow culverts discharging into floodplains must account
for tailwater. Tailwater corresponding to the one-hundred-year floodplain
elevation must be used for all design storms, or the applicant may
elect to determine flood elevations of the adjacent watercourse for
each design storm. The floodplain is assumed to be 50 feet from top
of stream bank in areas where a floodway is not designated, or no
other evidence is provided.
P.
The invert of all stormwater management facilities and underground
infiltration/storage facilities shall be located a minimum of two
feet above the seasonal high groundwater table. The invert of stormwater
facilities may be lowered if adequate subsurface drainage is provided.
Flows from underdrains need not be accounted for in volume or rate
control calculations.
Q.
Whenever possible, the side slopes and basin shape shall be amenable
to the natural topography. Vertical side slopes and rectangular basins
shall be avoided whenever possible.
R.
Impervious low-flow channels are not permitted within stormwater
management facilities.
S.
Unless specifically designed as a volume control or water quality
facility, all stormwater management facilities must empty over a period
of time not less than 24 hours and not more than 72 hours from the
end of the facility's inflow hydrograph. Infiltration tests performed
at the facility locations and proposed basin bottom depths, in accordance
with the BMP Manual, must support time-to-empty calculations if infiltration
is a factor.
T.
Energy dissipators and/or level spreaders shall be installed at points
where pipes or drainageways discharge to or from basins. Discharges
to drainage swales shall be dissipated, or piped, to an acceptable
point.
U.
Landscaping and planting specifications must be provided for all
stormwater management basins and be specific for each type of basin.
Minimal maintenance, saturation-tolerant vegetation must be provided
in basins designed as water quality/infiltration BMPs.
V.
Principal outlet structures. The primary outlet structure shall be
designed to pass all design storms (up to and including the one-hundred-year
event) without discharging through the emergency spillway. All principal
outlet structures shall:
(1)
Be constructed of reinforced concrete or an alternative material
approved by the Township. When approved for use, all metal risers
shall:
(a)
Be suitably coated to prevent corrosion.
(b)
Have a concrete base attached with a watertight connection.
The base shall be sufficient weight to prevent flotation of the riser.
(c)
Provide a trash rack or similar appurtenance to prevent debris
from entering the riser.
(d)
Provide an anti-vortex device, consisting of a thin vertical
plate normal to the basin berm.
(2)
Provide trash racks to prevent clogging of primary outflow structure
stages for all orifices.
(3)
Provide outlet aprons and shall extend to the toe of the basin slope
at a minimum.
W.
Emergency spillways. Any stormwater management facility designed
to store runoff shall provide an emergency spillway designed to convey
the one-hundred-year post-development peak rate flow with a blocked
primary outlet structure. The emergency spillway shall be designed
per the following requirements:
(1)
The top of embankment elevation shall provide a minimum one foot
of freeboard above the maximum water surface elevation. This is to
be calculated when the spillway functions for the one-hundred-year
post-development inflow, with a blocked outlet structure.
(2)
Avoid locating on fill areas whenever possible.
(3)
The spillway shall be armored to prevent erosion during the one-hundred-year
postdevelopment flow, with a blocked primary outlet structure.
(a)
Synthetic liners or riprap may be used, and calculations sufficient
to support proposed armor must be provided.
(b)
An earthen plug must be used to accurately control the spillway
invert if riprap is the proposed armoring material.
(c)
Emergency spillway armor must extend up the sides of the spillway,
and continue at full width to a minimum of 10 feet past the toe of
slope.
(4)
Millcreek may require use of additional protection when slopes exceed
4:1 and spillway velocities might exceed NRCS standards for the particular
soils involved.
(5)
Any underground management facility (pipe storage system) must have
a method to bypass flows higher than the required design (up to a
one-hundred-year postdevelopment inflow) without structural failure,
or causing downstream harm or safety risks.
(6)
To the extent this subsection conflicts with PPIC Figure SD-10, this
subsection shall control.
X.
Design of stormwater management facilities having three feet or more
of water depth (measured vertically from the lowest elevation in the
facility to the crest of the emergency spillway) shall meet the following
additional requirements:
(1)
The maximum water depth within any stormwater management facility
shall be no greater than eight feet when functioning through the primary
outlet structure.
(3)
A ten-foot-wide access to the basin bottom must be provided with
a maximum longitudinal slope of 10%.
(4)
Berms shall be constructed using soils that conform to the unified
soil classification of CH, MH, CL or ML. Soils used shall be tested
to determine their density analysis per ASTM 698. The embankments
shall be constructed in a maximum of six-inch lifts. The lifts will
each be compacted to a density at least 98% of their maximum dry density.
Each layer of compacted fill shall be tested to determine its density
per ASTM 2922 or ASTM 3017. One test per 50 cubic yards of material
placed (at least one per layer) shall be performed by an independent
testing agency.
(5)
A cutoff and key trench of impervious material shall be provided
under all embankments four feet or greater in height. The cutoff trench
shall run the entire length of the embankment and tie into undisturbed
natural ground.
(6)
Antiseep collars, or a PaDEP-approved alternative, must be provided
on all outflow culverts in accordance with the methodology contained
in the latest edition of the PaDEP E & S Control Manual. An increase
in seepage length of 15% must be used in accordance with the requirements
for permanent antiseep collars.
Y.
To provide an access point from which a detention facility can be maintained, a minimum twenty-four-inch-diameter access point must be provided on all sides of an underground detention facility's ends. See also § 29-40F(3), regarding manway access ports or structures, of Chapter 29, Code Enforcement, Part 2, Public and Private Improvements.
Z.
Other considerations to 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)
Landscaping shall be provided for the facility which harmonizes with
the surrounding area. Landscaping proposals, including type of vegetative
cover, seeding and fertilizer rates, shall be submitted to the Engineer
for approval. Landscaping shall not be considered complete until there
is a minimum of 85% fully established vegetative cover.
(6)
Facilities shall be located to facilitate maintenance, considering
the frequency and type of equipment that will be required.
(7)
Fencing shall be provided if required by the Township. Factors to
be considered shall include but not be limited to depth and/or slope,
whether the facility is to be used as an erosion and sedimentation
control basin and/or the length of time water is likely to remain
in the basin after a flood event.
AA.
Construction of stormwater management facilities.
(1)
Basins used for rate control only shall be installed prior to
or concurrent with any earthmoving or land disturbances which they
will serve. The phasing of their construction shall be noted in the
narrative and on the plan.
(2)
Basins that include water quality or recharge components shall
have those components installed in such a manner as to not disturb
or diminish their effectiveness.
(3)
Compaction test reports shall be kept on file at the site and
be subject to review at all times, with copies being forwarded to
the Township upon request.
(4)
Temporary and permanent grasses or stabilization measures shall
be established on the sides and base of all earthen basins within
15 days of construction.
BB.
Exceptions to these requirements may be made at the discretion of
the Township for BMPs that retain or detain water, but are of a much
smaller scale than traditional stormwater management facilities.
A.
All stormwater runoff collection or conveyance facilities, whether
storm sewers or other open or closed facilities, shall be designed
in accordance with the following basic standards:
(1)
All storm sewer pipes, grass waterways, open channels, swales and
other stormwater carrying or conveyance facilities that service drainage
areas within the site must be able to convey postdevelopment runoff
from the twenty-five-year design storm.
(2)
Stormwater management facilities that convey off-site water through
the site shall be designed to convey the twenty-five-year storm event
(or larger events, as determined by the Township).
(3)
All developments shall include provisions that allow for the overland
conveyance and flow of the postdevelopment one-hundred-year storm
event without damage to public or private property.
(4)
All sites shall be graded to provide drainage away from and around
the structure(s) and improvements in order to prevent any potential
flooding damage.
(5)
The developer shall be responsible for extending roof, sump and french
drains to a stormwater collection or conveyance control system, a
storm sewer or a natural watercourse in accordance with the approved
stormwater management or grading plan for the development. Subject
to these considerations:
(a)
Roof drains and downspouts may discharge to a lawn area subject
to Township approval; provided, that any discharge to a lawn area
will be authorized only where it will not result in discharge of stormwater
onto neighboring properties and the owner of the subject property
will be responsible for its maintenance.
(b)
Slope, soil type and distance from neighboring properties will
be factors in the Township's considerations.
(c)
In appropriate circumstances, the Township may authorize connection
of a sump or a french drain to a dry well instead of to a storm sewer.
(6)
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.
(7)
All collection and conveyance facilities shall be designed to convey
postdevelopment runoff without surcharging inlets for the twenty-five-year
storm event.
(8)
Where drainage swales or open channels are used, they shall be suitably
lined to prevent erosion and designed to avoid excessive velocities.
(9)
Within a delineated bluff recession hazard area, 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 Township to effect such protections.
B.
Storm sewers. Storm sewers shall comply with the following criteria:
(1)
Where practical, storm sewers shall be 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; provided, that such backfill shall be required
in the area within five feet of road paving in all cases and in the
area within 10 feet of road paving where the trench depth is in excess
of five feet. See Figure SD-1 of the Public and Private Improvements
Code for detail. [1]
(2)
Shall be 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 cover and/or adequate protection
during the fill construction.
(4)
Designed to adequately handle the anticipated stormwater flow and
be economical to construct and maintain. The minimum pipe size shall
be 12 inches in diameter.
(a)
Minimum pipe size of four inches in diameter at a minimum slope
of 0.5% will be allowed for conveyance of downspout flows from roof
areas not exceeding 900 square feet.
(5)
Drain pipe, trenching, bedding and backfilling requirements shall conform to Township specifications set forth in the PPIC and shall be installed in such manner as is required in the PPIC and/or Chapter 125, Subdivision and Land Development.
(6)
Inlets, manholes, pipes and culverts shall be constructed in accordance with standards therefor in the PPIC and/or Chapter 125, Subdivision and Land Development, or, if not provided for therein, as set forth in PennDOT's Publication 408 and/or Publication 72M or other detail approved by the Township.
(7)
Storm inlets and structures shall be designed to be adequate, safe,
self-cleaning and unobtrusive and consistent with the Township's
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)
Storm sewer outfalls shall be equipped with energy-dissipation
devices to prevent erosion and conform with requirements of PaDEP's
Chapter 105 regulations for stream encroachments.
(11)
Where a proposed storm conveyance system will connect with an
existing conveyance system, the applicant shall demonstrate that sufficient
capacity exists in the downstream system to handle the increased volume
of flows subject to the applicable release rate for peak flows.
(12)
Catch basins shall be spaced at appropriate locations so that
a spread of no more than 10 feet will occur during the design storm.
(13)
For inlets containing a change in pipe size, the elevation for
the crown of the pipes shall be the same or the smaller pipe's
crown and shall be at a higher elevation.
(14)
At all street low points, swales and easements shall be provided
behind the curb or swale and through adjacent properties to channelize
and direct any overflow of stormwater runoff away from dwellings and
structures.
(15)
All inlets in paved areas shall have heavy-duty bicycle-safe
grating. A note to this effect shall be added to the SWM site plan
or inlet details in the plan.
(16)
Inlets must be sized to accept the specified pipe sizes without
knocking out any of the inlet corners. All pipes entering or exiting
inlets shall be cut flush with the inside wall of the inlet. A note
to this effect shall be added to the SWM site plan or inlet details
in the plan.
(17)
Inlets shall have weep holes covered with geotextile fabric
placed at appropriate elevations to completely drain the subgrade
prior to placing the base and surface course on roadways.
(18)
Inlets, junction boxes or manholes greater than five feet in
depth shall be equipped with ladder rungs and shall be detailed on
the SWM plan.
(19)
Accessible drainage structures shall be located on continuous
storm sewer systems at all vertical dislocations, at all locations
where a transition in storm sewer pipe sizing is required, at all
vertical and horizontal angle points exceeding five degrees and at
all points of convergence of two or more storm sewer pipes.
(20)
All storm drainage piping shall be provided with either reinforced
concrete headwalls or end sections of compatible material as the pipe
involved at its entrance and discharge.
(21)
Stormwater roof drains and pipes shall not discharge onto impervious
areas or be connected to storm sewers without approval by the Township.
(22)
Inlets shall have a six-inch sump in the bottom.
C.
Swale conveyance facilities.
(1)
Swales must be able to convey postdevelopment runoff from a twenty-five-year
design storm with six inches of freeboard to top of the swale and
side slopes no steeper than 3:1.
(2)
All swales shall be designed, labeled on the SWM plan with details
provided in the SWM plan to adequately construct and maintain the
design dimension of the swales.
(3)
Swales shall be designed for stability using velocity or shear criteria.
Velocity criteria may be used for channels with less than 10% slope.
Shear criteria may be used for all swales. Documentation must be provided
to support velocity and/or shear limitations used in calculations.
(5)
Manning's "n" values used for swale capacity design must reflect
the permanent condition.
