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.
F. Any detention system which requires a dam safety permit or a water
obstruction permit from the PaDEP shall be designed consistent with
the provisions of the Dam Safety and Encroachments Act and the PaDEP Chapter 105 rules and regulations.
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.
(2) The top of embankment width shall be:
(a)
For embankments up to four feet, width shall be at least six
feet;
(b)
For embankments between four feet and six feet, width shall
be at least eight feet;
(c)
For embankments over six feet, width shall be at least 10 feet.
(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.