Any development involving the construction of any new impervious
or semipervious surfaces on a tract of land and all subdivisions and
land developments shall provide stormwater management facilities when
required by and in accordance with the requirements in this article.
Stormwater management facilities consist of two parts: stormwater
detention/storage facilities and stormwater conveyance facilities.
A.
Stormwater flow may not be transferred from one watershed to another,
concentrated and/or redirected unless any adverse effects of these
procedures are mitigated to the satisfaction of and specifically approved
by the Township Engineer.
B.
Stormwater flow that is concentrated and discharged shall be discharged
in one of the following manners:
(1)
The stormwater shall be discharged to an energy dissipator(s)
and shall then flow directly into a stream on the subject property.
(2)
The stormwater shall be conveyed and piped to an existing stormwater
system on an abutting property, including public property that is
capable of handling the flow, as determined by the Township Engineer.
C.
Stormwater facilities, especially detention basins, shall be designed
to utilize the natural contours of the land. When such design is impracticable,
the construction of the facilities shall utilize slopes as flat as
possible to blend the structure into the terrain.
E.
The design of stormwater facilities must facilitate regular maintenance,
mowing, desilting and reseeding.
F.
A deed restriction prohibiting removal or alteration of stormwater
facilities shall be provided for all stormwater facilities. This deed
restriction shall allow removal or alteration that is approved by
the Township.
G.
An easement granting the Township the right, but not the obligation,
to enter and improve stormwater facilities shall be provided.
H.
Sedimentation basins shall meet the requirements of the County Soil
Conservation District and Chapter 102 of Pennsylvania Code Title 25.
I.
Roof drains shall discharge to infiltration or vegetative BMPs unless
otherwise approved by the Township or Township Engineer. The outflow
from roof drains or sump pumps shall not adversely affect neighboring
properties. For roof drains that do not discharge to an infiltration
BMP, the discharge point for the roof drain must be located a maximum
five feet from the building unless another location is approved by
the Township or Township Engineer. The location of the discharge point
for roof drains that discharge to vegetative BMPs shall be reviewed
by the Township on a case-by-case basis to ensure no adverse impacts
on neighboring properties. The discharge point for sump pumps shall
be located a maximum of five feet from the building unless another
location is approved by the Township or Township Engineer. To address
safety concerns, roof drain and sump pump discharge shall not flow
over sidewalks.
J.
Whenever a watercourse, stream or intermittent stream is located
within a development site, it shall remain in its natural state and
locations and shall not be piped unless necessary state permits are
received and the applicant shows that no other alternatives are feasible.
K.
Construction and materials. All construction and materials shall
be in conformance with Pennsylvania Department of Transportation (PADOT)
Publication 408 unless otherwise approved and shown on an approved
land development/subdivision plan.
L.
Groundwater recharge and water quality.
(1)
Groundwater recharge. In general, all runoff control measures
outside of sinkhole-prone areas shall be designed to encourage groundwater
recharge and shall be permitted only if suitable subsurface conditions
are present. The on-site recharge of all stormwater runoff shall be
required if the Township determines that conditions so warrant.
M.
For all regulated earth disturbance activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual), No. 363-2134-008 (April 15, 2000), as amended and updated. (NOTE: See § 140B-29B.)
N.
Stormwater flows (including roof drains) and sump pump discharge onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s) by the developer/property owner. Such stormwater flows and sump pump discharge shall be subject to the requirements of this chapter. See also § 140B-12A and I above.
O.
For all regulated activities, SWM BMPs shall be implemented, operated,
and maintained to meet the purposes and requirements of this chapter
and to meet all requirements under Title 25 of the Pennsylvania Code,
the Clean Streams Law, and the Storm Water Management Act.
P.
Various BMPs and their design standards are listed in the PADEP Stormwater
BMP Manual.[2]
[2]
Editor's Note: The List of Stormwater Best Management Practices is included as an attachment to this chapter.
A.
Applicability of stormwater detention/storage requirements. Stormwater
detention/storage facilities or a fee in lieu of such facilities shall
be provided in accordance with the following provisions:
(1)
Subdivisions and land developments. Stormwater from subdivisions
and land developments, as defined in this chapter, shall be detained/stored
through one of the following mechanisms:
(a)
Provision of stormwater detention/storage facilities and/or
BMPs.
(b)
A combination of stormwater detention/storage facilities and payment of a fee in lieu for some of the required facilities, in accordance with § 140B-13C(3)(a) of this chapter, when the Township determines that fees used for regional stormwater control will work better than comprehensive stormwater detention/storage facilities on a particular site. The proposed comprehensive stormwater detention/storage facilities on the site shall, at a minimum, be designed to accommodate the stormwater flow from any proposed additional impervious coverage on the site.
B.
Stormwater control facilities.
(1)
General design requirements.
(a)
All plans and designs for stormwater management systems and
facilities submitted to the Township for approval shall determine
stormwater peak discharge and runoff by the use of the USDA Soil Conservation
Service (SCS)[1] Soil Cover Complex Method as set forth in Urban Hydrology
for Small Watersheds (Technical Release No. 55), with specific attention
given to antecedent moisture conditions, flood routing and peak discharge
specifications included therein and in Hydrology National Engineering
Handbook, Section 4, both by United States Department of Agriculture,
Soil Conservation Service. The Township Engineer, however, may permit
the use of the Rational Method for calculation of runoff on land developments
of five acres or less. For basins designed using the Rational Method,
the hydrograph and detention volumes shall, at minimum, equal the
volume derived from the routing process when using SCS TR55.
[1]
Editor's Note: Now the Natural Resources Conservation
Service.
(b)
When using the USDA SCS Soil Cover Complex Method of stormwater
management, the developer shall ensure that, after construction, peak
discharges from the site for a ten-year, twenty-four-hour storm of
5.0 inches shall not exceed the before-disturbance peak discharge
from a two-year, twenty-four-hour storm of 3.2 inches of rainfall.
Peak discharges for any storms of greater than ten-year frequency
up to and including a one-hundred-year storm shall not exceed 90%
of the peak discharges from the site of the same storms before disturbance
and shall be calculated based on the following storms:
(c)
When using the Rational Method of stormwater management, the
developer shall ensure that the after-construction peak discharges
from the site for a ten-year storm shall not exceed the before-disturbance
peak discharge from a two-year storm. Peak discharges for any storms
of greater than ten-year frequency, including twenty-five-year, fifty-year
and one-hundred-year storms, shall not exceed 90% of the peak discharges
from the site before disturbance for storms of the same duration.
(d)
Under certain conditions, the Township Engineer may impose the
following additional restrictions on stormwater discharges:
[1]
Peak discharge rates on storms in excess of the ten-year storm
may be further restricted when a probable risk to downstream structures
or unique natural areas exists or when an existing severe flooding
problem would be further aggravated.
[a]
The developer and/or landowner shall submit to
the Township an analysis of the impacts of the stormwater flows on
the downstream drainagecourses in the watershed. The analysis shall
include hydrologic and hydraulic calculations necessary to determine
the impact of the proposed development upon a dam, highway, structure
or existing point of restricted stream flow. The existing point downstream
of restricted flow shall be established with the concurrence of the
Township.
[2]
Where the nature of the soils underlying a runoff structure
or the type of business constitute actual or potential risks of contamination
to ground- or surface water, submission by the developer to the Department
of Environmental Protection to determine permitting requirements is
required. A copy of any required permit shall be supplied to the Township.
[3]
Where groundwater yields are very low or where a groundwater
supply already is heavily used, the Township may require that the
entire volume of a two-year storm be held on site and recharged where
feasible and where it will not impact on the geology of the area.
Refer to the Township Sinkhole Zone Map,[2] which identifies karstic topography identified as areas
with significance related to potential sinkhole development.
[2]
Editor's Note: The Township Sinkhole Zone Map is on file
in the Township offices.
[4]
In areas where the probability for subsidence activity and sinkhole
development exists, the developer shall submit to the Township a geological
report. If the potential for sinkholes exists within the site, design
conditions shall be required to reduce the potential of sinkhole formation.
(e)
In establishing the antecedent conditions for calculating runoff
and time of concentration prior to development, the following assumptions
shall apply:
[1]
A Type II storm distribution as identified in the latest edition
of Urban Hydrology for Small Watersheds (Technical Release No. 55).
[2]
Calculations shall be based on the whole tract and/or lot on
which development is occurring, even if only a portion of this tract
and/or lot is being developed.
[3]
The ground cover used in determining the predevelopment flow
rates shall be based upon current actual ground cover conditions or
ground cover which existed 18 months prior to the application, whichever
generates the lower runoff coefficient.
(f)
The design of all stormwater management facilities shall be
in accordance with this chapter and all other applicable local, state
and federal regulations.
(g)
Stormwater control systems may be planned and constructed in
coordination by two or more developments so long as they are jointly
in compliance with this chapter and other applicable ordinances, rules
and regulations.
(2)
Detention/storage systems. Stormwater detention/storage facilities
shall consist of detention/retention basins, underground stormwater
storage and/or BMPs.
(a)
Retention/detention basins.
[1]
Planning criteria.
[a]
All detention basins shall be designed to infiltrate
or empty within 72 hours.
[2]
Construction criteria.
[a]
In order to ensure proper drainage on the floor
of detention basins, a minimum grade of 2% shall be maintained for
areas of sheet and channel flow. For channel flow, a minimum grade
of 1% will be permitted with the addition of a low-flow channel. Under
certain circumstances, such as continuous seasonal flow, the Township
may require a low-flow channel or underdrain be constructed.
[b]
The minimum top width of the basin berm shall be
10 feet. A cutoff trench (key-way) of impervious material shall be
provided under all embankments that require fill material. The cutoff
trench shall be a minimum of eight feet wide, two feet deep, and have
side slopes of one to one.
[c]
The maximum slope of all slopes around a detention/retention
basin, including the slopes of earth detention embankments, shall
be three to one. The top and/or toe of any slope shall be located
a minimum of five feet from any property line. Whenever possible,
the side slopes and basin shape shall conform to the natural topography.
[d]
When requested by the Township Engineer, the Township
shall be supplied with information providing that all embankments
are structurally sound. This requires a geotechnical investigation.
Construction of the embankments must be in eight-inch (or less) lifts
and compacted to a density not less than 95% of the maximum dry density
as determined by American Society for Testing and Materials (ASTM)
D-1557. When requested by the Township, each eight-inch (or less)
layer shall be tested to determine its density per ASTM D-1556. Compaction
test results shall be certified by a Pennsylvania-licensed professional
engineer and kept on file at the site during construction and be subject
to review at all times, with copies being forwarded to the Township.
Polyvinyl chloride (PVC) liners, forty-mil thickness or alternate
acceptable to the Township Engineer, must be used in the construction
of any basin in sinkhole-prone areas as defined on the Township's
Sinkhole Zone Map.[4]
[4]
Editor's Note: The Sinkhole Zone Map is on file in the
Township offices.
[e]
The average detention basin length should be at
least twice the average width. The maximum depth shall be 10 feet,
measured from the invert to the emergency spillway. Inflow and outflow
facilities to the basin shall be the greatest possible distance from
each other and should, if possible, be at opposite ends of the basin.
[f]
All temporary sediment control basins shall meet
the requirements of the County Soil Conservation District and Chapter
102 of Pennsylvania Code Title 25.
[3]
Spillway design criteria. Emergency spillways shall be constructed
to withstand the pressures of impounded waters. Whenever possible,
the spillway shall be constructed on undisturbed ground. Emergency
spillways shall be constructed of reinforced concrete, vegetated earth,
riprap, turf reinforcing grids or other material accepted by the Township.
All emergency spillways shall be constructed so that the embankment
is protected against erosion. The emergency spillway water elevation
for the peak flow rate of the one-hundred-year design storm under
outlet block conditions shall be indicated on the design plans. The
minimum freeboard shall be one foot. "Freeboard" is the difference
between the spillway water surface elevation for the peak flow rate
of the one-hundred-year design storm under outlet block conditions
and the top of the basin embankment. The emergency spillway shall
not discharge over uncompacted earthen fill and/or easily erodible
material. Spillway velocities may not exceed those recommended for
each lining material in the latest edition of the Erosion and Sediment
Pollution Control Program Manual published by the Bureau of Soil and
Water Conservation. Flow from the spillway must be directed to a receiving
stream via a stable channel or controlled to restore existing drainage
patterns.
[4]
Outlet structure design criteria.
[a]
Outlet structures within the detention/retention
basins shall be constructed of reinforced concrete or an approved
alternate. Orifice, weir flow or a combination of both may be used
for stormwater control. Extreme care should be taken to assure that
outlet pipes are watertight.
[b]
Energy dissipators shall be installed at points
where pipes or drainageways discharge to or from basins.
[c]
Antiseep collars shall be installed around the
pipe barrel within the normal saturation zone of the detention basin
berms. The antiseep collars and their connections to the pipe barrel
shall be watertight. The antiseep collars shall extend a minimum of
two feet beyond the outside of the principal pipe barrel. The maximum
spacing between collars shall be 14 times the minimum projection of
the collar measured perpendicular to the pipe. A minimum of two antiseep
collars shall be installed on each pipe outlet.
[5]
Sinkhole-prone areas. In sinkhole-prone areas, as identified
on the Township's Sinkhole Location Map, the Township Engineer
may require applicants to install liners in retention and detention
systems. Such liners shall be constructed of PVC with a forty-mil
thickness sandwiched between two layers of eight-ounce geotextile
material or clay alternatives approved by the Township Engineer.
[6]
Fencing. Fencing is to be provided around detention basins as
determined by the Township Engineer. Fencing may be either three-rail
post and rail fence with wire mesh attached to post and rails, chain-link
fencing or other approved equal.
(b)
Underground stormwater storage.
[1]
General requirements. Underground stormwater storage is permitted
in pipes or tanks but is not permitted in stone-filled underground
storage pits. In sinkhole-prone areas, underground stormwater storage
shall only be permitted when approved by the Township Engineer and
only when no other type of stormwater storage facility is feasible.
See the Upper Merion Township Sinkhole Location Map revised April
2004 or most recent version for sinkhole-prone areas.
[2]
Structural design. Underground storage structures shall be designed
to prevent failure by internal or external pressures and forces, including,
but not limited to, hydrostatic uplift pressure and imposed surface
loads such as vehicles operated on or adjacent to the tank. Structural
design criteria must be based on sound and accepted engineering principles.
[3]
Site design and maintainability.
[a]
In acid or sulfitic soils, structure materials
shall be nonreactive with the soil or measures shall be taken to protect
the tank from the soil.
[b]
All underground storage structures shall be provided
with adequate access for inspection and maintenance. A sufficient
number of access points shall be provided by the applicant, as required
by the Township Engineer and determined by plan area or length of
storage pipe. Access must include a removable manhole cover(s) of
at least two feet in diameter to a vertical pipe or precast shaft
of at least four feet in diameter or an inlet grating on a precast
vertical shaft of at least two feet by four feet. Access must be provided
to the lowest point of water storage. Manhole steps that meet the
Township Engineer's design specifications shall be provided.
[c]
All underground storage structures shall be kept
free from sediment and debris. Sediment traps shall be designed to
be readily maintainable.
[d]
Underground storage volume, above the frost line
or below the water table, shall not be included in the calculations
for storage volume to satisfy stormwater runoff criteria.
[e]
In sinkhole-prone areas, all underground storage
structures shall have watertight connections and shall be separated
from all surrounding soil or overhead material by a PVC liner of forty-mil
thickness sandwiched between two layers of eight-ounce geotextile
material.
[f]
Underground storage structures shall not be located
in areas conflicting with present trees or future plantings or in
areas where slope instability could result.
[g]
All underground storage structures must be provided
with positive gravity flow outlets. Minimum outlet pipe diameter shall
be four inches. Discharge dissipation shall be handled as specified
for aboveground storage facilities.
(c)
BMPs.
[1]
Peak rate reduction BMPs can be installed to meet the peak rate reductions referenced in § 140B-13B(1). Please refer to the following publications for acceptable peak rate reduction BMPs and associated design guidelines. Appendix B of this chapter contains a sample list.[5]
[5]
Editor's Note: Appendix B is included as an attachment to this chapter.
C.
Fee-in-lieu alternative for detention/storage facilities.
(1)
In order to address flooding and erosion problems found in all the watersheds in Upper Merion Township, applicants may pay a fee in lieu of stormwater detention/storage facilities as an alternative to the construction of stormwater detention/storage facilities, with the approval of the Township Board of Supervisors. When fees are offered by an applicant, the Township may, but shall not be required to, accept the fees, provided that the proposed stormwater solution meets the requirements in § 140B-13A(1).
(2)
These fees shall be used to construct Township-sponsored stormwater
facilities located within the same watershed as the project paying
the fee. Additionally, such fees may be used for the acquisition of
land and rights-of-way, engineering, legal and planning costs and
all other costs, including debt service, related to the construction
of necessary stormwater control facilities.
(3)
Applicants shall pay the following fees in lieu of stormwater
facilities:
(a)
Subdivisions and land developments. Fee-in-lieu payments for
subdivisions and land developments may be permitted when the Township
Board of Supervisors determines that fees used for regional stormwater
control will work better than comprehensive stormwater detention/storage
facilities on a particular site. These fee-in-lieu payments shall
be calculated using the following procedure:
[1]
The amount and design of required stormwater detention/storage
facilities shall be determined for the whole site, based on a meadow
condition for the whole site.
[2]
The amount and design of proposed stormwater detention/storage
facilities shall be determined for the site. These proposed facilities
must be able to accommodate, at a minimum, all increased stormwater
flows from new impervious coverage on a site. The cost of such fees
shall be calculated using the following criteria:
[a]
The fee shall be equal to the sum of the cost that
would have been incurred for constructing the stormwater detention/storage
facilities that will not be built plus the fair market value of the
raw, undeveloped land that would have been required for the stormwater
facilities.
[b]
The Township and the applicant shall attempt to
agree on a figure for the costs of constructing stormwater facilities
and fair market value of land. If the applicant and Township cannot
agree on the construction costs and fair market value, then the applicant
shall provide the Township with a construction estimate and a land
value appraisal, at the applicant's sole expense; the Township
shall review said construction cost estimate and land value appraisal
and make a decision as to construction costs and fair market land
value. The stormwater construction cost estimate shall be prepared
by an independent third-party registered engineer with no interest,
financial or otherwise, in the affected property or application. The
appraisal shall be prepared by an MAI real estate appraiser with no
interest, financial or otherwise, in the affected property or application.
[3]
The difference between the required amount of stormwater facilities
and the proposed amount of stormwater facilities may be accommodated
through the payment of a fee in lieu of such facilities. Such fees
shall equal the cost that would have been incurred for constructing
the stormwater detention/storage facilities that will not be built,
as determined by the applicant and the Township's Engineer.
(4)
Fees collected by the Township shall be deposited in an interest-bearing
account in a bank authorized to receive deposits of Township funds.
Interest earned by the account shall be credited to that account and
shall be used for the same purposes as money deposited in the account.
(5)
Fee-in-lieu payments shall be collected when the land development
agreement is finalized or, if no land development agreement is required,
before final permits are issued.
(6)
Fee-in-lieu credit. Each cubic foot of stormwater storage capacity
compensated by a fee-in-lieu payment shall be credited as the actual
construction of this stormwater storage when sites are proposed for
development in the future.
A.
Applicability of stormwater conveyance requirements.
(1)
All developments providing comprehensive stormwater detention/storage
facilities shall meet all stormwater conveyance system requirements.
(2)
Subdivisions and land developments, as defined in this chapter,
providing a fee in lieu of stormwater detention/storage facilities
and/or BMPs shall meet the stormwater conveyance system requirements
in this section when the Township Engineer determines that such systems
are necessary.
B.
Design requirements.
(1)
Standard head walls shall be installed on all pipes. Trashbars
shall be installed when the Township Engineer determines that trashbars
are necessary for the proper functioning and maintenance of pipes.
(2)
Pipeline design.
(a)
Piping materials.
[1]
Storm sewer pipelines shall be designed based on the Manning
Equation and shall utilize the following friction factors:
|
n
|
=
|
0.013 concrete pipe
| |
|
n
|
=
|
0.02 polyethylene pipe (corrugated interior)
| |
|
n
|
=
|
0.012 polyethylene pipe (smooth interior)
|
[2]
All other types of piping materials must be approved by the
Township Engineer or designated person with the required "n" value
as per manufacturers' specifications.
(b)
Pipes within Township street rights-of-way or other Township
property shall be made of reinforced concrete or corrugated high-density
polyethylene with a smooth interior, in accordance with specifications
established by the Township Engineer.
(c)
The minimum allowable pipe size is 15 inches.
(d)
The minimum slope of any pipe shall be such that a minimum velocity
of 2.5 feet per second shall be maintained when the pipe is flowing
1/4 full and shall not be less than 0.005 foot per foot slope.
(e)
Calculations shall be provided for each sewer run, including
a determination of inlet and outlet control.
(3)
Inlets.
(a)
Single open-mouth inlets shall be used on streets with grades
of 4% or less. Double No. 1 open-mouth inlets shall be used on streets
of more than 4%.
(b)
Sufficient inlets shall be located and constructed so as to
collect the stormwater flow in the drainage area tributary to the
inlet.
(c)
Where surface water is collected from two directions at one
street corner, inlets shall be placed at, or near, the tangent points
of both ends of the radius. The use of an inlet in the radius shall
not be allowed.
(d)
Where pipe sizes change, an inlet, manhole or junction box must
be used. In the event that a pipe entering an inlet, manhole or junction
box is smaller than the pipe exiting, the top of each pipe shall be
the same elevation.
(e)
Calculations must be provided for inlet capacities. Inlet grates
shall be designed to intercept the design storm runoff.
(f)
Grates that are safe for bicycles shall be provided in streets.
(4)
Manholes shall be located at intervals of approximately 300
feet where pipe sizes of 24 inches or less are used and 400 feet apart
for larger sizes. Inlets may be substituted for manholes where they
will serve a useful purpose. Manhole covers shall have the word "STORM"
in four-inch raised block letters.
(5)
Open channel design shall be based on the following hydraulic
considerations:
(a)
Manning's Equation.
[1]
"N" value.
|
n
|
=
|
0.013 for best concrete-lined ditch
| |
|
n
|
=
|
0.025 for best unlined ditch
| |
|
n
|
=
|
0.03 to 0.15 for fair to poor natural streams and watercourses
|
[2]
Maximum velocity.
|
Excavation Material
|
Velocity
(feet per second)
| |
|---|---|---|
|
Fine sand and firm loam
|
2.50 to 3.5
| |
|
Grass-lined ditch
|
4 to 5
| |
|
Stiff clay and hardpan
|
3.75 to 6.0
| |
|
Concrete-lined ditch
|
10
|
(b)
Ample freeboard shall be provided on all channels as directed
by the most recent version of the Pennsylvania Erosion and Sediment
Pollution Control Program Manual.
(c)
The channel shall be designed to conform, wherever possible,
to the adjacent ground conditions. This means that it shall not be
projecting excessively above the surrounding ground or placed excessively
below the surrounding ground.
(d)
Continuous profiles for each reach of open channel shall be
plotted along with the adjacent average ground, type of channel lining,
the proposed designed maximum water depth and channel slope.
(e)
Cross sections of the open channel shall include adjacent average
ground and hydraulic information pertinent to each cross section.
This information shall include the type of channel lining, the width
of the channel bottom, the side slope, the proposed designed maximum
water depth, the designed capacity and velocity.
(f)
Open earthen channels shall have a maximum side slope of three
to one and shall have adequate slope protection as required by this
chapter.
(6)
Single-opening culverts are desirable. The design of culverts
shall be such as to minimize the probability of debris accumulation.
Bridges and culverts shall be designed to meet current Pennsylvania
Department of Transportation standards for expected loads.
(7)
Wherever possible, storm drains shall be located within the
streets. When not possible, the sewer line must be placed within the
confines of at least a twenty-foot-wide easement. A minimum clearance
of six feet must be maintained between the outside edge of the pipe
and the edge of the easement. Pipes shall be protected by a cover
of at least 18 inches. Additional cover will be required depending
on the diameter of the pipe and manufacturer's and/or PADOT's
recommendations.
(8)
Due to safety concerns, roof drains, sump pumps and storm pipes
immediately adjacent to a sidewalk shall not discharge water over
a sidewalk. Roof drains shall discharge to infiltration or vegetative
BMPs unless otherwise approved by the Township. For roof drains that
do not discharge to an infiltration BMP, the discharge point for the
roof drain must be located a maximum five feet from the building unless
another location is approved by the Township. The location of the
discharge point for roof drains that discharge to vegetative BMPs
shall be reviewed on a case-by-case basis to ensure no adverse impacts
on neighboring properties. The discharge point for sump pumps shall
be located a maximum of five feet from the building unless another
location is approved by the Township.
(9)
Computation of the rate of flow at any given location shall
be based on either TR55 or the Rational Formula, In setting the value
of the runoff coefficient C for the Rational Formula, consideration
will be given to the physical features of the drainage basin and the
best available data on the future density of development of the drainage
basin. In no case shall meadow have a C value of greater than 0.30.
In no case shall grass in a developed condition have a C value of
less than 0.40 in a developed area. Impervious coverage in a developed
condition shall have a minimum C value of 0.95.
(10)
The stormwater conveyance system shall be based on the following:
(a)
As a minimum, all stormwater conveyance structures, systems
and inlets shall be designed to carry the peak flow for the fifty-year
storm.
(b)
As a minimum, stormwater conveyance structures, systems and
inlets conveying water to a stormwater detention basin or other storage
facility shall also be designed to accommodate the peak flow for a
one-hundred-year storm, either through the structures, systems or
inlets themselves or through an overflow system leading to the stormwater
management facility.
A.
The total impervious surface on the site shall be reduced by at least
20% based upon a comparison of existing impervious surface (based
on actual ground cover conditions) to proposed impervious surface.
B.
Where site conditions prevent the reduction of impervious area, stormwater
BMPs shall be implemented to provide water quality volume (runoff
volume generated from a two-year-return-period storm event) equivalent
to a reduction in impervious area of at least 20%.
C.
Where site constraints prevent impervious area reduction or the implementation
of stormwater BMPs, practical alternatives may be used to result in
an improvement in water quality. Such practical alternatives may include
but not be limited to:
(1)
Fees paid in lieu of impervious area reduction or the installation of stormwater BMPs providing volume reduction. The fee shall be equal to the cost that would have been incurred for constructing the stormwater BMP(s) referenced in Section § 140B-13B. The Township and the applicant shall attempt to agree on a figure for the costs of constructing stormwater BMP(s). If the applicant and the Township cannot agree on the construction costs, the applicant shall provide the Township with a construction estimate at the applicant's sole expense. The Township shall review said construction cost estimate and make a decision as to construction costs. The construction cost estimate shall be prepared by an independent third-party registered engineer with no interest, financial or otherwise, in the affected property or application.
(2)
Off-site BMP implementation for a drainage area comparable to
that of the project;
(3)
Watershed or stream restoration;
(4)
Retrofitting an existing stormwater facility or BMP; or
(5)
Other practices/alternatives approved by the Township Engineer.
D.
If the developer proposes a reduction in impervious area, the developer
will be required to substantiate the reduction by submitting engineering
calculations for the proposed reduction.
(1)
Disconnect impervious cover. Impervious area is considered either
connected or disconnected depending on where stormwater runoff is
discharged. When stormwater runoff from an impervious area flows directly
to a stormwater management facility it is considered DCIA. However,
some impervious cover can be disconnected by directing the flow to
a pervious area which allows for infiltration. When this is done,
the area may be considered disconnected impervious cover (DIC). DIC
may be treated as pervious when determining whether a redevelopment
site has met the twenty-percent reduction in impervious surface.
(2)
Rooftop disconnection. An adjustment to DCIA is permitted when
a downspout is disconnected and then directed to a pervious area which
allows for infiltration. The roof area which drains to the disconnected
downspout may be considered DIC. DIC may be treated as pervious when
determining whether a redevelopment site has met the twenty-percent
reduction in impervious surface.
