A.
For any of the activities regulated by this Part 2, unless preparation
of a stormwater management (SWM) site plan is specifically exempted,
the preliminary or final approval of subdivision and/or land development
plans, the issuance of any building or occupancy permit, the commencement
of any earth disturbance activity may not proceed until the property
owner or applicant or his/her agent has received written approval
from the municipality of a SWM site plan that demonstrates compliance
with the requirements of this Part 2, and a written approval of an
adequate erosion and sediment (E&S) control plan from the municipality
or County Conservation District when required.
B.
The SWM site plan approved by the municipality shall be on site throughout
the duration of the regulated activity.
C.
The municipality may, after consultation with the Department of Environmental
Protection (DEP), approve measures for meeting the state water quality
requirements other than those in this Part 2, provided that they meet
the minimum requirements of, and do not conflict with, state law,
including but not limited to the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth disturbance activities, E&S control best
management practices (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
Part 2 and to meet all requirements under Title 25 of the Pennsylvania
Code and the Clean Streams Law.[2] DEP regulations require an erosion and sediment control
plan for any earth disturbance activity of 5,000 square feet or more,
under 25 Pa. Code § 102.4(b). In addition, under 25 Pa.
Code Chapter 92,[3] a DEP "NPDES construction activities" permit is required
for regulated earth disturbance activities. A copy of the erosion
and sediment control plan and any required permit, as required by
DEP regulations, shall be available on the project site at all times.
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.
E.
For all regulated activities, implementation of the water volume controls in § 172-60 is required.
F.
Impervious areas:
(1)
The measurement of impervious areas shall include all of the impervious
areas in the total proposed development even if development is to
take place in stages.
(2)
For development taking place in stages, the entire development plan
must be used in determining conformance with this Part 2.
(3)
For projects that add impervious area to a parcel, §§ 172-57
through 172-62 shall apply to the total impervious area within the
limits of earth disturbance.
G.
Stormwater discharges onto adjacent property shall not be created,
increased, relocated, or otherwise altered without written notification
of the adjacent property owner(s). Such stormwater discharges shall
be subject to the requirements of this Part 2.
H.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety and property;
(2)
Meet the water quality goals of this Part 2 by implementing measures
to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(3)
To the maximum extent practicable, incorporate the techniques for
low-impact development practices described in the Pennsylvania Stormwater
Best Management Practices Manual (BMP Manual).
I.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
J.
Infiltration BMPs should be dispersed on site, made as shallow as
practicable, and located to maximize use of natural on-site infiltration
features while still meeting the other requirements of this Part 2.
K.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
and not more than 72 hours from the end of the design storm.
L.
The design storm precipitation amounts to be used in the analysis
of peak rates of discharge shall be those from the upper limits of
the 90% confidence intervals for the twenty-four-hour precipitation
events in the Precipitation-Frequency Atlas of the United States,
Atlas 14, Volume 2, Version 3.0, United States Department of Commerce,
National Oceanic and Atmospheric Administration (NOAA), National Weather
Service (NWS), Hydrometeorological Design Studies Center (HDSC), Silver
Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
N.
Various BMPs and their design standards are listed in the BMP Manual.
O.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT), and shall require an agreement to indemnify and hold harmless
the Township and PennDOT from any adverse consequences or costs.
P.
Any stormwater management facility (i.e., detention basin BMP) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Part 2 shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed conditions. The height of embankment must be set as to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year proposed conditions' peak inflow. Should any stormwater management facility require a dam safety permit under Pennsylvania Code Title 25, Environmental Protection, Chapter 105, Dam Safety and Waterway Management, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than the one-hundred-year event.
Q.
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands as directed in DEP Chapter 105 regulations (as amended or replaced from time to time by DEP)[6] shall be designed in accordance with Chapter 105 and will require a permit from DEP. Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
R.
Any drainage/conveyance facility and/or channel that does not fall under Chapter 105 regulations[7] must be able to convey, without damage to the drainage
structure or roadway, runoff from the ten-year design storm. Conveyance
facilities to or exiting from stormwater management facilities (i.e.,
detention basins) shall be designed to convey the design flow to or
from that structure. Roadway crossings located within designated floodplain
areas must be able to convey runoff from a one-hundred-year design
storm. Any facility located within a PennDOT right-of-way must meet
PennDOT minimum design standards and permit submission requirements.
S.
Storm sewers must be able to convey proposed conditions' runoff
from a ten-year design storm without surcharging inlets, where appropriate.
T.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
U.
Detention facilities will be designed as follows:
(1)
The emergency spillway on detention facilities shall be designed
to convey the one-hundred-year storm after development with one foot
of freeboard to the top of berm elevation. Emergency spillways shall
be constructed with a suitable liner, as determined by the Township
Engineer, designed in accordance with the manufacturer's specifications,
backfilled with topsoil and seeded to protect the berm against erosion.
(2)
Basins and all other types of BMPs shall be naturalized with landscaping
as recommended in DEP's Best Management Practices Manual as last
revised. The plantings shall be in addition to all other landscaping
requirements.
(3)
Two antiseep collars shall be installed around the basin outlet pipe
and shall be centered within the normal saturation zone of the berm.
The antiseep collars and their connections to the pipe barrel shall
be watertight.
(4)
The minimum basin berm width at the design elevation shall be 10
feet. A cutoff trench (keyway) of impervious material shall be provided
under all embankments that require fill material. The cutoff trench
shall be a minimum of eight feet wide, three feet deep and have side
slopes of one horizontal to one vertical.
(5)
Any stormwater basin that is intended to maintain a permanent pool
shall be designed in accordance with DEP's Pennsylvania Best
Management Practices Manual as last revised. All wet basins shall
have a ten-foot-wide safety bench, be planted with wet-tolerant vegetation
as approved by the Township Engineer and Township Landscape Architect,
provide a minimum pond depth of four feet and be lined with clay or
other impermeable liner and provide safety bench planting soil above
the liner. Additionally, aerators shall be installed in all basins
that are designed to maintain a permanent pool.
(6)
When an applicant retains ownership of any stormwater management
facility, such entity shall be responsible for repair and maintenance
of the facility. The site plan shall contain a notation permitting
access to such facilities deemed critical to public welfare, for inspection
at any reasonable time by the Township or its designee.
(7)
The site plan shall also contain a notation that states the approval
of the final plans is conditioned upon the applicant agreeing to be
responsible for all repairs and maintenance of the stormwater management
facility and permitting access to such facilities deemed critical
to public welfare, for inspection at any reasonable time by the Township
or its designee. The failure of the applicant to properly maintain
any stormwater management facility shall be construed to be a violation
of this Part 2 and shall be declared to be a public nuisance, subjecting
the violator to any and all penalties provided by law.
V.
Location, maintenance and responsibility for detention facilities.
(1)
Detention basins may be located on individual lots if approved by
the Township. The maintenance and ownership of these basins will be
the responsibility of the lot owner. The responsibility for maintenance
of any such basin shall be clearly noted on the subdivision plans
and shall appear on any deed conveying said lot from developer to
another party.
(2)
Detention basins may be located to serve multiple lots. In this case,
the ownership, maintenance and responsibility of the basin may be
either that of a homeowners' association or lot owners'
association. The responsibility for maintenance of any such basin
shall be clearly noted on the subdivision plans and shall appear on
any deed conveying said lot from the developer to another party.
(3)
Lower Moreland Township will not accept ownership of detention facilities.
(4)
Lower Moreland Township may require the developer and owner to execute
an agreement promising to maintain any stormwater facility and granting
access to the Township to inspect any such facility.
W.
Underground detention facilities will be permitted if they meet the
following criteria:
(1)
Unless otherwise agreed by the Board of Commissioners, underground
detention basins shall be located outside of all public rights-of-way.
(2)
Underground detention basins shall be constructed of high-density
corrugated polyethylene pipe or approved equal.
(3)
Inlets or storm manhole structures with steps shall be located at
each corner of the underground basin to provide access for maintenance.
(4)
A minimum one foot of freeboard shall be provided between the one-hundred-year
water surface elevation and the top of all inlet grates or manhole
rims.
(5)
The underground basin shall be wrapped in permeable geotextile fabric.
(6)
Backfill between the top of the underground basin and the bottom
of the pavement section, or the bottom of the topsoil layer in unpaved
areas, must be full stone backfill.
(7)
When infiltration rates exceed the acceptable values in the Pennsylvania
Best Management Practices Manual, as last revised, a twelve-inch-thick
sand filter layer, wrapped in geotextile fabric, should be provided
for additional water quality treatment.
(8)
An emergency overflow weir shall be provided for the outlet structure
in order to convey basin inflow in excess of design flows or in the
event the outlet structure becomes blocked and is unable to convey
the design flow. The minimum capacity of the emergency overflow weir
shall be equivalent to the one-hundred-year peak inflow rate for the
post-development design storm.
(9)
Proposed site improvements and grading downstream of the underground
detention basin shall be designed to convey the one-hundred-year peak
inflow rate to a natural outfall, storm sewer system or other drainage
facility, without adversely affecting downstream properties.
(10)
Underground stormwater facilities that receive runoff from more
than one lot within a residential community shall require the creation
of a homeowners' association (HOA). The HOA will be responsible
for the ownership and continued maintenance of the underground facility.
X.
Detention/retention facility design and construction standards:
(1)
Detention basins shall be designed to facilitate regular maintenance,
periodic mowing and periodic silt removal and reseeding. Shallow broad
basins are preferred to steep-sided basins.
(2)
The maximum slope of the earth and detention basin embankment shall
be three to one, with the exception that any slope to be maintained
by the Township shall be four to one. The top or toe of any slope
shall be located a minimum of five feet from a property line. Whenever
possible, the side slope and basin shape shall conform to the natural
topography.
(3)
Unless permitted by the Zoning Hearing Board in accordance with Chapter 108, "Floodplain Conservation and Damage Prevention," detention basins shall not be located within floodplains.
(4)
If retention basins are used, the applicant shall demonstrate that
such ponds are designed to protect public health, safety and welfare.
(5)
The minimum top width of the detention basin berm shall be 10 feet.
A cutoff trench (keyway) of relatively impervious material shall be
provided beneath all embankments requiring fill material. The keyway
shall be a minimum eight feet wide, minimum three feet deep and have
one-to-one side slopes.
(6)
In order to ensure proper drainage on the basin bottom, a minimum
grade of 2% shall be maintained for sheet flow. Where a slope of 2%
cannot be maintained, infiltration channels or trenches will be required
and shall be constructed between all basin inlets and the basin outlet.
(7)
All detention and retention basin embankments shall be placed in
eight-inch maximum lifts to a minimum dry density of 95%. Prior to
proceeding to the next lift, compaction shall be checked by the Township
Engineer or an approved soils engineer, who shall provide the Township
Engineer with a written report. Compaction tests shall be performed
using the Modified Proctor Method in accordance with ASTM D-1557.
Compaction tests shall be run on the leading and trailing edge as
well as the top of the berm.
(8)
Emergency overflow facilities shall be provided for detention facilities
to accommodate runoff in excess of design flows. Whenever possible,
emergency spillways for the detention basins shall be constructed
on undisturbed ground. Emergency spillways shall be constructed of
concrete pavers, gabions or other similar materials approved by the
Township Engineer. All emergency spillways shall be constructed so
that the detention basin berm is protected against erosion. The construction
material of the emergency spillway shall extend along the upstream
and downstream berm embankment slopes. The downstream slope of the
spillway shall, as a minimum, extend to the toe of the berm embankment.
The emergency spillway shall not discharge over earthen fill or easily
erodible material.
(9)
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 barrels 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 outlet pipe.
(10)
All outlet pipes through the basin berm shall be reinforced
concrete pipe, designed to withstand the loading caused by a fully
saturated berm, and shall have watertight joints using zero-ring joint
pipe. Outlet pipe shall be backfilled with material similar to the
core material (semi-impervious).
(11)
The invert of the inlet pipe(s) into a basin shall be six inches
above the basin floor or lining so that it can adequately drain after
rainstorms. Inlet pipe(s) shall discharge to areas of the basin that
slope toward the outlet structure.
(12)
Energy dissipaters and/or level spreaders shall be installed
at points where pipe, or drainageways drain to or from the basin.
Energy dissipaters shall comply with the Montgomery County Soil Conservation
District/Department of Environmental Protection criteria. NCRS energy-dissipating
device calculations shall be submitted for Township Engineer review
and approval.
(13)
Inlet and outlet structures shall be located at a maximum distance
from one another in order to promote water quality benefits. The Township
Engineer may require sediment trap devices for entrapping sediments
carried in stormwater if sufficient separation of inlet and outlet
structures cannot be achieved.
(14)
A perforated riser or similar sediment control device shall
be provided at each outlet of all detention basins during construction
for sediment control. The riser shall be constructed of metal or concrete.
The riser shall extend to a maximum elevation of one foot below the
crest elevation of the emergency spillway. The perforated riser shall
be designed so that the rate of outflow is controlled by the pipe
barrel through the basin berm when the depth of water within the basin
exceeds the height of the riser. Circular perforations with a maximum
diameter of one inch shall be spaced 12 inches vertically. The horizontal
spacing shall be in accordance to DEP Soil Erosion and Sedimentation
Control Manual specifications. The perforations shall be clearly cut
and shall not be susceptible to enlargement. All metal risers shall
be suitably coated to prevent corrosion. A trash rack or similar appurtenance
shall be provided to prevent debris from entering the pipe. All risers
shall have a concrete base attached with a watertight connection.
The base shall be of sufficient weight to prevent flotation of the
riser. An antivortex device consisting of a thin vertical plate normal
to the base and berm shall be provided at the top of the riser. Unless
this structure is part of the permanent outlet control, it shall be
removed from the site when the site has been adequately stabilized,
as determined by the Township Engineer.
(15)
All drainage channels shall be designed to prevent erosion of
the bed and batiks. The maximum permissible flow velocity shall not
exceed the design requirements outlined in the current Design Manual
published by the Pennsylvania Department of Environmental Protection.
Suitable stabilization shall be provided where required to prevent
erosion of the drainage channels.
(16)
Any vegetated drainage channel requiring mowing of the vegetation
shall have a maximum grade of three horizontal to one vertical on
those areas to be mowed.
(17)
Because of the critical nature of vegetated drainage channels,
the design of all vegetated channels shall, as a minimum, conform
to the design requirements outlined in the current Design Manual published
by the Pennsylvania Department of Environmental Protection.
Y.
Stormwater conveyance system design and construction standards:
(1)
Storm sewers, culverts, bridges and related installation shall be
provided to:
(a)
Permit unimpeded flow of natural watercourses and in such a
manner as to protect the natural character of the watercourses and
to provide regulated discharge.
(b)
Ensure adequate drainage of all low points along the line of
streets,
(c)
Intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area drained and to prevent
substantial flow of water across intersections.
(2)
All storm sewer system components shall conform to current PennDOT
standards.
(3)
Drainage structures which drain watershed areas in the excess of
1/2 square mile (320 acres) or which have a span of eight feet or
more shall be designed using the Natural Resources Conservation Service
Technical Release 55, Urban Hydrology for Small Watersheds.
(4)
If a water obstruction permit is required from the Pennsylvania Department
of Environmental Protection, the design storm shall be a one-hundred-year
storm.
(5)
The culvert or bridge shall be as wide as the ultimate width of the
roadway. Additional width may be required to provide sidewalks on
one or both sides of the cartway.
(6)
Storm sewer design and construction requirements:
(a)
Minimum pipe size is 18 inches.
(b)
Minimum pipe slope shall be 0.5% per foot.
(c)
Minimum drop across junctions shall be two inches. At changes
in pipe diameter, pipe crowns shall be matched at junctions (manhole,
inlet or junction box).
(d)
Maximum distance between junctions shall be 300 feet.
(e)
Runoff to proposed storm sewers and inlets shall be calculated
using the Rational Method.
(f)
The time of concentration shall be assumed to be five minutes
for pipes under 30 inches. For pipes 30 inches or greater, the calculated
time of concentration can be utilized.
(g)
The time of concentration to inlets for grate capacity calculations
shall be assumed to be five minutes.
(h)
All storm sewer pipes shall be designed at a minimum to accommodate
a minimum of a ten-year storm.
(i)
The one-hundred-year storm shall be transported to the basin.
The plans shall reflect this requirement as required by the Township
Engineer.
(j)
All storm sewer pipes at inlets in sump condition shall be designed
to accommodate the fifty-year storm.
(k)
All storm sewer pipes and inlets intended to drain to detention
facilities shall be designed to accommodate the one-hundred-year storm
if the bypass or overflow runoff will not reach the basin by overland
flow.
(l)
All inlets in sump condition shall be six-foot inlets or dual
four-foot inlets, separated by eight feet, as needed.
(m)
All storm sewer systems shall be analyzed for both inlet and
outlet control (including tailwater effects) by using the equations
and nomographs as shown in the FHA's Hydraulic Design Services
No. 5. In lieu of this, computer programs that calculate the actual
hydraulic grade line for the storm sewer system can be used, provided
that all losses (friction, bend, junction, etc.) are taken into account.
Documentation for the program must be submitted for approval.
(n)
Minimum cover over pipes shall be two feet from the top of the
outside of pipe bell to the final grade.
(o)
Inlet capacities shall be calculated using PennDOT or manufacturer's
nomographs. Documentation for manufacturer's nomograph must be
provided to the Township Engineer.
(p)
All stormwater pipes must be reinforced concrete pipe (RCP),
high-density polyethylene pipe (HDPE) or other equivalent material
approved by the Township Engineer.
Approvals issued and actions taken under this Part 2 do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
A.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office or County Conservation
District must be provided to the municipality.
B.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
The design of all regulated activities should include the following
to minimize stormwater impacts:
A.
The applicant should find practicable alternatives to the surface
discharge of stormwater, the creation of impervious surfaces, and
the degradation of waters of the commonwealth and must maintain as
much as possible the natural hydrologic regime of the site.
B.
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
C.
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
A.
Infiltration best management practices (BMPs) shall meet the following minimum requirements unless the site qualifies for an exemption from the infiltration requirements of this Part 2 as listed in § 172-45:
(1)
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics.
(a)
A minimum soil depth of 24 inches between the bottoms of the
infiltration BMPs and bedrock or other limiting zones.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's qualified person.
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(d)
All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
(2)
The size of the infiltration facility shall be based upon the following
volume criteria:
(a)
Where practicable and appropriate, the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
inch of runoff (I) over all proposed impervious surfaces.
(b)
The Rev required shall be computed as:
Rev = (1/12) * (I)
|
Where:
| ||||
Rev
|
=
|
Recharge volume (cubic feet)
| ||
I
|
=
|
Impervious area within the limits of earth disturbance (square
feet)
| ||
An asterisk (*) in equations denotes multiplication.
|
B.
Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified person, and at a minimum address
soil permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
(1)
Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
(2)
Provide field tests such as double ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(3)
Design the infiltration structure for the required recharge volume
(Rev) based on field determined capacity at
the level of the proposed infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
qualified person, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
(5)
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
The low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, this Part 2 establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors. All regulated activities greater than one acre must use the Design Storm Method.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
The post-development total runoff volume for all storms equal to
or less than the two-year, twenty-four-hour storm event shall not
be increased.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre, or for projects that require design
of stormwater storage facilities. For new impervious surfaces.
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces. [Note: An asterisk (*)
in equations denotes multiplication.]
Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
|
(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow — i.e., it
shall not be released into the surface waters of the commonwealth.
Removal options include reuse, evaporation, transpiration, and infiltration.
Volume (cubic feet) = (1/12) * impervious surfaces (square feet)
|
(3)
Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff;
however, in all cases, at least the 0.5 inch of the permanently removed
runoff should be infiltrated.
If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel. The buffer area shall be established and maintained with appropriate native vegetation. (Refer to Appendix B of the BMP Manual for plant lists.) If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback to a minimum of 10 feet. If an existing buffer is legally prescribed (e.g., deed, covenant, easement, etc.) and it exceeds the requirements of this Part 2, the existing buffer shall be maintained. Applicants shall adhere to the following stream bank erosion/channel protection requirements:
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream bank erosion),
the primary requirement is to design a BMP to detain the proposed
conditions' two-year, twenty-four-hour storm event to the existing
conditions' one-year flow using the SCS Type II distribution. Additionally,
provisions shall be made (such as adding a small office at the bottom
of the outlet structure or a sand filter) so that the proposed conditions'
one-year twenty-four-hour storm event takes a minimum of 24 hours
to drain from the facility from a point when the maximum volume of
water from the one-year twenty-four-hour storm event is captured (i.e.,
the maximum water surface elevation is achieved in the facility).
Release of water can begin at the start of the storm (i.e., the invert
of the water volume control orifice is at the invert of the facility).
B.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small drainage areas
contributing to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake. It is recommended that the design,
to accommodate maintenance, include a sand or porous media filter.
A.
The Pennypack Creek Watershed has been divided into stormwater management
districts as shown on the Management District Map (Figure 172-62)[1].
(1)
In addition to the requirements specified in Table 172-62 below, the erosion and sedimentation control (§ 172-57), the nonstructural project design (§ 172-58), the groundwater recharge (§ 172-59), the water volume control (§ 172-60), and the stream bank erosion (§ 172-61) requirements shall be implemented.
(2)
Standards for managing runoff from each subarea in the Pennypack
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year storm events are shown in 172-62. Development sites
located in each of the management districts must control proposed
condition runoff rates to existing condition runoff rates for the
design storms in accordance with Table 172-62.
Table 172-62
Peak Rate Control Standards by Stormwater Management District
in the Pennypack Creek Watershed
| ||||
---|---|---|---|---|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
A
|
2-year
5-year
10-year
25-year
50-year
100-year
|
Reduce to
|
1-year
5-year
10-year
25-year
50-year
100-year
| |
B
|
2-year
5-year
10-year
25-year
50-year
100-year
|
Reduce to
|
1-year
2-year
5-year
10-year
25-year
50-year
|
NOTES:
| |
---|---|
In District C, development sites that can discharge directly
to the Pennypack Creek Main Channel (east of I-95) and to the Delaware
River main channel without use of city infrastructure may do so without
control of proposed conditions' peak rate of runoff.
| |
Projects that are required to obtain a NPDES permit for stormwater
discharges associated with construction activities are required to
show no increase in peaks from existing conditions.
| |
When adequate capacity in the downstream system does not exist
and will not be provided through improvements, the proposed conditions'
peak rate of runoff must be controlled to the predevelopment conditions'
peak rate as required in District A provisions for the specified design
storms. The predevelopment condition for new development is the existing
condition. For redevelopment purposes in Philadelphia County, the
predevelopment condition shall be determined according to the procedures
found in the Philadelphia Stormwater Guidance Manual.
|
C.
District boundaries. The boundaries of the stormwater management
districts are shown on an Official Map that is available for inspection
at the municipal and county planning offices. A copy of the Official
Map at a reduced scale is included as Figure 172-62. The exact location
of the stormwater management district boundaries as they apply to
a given development site shall be determined by mapping the boundaries
using the two-foot topographic contours (or most accurate data required)
provided as part of the drainage plan.
D.
Sites located in more than one district. For a proposed development
site located within two or more stormwater management districts, the
peak discharge rate from any subarea shall meet the management district
criteria in which the discharge is located.
E.
Off-site areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
F.
Site areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area utilizing stormwater management measures shall
be subject to the management district criteria. In other words, unimpacted
areas bypassing the stormwater management facilities would not be
subject to the management district criteria.
A.
Stormwater runoff from all development sites with a drainage area
of greater than 20 acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. The qualified person must consult with the municipality to
gain approval of design methods prior to design.
(1)
Table 172-63 summarizes acceptable computation methods, and the method
selected by the qualified person shall be based on the individual
limitations and suitability of each method for a particular site.
The municipality may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than 20 acres.
The Soil Complex Method shall be used for drainage areas greater than
20 acres.
Table 172-63
Acceptable Computation Methodologies for Stormwater Management
Plans
Bucks County and Montgomery County Portions of the Watershed
| |||
---|---|---|---|
Method
|
Method Developed by
|
Applicability
| |
WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
Rational Method or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than 20 acres, or as approved by the municipality
and/or Municipal Engineer
| |
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
|
NOTES:
| ||
---|---|---|
Successors to the above methods are also acceptable. These successors
include WinTR55 for TR-55 and WinTR20 for TR-20.
|
B.
If a hydrologic computer model such as HydroCAD or HEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference NRCS
Type II.
C.
For the purposes of existing conditions' flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
Rational "C" value (i.e., forest).
D.
For Montgomery County only, all calculations using the Rational Method
shall use rainfall intensities from the NOAA 14 Precipitation-Frequency
Atlas of the United States (2004, revised 2006). Times of concentration
for overland flow shall be calculated using the methodology presented
in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55
(as amended or replaced from time to time by NRCS). Times of concentration
for channel and pipe flow shall be computed using flow velocities
as determined by Manning's Equation.
E.
The Manning Equation is preferred for one-dimensional, gradually
varied, open channel flow. In other cases, appropriate, applicable
methods should be applied; however, early coordination with the municipality
is necessary.
F.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part 2 using the generally
accepted hydraulic analysis technique or method of the municipality.
G.
The design of any stormwater detention facilities intended to meet
the performance standards of this Part 2 shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than 20 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The municipality may approve the use of
any generally accepted full hydrograph approximation technique that
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.