[Ord. No. 129, 9/3/2015]
1.
All stormwater management, collection, conveyance, erosion control,
and floodplain considerations shall be accomplished in accordance
with the following provisions:
A.
Prior to the final approval or commencement of any regulated activity
within the jurisdiction of this chapter, the developer shall submit
a stormwater management site plan to the Township for approval.
(1)
When plan applications, whether preliminary or final, are submitted
in sections, a detailed stormwater management site plan for the entire
project site shall be submitted. This detailed plan shall demonstrate
how the stormwater of the proposed section will relate to the entire
development. The amount and velocity at the discharge point of the
section shall be included in the data submitted. If temporary facilities
are required for construction of a section, such facilities shall
be included in the submitted plans.
(2)
The type, location, and extent of all erosion and sedimentation
control measures shall be shown on an erosion and sedimentation control
plan that conforms to the requirements of the Soil Erosion and Sediment
Control Manual of the Pennsylvania Department of Environmental Protection
and the erosion and sedimentation control design standards of this
chapter.
(3)
An operation and maintenance agreement, in recordable form acceptable
to the Township, that clearly sets forth the ownership, operation
and maintenance responsibility of all temporary and permanent stormwater
management facilities and erosion and sedimentation control facilities
as provided for in Part 6. The intent of these regulations is to provide
private operation and maintenance of stormwater management and erosion
and sedimentation control facilities, including:
(4)
A written report shall be submitted that includes the following
information:
(a)
General description of the development site, including a description
of existing natural and hydrologic features and any environmentally
sensitive areas.
(b)
General description of the overall stormwater management concept
for the project, including a description of permanent stormwater management
techniques, structural and nonstructural BMPs to be employed.
(c)
Construction specifications of the materials to be used for
structural stormwater management facilities.
(d)
Stormwater runoff calculations for both predevelopment and post-development
conditions.
(e)
An erosion and sedimentation control plan narrative that conforms
to the requirements of the Soil Erosion and Sediment Control Manual
of the Pennsylvania Department of Environmental Protection and provides
a description of all erosion and sedimentation control measures, temporary
as well as permanent, including the staging of earth moving activities,
sufficient in detail to clearly indicate their function.
(f)
For all proposed detention basins and retention basins, except
temporary sedimentation basins, the documentation shall include a
plotting or tabulation of storage volumes with corresponding water
surface elevations and the outflow rates for those water surfaces.
(g)
For all proposed detention basins and retention basins, except
temporary sediment basins, documentation shall set forth the design
hydrograph, the routing method or a method of equal caliber acceptable
to the Township Engineer, utilized to determine the function of the
basin.
B.
All stormwater runoff flowing over the project site shall be considered
in the design of the stormwater management facilities.
C.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
D.
Where a development site is traversed by watercourses, a drainage
easement shall be provided conforming substantially to the line of
such watercourses.
E.
The PADEP Chapter 105 rules and regulations apply to the construction,
modification, operation or maintenance of both existing and proposed
water obstructions and encroachments throughout the watershed, including
work in wetlands. Inquiries on permit requirements or other concerns
shall be addressed to PADEP's regional office.
F.
When it can be shown that, due to topographic conditions, natural
drainageways on the development site cannot adequately provide for
drainage, open channels may be constructed conforming substantially
to the line and grade of such natural drainageways. Work within natural
drainageways shall be subject to approval by PADEP.
G.
Any stormwater management facilities or any facilities that constitute
water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures,
etc.) that are regulated by this chapter, that will be located in
or adjacent to waters of the commonwealth (including wetlands), shall
be subject to approval by PADEP. When there is a question whether
wetlands may be involved, it is the responsibility of the applicant
or his agent to show that the land in question cannot be classified
as wetlands; otherwise, approval to work in the area must be obtained
from PADEP.
H.
Should any stormwater management facility require a dam safety permit
under PADEP Chapter 105, 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 100-year
event.
I.
No detention basin, storm pipe, or swale or any other stormwater
management facility shall discharge directly onto any publicly used
Township, state or private road but shall discharge into a culvert
under or along the road.
J.
Any stormwater management facilities regulated by this chapter that
will be located on or discharged onto state highway rights-of-way
shall be subject to approval by PennDOT.
K.
Stormwater management facilities located within or affecting the
floodplain of any watercourse shall comply with the requirements of
the flood plain regulations provided in the Zoning Ordinance[1] or any future ordinances regulating construction or development
within areas of the Township subject to flooding.
L.
The minimum floor elevations for all structures that would be affected
by a basin, other temporary impoundments, or open conveyance systems
where ponding may occur shall be two feet above the 100-year water
surface.
(1)
If basement or underground facilities are proposed, detailed
calculations addressing the effects of the stormwater ponding on the
structure and waterproofing and/or floodproofing design information
shall be provided for review and approval.
M.
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.
N.
Stormwater management facilities intended to receive and infiltrate
runoff from regulated activities shall be selected based on suitability
of soils and site conditions and shall be constructed on soils that
have the following characteristics:
(1)
Infiltration testing shall be conducted in accordance with BMP
Manual.
(2)
Acceptable soil will have a percolation rate of at least 0.1
inch per hour and not more than 10 inches per hour (after application
of the appropriate safety factor from the BMP Manual).
(3)
A minimum separation of 24 inches of acceptable soil between
the bottom of the facility and the limiting zone, unless it is demonstrated
to the satisfaction of the Township that the selected BMP has design
criteria which allow for a smaller separation.
(4)
A stabilized infiltration rate sufficient to accept the additional
stormwater load and drain completely as determined by field tests
conducted by the applicant's professional designer.
(5)
The Township shall be notified 24 hours prior to infiltration
tests being conducted as to provide an opportunity for the Township
to witness the tests.
O.
Stormwater discharge points onto an adjacent property shall comply
with the following:
(1)
Stormwater flows onto adjacent property shall not be created,
relocated, or peak rate increased without written notification of
the adjacent property owner(s) by the developer including the establishment
of an easement.
(2)
Stormwater runoff from a project site shall flow directly into
a natural watercourse or into an existing storm sewer system. If neither
of these is available, the applicant shall obtain an easement from
the downstream landowner(s) to allow the site's runoff discharge
to reach a natural watercourse or an existing storm sewer system through
the easement. If an easement is obtained, post-developed flow characteristics
must be similar to the runoff characteristics (spread, velocity, and
peak rate) of the predeveloped flows. The easement from the downstream
property owner(s) shall be to allow for a piped storm sewer system,
an overland flow system, or a combination of the two. The downstream
system design shall conform to the design requirements of this chapter.
(3)
When the applicant provides verification that the downstream
landowner(s) refuses to grant an easement at reasonable terms in the
sole opinion of the Township, the site shall be designed such that
the discharge from the applicant's site shall be in a non-erosive,
sheet flow condition. For all design year storms, including the 100-year
storm, runoff from the applicant's site shall flow onto the adjacent
property in a manner similar to the runoff characteristics (spread,
velocity, and peak rate) of the predeveloped flow. The use of level
spreaders is prohibited.
(4)
Stormwater runoff shall not be transferred from one watershed
to another unless the watersheds are subwatersheds of a common watershed
which join together within the perimeter of the property, or both
of the following apply: 1) the effect of the transfer does not alter
the peak discharge (in conformance with the requirements of the Act
167 Plan) onto adjacent lands, and 2) drainage easements from the
affected landowners are provided.
(a)
For the purposes of this subsection, "affected landowners" shall
include all landowners receiving runoff that was transferred from
one watershed to another watershed that does not join together within
the perimeter of the project site, from the point of discharge from
the project site to the location in the watershed where the transferred
runoff discharges to a receiving watercourse located within the watershed
that the runoff was transferred from.
P.
Unless an alternate design is submitted to the Township for review,
and said design is prepared by a licensed (in the Commonwealth of
Pennsylvania) geologist or geotechnical engineer:
(1)
No stormwater facilities shall be placed in, over or within
a distance that will impact the following features:
(2)
The minimum isolation distance from stormwater management facilities
to the listed geologic features shall be as follows:
(3)
Stormwater runoff from any regulated activity shall not be discharged
into sinkholes.
(4)
It shall be the developer's responsibility to verify if
the development is underlain by carbonate geology. The certificate
provided in the Appendix[2] shall be attached to all stormwater management site plans
and signed and sealed by the developer's qualified professional.
[2]
Editor's Note: Said appendix is included as an attachment
to this chapter.
(5)
Whenever a stormwater facility will be located in an area underlain
by carbonate geology, a geological evaluation of the proposed location
by a registered professional geologist shall be conducted to determine
subsurface conditions including soil permeability, depth to bedrock,
subgrade stability, and susceptibility to sinkhole formation.
(6)
Impermeable liners may be used to reduce or eliminate the separation
distances listed in Subsections A and B above.
Q.
The calculated peak rates of runoff for stormwater originating on
the project site must meet one of the following conditions for all
watersheds flowing from the project site:
(1)
Match Predevelopment Hydrograph.
(a)
Developers and/or landowners are encouraged to provide infiltration
facilities or utilize other techniques which will allow the post-development
100-year hydrograph to match the predevelopment 100-year hydrograph,
along all parts of the hydrograph, for the development site.
(b)
To match the predevelopment hydrograph, the post-development
peak rate must be less than or equal to the predevelopment peak rate,
and the post-development runoff volume must be less than or equal
to the predevelopment volume for the same storm event.
(c)
A shift in hydrograph peak time of up to five minutes and a
rate variation of up to 5% at a given time may be allowable to account
for the timing effect of BMPs used to manage the peak rate and runoff
volume.
(d)
Incorporating the volume control requirements as given in Part
5 can be used as part of this option.
(2)
Where the predevelopment hydrograph cannot be matched, the calculated
peak rates of runoff for stormwater originating on the project site
must meet the following conditions, for all watersheds flowing from
the project site:
(a)
The two-year post-development rate of runoff (peak flow) from
any regulated activity shall be less than or equal to 50% of the calculated
two-year predeveloped rate (peak flow) except where:
(b)
Where an application meets one of the above exceptions, the
two-year post-developed rate of runoff (peak flow) shall be less than
or equal to the calculated two-year predeveloped rate of runoff (peak
flow).
(c)
Post-development runoff from any regulated activity shall not
exceed the peak rates of runoff prior to development for all other
design storms (five-, ten-, twenty-five-, fifty-, and 100-year storm
events).
(3)
Developers have the option to propose a regional stormwater
management plan or participate in a regional stormwater management
plan developed by others.
(a)
A regional stormwater management plan may include off-site volume
and rate control, as appropriate and supported by a detailed design
approved by the Township.
(b)
A regional stormwater management plan must meet all of the volume
and rate control standards required by this chapter for the area defined
by the regional stormwater management plan, but not necessarily for
each individual development site.
(c)
Appropriate agreements must be established to ensure the requirements
of this chapter and the requirements of the regional stormwater management
plan are met.
R.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity. Staging of earthmoving activities and selection of construction
equipment shall consider this protection.
S.
Infiltration BMPs shall not be constructed nor receive runoff from
disturbed areas until the entire contributory drainage area to the
infiltration BMP has achieved final stabilization.
T.
If structure foundations, basements, or other underground facilities
(whether on-site or off-site) would be affected by saturated soil
due to a nearby stormwater management facility, detailed calculations
addressing the impact of the saturated soil, and waterproofing design
information, where applicable, shall be submitted for approval.
2.
In the case of any dispute in the methodology used in the design
of any stormwater management site plan and/or in the presentation
of such information, the Supervisors shall make the final determination
on the design criteria, methodology, and form of presentation.
[Ord. No. 129, 9/3/2015]
1.
Methods shall be selected by the qualified person based on the individual
limitations and suitability of each method for a particular site.
The methods of computation used to determine peak discharge and runoff
shall be:
A.
The Soil-Cover-Complex Method (as set forth in the latest edition
of Urban Hydrology for Small Watersheds, Technical Release No. 55
as published by NRCS, formerly SCS) shall be used for all detention
facilities with a drainage area greater than or equal to 60 acres.
This method is recommended for design of stormwater management facilities
and where stormwater runoff volume must be taken into consideration.
B.
The Rational Method may be used for drainage areas up to 60 acres.
Extreme caution should be used by the qualified person if the watershed
has more than one main drainage channel, if the watershed is divided
so that hydrologic properties are significantly different in one watershed
versus the other, if the time of concentration exceeds 60 minutes,
or if stormwater runoff volume is an important factor. The combination
of Rational Method hydrographs based on timing shall be prohibited.
D.
Any other method approved by the Township Engineer.
E.
If the NRCS/SCS Method (also known as the "Soil-Cover-Complex Method")
is used, the design storm volumes to be used in the analysis of peak
rates of discharge shall be obtained from the Precipitation-Frequency
Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S.
Department of Commerce, National Oceanic and Atmospheric Administration
(NOAA), National Weather Service, Hydrometeorological Design Studies
Center, Silver Spring, Maryland. NOAA's Atlas 14 can be accessed
at http://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.
(1)
Provide the rainfall used for the two-, ten-, twenty-five-,
fifty-, and 100-year twenty-four-hour storm events. Rainfall values
vary throughout the county depending on location.
(2)
Provide the location (longitude and latitude) or a description
of the location for which the rainfall applies.
(3)
If rainfalls from more than one location are used, provide the
methodology by which the design rainfall was calculated.
(4)
Antecedent Moisture Condition 1 shall be used in areas of carbonate
geology.
(5)
Antecedent Moisture Condition 2 shall be used in all other areas.
(6)
A Type II distribution shall be used in all areas.
F.
If the Rational Method is used, the NOAA Atlas 14 data (see § 23-502, Subsection 1E, above) shall be used to determine the rainfall intensity in inches per hour based on the information for the five- through sixty-minute duration storm events.
G.
Hydrographs may be obtained from NRCS methods such as TR-55, TR-20,
or from use of the "modified" or "unit hydrograph" rational methods.
(1)
If "modified" or "unit hydrograph" rational methods are used,
the ascending leg of the hydrograph shall have a time of three times
the time of concentration (3 x Tc) and the descending leg shall have
a time of seven times the time of concentration (7 x Tc) to approximate
an NRCS/SCS Type II hydrograph.
I.
Times of concentration shall be based on the following design parameters:
(1)
Sheet flow: The maximum length for each reach of sheet or overland
flow before shallow concentrated or open channel flow develops is
150 feet. Flow lengths greater than 100 feet shall be justified based
on the actual conditions at each development site. Sheet flow may
be determined using the nomograph in the Appendix, or the Manning's
kinematic solution shown in the sheet flow section of Worksheet No.
1 in the Appendix.
(2)
Shallow concentrated flow: Travel time for shallow concentrated
flow shall be determined using average velocities for estimating travel
time for shallow concentrated flow nomograph from TR-55, Urban Hydrology
for small watersheds in the Appendix.
(3)
Open channel flows: At points where sheet and shallow concentrated
flows concentrate in field depressions, swales, gutters, curbs, or
pipe collection systems, the travel times and downstream end of the
development site between these design points shall be based upon Manning's
Equation and/or acceptable engineering design standards as determined
by the Township Engineer.
J.
For the purpose of calculating predevelopment peak discharges, all
runoff coefficients, both on site and off site, shall be based on
actual land use assuming summer or good land conditions. Post-development
runoff coefficients for off-site discharges used to design conveyance
facilities shall be based on actual land use assuming winter or poor
land conditions.
K.
Design of on-site conveyance systems calculations may use the Rational
Method of Q=CIA, where Q is the peak discharge of the watershed in
cubic feet per second, C is the coefficient of runoff, I is the intensity
of rainfall in inches per hour, and A is the area of the watershed
in acres; or any other method approved by the Township.
L.
Runoff calculations shall include a hydrologic and hydraulic analysis
indicating volume and velocities of flow and the grades, sizes, and
capacities of water carrying structures, sediment basins, and retention
and detention structures and sufficient design information to construct
such facilities. Runoff calculations shall also indicate both predevelopment
and post-development rates for peak discharge of stormwater runoff
from the project site.
M.
Runoff calculations will also be made to ensure that the runoff from
the upstream watershed area can be accommodated by the pipes, drainage
easements, watercourses, etc., on the site.
N.
Runoff calculations will also be made to ensure that the runoff from
the project site can be accommodated by the receiving pipes, drainage
easements, watercourses, etc., downstream from the site.
O.
Peak rate control is not required for off-site runoff. Off-site runoff
may be bypassed around the site provided all other discharge requirements
are met. If off-site runoff is routed through rate control facilities,
runoff coefficients for off-site discharges used to design those rate
control facilities shall be based on actual land use assuming winter
or poor land conditions.
[Ord. No. 129, 9/3/2015]
1.
The applicant shall meet requirements as contained in 25 Pa. Code
Chapters 92 and 102 as required and applicable as follows:
2.
Evidence of any necessary plan or permit approval for land disturbance
activities from PADEP or the Lancaster County Conservation District
must be provided to the Township.
3.
A copy of the notice of termination for NPDES permits shall be provided
to the Township once accepted by Lancaster County Conservation District.
4.
Construction of temporary roadways (e.g., for utility construction,
timber harvesting, etc.) shall comply with all applicable standards
for erosion and sedimentation control and stream crossing regulations
under 25 Pa. Code Chapters 102 and 105.
5.
The erosion and sedimentation control plan shall be submitted to
the Lancaster County Conservation District for approval and shall
address the following, as applicable:
A.
Design of the roadway system, including haul roads, skid roads, landing
areas, trails, and storage and staging areas.
B.
Runoff control structures (e.g., diversions, culverts, detention
ponds, etc.).
C.
Stream crossings for both perennial and intermittent streams.
D.
Access to public roadways, including design of rock construction
entrance for mud and debris control.
E.
A remediation plan for restoring the disturbed area through regrading,
topsoil placement, reseeding, and other stabilization techniques,
as required.
6.
The following principles shall be applied to the design plan and
construction schedule to minimize soil erosion and sedimentation:
[Ord. No. 129, 9/3/2015]
1.
All storm sewer pipes, grass waterways, open channels, swales, and
other water-carrying facilities that service drainage areas within
the site shall be designed to convey the twenty-five-year storm event
unless, in the opinion of the Township or Township Engineer, the character
of development and potential for damage warrant design for the fifty-
or 100-year storm.
2.
Stormwater management facilities that convey off-site water through
the site shall be designed to convey the fifty-year storm event.
3.
All developments shall include provisions that allow for the overland
conveyance and flow of the post-developed 100-year storm event without
damage to public or private property.
4.
Conveyance facilities shall comply with the design criteria in the
following table:
|
Conveyance Facility Design Criteria
| ||||
|---|---|---|---|---|
|
Location
|
Within Public Street Right-of-Way
|
Outside Public Street Right-of-Way
| ||
|
Loading
|
All
|
Vehicular Loading
|
Non-Vehicular-Loading
| |
|
(a)
|
Pipe design
| |||
|
[1]
|
Material
|
SLHDPE, RCP
|
SLHDPE, RCP
|
SLHDPE, RCP
|
|
[2]
|
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
|
|
[3]
|
Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 foot to surface
|
|
[4]
|
Diameter (minimum)
|
18 inches
|
18 inches
|
18 inches
|
|
[5]
|
Street crossing angle
|
90°
|
90°
|
N/A
|
|
[6]
|
Access/maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
400 feet
|
|
(b)
|
Inlet design
| |||
|
[1]
|
Material
|
Concrete
|
Concrete
|
N/A
|
|
[2]
|
Grate depression
|
N/A
|
N/A
|
6 inches with 20:1 approach slope
|
|
(c)
|
Manhole design
| |||
|
[1]
|
Material
|
Concrete
|
Concrete
|
Concrete
|
|
(d)
|
Swale design
| |||
|
[1]
|
Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
|
|
[2]
|
Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
|
|
[3]
|
Slope (minimum)
|
2%
|
2%
|
2%
|
|
[4]
|
Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
|
|
[5]
|
Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
|
|
[6]
|
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
|
|
(e)
|
Pipe Inlet/Outlet design
| |||
|
[1]
|
End treatment
|
Concrete headwall/endwall
|
Headwall/endwall or flared end section
|
Headwall/endwall or flared end section
|
|
[2]
|
Energy dissipater
|
Required
|
N/A
|
Required
|
5.
All storm sewer pipes, culverts, manholes, inlets, endwalls and end
sections shall be constructed in accordance with Pennsylvania Department
of Transportation Publication 408, as modified by West Cocalico Township.
6.
Storm sewer pipes, culverts, manholes, inlets, endwalls, and end
sections proposed for dedication or located along streets shall conform
to the requirements of the Pennsylvania Department of Transportation,
Bureau of Design, Standards for Roadway Construction, Publication
No. 72, in effect at the time the design is submitted, as modified
by West Cocalico Township.
7.
The roughness coefficient (Manning "n" values) used for conveyance
pipe capacity calculations shall be determined in accordance with
PennDOT Publication 584, PennDOT Drainage Manual, or per the manufacturer's
specifications.
8.
Inlets shall be placed along streets as follows:
A.
On both sides of streets at low spots.
B.
At all changes in the horizontal or vertical direction of storm sewers.
C.
At points where the flow in gutters exceeds three inches.
E.
Inlets shall be depressed below the grade of the roadside swale or
ground surface as indicated in the above design chart.
F.
An access/maintenance port may either be an inlet or manhole.
G.
Manholes may be substituted for inlets at locations where inlets
are not required to collect surface runoff.
9.
Material consistency and placement depths for storm sewer pipe backfill
shall be (at a minimum) per all applicable pipe manufacturer's
recommendations, further providing it should be free of large (not
exceeding six inches in any dimension) stone, rock, or other objectionable
or detritus material.
10.
Within the public street right-of-way, the gutter spread based on
the twenty-five-year storm shall be no greater than 1/2 of the travel
lane and have a maximum depth of three inches at the curbline. A parking
lane shall not be considered as part of the travel lane.
11.
Inlets or manholes shall be placed at all points of changes in the
horizontal or vertical directions of conveyance pipes. Curved pipe
sections are prohibited.
12.
All inlets placed in paved areas shall have heavy-duty bicycle-safe
grating consistent with PennDOT Publication 72M, latest edition. A
note to this effect shall be added to the stormwater management site
plan or inlet details therein.
13.
Inlets, junction boxes, or manholes greater than five feet in depth
shall be equipped with nonaluminum ladder rungs and shall be detailed
on the stormwater management site plan.
14.
Where the connecting pipe has a diameter 18 inches or greater, headwalls
and endwalls shall be provided with a protective barrier device to
prevent entry of the storm sewer pipe by unauthorized persons. Such
protection devices shall be designed to be removable for cleaning.
15.
Flow velocities from any storm sewer shall not result in a degradation
of the receiving channel.
16.
Energy dissipaters shall be placed at the outlets of all storm sewer
pipes where flow velocities exceed maximum permitted channel velocities.
17.
The capacities of swales shall be computed from the Manning Equation
using the following design parameters: Permissible open channel velocities
and design standards shall be in accordance with good engineering
practice as documented in the Engineering Field Manual for Conservation
Practices, USDA, SCS, or in Design Charts for Open-Channel Flow, Hydraulic
Design Series No. 3, U.S. Department of Transportation.
B.
The "n" factors to be used for paved or riprap swales or gutters
shall be based upon accepted engineering design practices, as approved
by the Township Engineer.
C.
All swales shall be designed to maximize infiltration and concentrate
low flows to minimize siltation and meandering, unless geotechnical
conditions do not permit infiltration.
[Ord. No. 129, 9/3/2015]
1.
Aboveground storage facilities shall consist of all stormwater facilities
which store, infiltrate/evaporate/transpire, clean, release, or otherwise
affect stormwater runoff and the top of which is exposed to the natural
environment. Aboveground storage facilities shall be located above
the finished ground elevation. Aboveground storage facilities do not
include stormwater management facilities designed for conveyance or
cisterns.
2.
Facilities with a facility depth greater than eight feet shall not
be permitted in residential areas.
3.
Aboveground storage facilities shall comply with the design criteria
in the following table:
|
Aboveground Storage Facility Design Criteria
| ||||
|---|---|---|---|---|
|
Facility Depth
| ||||
|
Less than 2 feet
|
2 feet to 6 feet
|
Greater than 6 feet
| ||
|
(a)
|
Embankment Geometry
| |||
|
[1]
|
Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
|
|
[2]
|
Interior side slope (maximum)
|
3:1
|
5:1
|
5:1
|
|
[3]
|
Exterior side slope (maximum)
|
3:1
|
3:1
|
3:1
|
|
(b)
|
Embankment Construction
| |||
|
[1]
|
Clay/impervious core
|
Not required
|
Required
|
Required
|
|
[2]
|
Pipe collar
|
Not required
|
Required
|
Required
|
|
[3]
|
Compaction density
|
Not required
|
Required
|
Required
|
|
(c)
|
Internal Construction
| |||
|
[1]
|
Dewatering feature
|
N/A
|
Required
|
Required
|
|
[2]
|
Pretreatment elements
|
Not required*
|
Required
|
Required
|
|
(d)
|
Outlet Structure
| |||
|
[1]
|
Pipe size (minimum)
|
12 inches
|
18 inches
|
18 inches
|
|
[2]
|
Pipe material
|
SLHDPE, PVC, RCP
|
SLHDPE, RCP
|
SLHDPE, RCP
|
|
[3]
|
Anticlogging devices
|
Required
|
Required
|
Required
|
|
[4]
|
Antivortex design
|
Not required
|
Required
|
Required
|
|
[5]
|
Watertight joints in piping?
|
Yes
|
Yes
|
Yes
|
|
(e)
|
Spillway Requirements
| |||
|
[1]
|
Spillway freeboard (minimum)
|
3 inches
|
6 inches
|
12 inches
|
|
[2]
|
Width (minimum)
|
5 feet
|
10 feet
|
20 feet
|
|
[3]
|
Width (maximum)
|
20 feet
|
50 feet
|
50 feet
|
|
[4]
|
Spillway channel design
|
Required
|
Required
|
Required
|
|
[5]
|
Routing of 100-year storm
|
Permitted
|
Permitted
|
Permitted
|
|
NOTES:
|
|---|
|
* Pretreatment is required for infiltration BMPs unless shown
to be unnecessary.
|
|
N/A = Not applicable.
|
|
SLHDPE = Smooth lined high density polyethylene pipe.
|
|
PVC = Polyvinyl chloride.
|
|
RCP = Reinforced concrete pipe.
|
4.
If required, pretreatment elements shall be designed according to
the BMP Manual.
5.
All aboveground storage facilities shall be structurally sound and
shall be constructed of sound and durable materials.
A.
All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless steel.
B.
Bolts/fasteners shall be stainless steel.
C.
The completed structure and the foundation of all basins shall be
stable under all probable conditions of operation.
D.
Spillways shall be capable of discharging the peak discharge of a
post-development 100-year storm event through the emergency spillway
facilities, in a condition that assumes the primary outlet(s) are
blocked, which will not damage the integrity of the facility or the
downstream drainage areas.
E.
Use of the spillway to convey flows greater than the fifty-year design
storm shall be permitted.
6.
The effect on downstream areas if the aboveground storage facility
embankment fails shall be considered in the design of all basins.
The basin shall be designed to minimize the potential damage caused
by such failure of the embankment.
7.
An easement shall be provided from the spillway outfall to a natural
or artificial watercourse.
8.
The maximum depth of water for aboveground storage facilities without
restricted access shall not exceed six feet.
A.
Access to basins with a maximum depth of water greater than six feet
shall be restricted by fencing that will discourage access.
9.
Aboveground storage facilities without restricted access shall have
impoundment areas with side slopes no greater than the horizontal
to vertical ratios in the design chart.
A.
Access to basins with steeper side slopes than those shown in the
chart shall be restricted by fencing that will discourage access.
10.
All detention basins shall include an outlet structure to permit
draining the rate control volume within 24 hours, exclusive of BMP
storage.
11.
All outlet structures and emergency spillways shall include a satisfactory
means of dissipating the energy of flow at its outlet to assure conveyance
of flow without endangering the safety and integrity of the basin
and the downstream drainage area.
12.
A clay/impervious core shall consist of a cutoff trench (below existing
grade) and a core trench (above existing grade).
A.
A clay/impervious core may not be required wherever the facility
depth is less than two feet.
B.
Materials used for the clay/impervious core shall conform to the
Unified Soil Classification GC, SC, CH, or CL and must have at least
30% passing the No. 200 sieve.
C.
The dimensions of the clay/impervious core shall provide a minimum
trench depth of two feet below existing grade, minimum width of four
feet and side slope of 1H:1V or flatter.
D.
The clay/impervious core should extend up to the twenty-five-year
water surface elevation or six inches below the emergency spillway
elevation, whichever is lower.
E.
The clay/impervious core shall extend four feet below any pipe penetrations
through the impervious core.
F.
The core shall be installed along or parallel to the center line
of the embankment.
G.
Compaction requirements for the clay/impervious core shall be the
same as those for the embankment to assure maximum density and minimum
permeability.
13.
All pipe collars, if required, shall be designed in accordance with Chapter 7 of the PADEP Erosion and Sediment Control Manual. The material shall consist of concrete or otherwise nondegradable material around the outfall barrel and shall be watertight.
14.
The embankment fill material shall be free of top soil, organic material,
roots, stumps, wood, rubbish, stones greater than six inches, frozen
or other objectionable materials.
15.
The minimum freeboard for spillways shall be provided above the 100-year
design elevation of the water surface at the emergency spillway in
a condition that assumes the primary outlet(s) is (are) blocked.
16.
The minimum bottom slope of facilities not designed for infiltration
shall be 1%. A flatter slope may be used if an equivalent dewatering
mechanism is provided.
17.
If required, dewatering shall be provided through the use of underdrain,
surface device, or an alternate approved by the Township Engineer.
If the facility is to be used for infiltration, the dewatering device
should be capable of being disconnected and only be made operational
if the basin is not dewatering within the required timeframe.
18.
Within basins designed for infiltration, a planting plan shall be
prepared in accordance with this chapter and the BMP Manual.
[Ord. No. 129, 9/3/2015]
1.
Stormwater runoff volume controls shall be designed using the Design
Storm Method (CG-1 in the BMP Manual) as follows:
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 shall not increase
for all storms equal to or less than the two-year twenty-five-hour
storm event.
(2)
For modeling purposes:
(a)
Existing (predevelopment) non-forested pervious areas must be
considered meadow in good condition.
(b)
Twenty percent of existing impervious area to be disturbed,
when present, shall be considered meadow in good condition in the
model for existing conditions.
(c)
The maximum loading ratio for volume control facilities in karst
areas shall be:
B.
Any portion of the volume control storage that meets the following
conditions may also be used as rate control storage:
(1)
Volume control storage that depends on infiltration and is designed
according to the infiltration standards in this chapter and the BMP
Manual.
(2)
The volume control storage which will be used for rate control
is that storage which is available within 24 hours from the end of
the design storm based on the stabilized infiltration rate and/or
the evapotranspiration rate.
C.
Volume control BMPs and pretreatment elements shall be designed in
accordance with the BMP Manual and the soil and infiltration testing
requirements of Part 5.
[Ord. No. 129, 9/3/2015]
1.
Subsurface storage facilities shall consist of all stormwater facilities
which store, infiltrate/evaporate/transpire, clean, release, or otherwise
affect stormwater runoff and the top of which is not exposed to the
natural environment. Subsurface facilities shall be located below
the finished ground elevation. Subsurface facilities shall not include
stormwater management facilities designed for conveyance.
2.
Subsurface storage facilities shall comply with the design criteria
in the following table:
|
Subsurface Storage Facility Design Criteria
| |||
|---|---|---|---|
|
Facility Type
| |||
|
Infiltration and Storage
|
Storage Without Infiltration
| ||
|
(a)
|
Facility Geometry
| ||
|
[1]
|
Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
|
|
[2]
|
Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
|
|
(b)
|
Distribution System Requirements
| ||
|
[1]
|
Pipe size (minimum)
|
4 inches
|
4 inches
|
|
[2]
|
Loading/balancing
|
Required
|
Required
|
|
[3]
|
Observation/access ports
|
Required
|
Required
|
|
NOTES:
| |
|---|---|
|
*
|
Unless otherwise determined by professional geologic evaluation.
|
3.
The facility shall be designed according to the BMP Manual to provide
pretreatment to eliminate solids, sediment, and other debris from
entering the subsurface facility.
4.
The facility shall be designed to provide a means of evenly balancing
the flow across the surface of the facility to be used for infiltration.
5.
Observation/access ports shall be provided for all subsurface storage
facilities as follows:
A.
For facilities with the bottom less than five feet below the average
grade of the ground surface, a cleanout shall be an acceptable observation
port.
B.
For facilities with the bottom five feet or more below the average
grade of the ground surface, a manhole or other means acceptable to
the Township shall be provided for access to and monitoring of the
facility.
C.
The number of access points shall be sufficient to flush or clean
out the system.
6.
Storage and distribution system piping shall be PVC, SLHDPE, or RCP.
7.
The stone used for infiltration beds shall be clean washed, uniformly
graded coarse aggregate. The void ratio for design shall be assumed
to be 40%.
8.
Material consistency and placement depths for backfill shall be (at
a minimum) per all applicable pipe manufacturer's recommendations.
A.
Backfill material shall be free of large (not exceeding six inches
in any dimension) stone, rock, or other objectionable or detritus
material.
B.
Select nonaggregate backfill material should be indigenous to the
surrounding soil material for nonvehicular areas.
C.
Backfill material within vehicular areas shall comply with the requirements
of the governing municipal road/street or subdivision and land development
ordinance.
D.
If the design concept includes the migration of runoff through the
backfill to reach the infiltration facility, the material shall be
well-drained, free of excess clay or clay-like materials and generally
uniform in gradation.
9.
Non-woven geotextiles shall be placed on the bottom, all sides and
top of subsurface infiltration facilities.
10.
When located under pavement, the top of the subsurface facility shall
be a minimum of three inches below the bottom of pavement subbase.
Where located under vegetative cover, the top of the subsurface facility
shall be a minimum of 12 inches below the surface elevation or as
required to establish vegetation.
11.
Subsurface facilities shall be designed to safely convey and/or bypass
flows from storms exceeding the design storm.
