[Ord. 2014-04, 4/28/2014, § 23-301]
1.
Preparation of a stormwater management plan is required for all regulated activities, unless preparation and submission of such a plan is specifically exempted according to this chapter or the activity meets the criteria in § 23-402.
2.
No regulated activities shall commence until the municipality issued
unconditional written approval of the stormwater management plan.
The approved plan shall be on-site during the duration of the regulated
activity.
3.
The Township may, after consultation with DEP, approve measures for
meeting the state water quality requirements other than those in this
chapter, provided that they meet the minimum requirements of, and
do not conflict with, state law including, but not limited to, the
Clean Streams Law. The Township shall maintain a record of consultations
with DEP pursuant to this subsection. Where an NPDES permit for stormwater
discharges associated with construction activities is required, issuance
of an NPDES permit shall constitute satisfaction of consultation with
DEP.
4.
For all regulated activities, erosion and sediment control and stormwater
management BMPs shall be designed, 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 and the Clean
Streams Law. Various BMPs and their design standards are listed in
the BMP Manual and E&S Manual.
5.
Applicants have the option to propose a regional stormwater management plan or participate in a regional stormwater management plan developed by others. 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 in accordance with Subsection 3. 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. Appropriate agreements must be established to ensure the requirements of this chapter and the requirements of the regional stormwater management plan are met.
6.
All stormwater management plans shall be designed and signed and
certified by a qualified person.
7.
Where applicable, stormwater management facilities shall comply with
the requirements of Chapter 105.
8.
Stormwater management facilities which involve a state highway or
impacts drainage facilities associated with a state highway shall
also be subject to the approval of the PennDOT.
9.
Stormwater management facilities located within or affecting the floodplain of any watercourse shall also be subject to the requirements of the Zoning Ordinance of Penn Township [Chapter 27], as amended, or any future zoning ordinance, regulating construction and development within areas of the Township subject to flooding and any other applicable requirements of the Pennsylvania Floodplain Management Act.
10.
Stormwater runoff from a project site shall flow directly into a
natural watercourse or into an existing storm sewer system, or onto
adjacent properties in a manner similar to the runoff characteristics
of the pre-development flow. The Township may require written notification
of downstream property owners. Where a concentrated flow cannot be
directly discharged into a natural watercourse or an existing storm
sewer system, the applicant shall be required to establish an easement
through such adjacent properties until said flow enters a natural
watercourse or existing storm sewer system.
11.
Stormwater runoff shall not be transferred from one watershed to
another unless the watersheds are sub-watersheds of a common watershed
which join together within the perimeter of the development site,
and the effect of the transfer does not alter the peak discharge onto
adjacent lands, or drainage easements from the affected landowners
are provided.
12.
All stormwater runoff flowing over the project site shall be considered
in the design of the stormwater management facilities.
13.
Stormwater management control facilities (i.e., detention, retention
basins, and BMPs) for all areas of the Township shall be provided
to provide rate control so that the peak discharge of the calculated
post-development runoff to an adjacent property does not exceed 50%
of the predevelopment peak discharge for all regulated design storms
(two-, ten-, twenty-five-, fifty- and one-hundred-year storm events),
with the exception of small projects when exempt from rate control.
14.
Runoff calculations for the pre-development and post-development
comparison shall consider five different storm frequencies; the two-,
ten-, twenty-five-, fifty- and one-hundred-year storm events.
15.
Stormwater management facilities shall be supplemented by BMPs to
minimize the impacts to water quality of receiving waters. Non-structural
BMPs shall be utilized for all regulated activities unless proven
to be impractical. Credit will not be provided for the use of soil
amendment and restoration, except within stormwater basin bottoms.
Credit will also not be provided for rain barrels.
16.
Innovative methods for the detention and control of stormwater runoff
may be used when approved by the Township. Various combinations of
methods should be tailored to suit the particular requirements of
the type of development and the topographic features of the project
site. The following is a partial listing of BMPs which can be utilized
in stormwater management systems where appropriate:
A.
Decreased impervious surface area.
B.
Routed and dispersed flows over grass areas.
C.
Grass channels and vegetated strips.
D.
Bio-retention areas and rain gardens.
E.
Concrete lattice block or permeable surfaces with underlying systems.
F.
Rooftop detention.
G.
Cisterns and underground reservoirs (with water budget analysis and
supplemental detention capacity acceptable to the Township).
H.
Parking lot detention.
I.
Retention basins.
J.
Detention basins.
K.
Other methods as may be found in the BMP Manual, when accepted by
the Township.
17.
Existing on-site natural and man-made facilities shall be used to
the maximum extent practicable. If existing stormwater management
facilities within a project site do not meet the design requirements
of this chapter, and if such facilities are affected by new development
being proposed within the site, then the existing stormwater management
facilities must be redesigned and improved to meet the requirements
of this chapter.
18.
Where proposed development and improvements to existing stormwater
management facilities will cause adverse impacts on adjacent downstream
properties, the applicant shall mitigate such impacts.
20.
Runoff from impervious areas shall be drained to pervious areas within
the development site, unless the site has 85% or more impervious cover
and is a redevelopment, in which case the portion of the development
site that discharges to pervious areas shall be maximized.
21.
When final plan applications are submitted in sections or phases,
and if temporary facilities are required for construction of a section,
such facilities shall be included in the submitted plans for that
section in accordance with Chapter 102.
22.
The applicant or his agent shall demonstrate that any facilities
intended to be installed and located on an individual or group of
individual lots can be adequately maintained by the homeowner(s) and/or
lot owner(s). When possible, stormwater management facilities shall
be designed with consideration that facilities be effectively accommodated
within the property of one private entity to simplify ownership and
maintenance.
23.
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 computed water
surface during the one-hundred-year storm event. If basement or underground
facilities are proposed, detailed calculations addressing the effects
of stormwater ponding on the structure and waterproofing design information
shall be submitted for approval.
24.
In areas of carbonate geology, a detailed geologic evaluation prepared
by a registered professional geologist (PG) must be submitted as part
of the stormwater management plan and shall certify the following:
A.
No stormwater facilities shall be placed in, over or immediately
adjacent to the following features:
(1)
Sinkholes.
(2)
Closed depressions.
(3)
Lineaments in carbonate areas.
(4)
Fracture traces.
(5)
Caverns.
(6)
Intermittent streams.
(7)
Ephemeral streams.
(8)
Bedrock pinnacles (surface or subsurface).
|
It shall be the applicant's responsibility to verify if the
development site is underlain by carbonate geology. Whenever a stormwater
facility will be located in an area underlain by carbonate geology,
a detailed geological evaluation of the proposed location shall be
conducted by a registered professional geologist to determine susceptibility
to sinkhole formation and/or other adverse impacts and a plan for
remediation of any identified karst features. The applicant shall
include following note on the stormwater management plans which shall
be signed and sealed by the applicant's professional geologist: "I,
__________, certify that the proposed stormwater management facility(s)
(utilize applicable one) are/are not underlain by carbonate geology."
|
B.
Stormwater management basins shall not be located closer than 100
feet from the rim of sinkholes or closed depressions, nor within 100
feet from disappearing streams; nor shall these basins be located
closer than 50 feet from lineaments or fracture traces; nor shall
these basins be located closer than 25 feet from surface or identified
subsurface pinnacles.
C.
Stormwater resulting from land development activities shall not be
discharged into sinkholes.
25.
A planting plan is required for all vegetated stormwater BMPs, subject
to the following:
A.
Native or naturalized/non-invasive vegetation suitable to the soil
and hydrologic conditions of the development site shall be used unless
otherwise specified in the BMP Manual.
B.
Invasive vegetation may not be included in any planting schedule.
C.
The limit of existing, native vegetation to remain shall be delineated
on the plan along with proposed construction protection measures.
D.
Prior to construction, a tree protection zone and protection measures
shall be delineated at the dripline of the tree canopy. All trees
scheduled to remain during construction shall be marked; however,
where groups of trees exist, only the tress on the outside edge need
to be marked. A forty-eight-inch high snow fence or forty-eight-inch
high construction fence mounted on steel posts located eight feet
on center shall be placed along the tree protection boundary. No construction,
storage of material, temporary parking, pollution of soil, or regrading
shall occur within the tree protection zone.
E.
All planting shall be performed in conformance with good nursery
and landscape practice. Plant materials shall conform to the standards
recommended by the American Association of Nurseryman, Inc., in the
American Standard of Nursery Stock.
[Ord. 2014-04, 4/28/2014, § 23-302]
1.
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 pre-developed
and post-developed rates for peak discharge of stormwater runoff from
the project site.
2.
Post development conditions shall be modeled to include reasonably supplemental allocations for possible future impervious areas such as additions, patios, sheds, etc. For projects involving high density residential development or compact nonresidential development the Township may require post development modeling to reflect maximum impervious lot coverage permitted by Chapter 27 [Zoning].
3.
The methods of calculation used to determine peak discharge and runoff
values for sizing stormwater management facilities shall be:
A.
For all projects with a total watershed area of 25 acres or more,
the USDA Soil Conservation Service Soil-Cover-Complex Method as set
forth in the latest edition of "Urban Hydrology for Small Watersheds,
Technical Release No. 55," published by SCS (SCS - TR #55) or SCS
TR-20, shall be used. An Antecedent Moisture Condition 1 and a Type
II Distribution shall be used. CN values shall be as shown in Appendix
23-4; rainfall values shall be based on the twenty-four-hour storm
events as determined through the National Oceanic and Atmospheric
Administration (NOAA) Atlas 14 data or PennDOT Publication 584 "PennDOT
Drainage Manual," 2008 Edition, or latest.
B.
For all projects with a total watershed area of less than 25 acres
the Rational Method may be used. The Rational Method is based on the
formula of Q=CIA where Q is the peak discharge of the watershed in
cubic feet per second, C is the coefficient of runoff as shown in
Appendix 23-5, "I" is the intensity of rainfall in inches per hour
as determined through the National Oceanic and Atmospheric Administration
(NOAA) Atlas 14 data or PennDOT Publication 584 "PennDOT Drainage
Manual," 2008 Edition, or latest, and "A" is the area of the watershed
in acres.
C.
Any other method approved by the Township Engineer.
4.
For the purpose of calculating pre-development peak discharges and
modeling, when the existing site contains impervious area, 20% of
the existing impervious area to be disturbed shall be considered meadow
in good condition.
5.
For the purpose of calculating pre-development peak discharges and
modeling, all existing non-forested pervious areas must be considered
meadow in good condition.
6.
Any areas designed to initially be gravel or crushed stone shall
be assumed to be impervious.
7.
The criteria and assumptions to be used in the determination of stormwater
runoff and design of management facilities are as follows:
A.
Runoff coefficients shall be based on the information contained in
Appendices 23-4 or 23-5 if the actual land use is listed in that Appendix.
If the actual land use is not listed in Appendices 23-4 or 23-5, runoff
coefficients shall be chosen from other published documentation deemed
acceptable by the Township, and a copy of said documentation shall
be submitted with the stormwater management report. 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.
B.
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. Sheet flow may be determined by using Manning's kinematic
solution as outlined in Chapter 3 of TR-55, "Urban Hydrology for Small
Watersheds."
(2)
Shallow Concentrated Flow. Guidance for the calculation of travel
time for shallow concentrated flow can be found in Figure 3-1 in TR-55,
"Urban Hydrology for Small Watersheds."
(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.
8.
Hydrographs may be obtained from NRCS methods such as TR-55, TR20,
or from use of the "modified" or "unit hydrograph" rational methods.
If "modified" or "unit hydrograph" rational methods are used, the
ascending leg of the hydrograph shall have a length equal to three
times the time of concentration (3xTc) and the descending leg shall
have a length equal to seven times the time of concentration (7xTc)
to approximate an SCS Type II hydrograph. The Township may require
longer time of concentration values or other alternative analysis
for rational method hydrographs to realistically evaluate potential
runoff volumes from impervious areas and resulting impact to the required
stormwater facility storage capacity.
9.
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, retention
and detention structures and sufficient design information to construct
such facilities. Runoff calculations shall also indicate both pre-development
and post-development rates for peak discharge of stormwater runoff
from all discharge points.
10.
Peak rate control is not required for off-site runoff. Off-site runoff
may be by-passed 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. 2014-04, 4/28/2014, § 23-303]
1.
All storm sewer pipes, culverts, bridges and stormwater conveyance
facilities (excluding BMPs, detention, retention and wetland basin
outfall structures), gutters and swales conveying water originating
only from within the boundaries of the development site shall be designed
for a twenty-five-year storm event (twenty-four-hour SCS Type II storm
or an IDF Curve Rational Method storm). All storm sewer pipes, culverts
and bridges (excluding detention, retention, and wetland basin outfall
structures) conveying water originating from off-site shall be designed
for a fifty-year storm event. If in the opinion of the Township Engineer
there is potential for damage to property or the loss of life due
to the failure of the system to carry flows above the minimum design
flows as outlined above, the design storm event shall be increased.
Conveyance facilities and associated drainage easements shall be provided
to safely contain and convey the one-hundred-year frequency flood
to appropriate peak rate control BMPs and throughout the development
site. Easements shall begin at the furthest upstream property line
of the proposed development site in a watershed.
A.
Distribution System Requirements.
(1)
Pretreatment Requirements. The facility shall be designed to
provide a method to eliminate solids, sediment, and other debris from
entering the subsurface facility.
(2)
Loading/Balancing. 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.
(3)
Observation/Access Ports.
(a)
For facilities with the bottom less than five feet below the
average grade of the ground surface, a clean-out 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 municipality shall be provided for access to and monitoring of
the facility.
(c)
The number of access points shall be sufficient to flush, vacuum
extract or otherwise clean out the system.
B.
Materials.
(1)
Pipe Material. Distribution system piping may be PVC, SLHDPE,
or RCP.
(2)
Stone for Infiltration Beds. The stone used for infiltration
beds shall be clean washed, uniformly graded coarse aggregate (AASHTO
No. 3 or equivalent approved by the municipality). The void ratio
for design shall be assumed to be 0.4. Clean AASHTO No. 57 shall be
permitted to be used on the top surface of beds to facilitate improved
stability of cover material.
(3)
Backfill Material. Material consistency and placement depths
for 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) objectionable or detritus material.
Select non-aggregate material should be indigenous to the surrounding
soil material for non-vehicular areas. Backfill within vehicular areas
shall comply with this section unless otherwise specified in governing
municipal road/street or subdivision and land development ordinances.
Furthermore, 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.
(4)
Lining Material. Non-woven geotextiles shall be placed on the
sides and top of subsurface infiltration facilities. No geotextiles
shall be placed on the bottom of subsurface infiltration facilities.
C.
Cover.
(1)
When located under pavement, the top of the subsurface facility
shall be a minimum of three inches below the bottom of pavement subbase.
(2)
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.
D.
Subsurface facilities shall be designed to safely convey and/or bypass
flows from storms exceeding the design storm. Overflow points shall
be strategically positioned to maximize dissipation of flow prior
to discharge onto downstream properties.
2.
Conveyance Facilities. Conveyance facilities consist of all stormwater
facilities which carry flow, which may be located either above or
below the finished grade. Conveyance facilities do not include stormwater
management facilities which store, infiltrate/evaporate/transpire,
or clean stormwater runoff.
A.
Design Criteria. Conveyance facilities shall comply with the design
criteria in the following table:
|
Conveyance Facility Design Criteria
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|---|---|---|---|---|---|
|
Location
|
Within Public Street Right-of-Way or Property Intended
for Dedication
|
Outside Public Street Right-of-Way
| |||
|
Loading
|
All
|
Vehicular Loading
|
Non-Vehicular Loading
| ||
|
(1)
|
Pipe design
| ||||
|
(a)
|
Material
|
SLHDPE, RCP
|
PVC, SLHDPE, RCP
|
PVC, SLHDPE, RCP
| |
|
(b)
|
Slope (minimum)
|
0.5%
|
0.5%
|
0.5%
| |
|
(c)
|
Cover
|
1 foot to stone subgrade
|
1 foot to stone subgrade
|
1 foot to surface
| |
|
(d)
|
Diameter (minimum)
|
15 inches
|
15 inches
|
8 inches
| |
|
(e)
|
Street crossing angle
|
75° to 90°
|
N/A
|
N/A
| |
|
(f)
|
Access/ maintenance port frequency (maximum)
|
400 feet
|
400 feet
|
600 feet
| |
|
(2)
|
Inlet design
| ||||
|
(a)
|
Material
|
Concrete
|
Concrete
|
Concrete, HDPE (when permitted by Township)
| |
|
(b)
|
Grate depression
|
2 inches
|
2 inches
|
1 inch minimum
| |
|
(3)
|
Manhole design
| ||||
|
(a)
|
Material
|
Concrete
|
Concrete
|
Concrete
| |
|
(4)
|
Swale design
| ||||
|
(a)
|
Freeboard (minimum)
|
6 inches
|
N/A
|
6 inches
| |
|
(b)
|
Velocity (maximum)
|
Stability check
|
N/A
|
Stability check
| |
|
(c)
|
Slope (minimum)
|
1%
|
N/A
|
1%
| |
|
(d)
|
Side slopes (residential area)
|
4:1 max
|
N/A
|
4:1 max
| |
|
(e)
|
Side slopes (nonresidential area)
|
4:1 max
|
N/A
|
3:1 max
| |
|
(f)
|
Bottom width to flow depth ratio
|
12:1
|
N/A
|
12:1
| |
|
(5)
|
Outlet design
| ||||
|
(a)
|
End treatment
|
Headwall/ endwall
|
N/A
|
Headwall/ endwall or flared end section
| |
|
(b)
|
Energy dissipater
|
Required
|
N/A
|
Required
| |
|
N/A = Not applicable or no criteria specified
|
|
HDPE = High density polyethylene
|
|
SLHDPE = Smooth lined high density polyethylene pipe
|
|
PVC = Polyvinyl chloride
|
|
RCP = Reinforced concrete pipe
|
B.
Conveyance pipes, culverts, manholes, inlets and endwalls within
the public street right-of-way or proposed for dedication shall conform
to the requirements of PennDOT Standards for Roadway Construction,
Publication No. 72M and PennDOT Form 408, latest editions. Conveyance
pipes, culverts, manholes, inlets and endwalls which are otherwise
subject to vehicular loading shall be designed for the HS-25 loading
condition.
C.
Conveyance Pipes.
(1)
Backfill Material Requirements. Material consistency and placement
depths for 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) objectionable
or detritus material. Select non-aggregate material should be indigenous
to the surrounding soil material for non-vehicular areas. Backfill
within vehicular areas shall comply with this section unless otherwise
specified in governing municipal road/street or subdivision and land
development ordinances.
(2)
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 unless specifically authorized by the
Township.
(3)
Access/Maintenance Ports. An access/maintenance port is required
may either be an inlet or manhole.
(4)
Watertight joints shall be provided where pipe sections are
joined, except for perforated pipe installed as pavement base drain.
(5)
The street crossing angle shall be measured between the pipe
centerline and the street centerline.
(6)
Elliptical pipe of an equivalent cross-sectional area may be
substituted in lieu of circular pipe where cover or utility conflict
conditions exist. Dual or multiple parallel pipe installations are
not permitted.
(7)
The roughness coefficient (Manning "n" values) used for conveyance
pipe capacity calculations should be determined in accordance with
PennDOT Publication 584, PennDOT Drainage Manual, or per the manufacturer's
specifications.
D.
Inlets.
(1)
All pipes must enter inlets completely through one of the sides.
No corner entry of pipes is permitted.
(2)
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 curb line.
A parking lane shall not be considered as part of the travel lane.
In the absence of pavement markings separating a travel lane from
the parking lane, the parking lane shall be assumed to be seven feet
wide if parking is permitted on the street.
(3)
Flow Depth Within Intersections. Within intersections of streets,
the maximum depth of flow shall be 1 1/2 inches based on the
twenty-five-year storm.
(5)
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 plan
or inlet details therein.
(6)
Inlets, junction boxes, or manholes greater than five feet in
depth shall be equipped with ladder rungs and shall be detailed on
the stormwater management plan.
(7)
Inlet capacity shall be based on design data provided by the
manufacturers and accepted by the Township. Where ponding occurs,
inlet capacity shall be based on accepted engineering design practices.
E.
Swales.
(1)
Inlets within swales shall have PennDOT Type "M" top units or
equivalent approved by the Township Engineer.
(2)
Swale capacities shall be computed using the Manning equation
using the following design parameters:
(a)
Vegetated Swales.
1)
Flow capacity for all swales shall be based on a Manning's "n"
value of 0.05. Maximum velocities shall be based on a Manning's "n"
value of 0.03. Allowable velocities shall be as outlined in the E&S
Manual.
2)
All vegetated swales shall have a minimum slope of 1% unless
otherwise approved by the Township Engineer.
(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.
(3)
All swales shall be designed to maximize infiltration and concentrate
low flows to minimize siltation and meandering, unless geotechnical
conditions do not permit infiltration.
F.
Culverts. In addition to the material requirements in this section,
culverts designed to convey waters of the Commonwealth may be constructed
with either a corrugated metal arch or a precast concrete culvert.
Culverts shall be evaluated for both inlet and outlet control.
I.
End Treatments.
(1)
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.
(2)
Headwalls and endwalls shall be constructed of concrete.
(3)
Flared end sections may be used in special cases at the discretion
of the Township, and shall be of the same material as the connecting
pipe and be designed for the size of the connecting pipe.
3.
The proposed design discharge at the perimeter of the development
site shall not be beyond the capacity of the existing storm sewer
system into which it flows.
4.
All existing and natural watercourses (including associated floodplains),
channels, drainage systems and areas of surface water concentration
shall be maintained in their existing condition unless an improvement
alteration is approved by the Township. If existing conditions currently
function in an erosive state, improvements shall be required as part
of the overall site improvements.
5.
Where proposed development is to be performed in phases, each phase
of the project shall be designed to accommodate the full development
of the project. If temporary facilities are required to properly implement
the development of a phase, the design and construction of such facilities
shall be included within the appropriate phase. In the event temporary
facilities cannot adequately handle the stormwater runoff, proposed
stormwater management facilities of future phases of development shall
be incorporated within the current phase of development as necessary
to adequately handle the runoff from the current phase of development.
[Ord. 2014-04, 4/28/2014, § 23-304]
1.
Above-Ground Storage Facilities. Above-ground storage facilities
consist of all stormwater facilities which store, infiltrate/evaporate/transpire,
clean or otherwise affect stormwater runoff and the top of which is
exposed to the natural environment. Above ground storage facilities
are located above the finished ground elevation. Above ground storage
facilities do not include stormwater management facilities designed
for conveyance or cisterns.
A.
Design Criteria. Above ground storage facilities shall comply with
the design criteria in the following table:
|
Above-Ground Storage Facility Design Criteria
| |||||
|---|---|---|---|---|---|
|
Facility Depth
| |||||
|
Less Than 2 Feet
|
2 feet to 8 feet
|
Greater Than 8 feet
| |||
|
(1)
|
Embankment Geometry
| ||||
|
(a)
|
Top width (minimum)
|
2 feet
|
5 feet
|
8 feet
| |
|
(b)
|
Interior side slope (maximum)
|
2:1
|
3:1
|
5:1
| |
|
(c)
|
Exterior side slope (maximum)
|
2:1
|
3:1
|
3:1
| |
|
(2)
|
Embankment construction
| ||||
|
(a)
|
Key trench
|
Not required
|
Required
|
Required
| |
|
(b)
|
Pipe collar
|
Not required
|
Required
|
Required
| |
|
(c)
|
Compaction density
|
Not required
|
Required
|
Required
| |
|
(3)
|
Internal Construction
| ||||
|
(a)
|
Dewatering feature
|
N/A
|
Required
|
Required
| |
|
(b)
|
Pretreatment elements
|
Not required*
|
Required
|
Required
| |
|
(4)
|
Outlet Structure
| ||||
|
(a)
|
Pipe size (minimum)
|
6 inches
|
12 inches
|
15 inches
| |
|
(b)
|
Pipe material
|
SLHDPE, PVC, RCP
|
SLHDPE, RCP
|
RCP
| |
|
(c)
|
Anticlogging devices
|
Required
|
Required
|
Required
| |
|
(d)
|
Antivortex design
|
Not required
|
Required
|
Required
| |
|
(e)
|
Watertight joints in piping
|
Yes
|
Yes
|
Yes
| |
|
(5)
|
Spillway Requirements
| ||||
|
(a)
|
Spillway freeboard (minimum)
|
Not required
|
3 inches
|
6 inches
| |
|
(b)
|
Width (minimum)
|
Not required
|
10 feet
|
20 feet
| |
|
(c)
|
Width (maximum)
|
Not required
|
50 feet
|
50 feet
| |
|
(d)
|
Spillway channel design
|
Not required
|
Required
|
Required
| |
|
(e)
|
Routing of one-hundred-year storm
|
Permitted
|
Permitted
|
Permitted
| |
|
*
|
Pretreatment 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
| |
B.
Facility Depth.
(1)
For the purposes of the design criteria, the facility depth
is defined to be the depth between the bottom invert of the lowest
orifice and the invert of the spillway. If there is no spillway, the
top of the berm shall be used. For basins with no orifices or outlet
structure, the bottom elevation of the basin shall be used.
(2)
Facilities with a facility depth greater than eight feet shall
not be permitted in residential areas.
(3)
Facilities with a facility depth greater than 15 feet require
a dam permit from DEP.
C.
Embankment Construction.
(1)
Impervious Core/Key Trench. An impervious core/key trench, when
required, shall consist of a cutoff trench (below existing grade)
and a core trench (above existing grade). A key trench may not be
required wherever it can be shown that another design feature, such
as the use of an impermeable liner, accomplishes the same purpose
to the satisfaction of the Township.
(a)
Materials. Materials used for the core shall conform to the
Unified Soil Classification GC, SC, CH, or CL and must have at least
30% passing the No. 200 sieve.
(b)
Dimensions.
1)
The dimensions of the 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.
2)
The core should extend up both abutments to the ten-year water
surface elevation or six inches below the emergency spillway elevation,
whichever is lower.
3)
The core shall extend four feet below any pipe penetrations
through the impervious core. The core shall be installed along or
parallel to the centerline of the embankment.
(c)
Compaction.
(2)
Pipe Collars. All pipe collars, when required, shall be designed in accordance with Chapter 7 of the E&S Manual. The material shall consist of concrete or otherwise non-degradable material around the outfall barrel and shall be watertight.
(3)
Embankment Fill Material. The completed structure and the foundation
of all embankments shall be stable under all probable conditions of
operation. The embankment fill material shall be taken from an appropriate
on- or off-site borrow area which shall be free of roots, stumps,
wood, rubbish, stones greater than six inches, frozen or other objectionable
materials.
(4)
Embankment Compaction. When required, embankments shall be compacted
by sheepsfoot or pad roller. The loose lift thickness shall be nine
inches or less, depending on roller size, and the maximum particle
size is six inches or less (two-thirds of the lift thickness). Five
passes of the compaction equipment over the entire surface of each
lift is required. Embankment compaction to visible non-movement is
also required. For berms six feet or greater, compaction testing may
be required.
D.
Internal Construction.
(1)
Bottom Slope. The minimum bottom slope of facilities not designed
for infiltration shall be one-percent. A flatter slope may be used
if an equivalent dewatering mechanism is provided.
(2)
Dewatering Features. When required, dewatering shall be provided
through the use of underdrain, surface device, or 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 time frame.
(3)
Pretreatment Elements. When required, pretreatment elements
shall consist of forebays, or alternate approved by the Township Engineer,
to keep silt to a smaller portion of the facility for ease of maintenance.
(4)
Infiltration Basins. Within basins designed for infiltration,
existing native vegetation shall be preserved, if possible. For existing
unvegetated areas or for infiltration basins that require excavation,
a planting plan shall be prepared in accordance with § 301.25.
and the BMP Manual which is designed to promote infiltration.
E.
Outlet Configuration.
(1)
For facilities with a depth of two feet or greater, a type D-W
endwall or riser box outlet structure shall be provided.
(2)
For facilities with a depth less than two feet, no outlet structure
is required. A protective concrete collar may be required.
(3)
All discharge control devices with appurtenances shall be made
of reinforced concrete and stainless steel. Bolts/fasteners shall
be stainless steel.
(4)
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.
F.
Spillway.
(1)
Material. The spillway shall be designed to provide a non-erosive,
stable condition when the project is completed. Special consideration
shall be given to protection of emergency spillways located in filled
areas.
(2)
Non-emergency Use. Use of the spillway to convey flows greater
than the fifty-year design storm is permitted.
(3)
Emergency Use. The spillway shall be designed to convey the
one-hundred-year post-development peak inflow.
(4)
When required, freeboard shall be measured from the top of the
water surface elevation for emergency use.
G.
Breach Analysis. The municipality may require a breach analysis based
on site-specific conditions and concern of threat for downstream property.
When required, the breach analysis shall be conducted in accordance
with the NRCS methodology, the US Army Corps of Engineers methodology
(HEC-1) or other methodologies as approved by the municipality.
2.
Subsurface Storage Facilities. Subsurface storage facilities consist
of all stormwater facilities which store, infiltrate/evaporate/transpire,
clean or otherwise affect stormwater runoff and the top of which is
not exposed to the natural environment. Subsurface facilities are
located below the finished ground elevation. Subsurface facilities
do not include stormwater management facilities designed for conveyance.
A.
Design Criteria. 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
| |||
|
(1)
|
Facility Geometry
| |||
|
(a)
|
Depth from surface (maximum)
|
2 feet less than limiting zone
|
N/A
| |
|
(b)
|
Loading ratio (maximum)
|
Per BMP Manual*
|
N/A
| |
|
(2)
|
Distribution System Requirements
| |||
|
(a)
|
Pipe size (minimum)
|
4 inches
|
4 inches
| |
|
(b)
|
Pretreatment
|
Required
|
Required
| |
|
(c)
|
Loading/balancing
|
Required
|
Not required
| |
|
(d)
|
Observation/access ports
|
Required
|
Required
| |
|
*
|
Unless otherwise determined by professional geologic evaluation.
|
3.
Stormwater management facilities which qualify as a dam per DEP regulations
or facilities deemed a potential threat to the life, safety or welfare
of the general public shall be subject to the following requirements:
A.
Stormwater management facilities which qualify as a dam per DEP regulation
shall obtain the required permit through DEP and design the facility
in accordance with DEP standards.
B.
Additional requirements and analysis may be required by the Township
to prove that the proposed facility has been designed to limit the
potential risk to the life, safety or welfare of the general public.
[Ord. 2014-04, 4/28/2014, § 23-305]
1.
All earthmoving activities shall be conducted in such a way as to
minimize accelerated erosion and resulting sediment pollution. Measures
to control erosion and sediment pollution shall, at a minimum, meet
the standards of the Conservation District, Chapter 102, and concerns
identified by the Township.
2.
The erosion and sediment pollution control plan must be available
at all times at the project site. When required, a permit allowing
earthmoving activity shall be obtained by the applicant before any
construction on the development site shall begin.
3.
Approval of an erosion and sediment pollution control plan by the
Township shall not be construed as an indication that the plan complies
with the standards of any agency of the Commonwealth.
4.
The erosion and sediment pollution control plan shall be submitted
to the Conservation District for its review and approval with copies
of materials submitted also provided to the Township and Township
Engineer. A copy of the approval shall be provided to the Township
prior to unconditional final plan approval.
[Ord. 2014-04, 4/28/2014, § 23-306]
No development or earthmoving activities shall involve uses,
activities or improvements which would entail encroachment into, the
regrading of, or the placement of fill in wetlands in violation of
state or federal regulations.
[Ord. 2014-04, 4/28/2014, § 23-307]
1.
The applicant shall reserve easements where stormwater management
facilities, floodplains or wetlands are existing or proposed, whether
located within or beyond the boundaries of the project site. If stormwater
management facilities, floodplains or wetlands are to be installed
or created beyond the boundary of the property, the applicant shall
provide the Township with all necessary easements, in a form acceptable
to the Township Solicitor, clearly demonstrating that the applicant
has the right to install and maintain stormwater management facilities
on such property and/or create floodplains or wetlands upon such property.
2.
Easements shall have a minimum width of 20 feet or 30 feet when in
close proximity to structures and shall be adequately designed to
provide area for (A) the collection and discharge of water, (B) the
maintenance, repair and reconstruction of all stormwater management
facilities, (C) the passage of machinery for such work, and (D) the
preservation of floodplains, riparian buffers and wetlands. The easements
shall clearly identify who has the right of access and the responsibility
of maintenance.
3.
Stormwater management facilities shall be centered within the easement.
4.
To the fullest extent possible, easements shall be centered on or
be adjacent to lot lines. Where the Township has concerns about shared
ownership and/or maintenance responsibilities, facilities and easements
may not be allowed to be centered on property lines.
5.
Nothing shall be placed, planted, set, or put within the area of
an easement that would adversely affect the function of the facility
within the easement or conflict with the easement agreement.
[Ord. 2014-04, 4/28/2014, § 23-308]
1.
Volume control BMPs are intended to maintain existing hydrologic
conditions for small storm events by promoting groundwater recharge
and/or evapotranspiration as described in this section. Runoff volume
controls shall be implemented using the Design Storm Method described
in Subsection 1A below, or through continuous modeling approaches
or other means as described in the BMP Manual. Small projects may
use the method described in Subsection 1B to design volume control
BMPs.
A.
The Design Storm Method is applicable to any size of regulated activity
and requires detailed modeling based on site conditions.
(1)
There shall be no increase in the post development total runoff
volume for all storms equal to or less than the two-year twenty-four-hour
storm event.
(2)
The maximum loading ratio for volume control facilities in Karst
areas shall be 3:1 impervious drainage area to infiltration area and
5:1 total drainage area to infiltration area. The maximum loading
ratio for volume control facilities in non-Karst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio may be approved by the municipality
if adequate justification is provided. Hydraulic depth may be used
as an alternative to an area based loading ratio if the design hydraulic
depth is shown to be less than the depth that could result from the
maximum area loading ratio.
(3)
In carbonate geology, a detailed geologic evaluation of the
development site shall be performed to determine the design parameters
of recharge facilities. The evaluation shall be performed by a registered
professional geologist in accordance with other applicable sections
of this chapter, and shall address loading rates, soil permeability,
depth to bedrock, susceptibility to sinkhole formation, and subgrade
stability, etc.
B.
Volume Control for Small Projects. At least the first three inches
of runoff from new impervious surfaces or an equivalent volume 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.
C.
Storage facilities, including normally dry, open top facilities,
shall completely drain the volume control storage over a period of
time not less than 24 hours and not more than 72 hours from the end
of the design storm. Any designed infiltration at such facilities
is exempt from the minimum twenty-four-hour standard, i.e. may infiltrate
in a shorter period of time, provided that none of this water will
be discharged into waters of this Commonwealth.
D.
Subregional (Combined Development Site) Storage. Runoff can be managed
regionally by one or more applicants, either on-site or off-site when
deemed acceptable by the Township. The design and release rate shall
be consistent with this chapter.
E.
Infiltration Design Standards.
(1)
If more than one hydrologic soil group (HSG) is present at a
development site, a composite recharge volume shall be computed based
upon the proportion of total development site area within each HSG.
(2)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and development
site conditions and shall be constructed on soils that have the following
characteristics:
(a)
A minimum depth of 48 inches between the bottom of the facility
and the seasonal high water table and/or bedrock (limiting zones)
may be required by the Township when concerns regarding the quality
of runoff exist or in close proximity of water supply sources.
(b)
An infiltration and/or percolation rate sufficient to accept
the additional stormwater load and drain completely as determined
by field tests conducted by the applicant's professional designer
at the location and elevation of the proposed facility.
(3)
Infiltration BMPs require a minimum depth of suitable soil of
24 inches between the bottom of the facility and the limiting zone.
(4)
The recharge volume provided at the development site shall be
directed to the most permeable HSG available.
[Ord. 2014-04, 4/28/2014, § 23-309]
1.
In order to protect and improve water quality, a riparian corridor
easement shall be created and recorded as part of any subdivision
or land development that encompasses a riparian corridor.
2.
The minimum riparian corridor easement shall extend to the widths identified in the Zoning Ordinance [Chapter 27], as amended, and encompass the entire one-hundred-year floodplain and adjacent wetland area as applicable, excepted when exceeded by the requirements of Chapter 102.
3.
Minimum Management Requirements for Riparian Corridors.
A.
Existing native vegetation shall be protected and maintained within
the riparian corridor easement.
B.
Whenever practicable, invasive vegetation shall be actively removed
and the riparian corridor easement shall be planted with native trees,
shrubs and other vegetation to create a diverse native plant community
appropriate to the intended ecological context of the site.
4.
The riparian corridor easement shall be enforceable by the Township and shall be recorded in the Lancaster County Recorder of Deeds Office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area as required by the Zoning Ordinance [Chapter 27], unless otherwise specified.
5.
Any permitted use within the riparian corridor easement shall be
conducted in a manner that will maintain the extent of the existing
ten-year floodplain, improve or maintain the stream stability, and
preserve and protect the ecological function of the floodplain.
