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 of any subdivision or land development
plan within the jurisdiction of this chapter, or the 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 generalized stormwater management site plan for the
entire project site shall be submitted in addition to the detailed
stormwater management site plan for the proposed section. This generalized
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 PADEP and the design standards of §
122-44 of this chapter.
(3) An operation and maintenance program, in recordable form acceptable
to the Township, that clearly sets forth the operation and maintenance
responsibility of all temporary and permanent stormwater management
facilities and erosion and sedimentation control facilities. The intent
of these regulations is to provide private operation and maintenance
of stormwater management and erosion and sedimentation control facilities,
including:
(a)
Description of temporary and permanent maintenance requirements;
(b)
Identification of a responsible individual, corporation, association
or other entity for operation and maintenance of both temporary and
permanent stormwater management and erosion and sedimentation control
facilities;
(c)
Provisions for permanent access or maintenance easements for all existing and proposed stormwater management facilities, as necessary to implement the operation and maintenance (O&M) plan described in §
122-54. All such agreements shall be duly recorded in the Office of the Recorder Deeds and shall constitute a binding permanent covenant upon the property, superior to all liens of record and not subordinate to any easement or restriction that would interfere with its provisions and the implementation thereof;
(d)
Where the Board of Supervisors accepts dedication of stormwater
management facilities, the Board of Supervisors may require the developer
to establish, at the time of dedication, a maintenance fund, in an
amount determined by the Township, adequate for the perpetual care
of such facilities.
(4) A written report shall be submitted that includes the following information:
(a)
Stormwater runoff calculations for both pre-development and
post-development conditions.
(b)
An erosion and sedimentation control plan narrative that conforms
to the requirements of the soil erosion and sediment control manual
of the PADEP 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.
(c)
A general description of the development site, including a description
of existing natural and hydrologic features and any environmentally
sensitive areas.
(d)
A general description of the overall stormwater management concept
for the project, including a description of permanent stormwater management
techniques, nonstructural BMPs to be employed and construction specifications
of the materials to be used for structural stormwater management facilities.
(e)
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.
(f)
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. 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.
C. Where a development site is traversed by watercourses other than
permanent streams, a drainage easement shall be provided conforming
substantially to the line of such watercourses. The terms of the easement
shall prohibit excavation, the placing of fill or structures, and
any alterations that may affect adversely the flow of stormwater within
any portion of the easement. Also, maintenance of vegetation within
the easement shall be required.
D. The PADEP Chapter 105 applies 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.
E. 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.
F. 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/her 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.
G. Should any stormwater management facility require a dam safety permit
under PADEP Chapter 105, the facility shall be designed in accordance
with PADEP Chapter 105 and meet the regulations of PADEP Chapter 105
concerning dam safety which may be required to pass storms larger
than one-hundred-year event.
H. Roof drains shall not be connected to sanitary sewers or public storm
sewers.
I. No outlet structure from a detention basin, storm pipe, or swale
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 which
will be located on or which will discharge onto state highway rights-of-way
shall be subject to approval by PennDOT. If PennDOT requires that
the Township be the permitee for such drainage facilities, the applicant
shall enter into an agreement, in recordable form, assuming all of
the obligations which PennDOT may place upon the Township as permittee,
including, but not limited to, long-term maintenance of any such facilities,
compliance with all conditions contained in the permit, and indemnification
of the Township for any costs or penalties which PennDOT may seek
to impose on the Township. The Township shall have no obligation to
make any applications to PennDOT.
K. Stormwater management facilities located within or affecting the
floodplain of any watercourse shall comply with the requirements of
the floodplain regulations provided in the Zoning Ordinance 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 one-hundred-year
water surface. 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. No outlet structure from a stormwater management facility, or swale,
shall discharge directly onto a Township or state roadway.
O. 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. The stabilized
infiltration rate is to be determined in the same location and within
the same soil horizon as the bottom of the infiltration facility.
The stabilized infiltration rate is to be determined as specified
in the BMP manual.
(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.
P. Stormwater discharge points onto an adjacent property shall comply
with the following:
(1) Stormwater flows onto adjacent property shall not be created, increased,
relocated, or otherwise altered without written notification of the
adjacent landowner(s) by the developer including the establishment
of an easement. Such stormwater flows shall be subject to the requirements
of this chapter. If, in the opinion of the Township, a decrease in
stormwater flow rate or volume will cause adverse impacts to downstream
properties, the applicant shall mitigate such impacts.
(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 or better than the runoff characteristics (spread,
velocity, and peak rate) of the pre-developed flows. The easement
from the downstream landowner(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 nonerosive, sheet flow condition.
For all design year storms, including the one-hundred-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 pre-developed 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: 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 drainage easements from the affected
landowners are provided.
Q. 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:
(c)
Lineaments in carbonate areas.
(h)
Bedrock pinnacles (surface or subsurface).
(2) The minimum isolation distance from stormwater management facilities
to the listed geologic features shall be as follows:
(a)
One hundred feet from the rim of sinkholes or closed depressions;
(b)
One hundred feet from disappearing streams;
(c)
Fifty feet from lineaments or fracture traces;
(d)
Twenty-five feet from surface or identified subsurface pinnacles.
(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 in Appendix C shall be attached to all stormwater management site plans
and signed and sealed by the developer's qualified professional.
(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 above.
R. All stormwater runoff flowing over the project site shall be considered
in the design of the stormwater management facilities.
S. 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 pre-development hydrograph. Developers and/or landowners are encouraged to provide infiltration facilities or utilize other techniques which will allow the post-development one-hundred-year hydrograph to match the pre-development one-hundred-year hydrograph, along all parts of the hydrograph, for the development site. To match the pre-development hydrograph, the post-development peak rate must be less than or equal to the pre-development peak rate, and the post-development runoff volume must be less than or equal to the pre-development volume for the same storm event. 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. Incorporating the volume control requirements as given in §
122-48 can be used as part of this option.
(2) Where the pre-development hydrograph cannot be matched, one of the
following shall apply:
(a)
Within the Little Conestoga Creek Watershed, post-development
rates of runoff from any regulated activity shall not exceed 50% of
the peak rates of runoff prior to development for all design storms
(two-, five-, ten-, twenty-five-, fifty- and one-hundred-year storm
events) unless the preexisting hydrograph is not exceeded at all points
in time. Where additional impervious coverage is proposed on lands
improved with existing structures, the 50% decrease in peak runoff
rates shall apply only to the new area of impervious coverage. Existing
land areas undisturbed, or disturbed and replaced to existing conditions
shall not exceed existing peak runoff rates. Existing stormwater facilities
with adequate capacity may be used to convey, store, and depeak runoff
from additional impervious coverage. Where existing stormwater basins
are shown to provide the necessary depeaking for existing impervious
coverage and the proposed impervious coverage, no additional facilities
need to be provided. See the release rate map in Appendix H for additional information.
(b)
Within all remaining watersheds of the Township, the calculated
peak rate of stormwater runoff from the project site shall not exceed
the peak rate of runoff from the project site prior to development
activities. Both the pre-development peak rate of runoff and the post-development
peak rate of runoff shall be calculated for the two-year, ten-year
and one-hundred-year storm events. See the release rate map in Appendix
H for additional information.
(3) Developers 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 offsite
volume and rate control, as appropriate and supported by a detailed
design approved by the Township. 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.
T. 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 should consider this protection.
U. 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.
V. 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.
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) may 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.
C. The Rational Method shall be used for all:
D. Any other method approved by the Township Engineer.
E. If the NRCS/SCS Method (also known as 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 NOAA Atlas 14.
(1) Provide the rainfall used for the two-, ten-, twenty-five-, fifty-,
and one-hundred-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.
F. If the SCS method is used, Antecedent Moisture Condition 1 is to
be used in areas of carbonate geology, and Antecedent Moisture Condition
2 is to be used in all other areas. A type II distribution shall be
used in all areas.
G. If the Rational Method is used, the NOAA Atlas 14 data (see Subsection
E 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.
H. 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 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 2 hydrograph.
I. Runoff coefficients "C" and curve numbers "CN" shall be based on
the charts contained in the Appendix.
J. 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 Appendix B-4, or the Manning's kinematic solution shown in the sheet
flow section of Worksheet No. 1 in Appendix B-5.
(2) Shallow concentrated flow: Travel time for shallow concentrated flow
shall be determined using Appendix B-6 from 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.
K. For the purpose of calculating pre-development 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.
L. 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.
M. 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-development
and post-development rates for peak discharge of stormwater runoff
from the project site.
N. 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.
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 offsite 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.
Stormwater runoff volume controls shall be implemented using
the Design Storm Method in Subsection A below.
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-four-hour storm event.
(2) For modeling purposes:
(a)
Existing (pre-development) nonforested pervious areas must be
considered meadow in good condition.
(b)
Twenty percent of existing impervious area, 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 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 nonkarst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio would be acceptable if proper
justification is submitted and approved.
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
evapo-transpiration 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 §
122-43O.
D. All applicable worksheets from Chapter 8 of the BMP manual must be
used when establishing volume controls.