All regulated activities shall be conducted in conformance with
the following minimum standards:
A. Post-development effects.
(1) Post-development peak discharge rates for the one-, two-, five-,
ten-, twenty-five-, fifty- and one-hundred-year recurrence interval
design storms from project sites that are not within the Susquehanna
River Tributaries Watershed (as delineated on Plates B-1 and B-2 in
Appendix B of this chapter) shall not exceed the respective predevelopment
peak discharge rates.
(2) Post-development peak discharge rates for the one-, two-, five-,
ten-, twenty-five-, fifty- and one-hundred-year recurrence interval
design storms from project sites (or portion of project sites) that
are within the Susquehanna River Tributaries Watershed (as delineated
on Plates B-1 and B-2 in Appendix B of this chapter) shall be subject
to the allowable release rates that are tabulated in Tables C-1 and
C-2 in Appendix C of this chapter.
(3) Stormwater runoff shall be managed so that no downstream increases
in flood damages or impairment of streets and other public facilities
occur. The Municipal Engineer may require that downstream impacts
be evaluated at critical locations such as dams, tributaries, existing
developments, undersized culverts and flood-prone areas. The municipality
and its Engineer shall make the final determination with respect to
the degree of management required for any site. The applicant/developer
shall evaluate the effects of the proposed plan on such critical locations
by providing computed water surface elevations (WSEL) for the ten-
and one-hundred-year recurrence interval design storms. Methods of
computation shall have prior approval of the Municipal Engineer. At
such downstream critical locations, stormwater management may be exercised
by:
(a)
Providing off-site improvements to downstream conveyances in
order to contain flow increases.
(b)
Providing downstream drainage easements with sufficient widths
to contain the flood limits.
B. Stormwater management plans for all regulated activities must include
infiltration and stormwater quality BMPs. For regulated activities
that do not require an NPDES construction permit, the design of infiltration
and stormwater quality BMPs shall be in accordance with Appendix A,
Section IV, of this chapter. Otherwise, the design of infiltration and stormwater quality
BMPs shall be in accordance with the NPDES construction permit.
C. In establishing the watershed conditions for calculating stormwater
runoff prior to development, the following assumptions shall apply:
(1) Woodland (in good condition) or meadow shall be used for all undeveloped
areas.
(2) Average antecedent moisture conditions as defined by the Natural
Resource Conservation Service shall be used for all areas.
(3) Drainage area reductions equal to the area of undrained depressions
or pond factor adjustments in accordance with the Urban Hydrology
for Small Watersheds, Technical Release No. 55 (TR-55, USDA, NRCS)
procedure shall be applied in determining predevelopment peak discharges
from karst geologic areas.
D. Hydrologic methods. All plans and designs for stormwater management
facilities shall be reviewed by the Municipal Engineer. Plans for
facilities other than storm sewers should determine stormwater peak
discharge and stormwater runoff by the use of the Soil-Cover-Complex
Method as set forth in Technical Release No. 55 (TR-55, USDA, NRCS)
and Technical Release No. 20 (TR-20, USDA, NRCS), with specific attention
given to antecedent moisture conditions, flood routing, and peak discharge
specifications included therein, and in the National Engineering Handbook,
Part 630 - Hydrology (USDA, NRCS, current edition). The Municipal
Engineer may permit the use of the Modified Rational Method or other
methods for calculation of the storage capacity of a stormwater management
facility from drainage areas of 20 acres or less.
(1) Acceptable runoff coefficient values for use in the Rational Method
equation, and permissible curve numbers for TR-55 and TR-20 are identified
in Tables A-1 and A-2 of Appendix A of this chapter. When applying the Rational Method coefficients in Table
A-2, "open space" coefficients shall be used for all predevelopment
pervious areas and for all undeveloped and undisturbed, densely vegetated
(nonforest) areas instead of "meadow" coefficients.
(2) The Rational Method may be used in lieu of the Soil-Cover-Complex
Method to compute design flows for the sizing of storm sewers, inlets
and swales. Methods approved by the Pennsylvania Department of Transportation
and/or the Pennsylvania Department of Environmental Protection may
be used to design the waterway areas of bridges.
(3) Design rainfall amounts for the specified recurrence intervals shall
be determined using the Precipitation-Frequency Atlas of the United
States (NOAA Atlas 14, Volume 2). Rainfall duration for the design
of stormwater detention basins shall not be less than 24 hours, unless
waived in writing by the municipality and its Engineer. All hydrologic
computations utilizing the Soil-Cover-Complex Method shall use a rainfall
duration of not less than 24 hours, unless waived in writing by the
municipality and its Engineer. Rainfall distribution shall be SCS
Type II, unless waived in writing by the municipality and its Engineer.
(4) Time of concentration shall be determined in accordance with methods
presented in Chapter 15 of the National Engineering Handbook, Part
630 - Hydrology (USDA, NRCS, current edition).
E. Stormwater management facilities and related installations shall
be provided:
(1) To permit unimpeded flow of natural watercourses. Such flow may be
redirected as required, subject to the approval of the Pennsylvania
Department of Environmental Protection and the municipality;
(2) To ensure adequate drainage of all low points along the curbline
of streets;
(3) To intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained, and to prevent substantial flow of water across intersections or flooded intersections during storms, in accordance with the procedures contained in the Design Manual Part 2 - Highway Design, Publication 13M (PennDOT), Chapter
10;
(4) To ensure adequate and unimpeded flow of stormwater under driveways
in, near or across natural watercourses or drainage swales. Suitable
pipes or other waterways shall be provided as necessary;
(5) To properly drain stormwater runoff from all project sites. All lot
and open areas shall be designed to drain to the nearest practical
street or drainage system, existing or proposed, as defined by the
Municipal Engineer, with no impact on adjoining properties, unless
an area specifically designed for stormwater detention is provided.
F. Storm sewers and related installations.
(1) Placement.
(a)
Storm sewers, where required by zoning and land use densities,
shall be placed under or immediately adjacent to the roadway side
of the curb, or as directed by the municipality and its Engineer,
when parallel to the street within the right-of-way.
(b)
When located in undedicated land, they shall be placed within
a drainage easement not less than 20 feet wide, as approved by the
Municipal Engineer.
(c)
The use of properly designed, graded, and turfed drainage swales
is encouraged in lieu of storm sewers in commercial and industrial
areas, and where approved by the Municipal Engineer, in residential
areas. Such swales shall be designed not only to carry the required
discharge without excessive erosion, but also to increase the time
of concentration, reduce the peak discharge and velocity, and permit
the water to percolate into the soil where appropriate. Criteria related
to the use and design of drainage swales are contained in Appendix
A, Section II, of this chapter.
(2) The design capacity of storm sewers shall be in accordance with Urban
Drainage Design Manual, Hydraulic Engineering Circular Number 22 (US
DOT, FHA). Storm drainage systems shall be designed without surcharging
inlets to provide conveyance of stormwater runoff into a detention
basin or similar facility utilized to manage the rate of stormwater
runoff. To avoid surcharging inlets and to ensure that inlets will
receive stormwater runoff, the hydraulic grade line at the inlet should
be at least six inches below the elevation of the inlet grate. Where
site grading will direct stormwater runoff from the one-hundred-year
recurrence interval design storm to a detention basin or similar facility
utilized to manage the rate of stormwater runoff, then the storm sewer
may be designed for the ten-year recurrence interval design storm.
Where site grading will not direct stormwater runoff from the one-hundred-year
recurrence interval design storm to a detention basin or similar facility
utilized to manage the rate of stormwater runoff, then the storm sewer
shall be designed for the one-hundred-year recurrence interval design
storm. Conveyance of storms to the detention basin, up to and including
the one-hundred-year recurrence interval design storm, shall be provided
so as not to endanger life or seriously damage property.
(3) Storm inlet types and inlet assemblies shall conform to the Standards
for Roadway Construction, Publication 72M (PennDOT) and the following
criteria:
(a)
Inlets shall, at a minimum, be located at the lowest point of
street intersections to intercept the stormwater before it reaches
pedestrian crossings or at sag points of vertical curves in the street
alignment which provide a natural point of ponding of surface stormwater.
(b)
Where the municipality and its Engineer deem it necessary because
of special land requirements, special inlets may be approved.
(c)
The interval between inlets collecting stormwater runoff shall be determined in accordance with the Design Manual Part 2 - Highway Design, Publication 13M (PennDOT), Chapter
10, Section 5, "Capacity of Waterway Areas," or Urban Drainage Design Manual (HEC-22, US DOT, FHA).
(d)
In curbed sections, the maximum encroachment of water on the
roadway pavement shall not exceed half of a through traffic lane or
one inch less than the depth of curb during the ten-year recurrence
interval design storm. Inlets shall be provided to limit the encroachment
of water on the pavement. When inlets are used in a storm system within
the right-of-way limits of a street in lieu of manholes, the spacing
of such inlets shall not exceed the maximum distance of 450 feet.
(e)
The design of storm inlets shall be in accordance with Drainage
of Highway Pavements, Hydraulic Engineering Circular Number 12 (US
DOT, FHA).
(4) Accessible drainage structures shall be located on a continuous storm
sewer system at all vertical dislocations, at all locations where
a transition in storm sewer pipe sizing is required, at all vertical
and horizontal angle points exceeding 5°, and at all points of
convergence of two or more influent storm sewers.
(5) When evidence available to the municipality and its Engineer indicates that existing storm sewers have sufficient capacity, as determined by hydrograph summation, and where such existing storm sewers are accessible, proposed stormwater facilities may connect to these existing storm sewers as long as the peak rate of discharge does not exceed the amount permitted by §
118-14A of this article.
G. Bridges and culverts shall have ample waterway to carry expected
flows, based on the following minimum design storms: ten-year recurrence
interval for driveways; twenty-five-year recurrence interval for local
streets; fifty-year recurrence interval for collector streets; and
one-hundred-year recurrence interval for arterials; or as otherwise
required by the municipality and its Engineer. Bridge and/or culvert
construction shall be in accordance with Pennsylvania Department of
Transportation specifications.
(1) The design criteria contained in this article are intended for use
in conjunction with the (Pennsylvania Code Title 25) Chapter 105 Regulations
of the Pennsylvania Department of Environmental Protection, entitled
"Water Obstructions and Encroachments." All information and regulations
contained in Chapter 105 shall be considered to be incorporated into
this article as if reproduced in full. A PA DEP permit in accordance
with Chapter 105 shall be required for any obstruction or encroachment
in regulated waters of the commonwealth prior to the approval of the
stormwater plan. All areas of the municipality shall be classified
as rural, suburban or urban, as determined by the municipality and
its Engineer. (See 25 Pa. Code § 105.161 for bridge and
culvert designs.) In the event any question or conflict arises between
this article and the PA DEP Chapter 105 Regulations, the design criteria
contained in the PA DEP regulations shall govern.
(2) Refer to Appendix A, Section II, of this chapter for additional design
criteria.
H. Detention or retention basins for the management of stormwater peak
discharges shall meet the following requirements:
(1) Basins shall be installed prior to or concurrent with any earthmoving
or land disturbances which they will serve. The phasing of their construction
shall be noted in the narrative and on the plan.
(2) The design of all facilities over limestone formations shall include
measures designed by a professional geologist to prevent groundwater
contamination and, where required, sinkhole formation. Soils used
for the construction of basins shall have moderate to low erodibility
factors (i.e., "K" factors of 0.32 or less).
(3) Energy dissipaters and/or level spreaders shall be installed at points
where pipes or drainageways discharge to or from basins.
(4) Outlets.
(a)
Outlet structures within stormwater detention basins shall incorporate
childproof, nonclogging trash racks or grates over all horizontally
oriented openings. All vertically oriented openings over 12 inches
or larger in any dimension where entry by a child could cause injury
or death shall be covered with childproof, nonclogging trash racks,
except where such openings carry perennial stream flows. Design openings
less than six inches in any dimension shall be covered with a pipe
screen (e.g., Neenah R-7512 or equivalent). Measures to completely
drain stormwater detention basins in the event of clogging of the
primary design opening(s) shall be incorporated into the design of
basin outlet structures. Basin outlet pipes shall have a minimum inside
diameter of 15 inches (or a cross-sectional area of 176 square inches),
except that pipes under 25 feet or greater fill shall not be less
than 24 inches (or a cross-sectional area of 453 square inches), and
shall be reinforced concrete pipe.
(b)
Outlet aprons shall be designed and shall extend, at a minimum,
to the toe of the basin slope. Where spillways will be used to manage
peak discharges in excess of the ten-year recurrence interval design
storm, such spillways shall be constructed to withstand the pressures
of impounded waters and convey flows at computed outlet velocities
without erosion.
(c)
Stormwater facilities shall be designed to release their total
volume within the following maximum time periods:
[1]
Roofs and parking lots: 24 hours.
[2]
Detention basin: 48 hours.
[3]
Infiltration facilities: 72 hours.
(5) When the Pennsylvania Department of Environmental Protection requires
facilities to be permitted, the designer shall submit all information
to the PA DEP regional office and obtain all necessary approvals and
permits pursuant to Pennsylvania Code, Title 25, Chapter 105, Dam
Safety and Encroachment Act.
(6) Downstream analysis.
(a)
Where deemed necessary by the municipality and its Engineer,
the applicant/developer shall submit an analysis of the impacts of
detained stormwater flows on downstream areas within the watershed.
The analysis shall include hydrologic and hydraulic calculations necessary
to determine the impact of peak discharge modifications on critical
locations such as dams, tributaries, existing developments, undersized
culverts and flood-prone areas.
(b)
Review and comment of the analysis by the engineer of a downstream
municipality shall be obtained as deemed necessary by Scott Township.
(7) Detention basins may be waived by the municipality and its Engineer at sites in close proximity to large receiving streams, depending on the hydrology of the watershed. It shall be incumbent upon the applicant/developer to demonstrate that no downstream increase in stream flooding or channel erosion will result, in accordance with § 118-14N(6) of this article, and that no increases in peak discharge within the receiving stream will occur, as outlined under §
118-14A of this article.
(8) Multiple-use basins. The design and construction of multiple-use
stormwater detention facilities are strongly encouraged. In addition
to stormwater management, multiple-use basins, where appropriate,
allow for recreational uses, including ball fields, play areas, picnic
grounds, etc. Provisions for parking facilities within basins and
permanent wet ponds with stormwater management capabilities may also
be appropriate. Prior approval and consultation with the municipality
and its Engineer are required before design. Multiple-use basins should
be constructed so that potentially dangerous conditions are not created.
(9) Multiple development basins. Stormwater management facilities designed
to serve more than one property or development in the same watershed
are encouraged. Staged construction of existing or proposed multiple-use
detention facilities by several applicants/developers within the watershed
development is encouraged. Each applicant/developer shall be responsible
for the incremental increase in stormwater runoff generated by the
respective development and incremental construction improvements necessary
for the overall detention facility. Prior approval and consultation
with the municipality and its Engineer is required before design of
such facilities.
(10)
Alternative detention facilities. Alternative stormwater detention
facilities, including rooftop storage, subsurface basins or tanks
and in-pipe detention storage, or other approved alternative designs
that may be permitted by the municipality and its Engineer, are encouraged.
(11)
Specific criteria related to the design of detention basins
is contained in Appendix A, Section III, of this chapter.
I. All calculations shall be submitted to the Municipal Engineer on
computation sheets for approval. If the Municipal Engineer determines
through review and independent computation that the size(s) of storm
pipes or detention basins is insufficient, the municipality and its
Engineer may require the applicant/developer to increase the size(s)
of said storm pipes or detention basins. If the storm drainage system
design is completed using computer software, sufficient supporting
data shall be provided to allow comprehensive review by the municipality
and its Engineer.
J. When the elevation of any existing or proposed entrance to a structure,
including windows, is lower than the elevation of the public cartway
serving that site, a grading plan shall be submitted to the municipality
for review and approval as part of the zoning permit process for the
proposed structure.
K. Natural drainageways shall be utilized to the maximum extent possible in carrying stormwater runoff, provided such use remains consistent with the purpose of this chapter specified in Article
I, §
118-4.
L. Stormwater management facilities located outside of an existing or
proposed public right-of-way shall be located within and accessible
by easements as follows:
(1) Drainage easements. Where a tract is traversed by a watercourse, drainageway, channel or stream, a drainage easement shall be provided that parallels the line of such watercourse, drainageway, channel or stream. The width of the drainage easement shall be adequate to preserve the unimpeded flow of natural drainage in the regulatory floodplain, in accordance with computed top widths for water surface elevations determined under §
118-14A of this article. Drainage easements shall provide for maintenance, and for the purpose of widening, deepening, improving, or protecting such drainage facilities.
(2) Access easements. Where proposed stormwater management facilities
will not be adjacent to an existing or proposed public right-of-way,
or where they will not be accessible due to physical constraints,
a twenty-foot-wide passable access easement specifying rights of entry
shall be provided. Access easements shall provide for vehicle ingress
and egress on grades of less than 10% for carrying out inspection
or maintenance activities.
(3) Maintenance easements. A maintenance easement shall be provided which
encompasses the stormwater facility and appurtenances and provides
for access for maintenance purposes. The maintenance easement must
be located at least 20 feet outside of the stormwater facility and
its appurtenances.
(4) Easements shall stipulate that no trees, shrubs, structures, excavation
or fill be placed, and no regrading be performed within the area of
the easement without written approval from the municipality upon review
by the Municipal Engineer. Upon approval of the Municipal Engineer,
such landscaping may be placed in maintenance easements, provided
it does not impede access.
(5) Whenever practicable, easements shall be parallel with and linked
to property lines.
(6) All easement agreements shall be recorded with a reference to the
recorded easement indicated on the stormwater management plan. The
format and content of the easement agreement shall be reviewed and
approved by the Municipal Engineer and the Solicitor.
Each stormwater management plan shall contain provisions which
clearly set forth the ownership responsibilities of all permanent
stormwater management and erosion and sediment pollution control facilities,
including the following items:
A. Description of ownership and maintenance requirements.
B. Establishment of suitable easements for access to all facilities in accordance with §
118-14L of this article.
C. Identification of the responsible party for ownership and maintenance
of both temporary and permanent stormwater management and erosion
and sediment pollution control facilities.
(1) Stormwater facilities within subdivisions may be part of an individual
lot (or lots) where the respective lot owner (or owners) will own
the facility; however, operation and maintenance shall be the responsibility
of a homeowners' association or similar entity. A description of the
facility or system and the terms of the required operation and maintenance
shall appear on the stormwater management plan drawing(s) and shall
be incorporated into the deed to the property. All stormwater management
plan drawings shall be recorded with the approved subdivision plan
among the deed records of Columbia County, Pennsylvania. In addition,
the approved subdivision plan and any deed written from said plan
for a lot or lots that will contain stormwater management facilities
shall contain a condition stating that it shall be mandatory for the
owner (or owners) of said lot (or lots) to be members of said homeowners'
association.
(2) For stormwater management facilities that are proposed as part of
a site development plan, the applicant/developer will be required
to execute a developer agreement and a maintenance agreement with
the municipality for the construction, operation and continued maintenance
of the facilities prior to approval of the final subdivision or land
development plan. Access for inspection by the municipality of all
such facilities deemed critical to the public welfare at any reasonable
time shall be provided.
(3) In the event the above priorities cannot be achieved or where it
is required, the facilities may be dedicated to the municipality in
accordance with this chapter. As a condition of municipal acceptance
of said facilities, the applicant/developer shall provide 15% of the
cost of improvements in the form of a maintenance bond, as estimated
by the applicant's/developer's registered professional and
approved by the Municipal Engineer, to cover contingency maintenance
costs for 18 months from the date that dedication of the stormwater
management facilities is accepted by the municipality. The fifteen-percent
bond shall be based on the construction costs of the stormwater management
facilities that will be offered for dedication to the municipality.