A.
In the interest of reducing the total area of impervious surface,
preserving existing features which are critical to stormwater management,
and reducing the concentration of stormwater flow, the designer should
consider imaginative design concepts and the flexibility provided
by the water process.
B.
Several examples of imaginative design controls are:
[Amended 7-17-2014 by Ord. No. 2014-02]
(1)
Minimization of directly connected impervious area.
(2)
Swales and filter strips.
(3)
Porous pavement, parking blocks, and porous patio pavers.
(4)
Infiltration devices, such as trenches, basins, and underground chambers.
(5)
Multistage basins.
(6)
Cisterns with soaker hoses or time clock release for recycling collected
rainwater.
C.
Maximum use should be made of the existing on-site natural and man-made
stormwater management facilities.
D.
The management of stormwater, both during and upon completion of
the disturbance associated with a land disturbance activity, shall
be accomplished in accordance with the standards of this chapter.
The management of stormwater, both during and upon completion
of the disturbance associated with a land disturbance activity, shall
be accomplished in accordance with the standards of this chapter.
A.
Predevelopment vs. post-development.
(1)
The peak discharge of the calculated post-development runoff to an
adjacent property shall be designed so that the peak discharge of
the calculated post-development runoff to an adjacent property does
not exceed the peak discharge of the calculated predevelopment runoff
for a Type II storm event. The twenty-four-hour rainfall values are
as follows:
[Amended 7-17-2014 by Ord. No. 2014-02]
|
Frequency
(years)
|
Rainfall
(inches)
| |
|---|---|---|
|
1
|
2.5
| |
|
2
|
3.0
| |
|
5
|
3.8
| |
|
10
|
4.5
| |
|
25
|
5.6
| |
|
50
|
6.6
| |
|
100
|
7.7
|
(2)
Runoff calculations for the pre- and post-development comparison
shall be based upon the peak flow from a one-, two-, five-, ten-,
twenty-five-, fifty-, and one-hundred-year storm frequency. The peak
discharges and volumes of runoff shall be determined by using the
Rational Method for watersheds less than five acres, and for larger
watersheds, the latest version of Technical Release No. 55, Urban
Hydrology for Small Watersheds, or an acceptable engineering method,
subject to Township approval.
(3)
For predevelopment computations, all runoff coefficients within the
project shall be based on meadow use in good condition. Off-site land
use conditions used to determine storm flows for designing storm facilities
shall be based on existing land uses.
B.
Design storm for collection facilities.
(1)
The design of stormwater management collection and conveyance facilities
that service drainage areas within the site shall be based upon the
peak flow from a twenty-five-year storm frequency event.
(2)
All developments shall include design provisions that allow for the
overland conveyance of the post-development one-hundred-year peak
flows through the site without damage to any private or public property.
C.
Method of calculations. Runoff calculations for on-site stormwater
conveyance facilities shall be based upon the Rational Method or an
acceptable engineering design method, subject to Township approval.
D.
Criteria, methodology and assumptions. Appropriate values for runoff
coefficients, time of concentration and rainfall intensity shall be
from the latest edition of the Commonwealth of Pennsylvania, Department
of Transportation, Design Manual, Part 2, Highway Design, Chapter
10. Use of other criteria, methodology, assumptions, references, calculation
methods and/or computer modeling may be accepted, provided detailed
design information and programming with references are submitted to
and approved by the Township.
E.
Coordination with adjacent lands.
[Amended 7-17-2014 by Ord. No. 2014-02]
(1)
Consideration shall be given to the relationship of the subject property
to the drainage pattern of the watershed. The proposed stormwater
discharge at the perimeter of the site shall not exceed the capacity
of any existing, immediately contiguous, stormwater management facility
into which it flows, unless the existing predevelopment flow at the
perimeter of the site already exceeds the capacity of the existing
facilities into which it flows and the post-development discharge
does not alter the calculated predevelopment discharge characteristics.
(2)
Stormwater shall not be transferred from one watershed to another,
unless:
(3)
Stormwater runoff from the subject property shall flow directly into
a natural watercourse or into an existing storm drainage system, or
onto adjacent properties in a manner comparable to the runoff characteristics
of the predevelopment flow or as sheet flow as agreed to by the adjoining
property owner.
F.
Stormwater basins.
(1)
Retention basins and detention basins shall be designed with a minimum
one-foot freeboard above the design elevation of the one-hundred-year
water surface at the emergency spillway. Additionally, basins shall
be designed to safely discharge the full one-hundred-year peak discharge
of a post-development storm event through an emergency spillway in
a manner which will not damage the integrity of the basin. Spillway
flow analysis is not to consider flow through the outlet control structure.
(2)
Basins shall not be located over or onto any existing or proposed
utility line or riparian buffer.
(3)
Stormwater basins located in known sinkhole-prone areas are required
to be lined to prevent infiltration into the ground. A man-made infiltration
bed with liner may be incorporated into a stormwater basin design
subject to approval by the Township Engineer.
(4)
The latest edition of Urban Hydrology for Small Watersheds, Technical
Release No. 55, United States Department of Agriculture, Natural Resources
Conservation Service, or an acceptable engineering design method,
subject to Township approval, shall be used for detention basins.
(5)
Basins shall be constructed and maintained to ensure the design capacity
after sedimentation has taken place.
(6)
Basin discharge outlets shall be designed to release runoff in a
condition which reflects predevelopment characteristics.
(7)
Basins which are not designed to release all stormwater shall be
specifically identified as retention basins or permanent pond basins.
All other basins shall have provisions for dewatering, particularly
the bottom, and shall not create swamp and/or other conditions that
are not maintainable. Low-flow channels shall be used to dewater noninfiltrating
basins using a minimum two-percent bottom slope or a paved bottom
of a minimum one-percent slope. Infiltration basins shall not have
a sloped basin bottom greater than 1%. Discharge structures shall
be designed to eliminate the possibility of clogging and blockage
during operation.
(8)
Basins which are located in or adjacent to a residential or school
zone and viewed by the Township as a potential hazard to the public
safety shall be completely surrounded by a fence or wall of not less
than six feet in height. The fence or wall shall not have an opening
or gap larger than two inches and shall be provided with a self-closing
and self-latching gate.
[Amended 7-17-2014 by Ord. No. 2014-02]
G.
Earth fill dams.
[Amended 7-17-2014 by Ord. No. 2014-02]
(1)
Basins which are designed with earth fill dams shall be designed
by a registered professional engineer with experience in earth-filled
dams and shall incorporate the following minimum standards:
(a)
The maximum water depth (measured from the base to the crest
of the emergency spillway) shall not exceed six feet, unless approved
by waiver of the Township.
(b)
The minimum top width of all dams up to 10 feet in height shall
be equal to 2/3 of the dam height, but in no case shall the top width
be less than five feet, unless approved by waiver of the Township.
(c)
The side slopes of earth fill dams shall not be steeper than
2 1/2 horizontal to one vertical on both sides of the embankment.
(d)
Basins without restricted access shall have impoundment areas
with side slopes no greater than three horizontal to one vertical.
(e)
A cutoff trench of impervious material shall be provided under
all dams.
(f)
All pipes and culverts through dams or impoundment fill embankments
must be reinforced concrete and have properly spaced concrete cutoff
collars. Rubber gaskets and mortared pipe joints are required to make
pipe joint watertight.
(g)
All permanent riser pipes and control structures shall be reinforced
cement concrete.
(h)
Minimum floor elevations for all structures that would be affected
by a basin, other temporary impoundment, or open conveyance systems
where ponding may occur shall be two feet above the one-hundred-year
water surface and/or one-foot above the five-hundred-year water surface.
If basement or underground facilities are proposed, detailed calculations
addressing the effects of stormwater ponding on the structure and
waterproofing and/or floodproofing design information shall be submitted
for approval.
(i)
No outlet structure of a basin or swale shall discharge directly
into a culvert under a public street.
(2)
The Township may, upon recommendation of the Township Engineer, impose
additional requirements on earth fill dams for the safety and welfare
of the Township.
H.
Capacities. The capacities of the pipes, gutters, inlets, culverts,
outlet structures, and swales shall consider all possible hydraulic
conditions. The following are minimum design standards:
(1)
Grass swales and roadside gutters shall consider both the channel
velocity and stability.
(2)
The "n" factors to be used for paved or riprap swales or gutters
shall be based on the latest edition of the Pennsylvania Department
of Transportation's Design Manual, Part 2, Highway Design, Chapter
10.
(3)
The "n" factors for corrugated metal, corrugated plastic pipe or
concrete pipe shall be based upon the manufacturer's standard.
(4)
The velocity to be used in the design of any piped stormwater conveyance
system shall be a minimum of 2 1/2 feet per second.
(5)
Inlets, culverts, and basin discharge systems shall be designed for
the worst-case condition. Inlet capacity shall be based on design
standards provided by latest edition of the Pennsylvania Department
of Transportation's Design Manual, Part 2, Highway Design, Chapter
10. If acceptable information is not available, inlets in nonponding
areas shall be designed for a maximum capacity of five cubic feet
per second. Where ponding occurs, inlet capacity shall be based on
accepted engineering design practices. Culvert design shall consider
either inlet/outlet control or a combination of hydraulic losses through
the system, whichever is greater. Basin discharge systems shall be
designed to the same standards as culverts. If it cannot be readily
determined which hydraulic condition controls, the basin discharge
rate shall be based on the highest possible discharge rating curve
with the basin capacity sized to store the excessive storm runoff
based on the lowest possible discharge rating curve.
I.
Stormwater flow along streets and access drives.
(1)
Inlets shall be along the curbline and are not permitted along the
curb radius at an intersection. When possible, inlets shall be located
away from the side lot property line to avoid conflicts with driveways.
For the purpose of inlet placement, curb, gutter, or roadside swale,
flow depths for a twenty-five-year storm frequency with a five-minute
duration shall not exceed three inches in a swale condition, two inches
in a gutter condition, and 1/2 inch across intersections and travel
lanes. In no case shall inlets be spaced more than 500 feet apart
and function at less than 65% efficiency based on the criteria in
the PennDOT Design Manual.
(2)
In general, inlets shall be spaced such that, based upon the Rational
Method, Tc=5 min., and a twenty-five-year rainfall intensity, the
area contributing to the inlet shall be spaced so that its efficiency,
based on efficiency curves proposed by the Pennsylvania Department
of Transportation, is not less than 65%. Additional inlets shall be
placed at the upper side of the street intersections to prevent stormwater
from flowing through intersections. Other devices such as high-efficiency
grates or perforated pipe may be required if conditions warrant.
J.
Manhole locations. Manholes shall not be spaced more than 500 feet
apart. Additionally, manholes shall be placed at points of change
in the horizontal or vertical direction of storm sewers. Inlets may
be substituted for manholes where they will serve a useful purpose.
K.
Alignment requirement. If less than a forty-eight-inch diameter,
curves in pipes or box culverts without an inlet or manhole are prohibited.
Tee-joints, cleanouts, and wyes are always prohibited.
L.
Minimum pipe size. Stormwater pipe collection and conveyance systems
shall have a minimum diameter of 18 inches. All stormwater pipes shall
be laid with a minimum cover of three feet.
M.
Material specifications. All material and installations must comply
with the latest edition of PennDOT Manual Form 408 and Standards for
Roadway Construction. Structural calculations that address the actual
design may be required.
N.
Surface flow characteristics. The maximum swale, gutter, or curb
velocity of stormwater runoff shall be maintained at levels which
result in a stable condition both during and after construction. The
following are considered characteristics of a stable condition:
(1)
It does not alter the condition beyond tolerable limits.
(2)
The channel banks do not erode to the extent that the channel cross
section is changed appreciably.
(3)
Sediment bars do not develop.
(4)
Erosion does not occur around culverts and bridges or elsewhere.
(5)
Gullies do not form or enlarge due to the entry of uncontrolled stormwater
runoff.
O.
Grass-lined channels.
(1)
Grass-lined channels shall be considered stable if the calculated
velocity does not exceed the allowable velocities shown below:
(a)
Three feet per second where only sparse vegetation can be established
and maintained because of shade or soil condition.
(b)
Four feet per second where normal growing conditions exist and
vegetation is to be established by seeding.
(c)
Five feet per second where a dense, vigorous sod can be quickly
established or where water can be temporarily diverted during establishment
of vegetation. Netting, turf reinforcement mats (TRMs) and mulch or
other equivalent methods for establishing vegetation shall be used.
(d)
Six feet per second where there exists a well-established sod
of good quality and a minimum five-inch mow height.
[Amended 7-17-2014 by Ord. No. 2014-02]
(2)
Where swale bends occur, the allowable velocities listed above shall
be divided by the following factors:
|
Degree of Bend
|
Velocity
| |
|---|---|---|
|
0° to 30°
|
1.50
| |
|
30° to 60°
|
1.75
| |
|
60° to 90°
|
2.00
| |
|
90° and over
|
2.50
|
(3)
The above grass-lined channel flows may be exceeded if the designer
can provide acceptable supportive design criteria as proof of erosion
prevention.
(4)
Where the velocity of stormwater runoff exceeds the allowable velocity,
erosion protection must be provided. The method of erosion protection
proposed must be supported by the appropriate design information and/or
references.
P.
Phasing plans. When SWM site plan applications are submitted in phases, and if temporary facilities are required for construction of a phase, such facilities shall be included in the submitted plans. All phases of development must comply with the provisions of this chapter. In the event temporary measures cannot adequately handle the stormwater runoff, the main outlet control and outfall systems shall be included as part of the construction of the proposed phase. All phased development must have fully functional stormwater collection, conveyance and management systems that meet all ordinance criteria of § 415-28 for each successive phase.
Q.
Erosion and sedimentation. All land disturbance facilities shall
conform to the requirements of the Pennsylvania Department of Environmental
Protection Soil Erosion and Sedimentation Control Manual. The following
principles shall be applied to the design plan and construction schedule
to minimize soil erosion and sedimentation:
(1)
Stripping of vegetation, grading, or other soil disturbance shall
be done in a manner which will minimize soil erosion.
(2)
Whenever feasible, natural vegetation shall be retained and protected.
(3)
The extent of the disturbed area and the duration of its exposure
shall be kept to a minimum, within practical limits.
(4)
Either temporary seeding, mulching, or other suitable stabilization
measures shall be used to protect exposed critical areas during construction.
(5)
Drainage provisions shall accommodate the stormwater runoff both
during and after construction.
(6)
Soil erosion and sedimentation facilities shall be installed prior
to any on-site grading.
(7)
The design plan and construction schedule shall incorporate measures
to minimize soil erosion and sedimentation.
(8)
Retention and/or detention basins and water-carrying facilities shall
be stabilized in accordance with current engineering and Natural Resources
Conservation Service practices.
R.
Floodplains. All stormwater management plans shall conform with the
floodplain standards specified in any other chapters of this Code
of Ordinances.
[Amended 7-17-2014 by Ord. No. 2014-02]
S.
Riparian buffers. Riparian buffers shall conform to the requirements of Chapter 490 of this Code of Ordinances, the Pennsylvania DEP BMP Manual, and the Chesapeake Bay Riparian Buffer Handbook. The five steps to establishing and maintaining a riparian buffer shall be outlined in the construction schedule. These include site preparation, tree planting, maintenance, survival inspections, and reinforcement planting. The following principles shall be applied to the planting schedule:
(1)
New trees shall be planted at a minimum rate of 15 feet on center
or one tree per 225 square feet in staggered rows or an equivalent
informal arrangement.
(2)
New trees shall be a variety of sizes ranging from a minimum four-
to five-foot branched whip to an approximate one-and-one-half-inch
balled and burlapped planting stock. New tree whips should be encased
in tubes to protect from deer damage.
(3)
New tree plantings shall be composed of native tree species.
(4)
Tree plantings shall be located along the stream bank to provide
shade for the stream, soil erosion control and stormwater benefits,
according to accepted stream bank practices.
(5)
Existing trees within the riparian buffer shall be preserved and
retained. Existing tree cover should be surveyed and inventoried to
assess the need for any new plantings. Existing tree species included
on the Department of Conservation and Natural Resources Invasive Plants
in Pennsylvania List may be removed where conditions warrant.
T.
Private roads and driveways. Private roads and driveways shall not
be sloped as to cause surface water to drain directly onto the street
or onto neighboring properties. All private roads and driveways (including
stoned) shall include a paved apron for a distance of 25 feet from
the street right-of-way with a minimum width of eight feet or the
final width of the driveway. A trench drain, or alternative BMP as
approved by the Township Engineer, shall be required across the driveway
or private road.
U.
Easements. Easements shall be provided where stormwater or surface
water drainage facilities are existing or proposed, whether located
within or beyond the boundaries of the property. Easements for maintenance
of pipes and culverts shall run from outlet to inlet. Normal lot grading
will not require easements. Swales which receive runoff from more
than one other lot must be provided with an easement. Easements shall
have a minimum width of 20 feet and shall be adequately designed to
provide area for: the collection and discharge of water; the maintenance,
repair, and reconstruction of the drainage facilities; and the passage
of machinery for such work. Easements shall include a description
of an ownership and maintenance program (to include esthetics, structural
integrity and functional integrity), in an appropriate form for recording
with the York County Recorder of Deeds, that clearly sets forth responsibility
for all temporary and permanent stormwater management facilities.[1]
[1]
Editor's Note: Former Subsection V, Ownership and maintenance
of stormwater management facilities, which immediately followed this
subsection, was repealed 7-17-2014 by Ord. No. 2014-02.
