A.
Any proposed storm drainage plans which affect the drainage basin
of any river or stream shall be approved by the Division of Dams and
Waterway Management of the Pennsylvania Department of Environmental
Protection if the affected drainage area is more than 100 acres.
B.
Easements shall be dedicated to the Township along all natural or man-made streams and watercourses (as described in § 152-8C) and/or detention basin areas within a subdivision or land development. These easements should be of sufficient width to convey a 100-year design storm, assuming the upstream drainage basin to be fully developed. Easements shall also be provided where storm drainage swales, culverts or other structures traverse, enter or discharge onto private property. On private property the entire easement area and fencing and landscaping (if any) shall be maintained by the property owner and the Township shall not maintain and/or repair any improvements within that easement, unless stormwater runoff from public roads or public park land crosses through the easement.
(1)
If stormwater runoff from public roads or public lands crosses through
the easement the Township shall, upon satisfactory installation of
improvements as specified in a developer's improvement agreement and
maintenance agreement, maintain and repair only the structural stormwater
management improvements within the easement such as:
(2)
The landowner shall be responsible for all other maintenance and
repairs within this easement. For example, the landowner must:
(a)
Mow the lawn.
(b)
Repair or replace fencing.
(c)
Repair or replace landscaping.
(d)
Repair sinkholes that may develop in the easement.
(e)
Keep the area free of obstructions, structures, vegetation or
accumulated sediment that may block or hinder the function and purpose
of the easement.
(f)
Keep the area free of litter or garbage.
(g)
Repair erosion and restore vegetation as necessary to keep the
easement in good repair.
C.
Storm sewers, culverts, and related installations shall be provided
to permit the flow of natural watercourses, to ensure the drainage
of all low points on the subdivided lots or developed land areas and
along the line of streets, and to intercept stormwater runoff along
streets at intervals related to the extent and grade of the area drained.
The system shall also be designed to accommodate or receive and discharge
all runoff from adjacent upstream properties, as if the upstream properties
were developed. Where adequate existing storm sewers are readily accessible,
the developer must connect new stormwater facilities to the existing
system.
D.
Flood protection. No stormwater runoff or natural drainage water
shall be so diverted as to overload existing drainage systems, or
create flooding or the need for additional drainage structures on
other private properties or public lands, without proper and approved
provisions being made for taking care of these conditions.
E.
When the runoff is increased, due to development, the developer must
secure written approval from the downstream property owners. In lieu
of such written approval, the developer shall follow the other requirements
of this chapter, in particular, the sections dealing with detention
basins and downstream facilities, and shall provide the Township with
a notarized statement satisfactory to the Township Solicitor, stating
that the Township shall be held harmless against any claim of damage
from the downstream property owners that may result from the proposed
development.
F.
Scour and erosion prevention. In areas in which street curbs are
not required by any ordinance of the Township, drainage may be accomplished
by natural or artificial swales and culverts. Special structures such
as check dams, drop-outlets, or other energy dissipating structures
or riprap may be required to prevent scour or erosion in locations
with large runoff quantities or steep slopes. In no case may a change
be made in the existing topography which would result in a slope exceeding
the normal angle of slippage of the material involved. No paved swales
will be allowed without the approval of the Township.
G.
Fixed pipe collection system. The Township may require a fixed pipe
collection system with catch basins. Such fixed pipe collection system
with catch basins shall be used in all areas with street curbs and
gutters, when required. The Township may require foundation pipe underdrains.
H.
Low point drainage.
(1)
Developer shall grade and install all necessary drainage facilities
to insure the drainage of all low points on subdivided lots or within
the subdivision or developed land areas.
(2)
Underdrains are required at a minimum for 50 feet upward from a road
low point along each approach road to the low point.
I.
Storm runoff detention facilities. Where the topography of the tract
being developed or the character of the downstream storm sewer indicate,
as determined by the Township, storm sewer detention facilities shall
be provided within the subdivision proposed. This provision may be
required even if downstream property owners have given written approval.
Such detention facility shall be designed in accordance with the following
requirements:
(1)
The detention facilities shall be designed to retard stormwater runoff rates after development to the peak runoff rate established for each district in § 152-7.
(2)
Spillways shall be provided for possible overflows, and such spillways
shall be protected from erosion. Spillways shall be designed for the
100-year storm.
(3)
Pipe outlet arrangements shall provide complete outletting of all
detained water, unless provisions for permanent ponding have been
approved. Privately owned infiltration practices meeting the DEP standards
for BMP'S, approved as part of a site's approved PCSM Plan shall be
considered to meet this requirement.
(4)
A detention basin, if utilized, shall be lined with a synthetic impervious liner, as more specifically called out in Subsection I(9).
(5)
The detention facility shall be planted with low-maintenance grass
or a substitute satisfactory to the Township. Detention basins shall
be planted with ground cover in accordance with Township standards.
(6)
To minimize the visual impact of detention basins, any required basin
shall be designed to avoid the need for safety fencing. To meet this
requirement, basins shall be designed to the following specifications:
(a)
Maximum depth of detained runoff shall be 24 inches for a ten-year
storm event; and
(b)
Maximum depth of detained runoff shall be 36 inches for a 100-year
storm event; and
(c)
Interior slopes shall not be steeper than a ratio of 4:1 horizontal
to vertical; and
(d)
A water surface limit shall be no closer than 100 feet to any
residential building and no closer than 25 feet to any other type
of building.
(7)
These depths and slopes may be exceeded by permission of the Board
of Supervisors of the Township, on a case-by-case basis if lot runoff,
topography, and/or existing downstream systems make the required pond
area unreasonably large. In such a case a deeper depth may be allowed
if the basin is designed to the following specifications:
(a)
A maximum depth of detained runoff shall be 24 inches for a
two-year storm event; and
(b)
A maximum depth of detained runoff shall be 36 inches for a
100-year storm event; and
(c)
A maximum depth of detained runoff shall be 48 inches for a
100-year storm event; and
(d)
Interior side slopes shall not be steeper than a ratio of 5:1,
horizontal to vertical; and
(e)
A water surface limit no closer than 100 feet to any residential
building and no closer than 25 feet to any other type building.
(8)
An access ramp of 10:1, 10 feet wide, shall be provided to allow
maintenance equipment to reach the basin floor.
(9)
All detention/retention facilities, whether ultimately privately
or publicly owned, shall be lined with a synthetic impervious liner
meeting the following minimum requirements:
(a)
The minimum liner allowed is 30 mil PVC. Actual individual liner
specifications shall be provided by the manufacturer for each individual
pond.
(b)
The liner must be placed on a layer of fine grained soil that
has been rolled with a smooth drum roller in both directions to produce
a smooth level base for the liner. The soil may not contain sharp
angular rock or other debris which could puncture the liner, and must
meet all manufacturer's specifications for a liner bedding. All vegetation,
roots, and grass must be removed and any cracks or voids shall be
filled.
(c)
If rock is encountered in the bedding area, this rock must be
excavated to a depth of six inches below the liner and backfilled
with a fine grained soil. This area should then be covered with geotextile
fabric, extending three feet beyond the limits of the rock outcrop
before placing the pond liner.
(d)
Installation of the liner may only take place when the ambient
temperature is within the manufacturer's recommended range. Installation
and testing shall be in accordance with manufacturer's specifications.
The number of field seams shall be minimized by requiring factory
fabrication of large panels. Any field seams performed must be in
accordance with the manufacturer's specifications.
(e)
All structures (i.e., headwalls, pipes, outlet structures) which
come in contact with the liner must have a waterproof seal installed
to prevent leaks around the structure. These seals shall be installed
per manufacturer's recommendations.
(f)
A minimum of 12 inches of earth cover shall be placed over the
lining. Soil containing sharp jagged rocks, roots, debris, or any
other material which may puncture the liner shall not be used as cover
material.
(g)
The liner must be installed to a minimum height of the 100-year
floodwater elevation in the facility.
(10)
The minimum slope of the bottom of a detention pond shall be
2% toward the outlet structure, except for privately owned infiltration
practices, meeting the DEP Standards for BMP's, approved as part of
a Site's approved PCSM Plan.
J.
All streets shall be designed so as to provide for the discharge
of surface water from their rights-of-way.
K.
Design criteria.
(1)
Unless a more conservative design is required by another regulation,
or is required because of conditions particular to an individual development,
the following storm criteria shall be used to design storm collection
and conveyance systems:
|
Area
|
Design Storm Return
(years)
|
|---|---|
|
Residential areas
| |
|
Fixed pipe
|
10
|
|
Total conveyance
|
100
|
|
Nonresidential areas
| |
|
Fixed pipe
|
25
|
|
Total conveyance
|
100
|
|
Culvert crossings of any road
| |
|
In culvert
|
25
|
|
Total conveyance
|
100
|
(2)
The preliminary plan or lot location plan shall indicate the areal
extent of the 100-year storm.
L.
Design preparation. Designs of storm drainage systems shall be prepared
by a licensed professional engineer. Complete detail calculations
shall be submitted to the Township for approval. Calculations shall
cover the entire drainage basin involved, including consideration
of areas outside the proposed subdivision or developed land areas.
All designs must be approved by the Township.
M.
The developer shall provide written assurance, satisfactory to the
Township, that the detention pond will be properly maintained. Such
assurances shall be in a form to act as a covenant that will follow
with the land, and shall provide Township maintenance in case of default,
and further provide for assessment of costs and penalties in case
of default in Township takeover.
N.
As an alternate to the above Subsection M, the Township may, at its own option, assume responsibility of the pond and require dedication of the pond by the developer. If the detention basin is dedicated or offered to the Township for long-term maintenance, the following regulations shall apply:
(1)
The dedicated area shall not be considered part of the open space and recreation dedication requirements as required in Chapter 159, Subdivision and Land Development.
(2)
The detention basin berm top shall be a minimum width of eight feet.
(3)
The developer shall provide for the special financial burden the
Township will be accepting if the Township accepts the detention basin
maintenance. To help mitigate this future financial burden the developer
shall give to the Township a cash payment in the amount of $60,000
per acre, on a pro-rata basis, for any detention basin site or area
dedicated to the Township and being accepted by the Township. This
requirement is minimum and can be modified by a resolution of the
Board of Supervisors, as necessary, to reflect actual long-term costs
of detention basin maintenance in the Township.
A.
The collection system shall be designed by the Rational Method of
Design in accordance with American Society of Civil Engineers Manual
No. 37 except where noted, using the formula Q = CiA, unless otherwise
approved by the Township.
(1)
Capacity: "Q" is the required capacity in cubic feet per second for
the collection system at the point of design.
(2)
Runoff coefficient: "C" is the runoff coefficient applicable to the
entire drainage area. It shall be based on consideration of soil conditions,
average slope of the drainage area and the ultimate development of
the entire drainage area according to comprehensive plans. For the
various types of ultimate development, the runoff coefficient shall
be taken from the table in Appendix C,[1] unless sufficient engineering data has been presented
to the Township Engineer by the developer which information in the
judgment of the Township Engineer and the Board of Supervisors is
sufficient to warrant the use of an alternate runoff coefficient.
[1]
Editor’s Note: Appendix C is included at the end of this chapter.
(3)
Rainfall intensity formula: "i" is the rainfall intensity in inches
per hour and shall be determined from rainfall intensity charts for
this area, based on time of concentrations, including overland flow
time, Manning's formulae for channelized flow time and pipe flow time.
The design rainfall frequency shall be taken from the PennDOT Intensity-Duration-Frequency
Field Manual, Region 4, as presented in Appendix C.
(4)
Drainage area: "A" is the drainage area, in acres, tributary to the
point of design, and shall include areas tributary from outside sources
as well as from within the subdivision or developed land area itself.
B.
Flow velocity. Storm drains shall be designed to produce a minimum
velocity of 3.0 feet per second when flowing full. The maximum permissible
velocity shall be 15.0 feet per second.
C.
Manholes. Manholes shall be spaced at intervals not exceeding 300
feet, and shall be located wherever branches are connected or sizes
are changed, and wherever there is a change in alignment or grade.
For drainage lines of at least 36 inches diameter, manholes may be
spaced at intervals greater than 300 feet with the approval of the
Township.
D.
Drain line location. Drain lines shall be located between the center
line of the street and the curbline, and shall parallel the center
line of the street as far as practical.
E.
Catch basin limits. Not more than two catch basins shall be interconnected
before being connected to a manhole or access-type catch basin.
F.
Open swales. Open swales shall be designed on the basis of Manning's
formula as indicated for collection systems with the following considerations:
(1)
Roughness coefficient. The roughness coefficient shall be 0.040 for
earth swales, and 0.015 for paved swales.
(2)
Bank slopes. Slopes for swale banks shall not be steeper than one
vertical for three horizontal.
(3)
Flow velocity. Design velocity in grass or vegetated swales shall
not exceed four feet per second.
(4)
Right-of-way. A swale right-of-way of sufficient minimum width to
include a ten-foot access strip, in addition to the width of the swale
from bank top, shall be shown as an easement for drainage purposes.
The Township may, under unusual conditions, require a wider swale
right-of-way.
G.
General design standards.
(1)
Curb inlets. Curb inlets shall be located at curb tangents on the
uphill side of street intersections. Design and location of curb inlets
shall be approved by the Township.
(2)
State approvals. Drainage structures that are located on state highway
rights-of-way shall be approved by the Pennsylvania Department of
Transportation, and a letter from that office indicating such approval
shall be directed to the Township.
(3)
Pipe materials. All storm piping shall be Class III reinforced concrete
piping with "O" ring joints. HDPE pipe with water-tight joints may
be utilized on privately owned stormwater facilities. Piping shall
be saw-cut at ends, as needed, and not hammered or broken.
(4)
Minimum pipe size. Minimum pipe size shall be 15 inches.
(5)
Inlet and manhole construction. Inlet and manhole castings and concrete
construction shall be equivalent to Pennsylvania Department of Transportation
design standards.
(6)
Roof drainage. Stormwater roof drains and pipes shall not discharge
water over a sidewalk, driveway or paved area. Where accessible, roof
drains can be connected with a storm drain system.
(7)
Discharge structures shall be designed to minimize the impact of
development on downstream properties. Flow retarding and dissipating
facilities are required to meet this goal. Such structures shall be
no closer than 20 feet to downstream off-site properties or drainage
easement boundaries.
(8)
Open pipe ends must be fitted with concrete end walls or wing walls.
(9)
No open pipe shall be allowed to end within the existing or proposed
Township road or state road right-of-way.
(10)
Any riprap dissipation area or riprap swale area shall be constructed
with mixed stone sizes in accordance with PennDOT criteria for riprap
and all such areas shall be lined with PennDOT approved geotextile
fabric.
(11)
A maximum of five cubic feet per second (cfs) will be permitted
along the curb or roadside swale, prior to discharge away from the
street surface by way of roadside swale or underground piping.
(12)
All pipe endwalls or wingwalls and all detention basin intake
and discharge structures shall be protected from frost-related movement
and scour activity by the construction of a concrete footer with a
minimum depth of 30 inches below ground level.
(13)
The minimum pipe slope is 0.5%.
(14)
All inlets shall be labeled with an embedded plastic disk indicating
a prohibition against pollutants.
A.
In accordance with the standards of this chapter, the developer shall
provide throughout his development, and adjacent to his development,
storm sewer facilities adequate to handle the expected storm flows.
This requirement pertains to collector systems or interceptor systems.
B.
The developer is required to install or improve the Township interceptor
system, either on-site, off-site, or both, by the value of at least:
C.
In those cases where the developer cannot make all or a portion of such required improvements as outlined in Subsection B, the Township shall accept money in lieu of improvements. Such money shall be in an amount equal to the difference between the minimum required improvement value; no credit shall be due the developer from the Township.
