The standards of design in this article shall
be used to judge the adequacy of development proposals and shall be
considered to be the minimum criteria in achieving the purpose and
objectives of this chapter. Where questions should arise regarding
the interpretation of these design standards, the determination of
the Board of Supervisors shall prevail.
Alleys shall be prohibited in residential districts
except where proven to be necessary. The applicant must submit to
the Planning Commission the justification with alternative explored
prior to review by the Planning Commission and Supervisors. All requests
must be reviewed by the Township Engineer prior to the Planning Commission
making recommendations.
Blocks shall not exceed 1,500 feet in length
and shall be of sufficient depth to permit two tiers of lots, except
as otherwise provided for in this chapter.
[Amended 12-2-2003 by Ord. No. 51; 3-6-2012 by Ord. No. 59]
A. The scope of a stormwater management plan (SWMP) shall be required
for each subdivision or land development plan at both the preliminary
and final plan submittal stage. As an integral part of the SWMP, erosion
and sedimentation control measures shall be included.
B. Basic requirements. All earthmoving activities and/or construction
activities shall be designed and constructed in a manner such that
the rate as measured in cubic feet per second of stormwater runoff
from a lot, tract or parcel shall not be greater after the activities
than prior to the activities. Such determination shall be made on
the basis of the following:
(1) Storm sewer design: ten-year storm event.
(2) Cross culverts design: twenty-five-year storm event.
C. General basis of facility design.
(1) All drainage facilities or stormwater facilities shall be designed
to carry, accommodate or handle peak runoff as tabulation in this
chapter. The computations of stormwater runoff for the event shall
be in accordance with the provisions of this chapter.
(2) All drainage facilities, stormwater facilities and erosion and sedimentation
control facilities shall be consistent, to the extent applicable,
with the standards and specifications of the "State Erosion and Sediment
Pollution Control Program Manual."
(3) Facilities shall be designed to accommodate existing and projected
on-site peak stormwater and projected peak stormwater from the site
of the proposed earthmoving activities or construction activities.
D. Prime hydraulic method and criteria.
(1) Design standard-computations for determining stormwater runoff and
for the design of stormwater management facilities shall be based
upon the Soil Cover Complex Method described in either TR-55, Urban
Hydrology for Small Watersheds; the United States Department of Agriculture,
Soil Conservation Engineering Field Manual; or the Soil Conservation
Service National Engineering Handbook, Section 4. Computations based
upon an alternative method may be accepted upon recommendation of
the Township Engineer and approval of Township Supervisors. The following
standards shall apply:
(a)
All pre-development calculations shall be based upon existing
land use features, excepting, however, that agricultural uses shall
be categorized by the following descriptions:
[1]
Cultivated land with conservation treatment pasture, good condition.
(b)
Release rates from storage structures shall be based on the
runoff from the one-, two-, five-, ten-, twenty-five-, fifty- and
one-hundred-year storm pre-developed storm events.
(c)
Storage structures shall be designed such that the post-developed
discharge will not exceed the pre-developed discharge for the like
storm event.
(d)
All storage structures or facilities will be designed with emergency
spillways sufficient to handle the one-hundred-year post-development
storm event.
(e)
Culverts, pipes, and other water-carrying structures shall be
designed to handle the runoff from the storm event.
(f)
The SWMP shall include calculations indicating velocities of
flow, grades, sizes and capacities of water-carrying structures, debris
or sedimentation basins, and retention and detention ponds and sufficient
design information to construct such facilities.
(g)
Stormwater runoff shall be based on the following twenty-four-hour
storm events:
|
Storm Frequency
(years)
|
Rainfall
(inches)
|
---|
|
2
|
2.5
|
|
5
|
3.8
|
|
10
|
4.7
|
|
25
|
5.1
|
|
50
|
5.8
|
|
100
|
6.4
|
(h)
Maximum permitted velocities are as follows:
[1]
For all proposed lined water-carrying channels, complete hydraulic
calculations shall be submitted to the Carroll Township Engineer for
review. The calculations shall determine that the protection proposed
will achieve results in this required article.
[2]
Three feet per second where only sparse vegetation can be established.
[3]
Four feet per second under normal conditions where vegetation
can be established by seeding.
[4]
Five feet per second where a dense, vigorous sod can be quickly
established or with establishment of vegetation.
[5]
Six feet per second where well-established sod is in existence.
[6]
The normal maximum velocity of open channel flows shall not
exceed 10 feet per second after channel lining adjustments are made.
(i)
Energy dissipaters/erosion control devices shall be placed at
the outlets of all pipes where flow velocities exceed maximum permitted
channel velocities. Such devices shall be designed and installed in
accord with the Pennsylvania Department of Environmental Protection's
Erosion and Sediment Pollution Control Program Manual.
(j)
Vertical pipes, inlets and other surface-water-receiving structures
shall be installed with trash racks or so designed to control trash
accumulation.
(k)
Stormwater runoff channels shall be designed and installed to
avoid trapping excess sediment, except if structures are so designed
to trap sediment.
E. Alternate hydraulic method and criteria.
(1) General. All methods and drainage details must be approved by the
Township Engineer.
(2)
(a)
The Rational Method shall not be used for drainage areas over
20 acres.
(3) Runoff determination.
(a)
Minor watercourse and storm sewers shall be designed using the
Rational Equation:
|
Where:
|
Q = CIA
|
|
|
C is a runoff coefficient based on future land use ( Table 1).
|
|
|
I is intensity of rainfall obtained from attached ( Chart 1).
|
|
|
A is the total area contributing runoff to the point under study.
|
(b)
Major watercourse design discharges can be determined by using
one of the following methods:
[1]
Pennsylvania Department of Environmental Protection Erosion
and Sediment Pollution Control Program Manual.
[2]
PennDOT Design Manual, Part 2, Chapter 10.
F. The SWMP plan shall contain the following:
(1) A stormwater management plan.
(2) A general description of the proposed project.
(3) Project location on a 7.5 minute USGS map or equivalent.
(4) Topographic features of the site and adjacent lands that are considered
to impact upon the stormwater management design.
(5) Runoff calculations and related design computations of the total
drainage basin necessary to substantiate the proposed temporary and
permanent stormwater management facilities.
(6) Design and specifications of temporary and permanent stormwater management
facilities.
(7) Staging or implementation schedule for constructing the proposed
stormwater control system.
G. Plan requirements are as follows:
(1) A plan or plans, drawn to a scale of one inch equals 50 feet, showing
at least the following:
(a)
Name, address and telephone number of the landowner or developer
submitting the plan.
(b)
Name, address and telephone number of the engineer preparing
the plan.
(c)
North arrow, scale, graphic scale, and date of plan preparation
with revision dates, if any.
(d)
Names of adjacent or surrounding landowners.
(e)
Site boundaries, with bearings and distances.
(f)
Existing topography, existing and proposed contours at an interval
of two feet and illustrating all natural features of the area.
(g)
The location and dimensions of all existing and proposed buildings
and associated impervious areas.
(h)
Location and dimensions of existing and proposed streets and
parking areas, including showing the difference between paved and
unpaved rights-of-way and specification of the type or types of paving
material to be used.
(i)
Location and dimensions of existing and proposed lots, tracts
or parcels.
(j)
Plans and profiles showing the locations, size and capacity
of existing and proposed surface and subsurface drainage facilities
and detailed plans and profiles thereof. The plan shall also show
how the proposed drainage facilities relate to or tie into existing
facilities, both on site and off site.
(k)
Location and description of erosion and sedimentation control
facilities and devices and a description of any vegetative material
to be used.
(l)
Stormwater runoff computations for the area of the earthmoving
activities or construction activities and for the entire site. The
computations shall be prepared in accordance with the provisions of
this chapter.
(m)
Such other certificates, affidavits, endorsements or dedications
as may be required by the Township in the administration and enforcement
of this chapter.
(n)
All plan sheets, details sheets, profiles and computations shall
be sealed by a registered professional engineer.
H. Preparation of stormwater runoff computations. All computations of
stormwater runoff shall be prepared in a clear, legible manner and
submitted to the Township, together with the herein-required stormwater
management plan. The Township shall forward the plan, together with
the computations, to the Township Engineer for review and recommendations.
I. Use of natural or man-made systems.
(1) Where natural swales, channels, drainageways, or easements are existing
or proposed to be used to convey stormwater, the landowner or developer
shall institute such measures as may be necessary to prevent erosion
of the natural systems. The landowner or developer shall be responsible
for maintenance of all on-site natural or man-made systems unless
or until said swales, channels, drainageways or easements have been
dedicated to and accepted by the Township. The Township Supervisors
may require dedication of swales, channels, drainageways or easements
if it is the option of the Township Supervisors that it is in the
best interests of the citizens of Carroll Township.
(2) No earthmoving activities or construction activities shall obstruct
or cause obstruction or otherwise impede the flow of natural watercourses,
swales, channels, drainageways or easements, nor shall plantings of
an obstructive nature be permitted herein.
J. Use of vegetative plantings. Where vegetative plantings and materials
are used or required to be used to assist in controlling erosion and
stormwater runoff or used to prevent accelerated erosion, such plantings
and materials shall be consistent with the standards and specifications
of the Soil Erosion and Sedimentation Control Handbook and the Section
on Erosion Control and Conservation Plantings of the Agronomy Guide.
K. Discharges.
(1) Stormwater shall not be discharged onto adjacent properties without
the written consent of the adjacent affected landowner, except that
stormwater may be discharged onto adjacent properties without said
written consent if the discharge is to natural watercourses or to
preexisting natural drainageways and if it can be demonstrated to
the satisfaction of the Township that the discharge will not result
in property damage or threaten public safety and is at a rate not
to exceed the existing rate discharging from the property.
(2) Where discharges are proposed to streams or watercourses, the landowner
or developer shall take any and all actions that are necessary to
assure channel stability. The landowner or developer shall obtain
all necessary permits and forward copies to Carroll Township prior
to any construction.
(3) To the maximum extent feasible, discharges from drainage facilities
shall be at nonerosive velocities. Where nonerosive velocities are
not feasible, erosion control facilities shall be constructed that
are in accordance with the applicable standards and specifications
of the Soil Erosion and Sedimentation Control Handbook of the Perry
County Conservation District and this chapter.
L. Other design criteria.
(1) Where on-site storm sewers are within 1,000 feet of an existing public
storm sewer system, the Township may require the landowner or developer
to connect thereto. Drainage facilities hereafter constructed shall
be separate from and independent of sanitary sewer systems.
(2) Inlets shall be provided in a manner so that surface water is not
carried across intersections and in a manner to prevent accumulation
of stormwater runoff in roadways.
(3) 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 Township-approved provisions being made by
the landowner or developer for properly handling and financing such
conditions.
(4) Opening channels shall be designed so that they provide two feet
of freeboard above projected peak flow water profiles.
M. General hydraulic design criteria.
(1) Drainage swales and channels.
(a)
Where vegetated drainage swales are used in lieu of or in addition
to storm sewers, they shall be designed to carry the ten-year discharge
without erosion, increase the time of concentration, reduce the peak
discharge and/or velocity and permit the water to percolate into the
soil.
(b)
Swales provided in cut areas shall not encroach upon the road
shoulder during a ten-year-frequency storm of five minutes' duration.
Frequent and/or sustained flooding of the subbase shall not be permitted.
(c)
The maximum velocity as determined by Manning's equation shall not exceed the allowable velocity in Subsection
D of this section. Inlets shall be provided to control the shoulder encroachment and/or water velocity.
(d)
Erosion prevention. All drainage swales and channels shall be
designed to prevent the erosion of the bed and bank areas. The flow
velocity in all vegetated drainage channels shall not exceed three
feet per second to prevent erosion. Suitable stabilization shall be
provided where required to prevent erosion during establishment of
vegetation.
(e)
The side slope for any vegetated drainage channel requiring
mowing of the vegetation shall have a maximum grade of three horizontal
feet to one vertical on those areas to be mowed.
(f)
Storm sewers or drainage swales shall discharge to a detention
or retention basin for the control of peak runoff discharge except
as provided in the plan.
(g)
Design standard. Because of the critical nature of vegetated
drainage channels, the design of all vegetated channels shall, as
a minimum, conform to the design procedures outlined in the Pennsylvania
Department of Environmental Protection's Erosion and Sediment
Pollution Control Program Manual.
(h)
Deed restrictions shall be required on property(ies) containing
drainage swales and/or perennial streams. These deed restrictions
shall specify that no property owner obstruct or alter any drainage
swale or perennial stream identified in the stormwater management/erosion
and sediment pollution control plan if such action would alter the
rational course of runoff, without the approval of the Township.
(2) Culverts and drainage channels.
(a)
Design flow standard. Culverts and drainage channels shall be
designed to carry flow rates as outlined in this article.
(b)
The maximum permissible flow velocity shall not exceed those
outlined in Pennsylvania Department of Environmental Protection's
Erosion and Sediment Pollution Control Program Manual.
(c)
A minimum grade of 1% shall be maintained for all channel flows.
(d)
Pipe capacity. The capacity of all pipe culverts shall, as a
minimum, provide the required carrying capacity as determined by the
Manning Equation; see Pennsylvania Department of Transportation's
Design Manual 2, Chapter 10.
(e)
Design of pipe culverts shall be in accordance with Pennsylvania
Department of Transportation's Design Manual 2, Chapter 10, and
FHWA's Hydraulic Design Series No. 5, "Hydraulic Design of Highway
Culverts."
(3) Drainage pipe criteria. Wherever possible, all storm drainpipes shall
be designed to follow straight courses. No angular deflections of
storm sewer pipe sections in excess of five degrees shall be permitted
without the installation of a storm inlet or manhole. No vertical
curves shall be permitted in the storm drainpipe system. Storm pipes
shall be either polymeric coated galvanized corrugated metal pipe,
reinforced cement concrete pipe, or PVC pipe.
(a)
Minimum grade and size. All storm drainpipes shall be designed
to maintain a minimum grade of 1/2%. All storm drainpipes shall have
a minimum inside diameter of 15 inches.
(b)
Pipe arches. Where depth or headroom is restricted, equivalent
pipe arches may be used in lieu of circular pipe. An equivalent pipe
size to a fifteen-inch circular pipe is the minimum arch permitted.
(c)
Minimum cover. A minimum of two feet of cover shall be maintained
over all storm pipes. In extreme cases, when approved by the Township
Engineer, this minimum may be reduced, but in no case shall the top
of the pipe be higher than 1/2 foot below the subgrade elevation of
the roadway.
(d)
Discharge velocity. All storm drainage piping discharging to
the ground surface shall be provided with either reinforced concrete
headwalls or end sections compatible with the pipe size involved.
A riprap apron of adequate length shall be provided at all surface
discharge points in order to minimize erosion, using the procedures
found in the Department of Environmental Protection's Erosion
and Sediment Pollution Control Program Manual.
(e)
Where the construction of endwalls is proposed at the outlet
of stormwater conveyance structures, these structures will be built
in accord with Pennsylvania Department of Transportation's Publication
408. Special care shall be used by the design engineer to select the
proper endwall to fit the conditions.
(f)
All storm sewer pipe within street cartway or other paved areas
shall be backfilled with tamped 2-RC stone placed in six-inch lifts
and compacted in accordance with PennDOT Form 408, as revised to date.
(g)
Where storm sewers and culvert pipes discharge into existing
drainage channels at an angle greater than 15° from parallel with
the downstream channel flow, the far side bank shall be stabilized
by the use of riprap or masonry and/or concrete walls. The stabilization
shall be designed to prevent erosion and frost heave under and behind
the stabilizing media.
(4) Inlet capacity and type.
(a)
The interval between inlets collecting stormwater runoff shall
be determined in accordance with DM-2, Section 10.5, "Capacity of
Waterway Areas." Inlets shall be provided to control the encroachment
of water on the pavement.
(b)
Inlets shall be standard precast types as indicated in Pennsylvania
Department of Transportation Design Standards Publication 72-Plate
RC-34.
(c)
On a curbed roadway, all street inlet tops shall be the combination
curb and gutter inlet referred to as Pennsylvania Department of Transportation
Type "C," with a ten-inch curb reveal to allow an automatic depressed
condition to exist when used with an eight-inch curb. All inlet tops
shall be precast concrete with heavy-duty steel grating. Weep holes
shall be provided on all inlet tops. In streets, private parking areas,
yard areas and drainage swales with no curbing, Pennsylvania Department
of Transportation Type "M" inlet tops shall be used.
(d)
In traffic lanes where grates are used, the grates must be bicycle
safe.
(5) Detention, retention and infiltration facilities.
(a)
Detention, retention and infiltration facilities, including
but not limited to seepage pits, ponds and drains, may be used to
control or reduce on-site runoff. Where such facilities are proposed
for use, detailed design drawings shall be submitted with the stormwater
management plan required in this chapter, and such facilities shall
be consistent with the standards and specifications of this chapter.
(b)
Where rooftop retention and/or detention of stormwater is proposed,
the landowner or developer shall assure that the structure will withstand
the stress and shall be responsible for maintaining any outflow structures.
In addition, the landowner or developer shall submit a signed statement
from a registered professional engineer that the structure can support
the roof loadings.
(c)
Where on-site detention, retention or infiltration facilities
are proposed, the Township Supervisors may require the landowner or
developer to pay a fee to the Township in lieu of installing the facilities
so that the Township can construct or have constructed off-site facilities
to control runoff from the site. Where such a fee is levied by the
Township, the fee shall be equal to no more than the estimated costs
that the landowner or developer would have incurred to install or
construct said on-site facilities. Such estimated costs shall be prepared
by the Township Engineer.
(6) Design of detention basins. All detention basins shall be designed
as per the procedures developed by the U.S. Department of Agriculture,
Soil Conservation Service, as outlined in its Technical Release No. 55, Urban Hydrology
for Small Watersheds, or by use of a method of plotting flood stages
based on the Rational Method, where drainage areas are less than 20
acres. All detention basins shall meet the general criteria as follows:
(a)
Detention basins shall not be located within floodplains, nor
within areas of floodplain soils, with the exception that areas of
alluvial soils may be utilized if proof is accepted that the area
is not subject to flooding.
(b)
Detention basins shall be designed to facilitate regular maintenance,
mowing, and periodic desilting and reseeding.
(c)
Whenever possible, the side slopes and basin shape shall conform
to the natural topography. When such design is impracticable, the
construction of the basin shall utilize slopes as flat as possible
to blend the structure into the terrain.
(d)
In residential developments, shallow broad basins may be provided
for recreational use.
(e)
Lakes and ponds:
[1]
Existing or proposed lakes and ponds may be used for stormwater
detention, provided that the design criteria herein are met and are
in accordance with DEP, Chapter 105, Permit Requirements.
[2]
All related basin structural components shall be constructed
of reinforced concrete.
[3]
The minimum level and the size of the permanent pool shall be
adequate to deter the growth of undesirable vegetation and mosquitoes.
(7) Basin design criteria.
(a)
Riser. A precast inlet riser may be provided at the outlet of
detention basins. The riser shall be constructed of metal or concrete
as approved by the Township Engineer. A one-foot minimum freeboard
shall be provided between the top of the riser and the crest elevation
of the emergency spillway. The riser shall be designed so that the
rate of outflow is controlled by the pipe barrel through the basin
berm when the depth of the water within the basin exceeds the height
of the riser or by accurately sized orifices. A trash rack or similar
appurtenance shall be provided to prevent children and debris from
entering the riser where openings greater than 12 inches in diameter
are used. The base shall be of sufficient weight to prevent flotation
of the riser.
(b)
Maximum depth of detention basins. In general, the maximum depth
of water in a detention basin measured to the invert of the emergency
spillway shall not exceed five feet.
(c)
Emergency spillways shall be designed and constructed in accord
with the Pennsylvania Department of Environmental Protection's
Erosion and Sediment Pollution Control Program Manual.
(d)
Antiseep collars. Antiseep collars shall be installed around
the principal pipe barrel within the normal saturation zone of the
detention basin berms. The antiseep collars and their connections
to the pipe barrel shall be watertight. The anitseep collars shall
extend a minimum of two feet beyond the outside of the principal pipe
barrel. The spacing between collars shall be designed by a professional
engineer and supporting calculation submitted to Carroll Township.
(e)
Freeboard. Freeboard is the difference between the design flow
elevations in the emergency spillway and the top of the settled detention
basin embankment. The minimum freeboard shall be one foot.
(f)
Slope of detention basin embankment. The top or toe of any slope
shall be located a minimum of 10 feet from any property line. Whenever
possible, the side slopes and basin shape shall be amenable to the
natural topography. Straight side slopes and rectangular basins shall
be avoided whenever possible.
[1]
Exterior slopes of compacted soil shall not exceed one foot
vertical for three feet horizontal and may be further reduced if the
soil has unstable characteristics.
[2]
Interior slopes of the basin shall not exceed one foot vertical
for three feet horizontal, except with approval of the Township.
(g)
Width of berm. The minimum top width of detention basin berms
shall be 10 feet.
(h)
Slope of basin bottom. In order to insure proper drainage of
the detention basin, a minimum grade of 2% shall be maintained for
all sheet flow.
[1]
Inlet and outlet structures shall be located at maximum distances
from one another. The Township Engineer may require a rock filter
berm or rock-filled gabions between inlet and outlet areas when the
distance is deemed insufficient for sediment trapping. All shall discharge
to areas of the basin which slope downward to lower elevations of
the basin.
[2]
A collecting swale and/or underdrain shall be provided to drain
basins intended for recreational use where the slope of the basin
bottom may be reduced to 1%.
[3]
The bottom of the detention pond may have a flat slope if infiltration
is proposed in the pond.
(i)
Energy dissipaters. Energy-dissipating devices (riprap, end
sills, etc.) shall be placed at all basin outlets. Level spreaders
shall be provided to spread discharges across drainage swales to prevent
concentrated and erosive flows.
(j)
The distance from the highest free water surface of any detention
basin or drainage facility to a dwelling unit shall be a minimum of
100 feet.
(k)
Fence or screening.
[1]
The developer shall submit a plan to address the protection
of all stormwater and erosion facilities, including temporary and/or
permanent fencing, gates and spillway protection.
[2]
The fence shall be chain-linked a minimum of four feet high
with access gates and protection of the emergency spillway from debris.
[3]
A vegetative screening of suitable landscaping plan material
in or around a detention basin may also be required. Vegetative screening
should generally provide a barrier to prevent entrance to and effectively
naturalize the appearance of the detention basin area.
(l)
Landscaping and grading of detention basins. All landscaping
and grading standards particularly applicable to detention basins
shall be approved by Carroll Township.
(m)
A soil investigation report shall be prepared for stormwater
detention basins and other structures as required by the Township
Engineer. The report shall identify unsuitable conditions, recommend
remedial actions, and determine potential risks to proposed improvements.
[1]
A minimum of three soil borings shall be required at the emergency
spillway, the pool or borrow area, and the center line of the dam
at the barrel. In any case, the Township reserves the right to require
additional soil borings.
[2]
The minimum soil boring depth shall be to the seasonal high
water table or five feet below the bottom of the stormwater management
facility, whichever is greater, or to refusal.
[3]
Soil boring data required:
[a] Bearing strength (number of blows) required for
pond only.
[b] Unified soil classification for each stratum.
[d] Depth to the seasonal high water table.
[e] USDA soil textual classification for each stratum.
(n)
A quality control program is critical for embankment fills.
Therefore:
[1]
Wherever embankment fill material in excess of three feet is
to be used, each layer of compacted fill shall be tested to determine
its density per ASTM D1556. The density of each layer shall not be
less than 95% of maximum dry density as determined by ASTM D1557.
[2]
Inspections shall be conducted by the Township Engineer.
[3]
Compaction test reports shall be kept on file at the site and
be subject to review at all times by the Township Engineer.
[4]
When rock is encountered during the excavation of a pond, it
shall be removed to an elevation of at least 12 inches below the proposed
basin floor.
[5]
Temporary and permanent grasses or stabilization measures shall
be established on the sides and base of all earth basins within 15
days of construction.
(o)
Design information. As part of the stormwater management/erosion
and sediment pollution control plan, all design information shall
be submitted, including, but not limited to, the following:
[1]
General description of proposed facilities and the operation
of the runoff control measures.
[2]
All computations of the stormwater runoff before, during, and
after construction, including all supporting material.
[3]
A detail of the berm embankment and outlet structure indicating
the embankment top elevation, embankment side slopes, top width of
embankment, emergency spillway elevation, perforated riser dimensions,
pipe barrel dimensions, and dimensions and spacing of antiseep collars.
[4]
Design computations for the pipe barrel and riser.
[5]
A plot of the stage storage and stage discharge (acre-feet versus
elevation) and all supporting computations.
[6]
Flood routing computations.
[7]
A detailed plan of the trash rack and antivortex device.
[8]
A plan, at a scale of one inch equals 50 feet, showing the grading,
landscaping, and fencing around the detention basin.
[9]
Soils investigation report, where required.
(8) Design of seepage pits and seepage trenches for infiltration of roof
drainage. The design may be in accordance with the guidelines in the
state BMP Manual or shall meet the following criteria. An operation
and maintenance plan shall be provided and notes added to the plan
to ensure the long-term integrity of these systems. Site-specific
soil investigations shall be required to demonstrate suitable soil
conditions exist for use of these systems; testing should be in accordance
with the guidelines of the BMP Manual. These structures shall be designed
to assimilate, in 72 hours, a volume of water equal to 0.2 cubic feet
per square foot of roof coverage (i.e., length times width of building
space covered) and to assimilate in two hours a volume equal to 0.1
cubic feet per square foot of roof coverage.
(a)
Runoff control capacity may be distributed among several seepage
pits, trenches, or runoff control berms so long as total assimilative
capacity of all structures equals the required volume.
(b)
Seepage pits connected to roof drains should be located at least
10 feet from basement walls and downhill from the building in the
direction of groundwater flow.
(c)
The bottom of a seepage pit should be at least two feet above
seasonal high water table and bedrock or be shown to be otherwise
capable of handling required design volumes.
(d)
The soils on which a seepage pit or trench are located shall
have a minimum infiltration rate of 0.27 inches per hour based upon
soils data obtained by direct testing methods.
(e)
The porosity of the gravel or rock to be used in seepage pits
must be specified on the plan. The rock or gravel shall be covered
with a ground stabilization fabric [trade names: Mirafi 500 (Monsanto),
Typer (Dupont), Bidim, Supsc, or equivalent].
(f)
Where adequate seepage pit capacity is difficult to achieve
with a rock-filled pit, a concrete (or equivalent material) culvert
pipe with a lid may be placed vertically over a stone bed to provide
storage capacity; alternatively, a septic tank-type structure operating
as a cistern with discharge to the seepage pit may be used.
(g)
The longer dimension of seepage pits or seepage trenches should
parallel the slope where slopes exceed 5%.
(h)
Seepage pits or seepage trenches shall not be installed on slopes
greater than 20% and shall be kept away from man-made grades.
(i)
The use of a perforated or porous pipe leading to the seepage
pit is encouraged.
(j)
In all cases, an overflow system should be provided to accommodate
heavy rains in excess of the design criteria.
(k)
Seepage pits or the drains to them must contain a sediment trap
which can be maintained regularly by the owner. All downspouts should
have leaf strains to prevent leaves from clogging the seepage pit.
(9) Design of seepage pits and seepage trenches, other than roof drainage.
The design may be in accordance with the guidelines in the state BMP
Manual or shall meet the following criteria. An operation and maintenance
plan shall be provided and notes added to the plan to ensure the long-term
integrity of these systems. Site-specific soil investigations shall
be required to demonstrate suitable soil conditions exist for use
of these systems; testing should be in accordance with the guidelines
of the BMP Manual.
(a)
Where seepage pits or trenches will be used for infiltration
of flows from grassed areas or street runoff, their design shall generally
follow the guidelines of this article. It is also suggested that:
[1]
Seepage trenches for such flows be located in diversion channels
where feasible.
[2]
Seepage pits be installed in drainage swales uphill of check
dams or small detention facilities.
(10)
Design of cisterns for water storage facilities. The design
may be in accordance with the guidelines in the state BMP Manual or
shall meet the following criteria. An operation and maintenance plan
shall be provided and notes added to the plan to ensure the long-term
integrity of these systems. Site-specific soil investigations shall
be required to demonstrate suitable soil conditions exist for use
of these systems; testing should be in accordance with the guidelines
of the BMP Manual.
(a)
These structures shall either be located within a building or
below frost level where they will be protected against freezing. They
shall be designed to hold 0.2 cubic feet of water per square foot
of roof coverage.
(b)
Access to the structures by insect or animal vectors shall be
controlled by screens or other obstructions.
(c)
Facilities should have a means of access for cleanout of accumulated
debris or sediment.
(d)
Facilities to be used for nonpotable water supply purposes shall
comply with plumbing code regulations for cross-connections.
(e)
Septic tank-type structures are recommended for smaller facilities.
(f)
Rooftop storage must comply with all building code regulations
on load limitations and other related factors.
(g)
All water storage facilities shall be equipped to divert flows
in excess of their holding capacity to appropriate areas of discharge.
If water stored in a cistern will be used for nonpotable water supply
purposes, the facility shall be designed to drain down through a seepage
bed within 72 hours.
In the event that water is to be provided by
a means other than private wells, owned and maintained by the individual
owner of lots within a subdivision or land development, the applicants
shall present evidence to the governing body and the planning agency
that the subdivision or development is to be supplied by a certified
public utility, a bona fide cooperative association, or by municipal
corporation authority or utility. A copy of the certificate of public
convenience under the Pennsylvania Public Utilities Commission, or
an application for such certificate, a cooperative agreement or a
commitment, or an agreement to serve the area in question, whichever
is appropriate, shall be evidence of the supply of water as set forth
above other than by means of private wells.