A.
General.
(1)
All land areas shall be graded to secure proper drainage
away from buildings. Drainage provisions shall be of such design as
to carry surface waters to the nearest practical adequate street,
storm drain or natural watercourse. Developers must carry surface
water to the practical storm drain or natural watercourse. The developer
shall construct and/or install such drainage structures and/or pipes
as required to prevent erosion, damage and siltation and to satisfactorily
carry off surface waters. The rate of stormwater runoff from any proposed
earth disturbance activity after full development shall not exceed
the peak rate of discharge prior to development.
(2)
Stormwater discharge shall be carried by conduit to
prevent excessive surface flow on or across streets, sidewalks, drives,
parking areas and any other paved surface or traveled way where infiltration
practices, such as pervious paving, would not be feasible. Culverts
or bridges shall be required at all stream crossings of any street
or roadway, using design criteria for a one-hundred-year peak flow
rate appropriate to the contributing watershed. Natural swales or
open drains of any type may be used only where there is no danger
to structures or abutting property.
(4)
The stormwater management plan for each earth disturbance
activity shall take into account and provide for upstream areas within
the entire watershed in computing discharge quantities, sizing of
pipes, inlets and other structures. The runoff from any proposed development
shall be subject to evaluation which includes the anticipated runoff
from other existing or proposed developments within the same watershed.
Stormwater management facilities designed to serve more than one property
or development in the same watershed are encouraged, in which case
consultation with the Township Engineer is required prior to design.
(5)
The Township Engineer may require stormwater retention
devices, basins, subsurface recharge facilities or other delayed-release
devices or designs to contain the excess storm discharge, prevent
downstream flow increases and encourage groundwater recharge.
B.
Storm sewer.
(1)
Storm sewers (pipes or other structures) shall be
reinforced concrete or HDPE pipe, have a minimum grade of 1/2% and
a minimum inside diameter of 18 inches or a cross-sectional area of
254 square inches. Roof drains or yard drains shall not connect to
any sanitary sewer and shall not discharge water directly over any
sidewalk. Storm drainage facilities should be located within the paved
areas of roadways whenever practical. Where storm sewers discharge
into existing drainage channels at an angle greater than 30° with
the downstream channel flow, the far side bank shall be stabilized
by the use of riprap, masonry, concrete walls and/or vegetative materials.
The stabilization shall be designed to prevent erosion and frost heave
under and behind the stabilizing media.
(2)
Storm sewer systems shall be constructed in accordance
with Township specifications, which are approved by resolution of
the Board of Supervisors. If a particular design or device is not
specifically approved within the Township specifications, the Township
Engineer must review the particular design or device for conformance
with the objectives of this section.
C.
Erosion and sedimentation control. Prior to the commencement
of any regulated earth disturbance activity, the following criteria
shall be met:
(1)
No changes shall be made in the contour of the land
and no grading, excavating, removal or destruction of the topsoil,
trees or other vegetative cover of the land shall be commenced until
such time as a plan for minimizing soil erosion and sedimentation
has been submitted and approved.
(2)
Measures used to control erosion and reduce sedimentation
shall meet or exceed the standards and specifications of the Conservation
District. The Township Engineer shall review the plan to ensure compliance
with the appropriate specifications, copies of which are available
from the Conservation District.
(3)
Plans shall preserve salient natural features, keep
cut/fill operations to a minimum and ensure conformity with natural
topography so as to create the least erosion potential and adequately
handle the volume and velocity of surface water runoff. Whenever feasible,
natural vegetation shall be retained, protected and supplemented.
The disturbed area and the duration of exposure shall be kept to a
practical minimum. Disturbed soils shall be stabilized as quickly
as practicable. Temporary vegetation and/or mulching shall be used
to protect critical areas during development.
(4)
The permanent (final) vegetation and mechanical erosion
control and drainage shall be installed as soon as practical. Provisions
shall be made to effectively accommodate the increased runoff caused
by changed soil and surface conditions during and after development.
Where necessary, the rate of surface water runoff will be mechanically
retarded. Sediment in all runoff water shall be trapped by the use
of debris basins, filters, sediment basins, silt traps or similar
measures until the disturbed area is fully stabilized. Such devices
shall be periodically cleaned of all silt and debris to ensure proper
operation.
(5)
During grading operations, necessary measures for
dust control must be exercised. Grading equipment will not be allowed
to cross flowing streams. Provision shall be made for the installation
of temporary culverts or bridges, including the procurement of appropriate
regulatory permits.
(6)
Permanent retention basins shall be provided when
required by the Board of Supervisors. The location of such basins
shall be approved by the Board of Supervisors upon advice from the
Township Engineer, subject to the approval of the DEP. Any requirement
for a permanent retention basin by the Board of Supervisors shall
not represent any liability on behalf of the Township.
(7)
No person, corporation or other entity shall block,
impede the flow of, alter, construct any structure or deposit any
material or thing or commit any act which will affect normal or flood
flow in any communal stream or watercourse without having obtained
prior approval from the Township and/or the Department of Environmental
Protection, whichever is applicable. It is the responsibility of any
person, corporation or other entity doing any act on or across a communal
stream, watercourse or swale or upon the floodplain or right-of-way
during the period of the activity to return it to its original or
equal condition after such activity is completed.
(8)
Whenever sedimentation is caused by stripping vegetation,
regrading or other development, it shall be the responsibility of
the person, corporation or other entity causing such sedimentation
to remove it from all adjoining surfaces, drainage systems and watercourses
and to repair any damage at his expense as quickly as possible.
(9)
Maintenance of all stormwater management facilities
and watercourses, both existing and proposed, within the proposed
site shall be the responsibility of the developer during construction
and any required maintenance guaranty period until such time as one
of the following is accomplished:
(a)
A right-of-way for these facilities is offered
for dedication by the developer and is accepted by the township, at
which time it shall then be the responsibility of the Township.
(b)
An easement acceptable to the Township is established.
In the case of a subdivision, the maintenance shall then be the responsibility
of the individual lot owners over whose property the easement passes.
For land developments, the maintenance shall thereafter be the responsibility
of the owner.
(c)
A homeowners' association, approved by the Township,
assumes responsibility for the maintenance of the development, including
the maintenance of the watercourses and/or drainage facilities, provided
that the Township Engineer approves the assumption of maintenance
of such facilities in writing.
(10)
The installation and design of the required
erosion and sediment control measures, listed below, shall be in accordance
with standards and specification found in the Erosion and Sediment
Pollution Control Program Manual available through the Conservation
District, Montgomery County Planning Commission or Township (see also
Township specifications) or the Pennsylvania Handbook of Best Management
Practices for Developing Areas.
(11)
Criteria contained in this chapter shall not
remove or alter the requirement for the developer to submit plans
and specifications to the Department of Environmental Protection under
the Clean Streams Act of 1972 where Department of Environmental Protection's
permits are required.
(12)
Three days prior to the commencement of any
operation in the construction or installation of any drainage facilities
or erosion and sedimentation control facility, the owner shall notify
the Township Engineer, who shall inspect the work, materials, construction
and installation for compliance with Township requirements. The Township
Engineer shall be authorized to have such tests of materials made
as are provided in the Skippack Township Specification and Design
Standards in order to have proper control of materials. Such fees
for tests of materials as provided above shall be in accordance with
the fee schedule as established by resolution of the Board of Supervisors.
(13)
Sediment basins, retention basins and detention
basins shall neither be located within nor discharge into a tree protection
zone.
A.
General.
(1)
Storm drainage required. A storm sewer system and
all appurtenances shall be required to be constructed by the owner
or subdivider in any area from which the surface or subsurface drainage
could impair public safety or cause physical damage to adjacent lands
or public property. The system shall be designed to collect water
at any point where three to five cubic feet per second is accumulated,
at the bottom of all vertical grades and immediately upgrade from
all street intersections. The system shall discharge to the nearest
practical natural drainage channel or storm system.
(2)
Off-site improvements. Any increase in runoff, if
permitted, may require necessary corrective measures, including deeds
of easement, deemed appropriate by the Board of Supervisors to alleviate
any off-site drainage problem affected by the earth disturbance activity.
The costs for such measures will be borne entirely by the developer
or subdivider.
(3)
Total watershed evaluation. The future use of undeveloped
areas upstream, as shown on the approved Township Comprehensive Plan,
shall be taken into account in the design of all storm sewer systems.
The capacity and maximum anticipated present flow of the body or system
receiving the proposed system(s) discharge shall be calculated to
verify its capability of receiving any additional flow caused by the
development. The runoff from any proposed development shall be subject
to an evaluation which includes the anticipated runoff from other
existing or proposed developments within the same watershed. Stormwater
management facilities designed to serve more than one property or
development in the same watershed are encouraged. Consultation with
the Township is required prior to the design of such facilities.
(4)
Contribution to Comprehensive Drainage Plan. In addition
to any other requirement of this chapter, the developer may be required
to participate in off-site storm drainage improvements in the drainage
basin within which the proposed development is located. The specific
off-site drainage improvements required shall be those specified by
the Township Board of Supervisors as part of the Skippack Township
Comprehensive Stormwater Drainage Plan.
B.
Underdrains.
(1)
All building foundations, grade slabs and cellar floors
located in soils that have a community development limitation degree
of moderate to severe seasonal high-water table, as defined by an
on-site soils test witnessed by the Township Engineer, shall be provided
with an underdrain system. This system shall provide for drainage
of the enclosed volume above the slab and relief of subsurface water
to a depth of not less than six inches below the slab or foundation
bottom. The system shall consist of a perforated pipe field of the
herringbone or gridiron configuration in coarse, gravel-filled trenches
that are in direct contact with the slab or foundation subbase. The
excavation shall provide a minimum of 0.005-foot/foot slope to the
gravel-filled trenches.
(2)
If required by the Township Engineer, proposed roadways
shall be constructed with an underdrain system consisting of six-inch
perforated pipe wrapped in a Class 1 geotextile material. The underdrain
shall be bedded with a minimum of six inches of 2B stone. The bedding
material shall extend to the bottom of the base course.
C.
Storm drain pipes and inlets.
(1)
All pipes and inlets shall meet material, capacity
and construction specifications as outlined in the latest revisions
of the Penn DOT Form 408 and the Penn DOT Design Manual or as outlined
in the township specification as approved by resolution of the Board
of Supervisors.
(2)
Design flow rate. The storm drain system shall be
designed to carry a fifty-year peak flow rate. The design fifty-year
peak flow rate into each inlet shall be indicated on the stormwater
drainage plan. The fifty-year flow rate shall be determined by the
rational formula as follows:
|
Q
|
=
|
CIA
| |
|
Where
| |||
|
Q
|
=
|
Peak runoff rate in cubic feet per second (cfs)
| |
|
C
|
=
|
Runoff coefficient equal to the ratio of the
peak runoff rate to the average rate of rainfall over a time period
equal to the time of concentration.
| |
|
I
|
=
|
Average rainfall intensity in inches per hour
for a time equal to the time of concentration.
| |
|
A
|
=
|
Drainage area in acres.
| |
|
NOTE:
|
Appropriate values for the runoff coefficient and rainfall intensity can be found in the Commonwealth of Pennsylvania Department of Transportation Design Manual, Part 2, Highway Design, Chapter 12.
| ||
(3)
Overflow system. An overflow system shall be provided
to carry flow to the detention basin when the capacity of the storm
drain pipe system is exceeded. The overflow system shall be of sufficient
capacity to carry the difference between the hundred-year and the
fifty-year peak flow rates.
(4)
Inlet capacity. All inlets must be designed to accommodate
the fifty-year peak flow rate. The capacity of all C-type, M-type
or S-type inlets shall be determined from the Commonwealth of Pennsylvania
Department of Transportation Design Manual, Part 2, Highway Design.
(5)
Straight pipe sections. Wherever possible, all storm
drain pipes shall be designed to follow straight courses. No angular
deflections of storm sewer pipe sections in excess of 5° shall
be permitted. No vertical curves shall be permitted in the storm drain
pipe system.
(6)
Minimum grade and size. All storm drain pipes shall
be reinforced concrete pipe designed to maintain a minimum grade of
1/2%. All storm pipes shall have a minimum inside diameter of 18 inches
or an elliptical equivalent, except that pipes under a fill of 25
feet or greater shall not be less than 24 inches or an elliptical
equivalent.
(7)
Pipe capacity. The capacity of all pipe culverts shall,
as a minimum, provide the required carrying capacity as determined
by the following sources:
(a)
United States Department of Commerce.
(b)
Bureau of Public Roads.
(c)
Hydraulic Engineering Circular No. 5.
(d)
Hydraulic Charts for the Selection of Highway
Culverts.
(e)
United States Department of Commerce.
(f)
Bureau of Public Roads.
(g)
Hydraulic Engineering Circular No. 10.
(h)
Capacity Charts for the Hydraulic Design of
Highway Culverts.
(8)
Pipe arches. Where headroom is restricted, alternative
culvert sections may be used in lieu of circular pipe, subject to
the approval of the Township Engineer.
(9)
Allowable headwater depth. At all inlets or manholes,
the maximum allowable headwater depth shall be one foot below the
top of the inlet grate or the manhole cover.
(10)
Horizontal pipe deflections. A manhole or inlet
shall be provided at all horizontal deflections in the storm pipe
system exceeding 5°.
(11)
Minimum and maximum cover. A minimum of three
feet of cover shall be maintained over all storm drain pipes. The
top of storm drain pipes shall be at least one foot below subgrade
elevation. The maximum cover over storm drain pipes shall be 10 feet
unless special conditions warrant otherwise, in which case structural
design calculations shall be submitted.
(12)
The elevation difference or invert of pipes
entering and exiting any drainage structure shall be a minimum of
0.2 feet.
(13)
Match crowns. The crowns of pipes of variable
size tying into an inlet or manhole shall be set at equal elevations.
(14)
Diversions of runoff. All storm drain pipes
shall be designed to carry the runoff into a detention basin or similar
facility utilized to control the rate of runoff.
D.
Runoff control.
(1)
The rate of stormwater runoff from any proposed subdivision
or land development after full development shall not exceed the peak
discharge prior to development. Where, in the judgment of the Township
Engineer, the quantity of stormwater runoff will cause detrimental
impact, a further reduction in quantity will be a consideration in
the method of stormwater regulations. This standard shall be maintained
for all storms, that is, both high frequency and low frequency.
(2)
Runoff control devices. The increased runoff which
may result from any earth disturbance activity shall be controlled
by permanent runoff control measures that will provide the required
runoff control specified above. All runoff control devices will be
evaluated for the effectiveness to maintain the above-mentioned standard
for all storms with a return period of up to 100 years.
E.
Detention basins.
(1)
The use of regional detention basins to combine and
eliminate numerous smaller basins is encouraged. Consultation with
the Township is required prior to design of a regional detention basin.
(2)
Design of detention basins. All detention basins shall
be designed as per the procedures developed by the United States Department
of Agriculture, Soil Conservation Service, as outlined in its Technical
Release No. 55, Urban Hydrology for Small Watersheds. The Township
Engineer may approve an alternative model.
(3)
Basin design criteria. The following design criteria
shall be used in the design of all detention basins and/or other stormwater
management facilities in the township. The basins shall be designed
to detain the quantity of water resulting from a one-hundred-year,
twenty-four-hour storm (see Soil Conservation Service Technical Release
No. 55) under full development conditions releases at a maximum outflow
rate no greater than the peak discharge prior to development. This
criteria shall be met for all frequency storms and must be analyzed
of the one-year, two-year, five-year, ten-year, twenty-five-year,
fifty-year and one-hundred-year storms. All flows in excess of the
above-mentioned standard shall flow over an emergency spillway.
(4)
Outflow rate control. A concrete structure shall be
provided at the outlet of all detention basins. The structure shall
extend to an elevation two feet below the crest elevation of the emergency
spillway and shall be designed so that the rate of outflow is controlled
through the basin berm when the depth of water within the basin exceeds
the height of the riser.
(5)
Maximum depth of detention basins. The maximum depth
of water in a detention basin shall not exceed four feet.
(6)
Emergency spillway. Whenever possible, the emergency
spillway for detention basins shall be constructed on undisturbed
ground. Emergency spillways shall be constructed of reinforced earth,
concrete pour blocks or other approved material. All emergency spillways
shall be constructed so that the detention basin berm is protected
against erosion. The minimum capacity of all emergency spillways shall
be equal to the peak flow rate from the one-hundred-year design storm.
The dimensions of the emergency spillways can be determined from the
DEP's Erosion and Sediment Pollution Control Program Manual. Emergency
spillways shall extend along the upstream and downstream berm embankment
slopes. The upstream edge of the emergency spillway shall be a minimum
of three feet below the spillway crest elevation. The downstream slope
of the spillway shall, as a minimum, extend to the toe of the berm
embankment. The emergency spillway shall not discharge over earthen
fill and/or easily erodible material.
(7)
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 antiseep collars shall
extend a minimum of two feet beyond the outside of the principal pipe
barrel. The maximum spacing between collars shall be 14 times the
minimum projection of the collar measured perpendicular to the pipe.
The use of an O-ring pipe is required for all detention discharge
pipes.
(8)
Freeboard. Freeboard is the difference between the
one-hundred-year design flow elevation in the emergency spillway and
the top of the settled detention basin embankment. The minimum freeboard
shall be one foot.
(9)
Slope of detention basin embankment. The maximum slope
of earthen detention basin embankments shall be 3 to 1. The top or
toe of any slope shall be located a minimum of five 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.
(10)
Width of berm. The minimum top width of detention
basin berms shall be 10 feet.
(11)
Slope of basin bottom. In order to ensure proper
drainage of the detention basin, and unless designed as a naturalized
basin providing BMPs, a minimum grade of 2% shall be maintained for
all sheet flow. A minimum grade of 1/2% shall be maintained for all
channel flow.
(12)
Low flow channel. Low flow channels shall be
constructed on the basin floor extending from all inflow structures
to the outflow structure. The low flow channel shall be constructed
of a minimum of four inches of concrete. The center of the low flow
channel shall be lower than the outside edges by six inches, thus
creating a swale to carry the flow to the outflow structure, causing
little or no erosion of the basin floor. A footer on both sides of
the channel shall be placed to a minimum depth of six inches where
subsurface conditions warrant as approved by Township Engineer.
(13)
Energy dissipaters. Energy dissipating devices
(riprap, end sills, etc.) shall be placed at all basin outlets. Forebays
or split flow controls are encouraged at the inlet to all basins.
(14)
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.
F.
Wet ponds and other facilities designed to retain
water year round.
(1)
Wet ponds and other facilities shall meet all design standards outlined in § 172-22E for detention basins.
(2)
The minimum permanent pool level shall be maintained
to allow sufficient depth throughout the year to reduce the growth
of unwanted vegetation and mosquitoes.
(3)
The pond shall be of sufficient size to allow the
appropriate aquatic community needed to maintain a healthy pond ecology.
(4)
An outlet structure shall be designed to allow complete
drainage of the pond for maintenance purposes.
(5)
The design of a detention facility with a permanent
pool shall include a determination of the proposed site's ability
to support a viable permanent pool. The design should account for
such factors as the required rate and quality of dry weather inflow,
the quality of stormwater inflow, seasonal and longer-term variations
in groundwater table and effects of expected pollutant loadings. The
Pennsylvania Fish Commission and the Conservation District should
be consulted during the design of these facilities.
G.
Stormwater wetland basins.
(1)
Stormwater wetland basins shall meet all design standards outlined in § 172-22E for detention basins.
(2)
Wetland areas within stormwater wetland basins shall
be designed in accordance with the Handbook of Constructed Wetlands,
Volumes 1 and 5, United States Department of Agriculture, 1995, as
may be revised.
H.
Naturalized detention basins.
(1)
Naturalized detention basins shall meet all design standards outlined in § 172-22E for detention basins.
(2)
The detention basin shall be very free-form and curvilinear
in shape so that from most edges of the basin, the whole basin will
not be within view. When the natural topography lends itself to a
straight-sided detention basin, the basin shall contain an island
or peninsula planted with trees which is large enough and appropriately
placed to reduce the perceived size of the detention basin. Such basins
shall have a perimeter length that is greater than or equal to the
length derived by the following formula, where ""x" equals the area
of the basin in square feet:
 | |
|---|---|
|
As an alternative, naturalized detention basins
may be created by constructing berms or using gabions to dam up stormwater
in a natural depression or valley, provided that the topography and
natural vegetation in the remainder of the basin is left in its natural
condition.
|
(3)
Naturalized planting materials, such as wildflowers
and nonaggressive meadow grasses, which tolerate wet, intermittently
wet and usually dry areas of the basin, shall be planted throughout
the basin. Trees and shrubs may be planted in the basin where they
will not interfere with the function of the basin.
I.
Culverts and drainage channels.
(1)
Design flow standard. All culverts and drainage channels
shall be designed to carry a flow rate equal to a fifty-year, twenty-four-hour
storm, except for culverts in streets which shall be designed to carry
a flow rate equal to a one-hundred-year, twenty-four-hour storm (Soil
Conservation Service, Technical Release No. 55).
(2)
Erosion prevention. All drainage 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 the maximum
permissible velocity to prevent erosion. Suitable bank stabilization
shall be provided where required to prevent erosion of the drainage
channels. Where storm sewers discharge into existing drainage channels
at an angle greater than 30° from parallel to the downstream channel
flow, the far-side bank shall be stabilized by the use of riprap,
masonry, concrete walls and/or vegetation. The stabilization shall
be designed to prevent erosion and frost heave under and behind the
stabilizing media.
(3)
Maximum side slope. Any vegetated drainage channel
requiring mowing of the vegetation shall have a maximum grade of three
horizontal to one vertical on those areas to be mowed.
(4)
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 Erosion and Sediment Pollution Control Program Manual. Several
acceptable sources outline procedures for nonvegetated drainage channels,
including the following:
J.
Groundwater recharge.
(1)
General. The difference between the two-year post-development
volume and the one-year pre-development volume of runoff must be infiltrated
for all subwatersheds at the site.
(2)
Subsurface recharge systems.
(a)
Soil investigation. In designing a subsurface
stormwater system, a key element is an analysis of the soil. The Township
Engineer shall require, witness and receive a certification from an
experienced soils scientist in accordance with commonly accepted professional
standards. Said certification shall include:
(b)
Permeability. A minimum infiltration rate of
0.27 inches per hour shall be required for all subsurface systems.
(c)
Infiltration capacity.
[1]
For any earth disturbance activity of less than
one acre, infiltration rates shall be determined from field investigation,
permeability testing, percolation testing or empirically from the
following table:
|
Hydraulic Soil Properties Classified by
Soil Textures*
| |||
|---|---|---|---|
|
Texture Class
|
Minimum Infiltration Rate (f)
(inches per hour)
|
Hydrologic Soil Groupings
| |
|
Sand
|
8.27
|
A
| |
|
Loamy sand
|
2.41
|
A
| |
|
Sandy loam
|
1.02
|
B
| |
|
Loam
|
.52
|
B
| |
|
Silt loam
|
.27
|
C
| |
|
Sandy clay loam
|
.17
|
C
| |
|
Clay loam
|
.09
|
D
| |
|
Silty clay loam
|
.06
|
D
| |
|
Sandy clay
|
.05
|
D
| |
|
Silty clay
|
.04
|
D
| |
|
Clay
|
.02
|
D
| |
|
NOTE:
| ||
|---|---|---|
|
*
|
Source: Rawls, W.J. Brakensiek, D.L., and Saxton,
K.E. Estimation of Soil Properties. Transactions of the American Society
of Agricultural Engineers. Volume 25, No. J, pages 1316-1320, 1982.
| |
[2]
For any earth disturbance activity over one
acre, infiltration capacity shall be determined by an infiltration
test.
(d)
Infiltration rate. Percolation tests shall be
accepted as a means of determining the infiltration rate of a soil.
The tests must be completed at the location and depth of the proposed
infiltration area. The percolation rate may be converted to infiltration
rate in inches per hour as follows:
|
Infiltration rate (inches per hour) = 60 ÷
(percolation rate, minutes per inch)
|
(f)
Sediment and debris. Various measures shall
be taken to guarantee that the subsurface recharge system does not
become filled with sediment during and after construction and that
the system is kept free of debris (leaves, twigs, etc.) to prevent
clogging. These measures include, but are not limited to, temporary
siltation berms and other erosion controls and gutter screens for
direct piping from roofs to the system.
(g)
An inspection/testing port shall be installed
by the developer to ensure that the postconstruction permeability
rate is preserved to the pre-development permeability testing rate.
(h)
Storage requirements.
[1]
Except in cases where infiltration is utilized
and designed as a separate system from the primary detention facility,
the amount of storage shall be designed to adequately store the quantity
of water resulting from the difference, measured in cubic feet, of
a one-hundred-year, twenty-four-hour post-development storm and a
one-hundred-year, twenty-four-hour pre-development storm.
[2]
The void rate of the aggregate shall be a maximum
of 40%.
(i)
Aggregate/filter material. Aggregate fill shall
be clean stone between two inches and four inches in diameter. Larger
stone shall be placed near the bottom of the system and smaller stone
near the top. An appropriate geotextile fabric shall line the bottom
and sides, with sufficient excess material left at the top to fold
over the aggregate fill when placed, prior to the placement of the
topsoil layer.
(j)
Prefabricated systems. When prefabricated systems
are proposed, the applicant shall adhere to all recommended manufacturer's
specifications, in addition to the requirements of this section.
(k)
Emergency overflow. Each subsurface recharge
system shall be designed with an emergency overflow/surcharge system
to protect the system in the event of a storm of higher intensity.
(l)
General note for all subsurface recharge systems.
The following note shall be placed on all plans which propose a subsurface
recharge system: "The subsurface recharge facilities shown on these
plans are a basic and perpetual part of the stormwater management
system of the proposed site located in Skippack Township, Montgomery
County, Commonwealth of Pennsylvania, and as such are to be protected
and preserved in accordance with the approved final plans by the owners,
their successors and assigns of these lands. Skippack Township and/or
its agents reserve the right and privilege to enter upon these lands
from time to time for the inspection of said facilities in order to
determine that the structural and design integrity is being maintained
by the owner."
A.
Cuts. No excavation shall be made with a cut face
steeper than three horizontal to one vertical, except under the condition
that the material in which the excavation is made is sufficiently
stable to sustain a slope of steeper than three horizontal to one
vertical. A written statement to that effect is required from a civil
engineer licensed by the Commonwealth of Pennsylvania having experience
in soils engineering and must be submitted to the Township Engineer
and approved by him. The statement shall affirm that the site has
been inspected and that the deviation from the slope should not result
in injury to persons or damage to property. Retaining walls will be
required if a stable slope cannot be maintained. Any retaining wall
design must be approved by the Township Engineer. The toe of the slope
or headwall of any cut must be located a minimum of five feet from
property lines.
B.
Fills. No fill shall be made which creates any exposed
surface steeper in slope than three horizontal to one vertical, except
where the fill is located so that settlement, sliding or erosion will
not result in property damage or be a hazard to adjoining property,
streets or buildings. A written statement is required from a civil
engineer licensed by the Commonwealth of Pennsylvania having experience
in soils engineering certifying that he has inspected the site and
that any proposed deviation from the slope specified above should
not endanger any property or result in property damage, and such statement
must be submitted to and approved by the Township Engineer. A concrete
or stone masonry wall designed and constructed in accordance with
these specifications and standards may be required to support the
face of the fill where the above specified slopes are exceeded. The
top of any fill or toe of the slope of any fill shall be located five
feet from any property line.
C.
Retaining walls. All retaining walls shall be designed
as per the procedures outlined in the American Association of State
Highway Officials, Standard Specifications for Highway Bridges, 1973.
D.
Landscaping.
(1)
Grassed areas or grass/ground cover combinations.
All such areas specified on proposed or approved plans shall be prepared,
installed and maintained in accordance with the Pennsylvania Department
of Transportation Form 408 specifications, as amended.
(2)
Drainage channels and retention areas. All storm drainage
channels and retention areas, whether existing or proposed, shall
be graded and planted to effectively naturalize area(s) so as to become
an integral and harmonious part of the landscape by contour and type
of plant materials used.
(3)
All detention basin side slopes and bottoms shall
be seeded with an appropriate nonmowable seed mixture specifically
suited to the levels of periodic inundation for which the facility
is designed.
(4)
A suitable, naturalized, native plant screening shall
be provided around all detention basins. Fences may be required at
the discretion of the Board of Supervisors. Native Plants acceptable
for use are listed as part of the Recommended Native Plants for Landscaping
in Pennsylvania, as issued by the Pennsylvania Department of Conservation
and Natural Resources. The use of native grasses and wetland tolerant
plants is highly encouraged.
(5)
Trees and shrubs shall be placed in informal irregular
groupings so as to provide a suitable visual barrier at the time of
planting. Trees may be placed in the basin bottom, subject to the
approval of the Township Engineer and Township Planner. Whatever materials
are used, they shall be placed sufficiently far back from the cut
or fill slope so as to allow ease in mowing.
(6)
(7)
The screening requirement shall be waived only with
the expressed consent of the Board of Supervisors.
E.
Topsoil. A minimum of six inches of topsoil material
shall be placed on all areas affected by the basin construction (bottom
of basin, side slopes, top of berm, etc.). The material must meet
the requirements of the Pennsylvania Department of Transportation
Form 408 specifications, as amended.
F.
Subsurface recharge system. In order to preserve the
integrity of the system, at no time shall any trees be planted in
or around a subsurface recharge system. As a general rule, the dripline
of a mature tree should not be located within five feet of a subsurface
recharge system.
G.
Grading design criteria.
(1)
Fill placement and compaction. All fill shall be placed
mechanically and shall be free of any debris or organic material,
roots and stumps. Fill shall be placed in consecutive, maximum eight-inch
layers and compressed or compacted mechanically with equipment weighing
not less than 10 tons or with similar sheepsfoot roller or compactors
having equivalent compression capability. The township may require,
at the owner's expense, a test certifying the adequate compaction
of fill materials as required above or in accordance with the requirements
of the Township Engineer if an on-site inspection was not performed
at the time of placement of the fill.
(2)
Large-scale removal of topsoil. Permanent removal
of topsoil shall be prohibited. The only exception to this prohibition
shall be under the circumstance where design of a large-scale development
permits the removal of excess topsoil in locations of large parking
areas or large buildings where the topsoil removed may be more appropriately
used elsewhere on the site or on a nearby site. A special permit for
such topsoil removal is required, as issued by the Board of Township
Commissioners.
H.
Tree protection standards.
(1)
General requirements shall be as follows:
(a)
Grade changes and excavations on any location
within the site shall not:
(b)
No toxic materials shall be stored within 100
feet of a tree protection zone, including petroleum-based and/or -derived
products.
(c)
The area within a tree protection zone shall
not be built upon or covered by impervious materials, either temporarily
or permanently.
(d)
The storage of equipment, materials, debris
or fill and the parking of vehicles shall be prohibited within a tree
protection zone.
(2)
Tree protection zone construction requirements shall
be as follows:
(a)
Prior to construction, any tree protection zone
shall be identified on the site in conformance with the approved subdivision
or land development plan and in the following manner:
[1]
All trees scheduled to remain shall be clearly
marked. Only trees on the outer edge of tree groups or forests are
required to be marked.
[2]
A four-foot-high wooden snowfence mounted on
steel posts located eight feet on center, or similar fencing materials
or barrier approved by the Township Arborist, shall be placed along
the boundary of the tree protection zone. Immediately following the
installation of said fencing, the Township Arborist shall inspect
and approve the same prior to the commencement of any development
activity on the site. The fencing shall be properly maintained until
all development activity has ceased, and damaged fencing shall be
repaired or replaced before any development may recommence.
[3]
In addition to such fencing, trees outside the
tree protection zone may be temporarily retained to the limits of
grading and utilized as an additional buffer area during construction.
Such trees shall be removed prior to the completion of development.
(b)
The operation of heavy equipment shall not occur
within a tree protection zone.
(c)
Trees being removed shall not be felled, pushed
or pulled into a tree protection zone or into any other trees which
are required to be retained.
(d)
Any tree stumps located within (ten) 10 feet
of a tree protection zone shall be removed by means of a stump grinder
to minimize their effect on surrounding root systems.
(e)
Tree roots which must be severed shall be:
[1]
Cut by a backhoe or similar equipment, aligned
radially to the tree, in such a manner so as to reduce the lateral
movement of roots during excavation.
[2]
Trimmed cleanly so that their edges are smooth
and are cut back to a lateral root, if exposed.
[3]
Covered temporarily with peat moss, moist burlap
or other moist biodegradable material within four hours of any severance
in order to avoid their drying out.
(f)
Damaged areas shall be treated promptly and
as may be required by the Township Arborist.
[1]
Damaged bark shall be cut back to a point where
the bark is intact and tight to the tree.
[2]
Exposed roots shall be cleaned up and covered
with topsoil.
[3]
Tree limbs shall be cut back in proportion to
root area loss.
[4]
Liquid or dry fertilizer shall be applied to
trees with disturbed root zones to compensate for any loss of roots.
[5]
All pruning shall be undertaken in accordance
with the standards of the National Arborist Association, as noted
in the National Arborist Association Book of Standards, as amended.
[6]
Trees shall not be used for roping, cables,
signs, fencing or lighting. Nails and spikes shall not be driven into
trees.
(g)
Fertilization.
[1]
All trees which have been disturbed or have
tree roots which have been damaged as a result of development shall
be fertilized.
[2]
Such trees shall be fertilized during the months
of September through October or April through May.
[3]
Fertilizer having approximately three parts
nitrogen to one parts phosphorus and potassium (3 to 1 to 1 ratio)
shall be broadcast over the soil surface in an area twice the size
of the tree protection zone at a rate equivalent to one pound nitrogen
per 1,000 square feet. A minimum of 1,000 square feet per tree shall
receive fertilization.
(h)
Retaining walls.
[1]
Any regrading which causes a change in the original
grade at the tree protection zone line shall require the installation
of a retaining wall outside of the tree protection zone, designed
to comply with township standards.
[2]
The top of such retaining wall shall be four
inches above the finished grade line.
[3]
The retaining wall shall be constructed of large
stones, brick, building tile, concrete blocks or treated woodbeams.
[4]
A means for drainage through the wall shall
be provided, as approved by the Township Engineer.
[5]
A layer of clean stone, sized 3/4 to one inch,
shall be placed one foot out from the retaining wall to aid in drainage.
(i)
Trenching and tunneling.
[1]
Utility lines shall be located outside tree
protection zones.
[2]
Tunneling shall be used for the installation
of any utility line approved by the Board of Supervisors for location
within a tree protection zone. Trenching may only be used where, in
the opinion of the Township Arborist, the survival of a tree or trees
would not be affected.
[3]
Upon the completion of the installation of any
utility line, the trench shall be filled immediately and tamped lightly
to avoid air spaces.
A.
In conjunction with the review of the BMP operations
and maintenance plan, the adequacy of existing or proposed off-site
stormwater drainage systems accommodating runoff from the development
site shall be determined. A contribution shall be paid towards that
portion of the cost for any off-site capital improvements and/or maintenance
costs of the existing stormwater drainage systems which may be necessitated
by the proposed earth disturbance activity or which may confer benefit
upon the proposed development site.
B.
Off-site stormwater drainage contributions shall be
as set forth in the fee schedule of Skippack Township.
C.
The off-site stormwater drainage contribution shall
be in addition to the obligation of the developer to construct and
install at its sole expense the on-site and off-site drainage improvements
required as a condition to subdivision or land development approval.
D.
When, in the discretion of the Board of Supervisors,
circumstances surrounding the development of a site warrant special
consideration, these contributions may be reduced or waived.
A.
The BMP operations and maintenance plan for the project
site shall establish responsibilities for the continuing operation
and maintenance of all permanent stormwater BMPs, as follows:
(1)
If a plan includes structures or lots which are to
be separately owned and in which streets, sewers and other public
improvements are to be dedicated to the Township, stormwater BMPs
may also be dedicated to, and maintained by, the Township;
(2)
If a Plan includes operation and maintenance by a
single ownership or if sewers and other public improvements are to
be privately owned and maintained, then the ownership and maintenance
of stormwater BMPs shall be the responsibility of the owner or private
management entity.
B.
The Township shall make the final determination on
the continuing operations and maintenance responsibilities. The Township
reserves the right to accept or reject the operations and maintenance
responsibility for any or all of the stormwater BMPs.
A.
The property owner shall sign an operations and maintenance
agreement with the Township covering all stormwater facilities and
BMPs that are to be privately owned. The agreement shall be substantially
the same as the agreement in Appendix B of this chapter.[1]
[1]
Editor's Note: Appendix B is at the end of
this chapter.
B.
Other items may be included in the agreement where
determined necessary to guarantee the satisfactory operation and maintenance
of all permanent stormwater facilities and BMPs. The agreement shall
be subject to the review and approval of the Township.
A.
Where stormwater or surface water will be gathered
within a proposed earth disturbance activity site and discharged or
drained in volume over lands within or beyond the boundaries of the
subdivision or land development, the applicant or owner shall reserve
or obtain easements over all lands affected thereby, provided that
no easement or right-of-way for drainage purposes exists on such affected
land. Such easements shall be adequate for such discharge of drainage
and for the carrying off of such water and for the maintenance, repair
and reconstruction of the same, including the right of passage over
and upon the same by vehicles, machinery and other equipment for such
purposes, and which shall be of sufficient width for such passage
and work and that would provide ingress to and egress from a public
right-of-way. The width of the easement shall be determined by the
Township Engineer. The owner shall convey, free of charge or cost,
such easements to the Township upon demand.
B.
Stormwater management easements shall be provided by the property owner if necessary for access for inspections and maintenance or preservation of stormwater runoff conveyance, infiltration and detention areas and other BMPs by persons other than the property owner. The purpose of the easement shall be specified in any agreement under § 172-26.
C.
All easements shall be fully described with metes
and bounds on the Plan.
