A.
Measures used to collect and carry stormwater on any site shall be
designed to meet the following minimum performance standards.
(1)
Prevent erosion damage and satisfactorily carry-off or detain and
control the rate of release of surface waters.
(2)
When subsurface soil conditions warrant, encourage runoff control
measures to percolate the stormwater into the ground to aid in the
recharge of groundwaters.
(3)
Carry surface water to the nearest adequate street, storm drain,
detention basin, natural watercourse, or drainage facility.
(4)
Take surface water from the bottom of vertical grades, to lead water
away from springs, and collect water upgrade of all street intersections.
(5)
Control/accommodate not only the anticipated peak discharge from
the on-site disturbed area, but also the existing runoff being contributed
from all land at a higher elevation in the same watershed.
(6)
Maintain the adequacy of the natural stream channels. Accelerated
bank erosion shall be prevented by controlling the rate and velocity
of runoff discharged to these watercourses, so as to avoid increasing
the occurrence of stream bank overflow.
(7)
Preserve the adequacy of existing culverts, and bridges by suppressing
the new flood peaks created by the new earth disturbances.
(8)
If in the course of reviewing the stormwater management plan, the
Township Engineer determines that off-site improvements are necessary
to satisfactorily control the stormwater from the site, the applicant
shall be responsible for such off-site improvements.
(9)
All stormwater detention/retention facilities shall be in place and
functioning prior to the creation of any impervious surface.
(10)
Whenever a watercourse, stream, or intermittent stream is located
within a grading site, it shall remain open in its natural state and
location and shall not be piped unless permitted by Pennsylvania Department
of Environmental Protection (DEP) and the Township Board of Supervisors.
(11)
The existing points of natural drainage discharge onto adjacent
property shall not be altered without the written approval/drainage
easement from the affected landowners.
(12)
No stormwater runoff or natural drainage 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.
(13)
All regulated earth disturbance activities are required to have
water quality protection measures after completion of the earth disturbance
activities and include operation and maintenance of the BMP.
A.
In addition to permit application requirements in § 58-17 of the Whitemarsh Township Code, a minor earth disturbance plan and a BMP operations and maintenance plan required under these regulations shall contain the following:
(1)
Mapping of the watershed area or areas in which the proposed earth
disturbance is located. (U.S.G.S. Quadrangle map or similar). All
drainage subareas analyzed in conjunction with the hydrologic study
shall be clearly delineated on the plan. Additionally, the assumed
flow path along the hydraulic length of each subarea used in the subarea
time of concentration calculations shall be indicated on the plans.
(2)
A study shall be performed of the watershed in which the earth disturbance
is located to assess the impact that the proposal will have on downstream
conditions. Stormwater management plans shall address all identified
impacts to the satisfaction of the Township Engineer.
(3)
Computations of the stormwater runoff for all points of runoff concentration
before, during, and after earth disturbance, including all supporting
data.
(4)
Complete drainage systems for the site. All existing drainage facilities
which are to be incorporated in the design shall be identified with
an explanation of the operations of the facilities.
(5)
Plans showing all existing and proposed drainage facilities affecting
the subject property.
(6)
Plan of the proposed stormwater drainage systems including storm
drain pipes and inlets, runoff control devices, and drainage channels.
(7)
Plan of the existing and proposed contours on vertical intervals
of not more than two feet.
(8)
Design computations for all BMPs and the sizing of the outlet control
structure.
(9)
A stage storage curve for each detention/retention basin.
(10)
Multistage flood routing model and/or storage requirement calculations.
(11)
A plan showing the stormwater basin berm embankment and outlet
structure. The plan shall also indicate the top of berm elevation,
top width of berm and side slopes, emergency spillway elevation, elevations
of the outlet structure including the riser, dimensions and spacing
of antiseep collars.
(12)
A cross-section through the outlet structure, emergency spillway,
and the berm embankment.
(13)
A detailed plan of the trash rack and antivortex device if applicable.
(14)
An overall plan of the basin area at a scale of one inch equals
50 feet minimum showing the grading and landscaping.
(15)
A detailed plan of all required off-site improvements.
(16)
Location and legal descriptions of rights-of-way, easements
or lands offered for dedication.
(17)
Inlet drainage area map. The extent of each inlet drainage area
shall be outlined on a map of highest order available and submitted
in duplicate to the Township Engineer with the design calculations
for storm sewers. The plans shall also indicate the stormwater runoff
anticipated at each proposed inlet and the slope of the street or
swale used to compute the inlet capacity. Care should be taken to
assure that all areas delivering runoff to the point under consideration
shall be included.
(18)
All pertinent details deemed necessary by the Township Engineer
for the proper construction of all erosion and sedimentation controls,
stormwater management, storm sewer, and BMP facilities.
A.
Stormwater management systems shall meet the following minimum design/construction
standards:
(1)
All earth disturbance activities shall limit the rate of stormwater runoff in accordance with the requirements of § 58-15.
(2)
The increased runoff which may result from earth disturbance activities
shall be controlled by permanent runoff control measures. All runoff
control measures shall be evaluated for their effectiveness to maintain
the above standards for all storms for the term period of up to 100
years.
(3)
All plans and designs for stormwater management systems and facilities
submitted to the Township for approval shall determine stormwater
peak discharge and runoff by use of the Soil Cover Complex Method
as set forth in the U.S. Department of Agriculture, Soil Conservation
Service Publication entitled "Urban Hydrology for Small Watersheds,"
Technical Release #55 (latest edition), with specific attention given
to antecedent moisture conditions, flood routing, and peak discharge
specifications included therein, and in the Hydrology National Engineering
Handbook, Section 4, both U.S. Department of Agriculture, Soil Conservation
Service.
(4)
The Township Engineer may permit the use of alternative methods for
calculation of runoff on earth disturbance sites of five acres or
less, and for the design of storm sewers.
B.
Stormwater shall not be transferred from one watershed to another
unless:
C.
All stormwater collection pipes, inlets, and swales shall be designed
to handle the peak flow rate for the 100-year storm. An inlet/outlet
control evaluation shall be utilized to determine all pipe sizes in
accordance with the method outlined in Hydraulic Design of Highway
Culverts, Federal Highway Administration, (latest edition).
D.
Storm duration.
(1)
A five-minute storm duration shall be used if this duration does
not result in a maximum expected discharge that exceeds the capacity
of a thirty-inch pipe.
(2)
If a five-minute storm duration results in a pipe size exceeding
30 inches, the time of concentration approach shall be used in determining
storm duration.
(3)
If a five-minute storm duration results in a pipe size exceeding
30 inches, within any run of pipe, the time of concentration approach
may be used for sizing of pipes from that point on by adjusting the
time of concentration.
E.
Limestone areas.
(1)
Whenever an infiltration BMP or a stormwater basin is suspected of
being located in an area underlain by limestone, a geological evaluation
of the proposed location shall be conducted by a registered professional
engineer experienced in geotechnical and soil engineering to determine
susceptibility to sinkhole formations.
(2)
The design of all BMPs over limestone formations shall include measures
to prevent groundwater contamination and, where necessary, sinkhole
formation.
(3)
It shall be the applicant's responsibility to verify whether
the site is underlain by limestone.
(4)
The following note shall be attached to all drainage plans and signed
and sealed by the applicant's registered professional engineer
experienced in geotechnical and soil engineering if a detention facility
or infiltration BMP is proposed: "I certify that the proposed facility
is/is not underlain by limestone."
A.
Stormwater detention/retention facilities shall meet the following
minimum design/construction standards:
(1)
Detention basin shall be designed to facilitate regular maintenance
mowing and periodic silt removal and reseeding. In residential subdivisions
and residential development earth disturbances, shallow broad basins
are preferred to steep sided basins.
(2)
The maximum slope of the earth and detention basin embankment shall
be three-to-one. The top or toe of any slope shall be located a minimum
of five feet from a property line. Whenever possible the side slope
and basin shape shall conform to the natural topography.
(3)
Unless permitted as a special exception by the Zoning Hearing Board, detention basins shall not be located within the Township's Floodplain Conservation or Riparian Corridor Conservation Districts as provided under the Whitemarsh Township Zoning Ordinance. Additionally, the location of detention/retention basins within any front, side or rear yard area shall be subject to the provisions of § 116-31.1 of the Whitemarsh Township Zoning Ordinance.
(4)
Detention basins shall be designed so that they provide for a twenty-four-hour
extended detention of the one-year, twenty-four-hour storm event (i.e.,
the stormwater runoff will be released over a minimum 24 hours for
the one-year, twenty-four-hour storm event).
(5)
If retention basins are used, the applicant shall demonstrate that
such ponds are designed to protect public health, safety and welfare.
(6)
The minimum top width of the retention or detention basin berm shall
be 10 feet. A cutoff trench (keyway) of relative impervious material
shall be provided beneath all embankments requiring fill material.
The keyway shall be a minimum eight feet wide, one foot deep, and
have 1:1 side slopes.
(7)
All detention/retention basin embankments shall be placed in eight-inch
maximum lifts to a minimum 95% dry density. Prior to proceeding to
the next lift, compaction shall be verified by a registered professional
engineer experienced in geotechnical engineering and acceptable to
the Township Engineer. Compaction tests shall be performed using the
Modified Proctor Method in accordance with ASTM 0-1557. Compaction
tests shall be run on the leading and trailing edge, as well as the
top of the berm.
(8)
Emergency overflow facilities shall be provided for detention facilities
to accommodate runoff in excess of design flows. Whenever possible,
emergency spillways for detention basins shall be constructed on undisturbed
ground. Emergency spillways shall be constructed of concrete pavers
or other materials approved by the Township Engineer. 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 the peak flow rate of the 100-year design storm after development.
The construction material of the emergency spillway 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 or easily erodible
material.
(9)
The minimum freeboard shall be one foot. The freeboard is the difference
between the design flow elevation in the emergency spillway and the
top of the compacted detention basin embankment.
(10)
Antiseep collars shall be poured-in-place and installed around
the pipe barrel within the normal saturation zone of the detention
basin berms. The antiseep collars and their connections to the pipe
barrels shall be watertight. The antiseep collars shall extend a minimum
of two feet beyond the outside of the outlet pipe. The maximum spacing
between collars shall be 14 times the minimum projection of the collar
measured perpendicular to the pipe. A minimum of two antiseep collars
shall be installed on each outlet pipe.
(11)
All outlet pipes through the basin berm shall be reinforced
concrete pipe, designed to withstand the loading caused by a fully
saturated berm and shall have watertight joints using zero-ring joint
pipe.
(12)
Energy dissipaters and/or level spreaders shall be installed
at points where pipes or drainage ways drain to or from the basin.
Energy dissipaters shall comply with criteria in the Erosion and Sediment
Control Program Manual, published by the Pennsylvania Department of
Environmental Protection or the Engineering Field Manual for Conservation
Practices, SCS. Energy dissipating device calculations shall be submitted
for Township review/approval.
(13)
Inlet and outlet structures shall be located at a maximum distance
from one another in order to promote water quality benefits. The Township
Engineer may require a rock filter or rock-filled gabion for entrapping
sediments carried in stormwater if sufficient separation of inlet
and outlet structures cannot be achieved.
(14)
A perforated riser, skimmer or similar sediment control device
shall be provided at each outlet of all detention basins during construction
for sediment control. The design shall conform to the Erosion and
Sediment Pollution Control Manual, as published by the Department
of Environmental Protection.
(15)
All drainage channels shall be designed to prevent erosion of
the bed and banks. The maximum permissible flow velocity shall not
exceed those outlined in Table 1 to these regulations. Suitable stabilization
shall be provided where required to prevent erosion of the drainage
channels.
(16)
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.
(17)
Because of the critical nature of vegetated drainage channels,
the design of all vegetated channels shall, as a minimum, conform
to the design requirements outlined in the current "Erosion and Sediment
Pollution Control Program Manual," published by the Pennsylvania Department
of Environmental Protection.
(18)
Landscaping.
(a)
Disturbed areas resulting from the detention/retention basin
construction shall be planted with cover vegetation such as grass,
native grasses and appropriate shrubs and trees. The choice of cover
vegetation must be approved by the Township and will be based upon
the intended use of the basin, maintenance requirements, structural
integrity of the berm area and conformity with surrounding landscaping.
All seeding shall be applied by hydroseeding disturbed areas, unless
otherwise approved by the Township Engineer.
(b)
Specialized plantings suitable for wet conditions should be
planted in portions of the basin which retain water or which contain
soils under saturated conditions.
(c)
Landscaping design should reduce overall maintenance requirements
for the basin. The use of native grass and shrubs are encouraged.
(d)
Stormwater management facilities shall be appropriately screened
in a manner which compliments the existing landscape and provides
sufficient access for maintenance. The Township Engineer may waive
this requirement if existing vegetation is adequate to ensure complete
perimeter screening.
(e)
Plantings shall consist of clusters of evergreens interspersed
with groupings of deciduous trees and shrubs. Single species planting
is prohibited. Where stormwater management facilities adjoin wooded
areas, trees and shrubs shall be selected and planted so as to blend
with existing surroundings.
(f)
Plants that attract wildlife are encouraged. Landscape designs
should not encourage burrowing animals.
(g)
No woody plant material or trees shall be located on a constructed
or natural berm acting as the impoundment structure of a detention/retention
basin. Trees shall be located on the downstream side of an impoundment
berm a sufficient distance from the toe of the constructed slope to
assure that the toe of the slope is outside the drip line of the mature
species planted.
(h)
In order to reduce runoff and erosion of existing topsoil, trees
and shrubs should be preserved within, and surrounding, the stormwater
basin, excluding the earth berm.
(i)
Vegetation shall be planted during appropriate times of the
year, predominantly between late March and mid-May or from early October
until evidence of ground freezing, depending upon the species selected.
Most deciduous trees and shrubs can be planted in either spring or
fall. Evergreens are best planted in late summer or early fall.
(j)
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.
A.
General.
(1)
Storm sewers, culverts, and related installations shall be provided:
(a)
To permit unimpeded flow of natural watercourses and in such
a manner as to protect the natural character of the watercourses and
to provide regulated discharge;
(b)
To ensure adequate drainage of all low points along the line
of streets; and
(c)
To intercept stormwater runoff along streets at intervals reasonably
related to the extent and grade of the area drained and to prevent
substantial flow of water across intersections.
(2)
All storm sewer system components shall conform to current PennDOT
standards.
(3)
Proposed drainage structures, which drain watershed areas in excess
of 1/2 square mile (320 acres), or which have a span of eight feet
or more, shall be designed for a maximum expected runoff as calculated
using the Soil Conservation Service Technical Release 55 "Urban Hydrology
for Small Watersheds (less than 2,000 acres)." The design storm shall
be a minimum 100-year storm. A water obstruction permit shall be obtained
from the Pennsylvania Department of Environmental Protection for the
waterway opening before final design is undertaken.
(4)
The cartway area over the bridge shall be as wide as the widest road
connecting with the bridge, or if the character of the road is expected
to change for future planning, the cartway of the bridge shall be
made to anticipate this condition.
B.
Specific storm drainage features.
(1)
Shoulders in cut areas (without swales).
(a)
Water flowing in the shoulder shall not encroach more than 2/3
the shoulder width during a 100-year frequency storm of five-minute
duration.
(b)
The maximum velocity as determined by Manning's equation
shall not exceed the allowable velocities as shown in Table 1 for
the specific type of shoulder material.
(c)
Inlets shall be provided to control the shoulder encroachment
and water velocity.
(2)
Swales adjacent to shoulders.
(a)
Swales in cut areas shall be designed to prevent the passage
of water on the cartway during a 100-year frequency storm of five-minute
duration.
(b)
The maximum velocity, as determined by Manning's equation,
shall not exceed the allowable velocities as shown in Table 1 for
the specific type of shoulder material.
|
Table 1
Allowable Water Velocities
| ||
|---|---|---|
|
Material
|
Allowable Velocity
(Feet per Second)
| |
|
1.
|
Well established grass on good soil
| |
|
Short pliant bladed grass
|
5-6
| |
|
Bunch grass - soil exposed
|
2-4
| |
|
Stiff stemmed grass
|
2-3
| |
|
2.
|
Earth without vegetation
| |
|
Fine sand or silt
|
1-2
| |
|
Ordinary firm loam
|
2-3
| |
|
Stiff clay
|
3-5
| |
|
Clay and gravel
|
4-5
| |
|
Course gravel
|
4-5
| |
|
Soft shale
|
5-6
| |
|
3.
|
Shoulders
| |
|
Earth
|
See 2 Above
| |
|
Stabilized
|
6
| |
|
Paved
|
10-15
| |
(4)
Inlets; general.
(a)
At street intersections, inlets shall be placed in the tangent
portion rather than the curved portion of the curbing.
(b)
When there is a change in pipe size in the inlet, the elevation
for the top of pipes should be the same or the smaller pipe higher.
A minimum drop of two inches should be provided in the inlet between
the lowest inlet pipe invert elevation and the outlet pipe invert
elevation.
(c)
If the capacity of the shoulder, swale, curb section or depressed
median section exceeds the assumed inlet capacities, the inlet capacities
shall govern the spacing of inlets.
(d)
If the capacity of the shoulder, swale, curb section, or depressed
median section is less than the inlet capacities, then the shoulder,
swale, curb section or depressed section capacity shall govern the
spacing of inlets.
(5)
Type C inlets.
(a)
This type inlet is designated for installation in nonmountable
curb.
(b)
In order to achieve greater efficiency, Type C inlets shall
be spaced so as to permit 5% of the gutter flow to bypass the inlet.
(c)
Inlet capacities shall be based on a maximum flow of 5.5 cfs.
(d)
The capacity of an inlet at a low point of a street vertical
curve may be designed to accept 4.0 cfs from each direction, or a
maximum of 8.0 cfs.
(6)
Type M and S inlets.
(a)
Type S inlets shall be installed in shoulder swale areas with
back slopes 6:1 and steeper. Type M inlets shall be used in swale
areas where the back slope is flatter than 6:1.
(b)
Inlet capacities shall be based on a maximum flow of 5.5 cfs,
except in sump conditions where the inlets may be designed to accept
4.0 cfs from each direction, or a maximum of 8.0 cfs.
(c)
Where a drainage dike is used, the side slope of the dike shall
be 8:1 or flatter.
(7)
Storm pipes.
(a)
Pipes shall be sized by use of inlet/outlet control calculations.
(b)
The roughness coefficient shall be in compliance with the Pennsylvania
Department of Transportation Design Manual, Part 2, as amended.
(c)
Where headroom is restricted, equivalent pipe arches may be
used in lieu of circular pipe.
(d)
The minimum diameter of all storm drainage pipes shall be 18
inches or equivalent thereto. All storm pipe shall be reinforced concrete
pipe which conforms to the Pennsylvania Department of Transportation
specifications, or Whitemarsh Township specifications, whichever are
more stringent.
(e)
Storm pipes will be provided in industry standard increments
of diameter.
(f)
Abrupt changes in direction or slope of pipe shall be avoided.
Where such abrupt changes are required, an inlet or manhole shall
be placed at the point of change. In no case will pipes be permitted
to enter a structure at an angle less than 90 degrees to the outflow
pipe.
(g)
The minimum slope in a pipe shall provide a minimum velocity
of 2.5 fps, but shall not have a minimum slope of less than 0.5%.
(h)
Storm pipes shall have a minimum eighteen-inch depth of cover
over the top of pipe.
(i)
Storm pipes shall be designed to maintain a minimum of one foot
of freeboard within each inlet box/manhole.
A.
No regulated earth disturbance activities within the Township shall
commence until approval by the Township of a BMP operations and maintenance
plan which demonstrates compliance with state water quality requirements
after construction is complete.
B.
The BMPs must be designed, implemented and maintained to meet state
water quality requirements, and any other more stringent requirements
as determined by the Township.
C.
To control post-construction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can be met
by BMPs, including site design, which provide for replication of pre-construction
stormwater infiltration and runoff conditions, so that post-construction
stormwater discharges do not degrade the physical, chemical or biological
characteristics of the receiving waters, as follows:
(1)
Any regulated earth disturbance shall specify permanent stormwater
BMPs to be implemented, operated, and maintained to meet legal water
quality requirements.
(2)
In order to protect and maintain water quality, additional stormwater
runoff created by the development project must be captured, stored
and treated. In addition, post-construction stormwater infiltration
of runoff must replicate pre-construction infiltration of runoff to
the maximum extent possible.
(3)
Infiltration.
(a)
Prevention of stormwater runoff is key objective of Chapter 93 of the DEP regulations, because runoff can change the physical, chemical and biological integrity of waterbodies thereby impacting water quality.
(b)
Any regulated earth disturbance activities shall describe how
these water quality protection requirements will be met. Infiltration
BMPs shall be evaluated and utilized to the maximum extent possible
to manage the net change in stormwater runoff generated so that post
construction discharges do not degrade the physical, chemical or biological
characteristics of the receiving waters.
(c)
Post construction stormwater infiltration of runoff shall replicate
pre-construction infiltration of runoff to the maximum extent possible.
(d)
In selecting the appropriate infiltration BMPs, the applicant
shall consider the following:
(e)
A detailed soils evaluation of the project site shall be performed
to determine the suitability of infiltration BMPs. The evaluation
shall be performed by a registered professional engineer experienced
in geotechnical engineering or a certified professional soil scientist,
and at a minimum, address soil permeability, depth to bedrock, susceptibility
to sinkhole formation, and subgrade stability. The general process
for designing the infiltration BMP shall be:
[1]
Analyze hydrologic soil groups, as well as natural and manmade
features within the watershed, to determine general areas of suitability
for infiltration BMPs.
[2]
Provide field testing data to determine appropriate percolation
rate and/or hydraulic conductivity.
[3]
Design infiltration BMPs for required stormwater volume based
on field determined capacity at the level of the proposed infiltration
surface.
(f)
Soil characteristics:
[1]
Infiltration BMPs are particularly appropriate in hydrologic
soil groups A and B.
[2]
Low-erodibility factors ("K" factors) are preferred for the
construction of basins.
[3]
There must be a minimum depth of 48 inches between the bottom
of any infiltration BMP and the seasonal highwater table and/or bedrock
(limiting zones), except for infiltration BMPs receiving only roof
runoff which shall be placed in soils having a minimum depth of 24
inches between the bottom of the facility and the limiting zone.
[4]
There must be an infiltration and/or percolation rate sufficient
to accept the additional stormwater load, and to drain completely
as determined by field tests.
[5]
Infiltration BMPs shall be located a minimum of 10 feet away
from the foundation wall of any building.
[6]
The infiltration system shall have positive overflow controls
to prevent storage within one foot of the finished surface or grade.
[7]
Infiltration rates shall not be used in computing the storage
volume of the infiltration system.
[8]
Surface inflows shall be designed to prevent direct discharge
of sediment into the infiltration system.
(g)
The recharge volume provided at the site shall be directed to
the most permeable HSG available, except where other considerations
apply such as in limestone geology.
(h)
Any infiltration BMP shall be capable of completely infiltrating
the impounded water within 96 hours.
(i)
(j)
During the period of land disturbance, runoff shall be controlled
prior to entering any proposed infiltration area, areas proposed for
infiltration BMPs shall be protected from sedimentation and compaction
during the construction phase, so as to maintain their maximum infiltration
capacity.
(k)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has received final stabilization.
