A.
All stormwater management site plans shall be designed and certified
by individuals registered in the Commonwealth of Pennsylvania and
qualified to perform such duties.
B.
Where applicable, stormwater management facilities shall comply with
the requirements of Chapter 105 (Dam Safety and Waterway Management)
of Title 25, Rules and Regulations, as amended, of the Pennsylvania
Department of Environmental Protection.[1]
C.
Stormwater management facilities which involve a state highway shall
also be subject to the approval of the Pennsylvania Department of
Transportation.
D.
Stormwater management facilities located within or affecting the
floodplain of any watercourse shall also be subject to the requirements
of the Manor Township floodplain regulations, as amended, or any future
ordinances, regulating construction and development within areas of
the Township subject to flooding.
E.
Stormwater discharge points onto an adjacent property shall comply
with the following:
(1)
Stormwater runoff from a project site shall flow directly into a
natural watercourse or into an existing storm sewer system. If neither
of these is available, the applicant shall obtain an easement from
the downstream landowner to allow the runoff discharge from the ten-year
storm to be piped to a natural watercourse or existing storm sewer
system.
(2)
Where the downstream landowner will not grant such an easement, the
plan shall be designed so that the discharge from the applicant's
site shall be in a non-erosive, sheet flow condition. The use of a
level spreader, concrete or otherwise, is expressly prohibited. Runoff
from the applicant's site shall flow onto the adjacent property in
a manner similar to the runoff characteristics of the predevelopment
flow. Where an easement is not granted, the ten- through one-hundred-year
discharge from the basin shall flow onto the adjacent property in
a manner similar to the runoff characteristics of the predevelopment
flow.
F.
Stormwater runoff shall not be transferred from one watershed to
another unless the watersheds are subwatersheds of a common watershed
which join together within the perimeter of the property, or the effect
of the transfer does not alter the peak discharge onto adjacent lands,
or drainage easements from the affected landowners are provided.
G.
All stormwater runoff flowing over the project site shall be considered
in the design of the stormwater management facilities.
H.
The calculated peak rates of runoff for stormwater originating on
the project site must meet the following conditions, for all watersheds
flowing from the project site:
(1)
Postdevelopment runoff from any regulated activity shall not exceed 50% of the peak rates of runoff prior to development for all design storms (two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year storm events), unless the predevelopment hydrograph is not exceeded at all points. To match the predevelopment hydrograph, the postdevelopment peak rate must be less than or equal to the predevelopment peak rate, and the post development runoff volume must be less than or equal to the predevelopment volume for the same storm event. A shift in hydrograph peak time of up to five minutes and a rate variation of up to 5% at a given time may be allowable to account for the timing affect of BMPs used to manage the peak rate and runoff volume. Volume control facilities as required in § 373-22 below may be used as part of this option.
I.
Innovative methods for the detention and control of stormwater runoff
may be used when approved by the Township. Various combinations of
methods should be tailored to suit the particular requirements of
the type of development and the topographic features of the project
site. The following is a partial listing of detention and control
methods which can be utilized in stormwater management systems, where
appropriate:
(1)
Detention basins and retention basins.
(2)
Rooftop storage.
(3)
Parking lot ponding.
(4)
Seepage pits, seepage trenches or other infiltration structures.
(5)
Concrete lattice block surfaces.
(6)
Grassed channels and vegetated strips.
(7)
Cisterns and underground reservoirs.
(8)
Routed flow over grass.
(9)
Decreased impervious surface coverage.
J.
Runoff can be managed regionally, by one or more developers, either
on-site or off-site. The design and release rate shall be consistent
with the Act 167 Plan. Groundwater recharge and water quality must
be addressed in this option.
K.
Unless an alternate design is submitted to the Township for review,
and said design is prepared by a geologist or geotechnical engineer
licensed in the Commonwealth of Pennsylvania:
(1)
No stormwater facilities shall be placed in, over or within a distance
that will impact the following features:
(2)
The minimum isolation distance from stormwater management basins
to the listed geologic features shall be as follows:
(3)
Stormwater runoff from any subdivision or land development activities
shall not be discharged into sinkholes.
(4)
It shall be the developer's responsibility to verify if the development
is underlain by carbonate geology. The certificate in the Appendix[2] shall be attached to all stormwater management site plans
and signed and sealed by the developer's qualified professional.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(5)
Whenever a stormwater facility will be located in an area underlain by carbonate geology, a geological evaluation of the proposed location by a registered professional geologist shall be conducted to determine susceptibility to sinkhole formation. The evaluation may include the use of impermeable liners to reduce or eliminate the separation distances listed in Subsection K(2).
The Township shall comply with the applicable provisions of
the Little Conestoga Creek Watershed Act 167 Stormwater Management
Plan, approved by the Lancaster County Commissioners on April 29,
1998, and by the DEP on June 28, 1998, and the Conestoga River Watershed
Act 167 Stormwater Management Plan, approved by the Lancaster County
Commissioners on June 8, 2005, and by the DEP on August 29, 2005.
In Manor Township, all lands within the Township are required to comply
with the provisions of these plans, unless there is a subsequent plan
adopted with more restrictive provisions. In that case, the more restrictive
provisions will apply within the applicable watershed(s).
The methods of computation used to determine peak discharge
and runoff shall be:
A.
All stormwater management predevelopment versus postdevelopment calculations
shall use one of the following:
(1)
The Soil-Cover-Complex Method (as set forth in the latest edition
of Urban Hydrology for Small Watersheds, Technical Release No. 55,
as published by NRCS) is the recommended and preferred method for
all facilities with a drainage area greater than or equal to 100 acres
or a time of concentration greater than 60 minutes.
(2)
The Rational Method is the recommended and preferred method for all
facilities with drainage areas less than 100 acres.
(3)
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
If the Rational Method is used, the NOAA Atlas 14 data shall be used
to determine the rainfall intensity in inches per hour based on the
information for the five- through sixty-minute duration storm events.
(4)
Runoff coefficients "C" and curve numbers "CN" shall be based on
the charts contained in the Appendix.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(5)
For the purpose of calculating peak discharges, all agricultural
lands that contribute storm drainage to or from the project site shall
be considered cultivated lands with conservation measures in good
hydrologic condition.
(6)
Hydrographs may be obtained from NRCS methods such as TR-55, TR-20,
or from use of the "modified" or "unit hydrograph" Rational Methods.
If "modified" or "unit hydrograph" Rational Methods are used, the
ascending leg of the hydrograph shall have a time of three times the
time of concentration (3 x Tc) and the descending leg shall have a
time of seven times the time of concentration (7 x Tc) to approximate
an NRCS type II hydrograph.
B.
Design of on-site conveyance systems calculations may use the Rational
Method of Q=CIA, where Q is the peak discharge of the watershed in
cubic feet per second, C is the coefficient of runoff, I is the intensity
of rainfall in inches per hour, and A is the area of the watershed
in acres; or any other method approved by the Township. When the Rational
Method is used, the NOAA Atlas 14 data shall be used to determine
the rainfall intensity in inches per hour based on the information
for the five- through sixty-minute duration storm events.
C.
Runoff calculations shall include a hydrologic and hydraulic analysis
indicating volume and velocities of flow and the grades, sizes, and
capacities of water-carrying structures, sediment basins, retention
and detention structures and sufficient design information to construct
such facilities. Runoff calculations shall also indicate both predevelopment
and postdevelopment rates for peak discharge of stormwater runoff
from the project site.
D.
Flow calculations for water carrying structures shall be presented
in tabular form using the flow tabulation form attached (or equal)
hereto. See the Appendix.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
E.
Permanent detention basins shall be designed with a primary outlet discharge that is less than or equal to the requirements for postdevelopment peak rate of runoff established by § 373-15H of this chapter.
F.
Runoff calculations will also be made to ensure that the runoff from
the upstream watershed area can be accommodated by the pipes, drainage
easements, watercourses, etc., on the site.
G.
For the NOAA Atlas 14 rainfall, provide the following:
(1)
Provide the rainfall used for the two-, ten-, twenty-five-, fifty-,
and one-hundred-year, twenty-four-hour storm events. Rainfall values
vary throughout the county depending on location.
(2)
Provide the location (longitude and latitude) or a description of
the location for which the rainfall applies.
(3)
If rainfalls from more than one location are used, provide the methodology
by which the design rainfall was calculated.
H.
Sheet flow may be determined using the nomograph found in the Appendix,
or the TR-55 Manning's kinematic solution shown in the sheet flow
section of Worksheet No. 1 in the Appendix.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
A.
Storm sewer pipes and culverts shall be reinforced concrete pipe
(RCP) or smooth-lined corrugated polyethylene pipe (SLCPP), shall
have a minimum diameter of 15 inches, and shall be installed on a
sufficient slope to provide a minimum velocity of three feet per second
when flowing full. Pipes and culverts located outside of public street
rights-of-way and not subject to vehicular loading shall have a minimum
diameter of eight inches.
B.
Inlets or manholes shall be placed at all points of changes in the
horizontal or vertical directions of conveyance pipes. Curved pipe
sections are prohibited.
C.
The roughness coefficient (Manning "n" values) used for conveyance
pipe capacity calculations should be determined in accordance with
PennDOT Publication 584, PennDOT Drainage Manual, or per the manufacturer's
specifications.
D.
Within the public street right-of-way, the gutter spread based on
the twenty-five-year storm shall be no greater than one-half of the
travel lane and have a maximum depth of three inches at the curbline.
A parking lane shall not be considered as part of the travel lane.
In the absence of pavement markings separating a travel lane from
the parking lane, the parking lane shall be assumed to be seven feet
wide if parking is permitted on the street.
E.
All inlets placed in paved areas shall have heavy-duty bicycle-safe
grating consistent with PennDOT Publication 72M, latest edition. A
note to this effect shall be added to the stormwater management site
plan or inlet details therein.
F.
Inlets, junction boxes, or manholes greater than five feet in depth
shall be equipped with non-aluminum ladder rungs and shall be detailed
on the stormwater management site plan.
G.
The "n" factors to be used for paved or riprap swales or gutters
shall be based upon accepted engineering design practices, as approved
by the Municipal Engineer and as follows:
(1)
For vegetated swales, the first condition shall consider swale stability
based upon a low degree of retardance ("n" = 0.03);
(2)
For vegetated swales, the second condition shall consider swale capacity
based upon a higher degree of retardance ("n" = 0.05); and
(3)
All vegetated swales shall have a minimum slope of 2% unless otherwise
approved by the Municipal Engineer.
H.
All swales shall be designed to maximize infiltration and concentrate
low flows to minimize siltation and meandering, unless geotechnical
conditions do not permit infiltration.
I.
Where the connecting pipe has a diameter 18 inches or greater, headwalls
and endwalls shall be provided with a protective barrier device to
prevent entry of the storm sewer pipe by unauthorized persons. Such
protection devices shall be designed to be removable for cleaning.
J.
All storm sewer pipes, grass waterways, open channels, swales and
other water-carrying facilities shall be designed for a twenty-five-year
storm event unless in the opinion of the Township or Township Engineer
the character of development and potential for damage warrant design
for the fifty- or one-hundred-year storm.
K.
All storm sewer pipes, culverts, manholes, inlets, endwalls and end
sections shall be constructed in accordance with Pennsylvania Department
of Transportation, Form 408, as amended.
L.
All storm sewer pipes, culverts, manholes, inlets, endwalls, and
end sections proposed for dedication or located along streets shall
conform to the requirements of the Pennsylvania Department of Transportation,
Bureau of Design, Standards for Roadway Construction, Publication
No. 72, in effect at the time the design is submitted, as modified
by the Township.
M.
Storm sewer pipes and culverts shall be reinforced concrete pipe
(RCP) or smooth-lined corrugate polyethylene (SLCPP), shall have a
minimum diameter of 18 inches, and shall be installed on a sufficient
slope to provide a minimum velocity of three feet per second when
flowing full.
N.
All storm sewer pipe shall be laid to a minimum depth of one foot
from subgrade to the crown of pipe.
O.
Endwalls and end sections shall be used where stormwater runoff enters
or leaves the storm sewer horizontally from a natural or man-made
channel.
P.
Inlets shall be placed on both sides of the street at low spots,
at a maximum of 600 feet apart along a storm sewer pipe, at points
of abrupt changes in the horizontal or vertical directions of storm
sewers, and at points where the flow in gutters exceeds three inches.
Inlets shall normally be along the curbline at or beyond the curb
radius points. For the purpose of inlet location at corners, the depth
of flow shall be considered for each gutter. At intersections, the
depth of flow across the through streets shall not exceed one inch.
Manholes may be substituted for inlets at locations where inlets are
not required to collect surface runoff.
Q.
Stormwater roof drains and pipes, wherever possible, shall discharge
water into a stormwater runoff dispersion and infiltration control
device and not into storm sewers or street gutters.
R.
All existing and natural watercourses, channels, drainage systems
and areas of surface water concentration shall be maintained in their
existing condition unless an alteration is approved by the Township.
S.
Flow velocities from any storm sewer shall not result in a degradation
of the receiving channel.
T.
Energy dissipaters shall be placed at the outlets of all storm sewer
pipes where flow velocities exceed maximum permitted channel velocities.
U.
The capacities of grassed waterways shall be computed from the Manning
Equation. Permissible open channel velocities and design standards
shall be in accordance with good engineering practice as documented
in the Engineering Field Manual for Conservation Practices, USDA,
NRCS, or in Design Charts for Open-Channel Flow, Hydraulic Design
Series No. 3, U.S. Department of Transportation.
V.
Grassed waterways may be utilized in place of conduit piping in those
areas where soil conditions allow recharge of groundwater. All newly
installed grassed waterways must be well established sod of good quality
or matted with an approved stabilizing material. The usage of grassed
waterways is not recommended in areas of year-round or seasonally
high groundwater table, unless provisions are made to handle long-duration
flows, for example, by means of subsurface drainage of stone-centered
waterways.
(1)
The shape of the waterway shall permit hydraulic efficiency and ease
of maintenance. Allowable velocities within the waterway shall be
limited to those values which would not cause erosion of the soil
or cover material. Vegetation or durable materials shall be established
on all channels where design velocities exceed the maximum values
for base earth channels. Permanent channels should be designed using
grass or other suitable material.
(2)
The following information should be utilized in selecting adequately
sized channels that do not exceed maximum velocities. The maximum
permissible velocity shall be selected as the lowest value from SubsectionV(2)(a)
and (b) which follow:
(a)
Guidelines for maximum permissible velocities relevant to individual
site conditions:
[1]
Three feet per second where only sparse vegetation can be established
and maintained because of shade or soil conditions;
[2]
Three feet to four feet per second should be used under normal
conditions where the vegetation is to be established by seeding;
[3]
Four feet to five feet per second should be used only in areas
where a dense vigorous sod is obtained quickly or where water can
be diverted out of the waterway during establishment of vegetation.
(Use where netting and mulch or other special methods of establishing
vegetation are used.)
[4]
Five feet to six feet per second may be used on well-established
sod of good quality (use where establishment of vegetation is by sodding
or water is introduced to a previously sodded channel).
(b)
Soil characteristics, design velocities and the level of desired
maintenance should be considered in determining seed mixtures and
methods of establishment of vegetation. Soils information for various
soil types is contained in the "Lancaster County Soil Survey." Maximum
permissible velocities in feet per second based on vegetation, slope
of waterway and soil erodibility are as follows:
|
*For seeding with Kentucky Bluegrass, Kentucky 31 Tall
Fescue, smooth brome grass or a mixture of Kentucky 31 Tall Fescue
and Birdsfoot Trefoil
(mow occasionally)
| ||||
|---|---|---|---|---|
|
Channel Grade
| ||||
|
0% to 5%
|
5% to 10%
|
Greater Than 10%
| ||
|
Easily eroded soils
(**k less than 0.37)
|
5 fps
|
4 fps
|
3 fps, recommended only with special engineering
| |
|
Erosion-resistant soils
|
6 fps
|
6 fps
|
5 fps, consideration (k=0.7 or greater)
| |
|
*For seeding with Red Fescue or similar lawn mixtures
(mow frequently)
| ||||
|---|---|---|---|---|
|
Channel Grade
| ||||
|
0% to 5
|
5% to 10%
|
Greater than 10%
| ||
|
Easily eroded soils
|
2.5
|
K<0.37
|
Not recommended over 5%
| |
|
Erosion-resistant soils
|
3.5
|
K >.37
|
Not recommended over 5%
| |
|
*
|
Redtop is recommended for use as a companion seeding.
| |
|
**
|
K is the erosion factor found in Table 16 of the "Soil Survey
of Lancaster County, PA," issued May 1985.
|
|
Velocities for other channels are as follows:
| ||
|---|---|---|
|
Channel Lining
|
Maximum Permissible Velocity
(feet per second)
| |
|
6" riprap
|
4
| |
|
9" riprap
|
8
| |
|
Durable bedrock
|
8
| |
|
Asphalt
|
7
| |
|
12" riprap
|
9
| |
|
Concrete or steel
|
12
| |
For aboveground facilities which include rate control and have a facility depth less than two feet, the standards in § 373-23 may be used. All other facilities that include rate control shall meet the standards in this section. Facilities that include volume control shall also meet the requirements of § 373-22.
A.
An impervious core/key trench, when required, shall consist of a
cutoff trench (below existing grade) and a core trench (above existing
grade). A key trench may not be required wherever it can be shown
that another design feature, such as the use of an impermeable liner,
accomplishes the same purpose.
B.
The core should extend up both abutments to the ten-year water surface
elevation or six inches below the emergency spillway elevation, whichever
is lower.
C.
All pipe collars (antiseep collars), shall be designed in accordance with Chapter 7 of the DEP E&S Manual. The material shall consist of concrete or otherwise nondegradable material around the outfall barrel and shall be watertight.
D.
Embankment fill material. The embankment fill material shall be taken
from an appropriate borrow area which shall be free of roots, stumps,
wood, rubbish, stones greater than six inches, frozen or other objectionable
materials.
E.
Bottom slope. The minimum bottom slope of facilities not designed
for infiltration shall be 1%. A flatter slope may be used if an equivalent
dewatering mechanism is provided.
F.
Pretreatment elements. When required, pretreatment elements shall
consist of forebays, or alternate approved by the Municipal Engineer,
to keep silt to a smaller portion of the facility for ease of maintenance.
G.
Infiltration basins. Within basins designed for infiltration, existing native vegetation shall be preserved, if possible. For existing unvegetated areas or for infiltration basins that require excavation, a planting plan shall be prepared in accordance with § 373-9 and the BMP Manual, which is designed to promote infiltration.
H.
All discharge control devices with appurtenances shall be made of
reinforced concrete and stainless steel. Bolts/fasteners shall be
stainless steel.
I.
Use of the spillway to convey flows greater than the fifty-year design
storm is permitted.
J.
Emergency use. The spillway shall be designed to convey the one-hundred-year
peak inflow.
K.
When required, freeboard shall be measured from the top of the water
surface elevation for emergency use.
L.
All basins shall be structurally sound and shall be constructed of
sound and durable materials. The completed structure and the foundation
of all basins shall be stable under all probable conditions of operation
and shall be capable of discharging the peak discharge of a postdevelopment
one-hundred-year storm event through the primary emergency and/or
spillway facilities, in a condition that assumes the primary outlet(s)
are blocked, which will not damage the integrity of the facility or
the downstream drainage areas.
M.
The effect on downstream areas if the basin embankment fails shall
be considered in the design of all basins. Where possible, the basin
shall be designed to minimize the potential damage caused by such
failure of the embankment.
N.
All detention basins shall include an outlet structure to permit
draining the rate control volume in the basin to the level of a two-year
storm within 24 hours (exclusive of BMP or volume control storage).
O.
All outlet structures and emergency spillways shall include a satisfactory
means of dissipating the energy of flow at its outlet to assure conveyance
of flow without endangering the safety and integrity of the basin
and the downstream drainage area.
P.
A cutoff trench of relatively impervious clay material shall be provided
within all basin embankments, except for those embankments with side
slope ratios of three horizontal to one vertical or flatter. Embankments
with flatter side slopes shall have a key trench.
Q.
All culverts through basin embankments shall have properly spaced
concrete cutoff collars or welded antiseep collars.
R.
A minimum one-foot freeboard above the design elevation of the water
surface at the emergency spillway shall be provided.
S.
No outlet structure from a detention basin or swale shall discharge
directly onto a Township road, but shall discharge into a culvert
under a Township road.
T.
The minimum top width of dams up to 10 feet in height shall be equal
to 2/3 of the dam height, but in no case shall the top width be less
than five feet.
A.
All earth disturbance activities shall be conducted in such a way
as to minimize accelerated erosion and resulting sedimentation. Measures
to control erosion and sedimentation shall, at a minimum, meet the
standards of the Lancaster County Conservation District and all requirements
of Title 25, Rules and Regulations of the Pennsylvania Department
of Environmental Protection.
B.
The erosion and sedimentation control plan must be available at all
times at the project site. When required, a permit allowing earthmoving
activity shall be obtained by the developer before any construction
on the project site shall begin.
C.
Approval of an erosion and sedimentation control plan by the Township
shall not be construed as an indication that the plan complies with
the standards of any agency of the commonwealth.
D.
The erosion and sedimentation control plan shall be submitted to
the Lancaster County Conservation District for its review and recommendations.
E.
The following principles shall be applied to the design plan and
construction schedule to minimize soil erosion and sedimentation:
(1)
Stripping of vegetation, grading or other soil disturbance shall
be done in a manner which will minimize soil erosion.
(2)
Whenever feasible, natural vegetation shall be retained and protected.
(3)
The extent of the disturbed area and the duration of its exposure
shall be kept to a minimum, within practical limits.
(4)
Either temporary seeding, mulching or other suitable stabilization
measures shall be used to protect exposed critical areas during construction.
(5)
Drainage provisions shall accommodate the stormwater runoff both
during and after construction.
(6)
Soil erosion and sedimentation facilities shall be installed prior
to any on-site grading.
A.
All large impervious surface areas, excluding buildings, but including
paved areas that are constructed with bituminous, concrete or other
impervious or semi-impervious surfaces which are in excess of 150,000
square feet, either initially or cumulatively as of May 19, 2004,
or any activity which when conducted on the same tract of land or
on adjacent tracts of land results in an aggregate paved area constructed
with bituminous, concrete or other impervious or semi-impervious surface
that exceeds 150,000 square feet, either initially or cumulatively
as of May 19, 2004, whether or not the paved areas are contiguous,
shall be subject to the provisions of this section.
B.
The expansion of any existing site or the redevelopment of previously developed sites that include facilities or activities defined by Subsection A shall meet the standards of this section.
C.
Any stormwater that comes into contact with the facilities or activities defined by Subsection A is considered to be contaminated by pollutants and in need of subsequent pretreatment. All stormwater shall be directed to a minimum of two separate BMPs for pretreatment prior to discharge from the site.
D.
Within the areas and activities that are the subject of this section,
the following additional stormwater management requirements must be
met:
(1)
Source reduction practices must be used which are intended to minimize
the contact between rainfall and the area/activity in order to minimize
the amount of stormwater that must be treated.
(2)
Pretreatment must be accomplished by the use of water quality inlets,
water quality swales, wet ponds, constructed stormwater wetlands,
bioretention, and extended detention basins. Other innovative methods
will be considered on an individual site basis.
(3)
Stormwater runoff shall be treated to provide hydrocarbon removal
and a reduction of thermal pollution.
(4)
The use of infiltration practices without pretreatment for total
suspended solids or hydrocarbons is prohibited.
E.
BMP systems required by this section shall incorporate designs which
allow for the shutdown, containment and isolation of stormwater from
the site in the event of an emergency spill or other unexpected contamination.
BMPs with inlets and outlets can be designed with a shutoff and containment
mechanism, using available storage in the BMP. Generally, a shutoff
valve or gate valve should be installed at the lowest invert point.
F.
Water quality design storm.
(1)
The runoff volume to be treated by BMPs (prescribed runoff) is to
be calculated based upon the two-year, twenty-four-hour storm event
for all impervious areas of the postdevelopment project site.
G.
Stormwater management BMP systems must be designed to remove 80%
of the average annual load (postdevelopment conditions) of total suspended
solids (TSS) after construction is complete and the site is stabilized.
It is presumed this standard is met when:
(1)
Water quality inlets shall be utilized for treatment of all runoff
prior to discharge to other BMPs;
(2)
Runoff from all impervious areas are directed to BMPs for pretreatment
prior to discharge from the site;
(3)
Stormwater management BMPs are sized to capture the prescribed runoff
volume;
(4)
Suitable nonstructural practices for source control and pollution
prevention are implemented, including but not limited to sweeping
and vacuuming of paved areas, litter control and spill containment;
and
(5)
Stormwater BMPs are maintained in a first class design condition.
H.
Design standards and specifications for water quality BMPs.
(1)
Extended detention basin.
(a)
The minimum contributing watershed shall be 10 acres;
(b)
Provide a minimum forty-eight-hour detention time for the prescribed
runoff;
(c)
Provide a sediment forebay at the inlet;
(d)
Provide a micropool located near the outlet structure to reduce
resuspension of sediments;
(e)
A minimum length-to-width ratio of 3:1 needs to be provided
with the inlet structure(s) placed at the maximum distance from the
outlet structure; and
(f)
Establish and maintain water-tolerant or wetland vegetation
on the basin bottom.
(2)
Wet extended detention basin.
(a)
The minimum contributing watershed shall be 25 acres or provide
other measures to maintain a permanent pool of water;
(b)
A permanent pool with a volume equal to 40% of the prescribed
runoff needs to be provided;
(c)
A sediment forebay with a minimum volume of 13% of the prescribed
runoff needs to be provided;
(d)
The extended detention storage volume shall be a minimum of
47% of the prescribed runoff;
(e)
Provide a maximum depth of 2.5 feet;
(f)
Wetland vegetation needs to be established along the basin edges;
and
(g)
A minimum length-to-width ratio of 3:1 needs to be provided
with the inlet structure(s) placed at the maximum distance from the
outlet structure.
(3)
Bioretention.
(a)
Can be designed online or offline of the stormwater facilities;
(b)
Bioretention components shall include grass buffer strips, sand
bed, ponding area, organic or mulch layer, planting soil and plants;
(c)
The facility shall be lined and treated runoff shall be drained
to downslope facilities;
(d)
Minimum contributing watershed of 1/2 acre;
(e)
Maximum contributing watershed of five acres;
(f)
Shall be designed to carry and filter the runoff from the two-year,
twenty-four-hour storm event. Additional flows must be conveyed in
safe and nonerosive manner to additional facilities;
(g)
The minimum surface area shall be equal to 7% of the product
of the drainage area multiplied by the Rational Method runoff coefficient
"c" determined for the site;
(h)
Provide a minimum width of 15 feet, minimum length of 40 feet;
(i)
The maximum ponding depth shall not exceed six inches;
(j)
The planting soil shall be a sandy loam with a clay content
ranging from 10% to 25%;
(k)
The area shall be planted in understory trees and shrub with
a shrub to tree ratio of 3:1; and
(l)
The area shall be fully stabilized to a nonerosive condition
with a dense vegetative cover prior to accepting any stormwater runoff.
(4)
Water quality inlets (WQI).
(a)
Shall be utilized for treatment of all runoff prior to discharge
to other BMPs;
(b)
The inlets must utilize three chambers (sediment, oil separation,
and discharge);
(c)
A separate manhole shall be provided for access to each chamber;
(d)
Shall utilize a screen between the first and second chamber,
and a pipe designed to pass along the design storm;
(e)
An inverted elbow shall be utilized between the second and third
chamber;
(f)
A coalescing unit designed to maximize oil/water separation
is required for each WQI;
(g)
The maximum contributing watershed to a WQI shall be one acre;
(h)
Diffusion baffles shall be utilized to reduce turbulent flow
and the resuspension of settled pollutants;
(i)
WQIs shall be reinforced concrete and watertight;
(j)
WQIs shall maintain permanent pools to maximize pollutant retainage;
and
(k)
The WQIs shall be cleaned at the beginning of each season (summer,
fall, winter, spring). Each WQI shall be inspected twice during each
season for proper operation. In the event that the WQI is not operating
effectively, it shall be repaired/cleaned immediately.
(5)
Water quality swales.
(a)
Provide maximum side slopes of 4:1;
(b)
The longitudinal slope shall range from 0.5% to 4%; and
(c)
Velocity checks shall be provided at maximum one-hundred-foot
intervals. Longer spacing may be permitted, provided calculations
are provided that document flow velocities for the two-year, twenty-four-hour
storm event do not exceed two feet per second.
(6)
Constructed stormwater wetlands. Constructed stormwater wetlands
shall be designed, constructed, operated and maintained in accordance
with the U.S. EPA Stormwater Technology Fact Sheet No, EPA 832-F-99-025,
Stormwater Wetlands, as amended, updated or rewritten from time to
time.
I.
An operation and maintenance plan for the proposed BMPs is required
which at a minimum includes:
(1)
Stormwater management system owner(s) and address;
(2)
The party or parties responsible for operation and maintenance;
(3)
A detailed schedule for the inspection and maintenance of all BMPs,
which shall include the routine maintenance and non-routine maintenance
tasks to be undertaken;
(4)
A detailed cost estimate for the operation, inspection and maintenance
of the stormwater management BMPs;
(5)
In addition to any other financial security required by this chapter,
financial security, in amount equal to 110% of the approved inspection,
operation and maintenance cost estimate, and a financial security
agreement shall be provided.
(a)
For the purposes of this subsection, the financial security shall be provided in accordance with Article V of the Pennsylvania Municipalities Planning Code.[1] The Township may annually readjust the amount of the financial
security by redetermining the costs of the inspection, operation,
and maintenance of the stormwater management BMPs to maintain financial
security in an amount equal to amount equal to 110% of the redetermined
inspection operation and maintenance cost estimate.
[1]
Editor's Note: See 53 P.S. § 10501 et seq.
(6)
The operation and maintenance plan is to be updated annually and
a revised version forwarded to the Township; and
(7)
Where the NPDES permit for the project requires that BMPs be installed,
annual written reporting of the inspection and maintenance of those
BMPs shall be required by the operation and maintenance plan.
A.
For those project sites that do not meet the applicability criteria of § 373-21, stormwater management facilities shall be supplemented by BMPs as outlined in this section. Required storage volume shall be provided to minimize impacts to water quality of receiving waters.
B.
Water volume controls shall be implemented using the design storm method described in Subsection C below.
C.
The design storm method is applicable to any size of regulated activity not already exempted in § 373-10. This method requires detailed modeling based on site conditions.
(1)
Do not increase the postdevelopment total runoff volume for all storms
equal to or less than the two-year, twenty-four-hour storm event.
(2)
For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
Twenty percent of existing impervious area, when present, shall be considered meadow in good condition in the model for existing conditions, subject to the limit stated in Subsection D.
(c)
The maximum loading ratio for volume control facilities in Karst
areas shall be 3:1 impervious drainage area to infiltration area and
5:1 total drainage area to infiltration area. The maximum loading
ratio for volume control facilities in non-Karst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio may be approved by the municipality
if justification is provided.
D.
For projects that add impervious area to a parcel, an amount of existing
impervious area not to exceed 20% of the proposed impervious area
is subject to the volume control requirements of this chapter.
E.
Any portion of the volume control storage that meets the following
criteria may also be used as rate control storage:
(1)
Volume control storage that depends on infiltration is designed according to the infiltration standards in § 373-25.
(2)
The volume control storage is located within a rate control facility.
(3)
The volume control storage which will be used for rate control is
that storage which is available within 24 hours based on the stabilized
infiltration rate and/or the evapotranspiration rate.
F.
A detailed geologic evaluation of the development site shall be performed
in areas of carbonate geology to determine the design parameters of
recharge facilities. The evaluation shall be performed by a state
licensed/certified professional geologist (PG), and shall, at a minimum,
address soil permeability, depth to bedrock, susceptibility to sinkhole
formation, and subgrade stability.
A.
Discharge pipes, where used, shall be reinforced concrete pipe (RCP)
or smooth lined corrugate polyethylene pipe (SLCPP), or PVC pipe,
and shall have a minimum diameter of six inches.
B.
Facilities shall have impoundment areas with side slopes no greater
than two horizontal to one vertical.
C.
All facilities shall drain any rate control volume within 24 hours. All other storage shall meet the drainage time requirements of § 373-15.
D.
The minimum top width shall be two feet.
A.
The maximum depth from the surface shall be two feet less than the
limiting zone.
B.
The maximum loading ratio shall be as specified in the BMP manual
unless otherwise determined by professional geologic evaluation.
C.
Pretreatment requirements. The facility shall be designed to provide
a method to eliminate solids, sediment, and other debris from entering
the subsurface facility.
D.
Stone for infiltration beds. The stone used for infiltration beds
shall be clean washed, uniformly graded coarse aggregate (AASHTO No.
3 or equivalent approved by the municipality). The void ratio for
design shall be assumed to be 0.4.
E.
Backfill material. Material consistency and placement depths for
backfill shall be (at a minimum) per all applicable pipe manufacturer's
recommendations, further providing it should be free of large (not
exceeding six inches in any dimension) objectionable or detritus material.
Select non-aggregate material should be indigenous to the surrounding
soil material for nonvehicular areas. Backfill within vehicular areas
shall comply with this section unless otherwise specified in governing
municipal road/street or subdivision and land development ordinances.
Furthermore, if the design concept includes the migration of runoff
through the backfill to reach the infiltration facility, the material
shall be well drained, free of excess clay or clay-like materials
and generally uniform in gradation.
F.
Lining material. Nonwoven geotextiles shall be placed on the sides
and top of subsurface infiltration facilities. No geotextiles shall
be placed on the bottom of subsurface infiltration facilities.
G.
When located under pavement, the top of the subsurface facility shall
be a minimum of three inches below the bottom of pavement subbase.
H.
Where located under vegetative cover, the top of the subsurface facility
shall be a minimum of 12 inches below the surface elevation or as
required to establish vegetation.
A.
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on the suitability of the soils and site conditions
and shall be constructed on soils that have the following characteristics:
(1)
A minimum depth of 24 inches between the bottom of the facility and
the seasonal high water table and/or bedrock (limiting zones).
(2)
An infiltration and/or percolation rate sufficient to accept the
additional stormwater load and drain completely as determined by field
tests conducted by the owner's design professional.
B.
Extreme caution shall be exercised where infiltration is proposed
in geologically susceptible areas such as carbonate or limestone areas,
and where salt or chloride would be a pollutant, since soils do little
to filter this pollutant, and it may contaminate groundwater. A detailed
geologic investigation may be required to determine the suitability
of recharge facilities and to specifically address soil permeability,
depth to bedrock, susceptibility to sinkhole formation, and subgrade
stability.
C.
For single-family residential subdivisions with on-lot BMPs, one
probe test pit per lot is required, within 25 feet of the proposed
BMP area. Verification testing shall take place when BMPs are sited
at greater distances.
D.
For multifamily and high-density residential developments, one test
pit per BMP area or acre is required.
E.
For large infiltration areas (basins, commercial, institutional,
industrial, and other proposed land uses), multiple test pits shall
be evenly distributed at the rate of four to six tests per acre of
BMP area.
F.
Additional tests shall be conducted if local conditions indicate
significant variability in soil types, geology, water table levels,
bedrock, topography, etc. Similarly, uniform site conditions may indicate
that fewer test pits are required. Excessive testing and disturbance
of the site prior to construction is not recommended.
G.
At least one percolation/infiltration test shall be conducted at
the proposed bottom elevation of an infiltration BMP, and a minimum
of two tests per probe test pit is required. More tests may be warranted
if the results for first two tests are substantially different.
H.
The highest rate (inches per hour) for test results shall be discarded
when more than two are employed for design purposes. The geometric
mean shall be used to determine the average rate following multiple
tests.
I.
Percolation/infiltration testing shall be done using a double-ring
infiltrometer or an equal method.
