[Ord. 1018, 6/6/2016]
1.
All regulated activities in the Township which do not fall under the exemption criteria shown in § 26-110 of this chapter shall submit a stormwater management site plan to the Township for review. This plan must be consistent with the Spring Creek Watershed Stormwater Management Plan. These criteria shall apply to the total proposed development even if development is to take place in phases. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas, and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the waiver criteria. The DEP Stormwater BMP Manual, as amended and updated, shall be used for design of stormwater management facilities. No disturbance may be performed prior to approval of a stormwater management site plan. All activities (especially earth disturbance activities) must comply with this chapter, Title 25 of the Pa. Code and the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
2.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
A.
Stormwater management facilities and related installations also shall
be provided:
(1)
To ensure adequate drainage of all low points along the curbline
of streets.
(2)
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 or flooded intersections during storms, in accordance with the procedures contained in Design Manual Part 2 (DM-2), Chapter 10, of the Pennsylvania Department of Transportation (PADOT).
(3)
To ensure adequate and unimpeded flow of stormwater under driveways
in, near, or across natural watercourses or drainage swales. Suitable
pipes or other waterways shall be provided as necessary.
(4)
To properly drain stormwater runoff from all land development
projects, except as required by recharge criteria. All lot and open
areas shall be designed to drain to the nearest practical street or
drainage system, existing or proposed, as defined by the Township
Engineer, with no impact on adjoining properties, unless an area specifically
designed for stormwater detention is provided.
3.
The existing points of concentrated drainage that discharge onto
adjacent property shall not be altered without permission of the altered
property owner(s) and shall be subject to any applicable discharge
criteria specified in this chapter.
4.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If diffused
flow is proposed to be concentrated and discharged onto adjacent property,
the developer must document that adequate downstream conveyance facilities
exist to safely transport the concentrated discharge, or otherwise
prove that no erosion, sedimentation, flooding or other harm will
result from the concentrated discharge. If, in the opinion of the
Township Engineer, the approved analysis identifies there will be
an impact on the downstream adjacent property, the Township may require
that the developer obtain a stormwater easement. Where a development
site is traversed by watercourses or significant drainageways, drainage
easements shall be provided conforming to the line of such watercourses
or significant drainageways. The terms of the easement shall prohibit
excavation, the placing of fill or structures, and any alterations
that may adversely affect the flow of stormwater within any portion
of the easement. Developers are encouraged to allow stormwater easements
to revert to a natural condition. Mowing or similar types of vegetative
control are discouraged while removal of invasive species are encouraged.
5.
When it can be shown that, due to topographic conditions, natural
drainageways on the site cannot adequately provide for drainage, open
channels may be constructed conforming substantially to the line and
grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by PADEP through the joint permit application
process, or, where deemed appropriate by PADEP, through the general
permit process.
6.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth (any
and all rivers, streams, creeks, rivulets, ditches, watercourses,
storm sewers, lakes, dammed water, wetlands, ponds, springs, and all
other bodies or channels of conveyance of surface and underground
water, or parts thereof, whether natural or artificial, within or
on the boundaries of this commonwealth) or wetlands shall be subject
to approval by PADEP through the joint permit application process,
or, where deemed appropriate by PADEP, the general permit process.
When there is a question whether wetlands may be involved, it is the
responsibility of the developer or his agent to show that the land
in question cannot be classified as wetlands, otherwise approval to
work in the area must be obtained from PADEP.
7.
Any stormwater management facilities regulated by this chapter that
would discharge to or be located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PADOT).
8.
Low-impact development (LID) is to be used to the maximum extent
practicable. Minimization of impervious surfaces and infiltration
of runoff through seepage beds, recharge trenches, etc., are encouraged,
where soil conditions permit, to reduce the size or eliminate the
need for detention facilities.
9.
To promote over-land flow and infiltration/percolation of stormwater,
roof drains should not be connected to streets, sanitary or storm
sewers, or roadside ditches unless approved by the Township Engineer
on a case-by-case basis.
10.
Where deemed necessary by the Township Engineer, the applicant shall
submit an analysis of the impacts of detained stormwater flows on
downstream areas within the watershed. These impacts shall be identified
with concurrence from the Township Engineer. The analysis shall include
hydrologic and hydraulic calculations necessary to determine the impact
peak discharge modifications from the proposed development have on
critical locations such as dams, tributaries, existing developments,
undersized culverts, floodprone areas, etc. The analysis shall include
the impact of the modified quantity of discharged water to developed
downstream areas that experience flooding. The applicant must demonstrate
that the additional flow does not adversely impact downstream properties
or structures and the limits of said flooded area are not increased
by the proposed activity. ANY REGULATED ACTIVITY THAT ADVERSELY IMPACTS
DOWNSTREAM PROPERTIES, STRUCTURES, PUBLIC FACILITIES, THREATENS THE
PUBLIC HEALTH OR SAFETY SHALL BE PROHIBITED BY THE TOWNSHIP.
11.
When stormwater management facilities are proposed within 1,000 feet
of a downstream municipality, the analysis of downstream impacts shall
be submitted to the downstream municipality's engineer for review
and comment concurrent with the Township review. Proof of written
notification from the applicant to the downstream municipality's
engineer shall be provided with the submission.
12.
When stormwater flows onto an adjacent property may be altered, proof
of written notification from the developer to the adjacent property
owner shall be provided. The notification shall be mailed by certified
mail, return receipt, to document delivery of the notification. The
notification shall advise the adjacent property owner of the specific
proposal and provide appropriate contact information for the developer's
engineer and Township Engineer.
13.
Approved stormwater management site plans must be on site at all
times during construction.
14.
Use of alternate and/or new stormwater management controls will be
considered by the Township. A description of the proposed alternate
controls must be submitted to the Township, the Centre County Conservation
District, and to DEP. The Township will coordinate any approvals with
the CCCD and/or DEP.
15.
The stormwater management site plan must contain a proper long-term ownership, operation and maintenance plan in accordance with Part 7 of this chapter.
16.
No habitable structure shall be built within 50 feet of the one-hundred-year
floodplain. All floors of any living space shall be at least one foot
above the one-hundred-year floodplain water surface elevation. Any
access including doors, windows, openings to an uninhabited basement
or floor shall be at least one foot above the one-hundred-year floodplain
water surface elevation. Plans shall clearly state if the basement
or lowest floor is habitable.
17.
Landscaping and grading of lots shall provide positive drainage away
from all structures.
[Ord. 1018, 6/6/2016]
1.
Water Quality Sensitive Area Districts (Sensitive Area Districts)
and Water Quality Sensitive Developments (Sensitive Developments)
have been identified which require special consideration with regard
to stormwater management.
A.
Water quality sensitive area districts are defined as those areas
that, if developed, have the potential to cause catastrophic loss
to a Water Authority well field. These areas consist of all Zone 2
wellhead protection areas defined by DEP within Township boundaries
(see Appendix B, Exhibit 1).[1] The Township may update the Sensitive Area District boundaries
based on new research or studies as required.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
B.
Water quality sensitive (WQS) developments (sensitive developments)
are defined as a land development project that has a high potential
to cause loss to local water quality, and could potentially threaten
groundwater reservoirs as determined by the Township. Water quality
sensitive developments include, but are not limited to:
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Vehicle fueling stations
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Industrial manufacturing sites*
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Salvage yards
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Recycling centers
| |
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Hazardous material storage areas*
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Interstate highways
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*
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The Township Engineer will determine if a development is a Sensitive
Development on a case-by-case basis. The Pennsylvania DEP wellhead
protection contaminant source list shall be used as a guide in these
determinations. Industrial manufacturing site and hazardous material
storage areas must provide NPDES SIC codes.
|
2.
In Sensitive Area Districts and Developments, the capture volume
shall be managed in a stormwater facility separate from the peak rate
control facility. Peak rate control facilities in sensitive areas
and developments shall be lined with an impervious liner. For sites
less than two acres in size, or in cases where BMPs are limited to
a total depth of 24 inches, separate peak rate control and capture
volume facilities are not required. In addition, the developer of
any project located within a defined Zone II wellhead protection area,
as defined by PADEP, and/or the owner of a public water supply shall
invite the governing body of the water provider (or appointed representative)
to a preapplication meeting at the conceptual design stage to solicit
input relative to providing appropriate protection for the associated
groundwater resource.
[Ord. 1018, 6/6/2016]
1.
General. Postdevelopment rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff prior to development
for the design storms specified.
2.
Sensitive Area District Boundaries. The location of sensitive areas
or sensitive area districts (SAD) within the watershed are illustrated
on a map included as Exhibit 1 in Appendix B[1] of this chapter.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
3.
Sites Located in More Than One District. For a proposed development
site which is traversed by a SAD boundary, the design criteria for
sensitive areas must be applied if postdevelopment runoff is directed
towards the sensitive area.
4.
Off-Site Areas. Off-site areas that drain from sensitive areas through
a proposed development site that is located entirely in a nonsensitive
area are not required to use or apply the sensitive area criteria.
5.
Site Areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area shall be subject to the design criteria.
6.
Downstream Hydraulic Capacity Analysis. Any downstream or off-site
hydraulic capacity analysis conducted in accordance with these standards
shall use the following criteria for determining adequacy for accepting
increased peak flow rates:
A.
Natural or man-made channels or swales must be able to convey the
postdevelopment runoff associated with a two-year return period event
within their banks at velocities consistent with protection of the
channels from erosion. Acceptable velocities shall be based upon criteria
included in the DEP Erosion and Sediment Pollution Control Program
Manual.
B.
Natural or man-made channels or swales must be able to convey the
postdevelopment twenty-five-year return period runoff without creating
any hazard to persons or property.
C.
Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area must be designed in accordance
with DEP, Chapter 105, regulations (if applicable) and, at a minimum,
pass the postdevelopment twenty-five-year return period runoff.
D.
It must be demonstrated that the downstream conveyance channel, other
stormwater facilities, roadways, or overland areas are capable of
safely conveying the one-hundred-year design storm without causing
damage to buildings or other infrastructure.
E.
Where the downstream conveyance channel or other facility is located
within a special flood hazard area as documented on the Department
of Housing and Urban Development, Federal Insurance Administration
Flood Hazard Boundary Maps for Ferguson Township, it must be demonstrated
that the limits of said flood hazard area are not increased by the
proposed activity.
F.
Stormwater management ponds that fall under the DEP Chapter 105 criteria
of a "dam" must meet the criteria within Chapter 105.
[Ord. 1018, 6/6/2016]
1.
Design criteria and calculation methodologies have been classified
by functional group for presentation as follows: 1) peak runoff rate
discharge requirements; 2) stormwater pond capture volumes; 3) recharge
volumes; 4) storm drain design including conveyance, channel protection,
and stability; 5) water quality standards; and 6) infiltration test
requirements.
2.
These criteria and calculation methodologies have been developed
to simplify stormwater management designs, unify methods, remove model
parameter subjectivity, remove improperly used methods, and to ensure
that stormwater management decisions are based more realistically
on hydrologic processes. In addition, common sense should always be
used as a controlling criteria.
3.
These standards provide consistent and process-oriented design procedures
for application by land development professionals. It is recognized
that in an attempt to generalize the computational procedures, assumptions
have been made which on some occasions may be violated. If such a
violation is identified, alternate standards and procedures may be
applied. Both the violation and the alternate procedures to be applied
must be documented by a hydrologist or hydrogeologist. Any request
for use of alternate standards or procedures under this provision
must be agreed to by the Township Engineer prior to formal submission
of plans for consideration by the Township.
A.
Peak Runoff Rate Control.
(1)
Exemptions.
(a)
Any site where the increase in postdevelopment peak runoff rates
is determined to be negligible by the Township Engineer is exempt
from the requirement to provide stormwater detention. In support of
this exemption, it must be shown that the downstream conveyance systems
have adequate capacity to convey the additional discharge without
adversely affecting downstream properties. This does not exempt the
requirement for implementation of designs for water quality, stormwater
conveyance, and/or recharge as required. A stormwater management site
plan and report documenting these design elements is also required.
The Township Engineer shall use a five-percent increase as a general
benchmark for defining "negligible." The final definition of negligible
shall be at the Township Engineer's discretion. Prior to using
this exemption (and prior to any land development plan submission),
the design engineer must provide written documentation and computations
as to why no peak runoff control should be required. The Township
Engineer has the right to reject any plan that uses this assumption
without prior approval of the Township Engineer. The intent of this
exemption is to eliminate the need for multiple or "piggyback" detention
facilities as a result of minor changes in imperviousness or land
use upstream of existing stormwater control facilities.
(b)
Small sites (less than one acre) located directly adjacent to
the main stem of creeks, or within the floodplain, are not required
to provide stormwater detention unless directed to do so by the Township
Engineer as a result of a documented drainage problem. All other stormwater
management standards must be implemented including water quality,
adequate stormwater conveyance, and/or recharge as required. The Township
Engineer has the right to reject any plan that uses this exemption
without prior approval of the Township Engineer.
(2)
Stormwater management analysis must be performed using the following
models. The size criteria are based on drainage area size including
site area and all off-site area draining across the development.
|
Drainage Area Size
(acres)
|
Model
| |
|---|---|---|
|
Up to 100
|
NRCS's TR-55 or TR-20
| |
|
Over 100
|
NRCS's TR-20 or HEC-1 (HEC-HMS)
|
(a)
The Modified Rational Method using the Gert Aron Curves may
be used for any site less than or equal to two acres in size without
prior authorization from the Township Engineer. The Modified Rational
Method may also be used for sites between two acres and five acres
in size where the Township Engineer has approved the method's
use. In this case the design engineer must make a written request
to the Township Engineer explaining why the use of the Modified Rational
Method is more appropriate than the NRCS's (Natural Resource
Conservation Service) methods for the site in question. The design
engineer should keep in mind that the Modified Rational Formula methodology
was not calibrated to account for the karst nature of the Spring Creek
Drainage Basin; and, therefore, its use should be limited to the special
cases identified above. In addition, since the minimum discharge criteria
are based on a calibration of the NRCS runoff model, their use is
not appropriate if the Modified Rational Method is used for runoff
computations.
(b)
The Township Engineer has the right to reject any stormwater
management design that uses hydrograph combinations with the Modified
Rational Method where the designer has not validated that the effects
of the timing differences are negligible. In addition, the Township
Engineer has the right to reject any stormwater management design
that improperly uses the method for determining runoff volumes or
does not properly apply the method.
(c)
More intensive physically based models may be used at the discretion
of the Township Engineer, but only for sites greater than 100 acres
in size.
(d)
Commercial software packages that use the basic computational
methods of TR-55 or TR-20 are permitted.
(e)
The NRCS models and methods recommended above are based on data
collected from actual watersheds. In contrast to this, stormwater
management analysis for land development activities is often conducted
using property lines to define drainage boundaries. Drainage areas
based on property boundaries are not true watersheds and are referred
to here as "hypothetical" drainage areas. It is known that these hypothetical
drainage areas do not respond like natural watersheds. Peak runoff
rates from hypothetical drainage areas are much smaller than comparable
runoff rates from natural watersheds of the same size. Therefore,
wherever possible, pre- and postdevelopment stormwater analysis should
be conducted for watersheds that are as nearly natural as possible.
Also, conducting stormwater analysis for a lot by lot comparison,
such as within residential developments is not permitted. Partitioning
drainage areas into different subwatersheds for the postdevelopment
scenarios is acceptable.
(f)
It is noted that natural watershed boundaries should not be
used where the relative size of the watershed compared to the site
size would inappropriately distort the pre- to postdevelopment runoff
comparison. In these cases a hypothetical drainage area defined by
the property boundary should be used because it will allow for a better
estimate of runoff changes directly downstream of the site. In addition,
the designer should recognize that, within the Spring Creek Watershed,
typical hypothetical drainage areas, in their predevelopment or natural
condition, do not produce surface runoff during minor to moderate
rainfall events. Available hydrologic models do not accurately reflect
this condition. This often results in postdevelopment nuisance flooding
since the models overestimate the predevelopment runoff magnitude.
(3)
Major natural drainage divides may not be altered without the
prior consent of the Township Engineer.
(4)
Pre- and postdevelopment stormwater management analysis shall
be conducted using the following design storms: one-year, two-year,
ten-year, one-hundred-year.
(5)
The twenty-four-hour precipitation depths as obtained from NOAA
Atlas 14 shall be used for stormwater management analysis for all
of Ferguson Township. PennDOT IDF curves may be used only if required
by PennDOT.
(6)
The NRCS's Type II precipitation distribution is required
for all stormwater management analyses.
(7)
The NRCS's dimensionless unit hydrograph "k" factor shall
be 484 for both pre- and postdevelopment stormwater analyses.
(8)
All undeveloped areas are to be modeled as meadow or woods in
good hydrologic condition. Existing impervious areas may be modeled
as being impervious for predevelopment conditions. The Township may
require a maximum of 20% of the existing impervious be modeled as
meadow in areas where there are known existing stormwater concerns
downstream of the project area or where the site being developed has
either deficient or nonexisting stormwater management facilities.
Developers of sites with existing impervious are highly encouraged
to set up a meeting with the Township prior to design so that any
additional requirements are identified prior to plan submission.
(9)
The NRCS's curve number (CN) shall be used as the rainfall
to runoff transformation parameter for all stormwater management analyses.
(10)
Curve numbers should be rounded to tenths for use in prepackaged
hydrologic models. It should be recognized that the CN is only a design
tool with a large degree of statistical variability. For large sites,
CNs should realistically be rounded to the nearest whole number.
(11)
The NRCS's method to determine unconnected impervious area
adjustments for CN may be used for distinctly defined impervious land
areas that flow onto pervious areas in a dispersed manner. The method
may only be used to calculate runoff from site impervious areas that
actually flow across pervious areas. The method cannot be applied
to the entire site using average weighted CN values.
(12)
Soils underlain by carbonate geology (limestone or dolomite)
shall have a hydrologic soil group (HSG) B used for both pre- and
postdevelopment conditions regardless of the NRCS or Centre County
Soil Survey's description, except for the following two conditions:
(a)
With the exception of minimally disturbed areas, the HSG for
all disturbed pervious areas shall be reduced by a minimum of 1 HSG
level to reflect an increase in runoff associated with the loss of
natural soil structure. No reduction in postconstruction HSG level
is required in areas where soil restoration techniques are applied.
Acceptable soil restoration techniques include those defined in the
current version of the Pennsylvania Stormwater Best Management Practices
Manual. Other techniques may be accepted if approved by the Township
Engineer.
(b)
Soils identified as "on floodplains" or "on terraces above floodplains"
in the Centre County Soil Survey will use the HSG as designated in
the Soil Survey. Refer to Appendix A[1] for a list of the soils.
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
(13)
Soils not underlain by carbonate geology shall use the HSG as
specified by the NRCS or Centre County Soil Survey's description,
except for the following two conditions:
(a)
Wooded areas on HSG C and D soils shall be treated as HSG B
for predevelopment conditions. Disturbed postdevelopment wooded areas
shall carry the NRCS or Centre County Soil Survey's defined HSG
with a minimum HSG of B.
(b)
Highly compacted structural fill areas shall use a minimum of
HSG C for postdevelopment conditions regardless of the NRCS or Centre
County Soil Survey's description. For most developments these
areas are normally covered with impervious surfaces, but may include
islands within parking areas, fringe land, etc. A HSG of C shall also
be used for large projects that clear and grade land for later phases
of development. The Township Engineer shall make the final determination
as to what areas of a land development site constitute compacted structural
fill. The intent is to account for large compacted areas, and not
minor grading within lawn areas or small areas around buildings, etc.
(14)
Areas draining to closed depressions must be modeled by removing
the storage volume from the predevelopment condition. The designer
may assume that infiltration in the closed depression does not occur
during a design runoff event. Areas draining to closed depressions
may also be used to adjust peak runoff rates to stormwater management
ponds for the postdevelopment analysis. This allowance has been developed
to entice designers to intentionally design or leave in place small
closed depressions that can reduce the total volume required from
a stormwater management pond. The site designer is responsible to
document downstream impacts if the closed depression were removed.
(15)
Drainage areas tributary to sinkholes shall be excluded from
the modeled point-of-interest drainage areas defining predevelopment
peak flows. Assumptions that sinkholes spillover during some storm
events must be supported by acceptable documentation as determined
by the Township Engineer. In addition, the design professional must
be aware that bypassing or sealing sinkholes will frequently result
in downstream flooding and should not be done if existing downstream
flooding already occurs. The site designer is responsible to document
downstream impacts if the sinkhole were to stop taking stormwater
runoff.
(16)
Ponds or other permanent pools of water are to be modeled by
the methods established in the NRCS's TR-55 Manual (1986). However,
more rigorous documented methods are acceptable as determined by the
Township Engineer.
(17)
The NRCS antecedent runoff condition II (ARC II, previously
AMC II) must be used for all simulations. The use of continuous simulation
models that vary the ARC are not permitted for stormwater management
purposes. In addition, prior to any continuous simulation model being
used in the Spring Creek Basin for any other purposes, the model unit
hydrograph must be modified for common events in additional to extreme
events based on an in-depth analysis of historical data from the basin.
(18)
Time of Concentration.
(a)
The following time of concentration (Tc) computational methodologies
shall be used unless another method is preapproved by the Township
Engineer:
1)
Predevelopment: NRCS's Lag Equation.
2)
Postdevelopment; commercial, industrial, or other areas with
large impervious areas (greater than 20% impervious area): NRCS's
Segmental Method.
3)
Postdevelopment; residential, cluster, or other low-impact designs
less than or equal to 20% impervious area: NCRS's Lag Equation.
(b)
The time of concentration is to represent the average condition
that best reflects the hydrologic response of the area. For example,
large impervious areas bordered by small pervious areas may not consider
the effect of the pervious areas in the Tc computation. If the designer
wants to consider the effect of the pervious area, runoff from the
pervious and impervious areas must be computed separately with the
hydrographs being combined to determine the total runoff from the
area.
(c)
Under no circumstance will the postdevelopment Tc be greater
than the predevelopment Tc for any watershed or subwatershed modeling
purposes. This includes when the designer has specifically used swales
to reduce flow velocities. In the event that the designer believes
that the postdevelopment Tc is greater, it will still be set by default
equal to the predevelopment Tc for modeling purposes.
(19)
Postdevelopment Minimum Discharges.
(a)
The following postdevelopment minimum discharges are permitted
for use with the NRCS (CN) runoff model*:
|
Return Period
|
Minimum Discharge
| |
|---|---|---|
|
1-year
|
Qpmin = 0.018 (DA) + 0.2
| |
|
2-year
|
Qpmin = 0.03 (DA) + 0.4
| |
|
10-year
|
Qpmin = 0.09 (DA) + 1.0
|
|
Where:
| ||||
|
DA
|
=
|
the drainage area in acres
| ||
|
Qpmin
|
=
|
minimum allowable peak runoff rate in cfs
| ||
(b)
For return periods greater than 10 years, the minimum discharge
shall be equal to the computed predevelopment peak runoff rate.
(c)
The minimum discharge criteria above are not appropriate for
use with the Modified Rational Method. This is because these values
were developed based on NRCS model corrections and do not actually
represent a true physical process or discharge. However, common sense
should be used by both the designer and reviewer in the evaluation
of acceptable minimum discharges for use with the Modified Rational
Method.
(d)
The intent of the minimum discharge is to allow reasonable runoff
release from a site when a hydrologic model has produced a predevelopment
runoff rate close to zero. The method is not permitted for areas that
previously drained completely to sinkholes in order to bypass the
sinkhole after development.
(e)
These minimum discharge values include the total of all stormwater
management facilities discharges and undetained area discharges. Peak
runoff rates for undetained fringe areas (where the designer has made
a realistic effort to control all new impervious areas) will be computed
using the predevelopment time of concentration for the drainage areas
tributary to them. Undetained areas are those portions of the site
that cannot be routed to a stormwater management facility due to topography
and typically include lower pond berms, or small areas around entrance
drives. The site drainage areas used shall represent the predevelopment
condition, even if drainage areas are altered following development.
(20)
All lined stormwater management ponds in carbonate and noncarbonate
areas must be considered impervious and may not be used as pervious
areas for stormwater management computations. "Lined" here means lined
with synthetic liners or Bentonite. All other compacted soil liners
will be considered to be HSG D for hydrologic computations.
(21)
Stormwater management ponds that have a capture depth for the
purposes of water quality or volume capture shall assume a negligible
discharge from these structures during design event routing. Only
discharges from the primary principal spillway or emergency spillway
need to be considered. Discharges from subsurface drains that tie
into a principal spillway should not be considered during design event
routing. All subsurface drains are to be equipped with a restrictor
plate with a one-inch opening in order to prevent the subsurface drain
from functioning as a primary orifice. A restrictor plate orifice
may be increased if necessary to the smallest size required to drain
the facility within the required 72 hours.
(22)
Stormwater management ponds that have a pond capture, recharge,
or water quality component shall assume that the basin is full to
the controlling component volume at the beginning of design event
routing.
(23)
Stormwater management ponds must provide safe passage of the
one-hundred-year return period peak runoff rate assuming that all
of the principal spillway orifices are fully clogged, and the principal
spillway overflow is 50% clogged. A minimum of a six-inch freeboard
must also be maintained above the resulting maximum water surface
elevations (W.S.E.). Any embankment emergency spillway may be assumed
to be unclogged. Stormwater management ponds with embankments completely
made up of natural undisturbed soils (fully in "cut") or where roadways
act as the emergency spillway are permitted. However, the design engineer
must verify downstream stability and control.
(24)
All pre- and postdevelopment comparisons of peak flows shall
be rounded to tenths of a cfs. The intent here is to recognize the
accuracy and precision limitations of hydrologic modeling procedures.
Again, small differences between pre- and postdevelopment discharge
rates should be permitted when no negative downstream impacts will
result.
(25)
The full Modified Puls routing method must be used for stormwater
management pond analyses. Simplified methods of determining pond size
requirements such as those in TR-55 (1986) may only be used for preliminary
pond size estimates. The full Modified Puls routing method must be
used for stormwater management pond analyses.
(26)
Prepackaged hydraulic programs are not approved for the analysis
of underground stormwater management facilities unless it can be verified
that the program rounding subroutines used for the stage/storage data
do not affect the results. This is because, for very small storage
volumes, the program may round off the volume to a significant percentage.
(27)
Full supporting documentation must be provided for all stormwater
management designs.
(28)
Designs must be checked for impervious area flash (IAF). This
check is used to determine if flooding may occur due to poor modeling
choices specifically related to the time of concentration. This analysis
requires that the watershed impervious area be modeled without the
pervious areas. The time of concentration should also be determined
from the impervious areas only. If the IAF analysis results in a higher
peak runoff rate at a culvert or discharge from a pond, this higher
rate must be used for the final design/comparison. The check will
frequently yield higher values if a watershed's impervious area
is located primarily near the watershed outlet or point of interest.
B.
Capture Volumes (Cv).
(1)
To minimize nuisance flooding from small precipitation events,
a runoff capture volume is required for all stormwater management
ponds that do not discharge directly to natural, well-defined (with
bed and banks) perennial streams. In general, natural well-defined
streams in the Spring Creek Basin and all of Ferguson Township are
limited to those delineated as USGS (U.S. Geological Survey) perennial
streams. This should be treated as a guideline and not a steadfast
rule. The final determination is at the discretion of the Township
Engineer. The pond capture volume is a volume of runoff that will
be retained in a pond below the elevation of any free surface principal
spillway orifice. In addition to the minimum required capture volume
identified within this section, the capture volume may also include
all or portions of the recharge volume and water quality volume (see
Subsection 3C and E below). No principal spillway orifice (except
those connected to subsurface drains), regardless of how small, shall
be below the pond elevation equivalent to this volume.
(2)
The Centre County Conservation District (CCCD) receives numerous
complaints regarding ponds that are located at the downslope edge
of a property that result in discharging runoff onto downstream properties
in an uncontrolled manner or where no existing defined outlet channel
exists. This is a very common problem in areas underlain by carbonate
rock. These discharges can cause erosion and flooding downstream.
While the pond capture volume is intended to minimize some of these
negative effects, it cannot deter or reduce the impacts from poor
design practices. Therefore, whenever possible, the CCCD recommends
that the designer consider the downstream morphological changes that
may occur and, when possible, consider constructing conveyance systems
to a stable natural channel. In some cases this may require cooperation
between landowners.
(3)
A minimum capture volume is required to mitigate nuisance flooding
from small precipitation events. The minimum required capture volume
is defined as a runoff depth of 0.25 inch from all impervious areas
tributary to the stormwater management facility. This volume will
be allowed to infiltrate, evaporate, or dewater from a subsurface
drain system connected directly to the facility's principal spillway.
(4)
The maximum capture depth shall be 24 inches. [NOTE: Twenty-four
inches is identified as the maximum depth of water recommended for
infiltration facilities in the PA BMP Manual (Appendix C, Protocol
2 - Infiltration System Design and Construction Guidelines).] The
Township Engineer reserves the right to require that the capture and
peak rate control be managed in separate facilities in cases where
potential sinkhole damage could cause impacts to public facilities
and/or off-site areas. Special consideration with regard to allowable
capture depth and dewatering time shall be given to stormwater designs
relying on total infiltration, or where capture of the ten-year event
or larger is proposed to be infiltrated due to site constraints (on
site or off site). In these cases a preapplication meeting shall be
held with the Township Engineer to identify an appropriate design
capture depth and dewatering time. Any increase in capture depth above
24 inches must be supported by a detailed geotechnical report in support
of the increased capture depth.
(5)
Supporting computations that show that 90% of the capture volume
can dewater in a maximum of 72 hours must be provided. The dewatering
time must also provide for sufficient passive water quality treatment.
Infiltration rates are to be maintained at less than six inches per
hour unless it is demonstrated that the soils cation exchange rate
is greater than five meg/100 g in which case the maximum infiltration
rate shall be 10 inches per hour. Basins outfitted with underdrains
for dewatering shall incorporate an orifice at the underdrain outlet
to maintain dewatering times as specified above.
(6)
To simplify computational requirements for design event analysis,
designers do not need to calculate discharges from subsurface drains
related to the capture volumes if the filter media is sand, or smaller
than AASHTO (American Association of State Highway and Transportation
Officials) 57 stone. The capture volume is to control runoff rates
from impervious areas and is not related to water quality. However,
pond designs that include a water quality volume that is greater than
the required capture volume are assumed to have also met the required
capture volume as long as it dewaters as required.
(7)
Designs that rely on the natural infiltration of insitu soils
must provide documentation supporting the infiltration rates used
for analysis. Infiltration rates reported in the Soil Survey of Centre
County or other published rates may be used at the discretion of the
Township Engineer.
(8)
The pond capture volume should always be used when up-slope
areas are developed where the pond's design creates a point discharge
that did not previously exist.
(9)
Stormwater management detention facilities that connect directly
to storm drainpipe networks discharging to natural well-defined channels
do not require a capture volume.
C.
Recharge Volumes (Rv).
(1)
The purpose of the recharge portion of the Part is two-fold.
First, the recharge requirement is to mitigate the loss of groundwater
recharge associated with the creation of impervious surfaces. In addition
the recharge criterion is to mitigate the increase in runoff volume
associated with the creation of impervious surfaces. This increase
in runoff volume has significant impacts on downstream landowners.
These impacts are most often exhibited in the form of increased nuisance
flooding and channel or drainageway erosion and instability. According
to local municipal engineers and representatives of the CCCD, these
problems are of significant local concern. The magnitude of these
problems increases with the percentage of impervious coverage created
on a site.
(2)
Recharge mitigation shall be provided for runoff from all proposed
impervious areas. The required recharge volume shall be computed as
0.5 inch of runoff from all proposed impervious areas. It is noted
that lined detention ponds and compacted fill areas are considered
to be impervious when calculating site impervious area for recharge
considerations. In addition, land areas covered by paver blocks, pervious
pavement, and other structural surface treatments which permit surface
infiltration may be treated as pervious areas when calculating the
site impervious area for recharge considerations as long as the structural
infiltration practice is supported by sound design and appropriate
construction specifications. The Township Engineer may require submission
of supporting documentation prior to approving structural infiltration
areas as pervious areas. It must also be demonstrated that the runoff
from the area directly tributary to recharge BMPs will satisfy the
recharge volume requirement during the one-year twenty-four-hour storm.
(3)
The following design practices may be used as credits to reduce
the recharge volume requirement:
(a)
Residential roof areas (detached, duplex, and townhouse dwellings)
and commercial/industrial buildings with roof areas less than 5,000
square feet may be removed from the computed impervious area when
these roof areas are sumped to dry wells designed in accordance with
the following minimum standard:
1)
Sump Design Criteria. To meet the recharge criteria, sump storage
or voids volume shall be equal to 0.04 cubic feet per square foot
of roof area (0.5 inch rainfall depth). If sump stone has a voids
ratio of 40%, the total sump volume will be 0.10 cubic feet per square
foot of roof area. When designed only to meet this recharge criteria,
the maximum size for a single sump is 100 cubic feet, and the minimum
sump surface area (A) to depth (D) ratio (A/D) must be a minimum of
4/1. The sump depth less any freeboard shall not exceed 24 inches.
This roof sump standard shall apply unless the Township adopts a separate
roof sump standard for water quantity or peak control.
(b)
All or portions of driveways, roadways, and parking areas may
be removed from the impervious area calculation when sheet flow from
these areas is directed to undisturbed natural buffer/filter areas
or constructed filter strips. This flow must be dispersed as sheet
flow as it crosses the buffer/filter area. Sheet flow velocities should
be nonerosive as they cross the impervious area/filter interface.
1)
To ensure proper infiltration characteristics
the natural soil profile within natural buffer/filter areas cannot
be disturbed during construction. Completely undisturbed natural recharge
areas serve this function best. However, minor surface scaring, seeding,
and landscaping is permitted in these areas as long as natural grades
are not altered. In special cases, when approved by the Township Engineer,
minor grading, combined with soil profile reconstruction may be permitted
in natural buffer/filter areas. In addition, the following standards
apply to natural filter/buffer areas:
a)
Natural filter/buffer areas must have a minimum
width of five feet or one-half of the impervious area drainage length
immediately tributary to the buffer area, whichever is greater. This
width is measured parallel to the direction of sheet flow.
b)
To qualify for a recharge volume credit, the surface
slope of natural filter/buffer areas must be conducive to recharge,
and not result in flow concentration or erosion. To meet this intent,
the surface slope of the area tributary to the natural buffer/filter
area, and the surface slope of the natural buffer/filter area itself,
may not exceed 5%. In special cases steeper slopes may be used if
specifically authorized by the Township Engineer.
c)
The total impervious area tributary to a natural
buffer/filter area cannot exceed twice the buffer/filter area.
2)
To qualify for a recharge volume
credit, constructed filter strips shall be designed to the following
standards:
a)
The minimum filter strip width shall be five feet
or one-half of the impervious area drainage length immediately tributary
to the constructed filter strip, whichever is greater. This width
is measured parallel to the direction of sheet flow.
b)
The total impervious area tributary to a constructed
filter strip area cannot exceed twice the constructed filter strip
area.
c)
The surface slope of the area tributary to the constructed
filter strip area, and the surface slope of the constructed filter
strip area itself may not exceed 5% and 3% respectively. In special
cases steeper slopes may be used if specifically authorized by the
Township Engineer.
d)
The filter strip surface shall consist of a minimum
of six inches of natural or reconstructed topsoil with a stable grass
surface treatment. Reconstructed topsoil designs must be approved
by the Township Engineer prior to application. Reconstructed topsoil
consists of soils augmented by tillage and the addition of soil amendments
such as fertilizers and organic material. Fertilizers include any
organic or inorganic plant nutrients or soil amendments. Organic material
is any nutrient or compound containing animal or plant materials such
as manure or compost.
e)
To minimize erosion of the topsoil layer during
construction, it is recommended that these areas be sodded. However,
the Township Engineer may permit the use of an acceptable erosion
control seeding application. In this later case, any loss of topsoil
and seed must be replaced until a permanent vegetative stand is achieved.
(c)
Sidewalks separated from roadways and/or other impervious surfaces
by a grass strip of equal or greater width than the sidewalk itself
may be removed from the impervious area calculation when the sidewalks
are graded so that sheet flow from the walk is directed to the grass
strip. Sidewalks with steep longitudinal slopes that themselves would
act as channels during runoff events cannot take advantage of this
credit. A five-percent longitudinal sidewalk slope shall be used as
the benchmark defining steep slopes.
(d)
Impervious areas tributary to natural closed depressions may
be subtracted from the total site impervious area used in the recharge
volume calculation as long as a qualified geotechnical engineer or
soil scientist certifies to the soundness of these site-specific applications.
Water quality pretreatment may be necessary prior to the direct discharge
of runoff to existing closed depressions or sinkholes.
(e)
Impervious areas tributary to man-made closed depressions may
be subtracted from the total site impervious area as long as a qualified
geotechnical engineer or soil scientist certifies to the soundness
of these site-specific applications. Man-made closed depressions may
be created through the use of low head berms one foot or less in height.
(f)
Additional credits may apply for undisturbed land areas that
are known to have high infiltration capacity and that are maintained
or enhanced. These areas must be defined and quantified from actual
site data collection.
(4)
After credits, the remaining recharge volume shall be directed
to a recharge BMP such as infiltration trenches, beds, etc. These
facilities may be located in open areas or under pavement structures.
The appropriateness of the particular infiltration practice proposed,
as well as the design parameters used, shall be supported by a geotechnical
report certified by a qualified professional (soil scientist, geologist,
hydrogeologist, geotechnical engineer).
(5)
Stormwater recharge requirements or credits affect stormwater
management design requirements. For stormwater management computations,
the reduction of site CNs based only on a weighting type analysis,
as is sometimes done for cluster type developments, is not permitted.
However, for stormwater management purposes, the CN for recharged
areas may be computed using the NRCS method for disconnected impervious
areas. The actual hydrologic process that occurs within the basin
must be stressed in all recharge situations.
(6)
These recharge requirements must be met on all sites unless
it can be demonstrated that recharge would be inappropriate. Any request
for such a waiver from these recharge requirements must be accompanied
by a supporting report certified by a qualified professional (soil
scientist, geologist, hydrogeologist, geotechnical engineer).
(7)
Developers and site design professionals are encouraged to use
a higher standard for recharge volume on sites where local site conditions
do not restrict a higher standard.
(8)
Water quality sensitive (WQS) developments must use an acceptable
pretreatment BMP prior to recharge. Acceptable pretreated BMPs for
these developments include BMPs that are based on filtering, settling,
or chemical reaction processes such as chemical coagulation. If infiltration
devices, including subsurface exfiltration devices, are to be utilized
as a part of the SWM design for a development in the WQS district,
the infiltration rate should be a maximum six in./hr. to allow proper
soil filtration/passive treatment of the runoff. A maximum infiltration
rate of 10 in./hr. may be used for soils with demonstrated cation
exchange rates greater than five. This infiltration rate may be achieved
through soil amendments or engineered soil media.
(9)
Accounting for recharge within lined stormwater management ponds
is not permitted. However, if unlined, uncompacted ponds and/or depressed
lawn areas are used to satisfy water quality or capture volume criteria,
these areas and volumes may also be used to meet recharge requirements
as previously defined. Additional recharge volume may be credited
to these areas as long as it is demonstrated by a qualified professional
that recharge processes can naturally occur in these areas.
(10)
Finally, because this analysis is concerned with trying to adequately
represent real processes that occur within the watershed, there will
be areas that cannot physically recharge stormwater. These areas include
exfiltration areas that are commonly found at the base of wooded hillsides
where clay pans exist, and saturation areas near major streams or
floodplains. These areas may not accept recharge during most runoff
events. These areas are exempt from recharge requirements when these
conditions are documented and certified by a qualified professional
(soil scientist, geologist, hydrogeologist, or geotechnical engineer).
In addition, stormwater management techniques relying on infiltration
techniques are not permitted in these areas.
(11)
The Township Engineer may waive the recharge requirement in
the following situations:
(a)
The Township Engineer may waive the recharge requirement in
highly developed areas or areas undergoing redevelopment where the
Township Engineer has determined that forced recharge could have adverse
impacts on adjacent landowner structures, property, or Township infrastructure.
These waivers should be limited to small land areas (generally less
than five acres in size), where the ability to place recharge beds
may be limited or may hinder redevelopment.
(b)
The Township Engineer may waive the recharge requirement in
areas where a qualified soils scientist or geologist has determined
that none of the site soils are suitable for recharge, or that the
location of the suitable soils is such that harm to adjoining properties
could occur as stated under Subsection 11A above.
(c)
The Township Engineer may waive the recharge requirement in
areas where recharge cannot physically occur as documented by a qualified
soil scientist, geologist, or hydrologist. These areas include:
(12)
As identified above, recharge analysis and/or waiver requests
must be supported by a geotechnical report sealed by a qualified professional
(soil scientist, geologist, hydrogeologist, or geotechnical engineer).
The intent of this report will be to establish the suitability of
a particular parcel of land or area for recharge, and to identify
areas on a development site appropriate for recharge. It is recommended
that the geotechnical/soils consultant discuss the extent and approach
to the analysis with the Township Engineer prior to initiating the
field investigation. At a minimum this report should include the following
information:
(a)
A description of the geotechnical site investigation performed
including the methods and procedures used.
(b)
Data presentation.
(c)
Analysis results including the following minimum information:
1)
A map identifying site areas inappropriate for recharge along
with supporting justification. In addition to illustrating topographic
features, significant geologic and hydrologic features should be identified
(rock outcrops, sinkholes, closed depressions, etc).
2)
Determination of the permeability coefficient for potential
recharge areas.
3)
Determination of the infiltration capacity of natural site soils.
4)
Location, depth, and permeability coefficient for any restrictive
layers identified.
5)
Soil uniformity.
6)
Depth to bedrock in potential recharge areas, and a statement
reflecting the uniformity of the depth to bedrock across the site.
7)
A statement relating to the site's proximity to fracture
zones within the bedrock.
8)
Additional information deemed pertinent by the geotechnical
engineer.
9)
In sensitive area
districts, projects shall also comply with the following requirements
for a preliminary geologic investigation:
a)
A qualified professional (soil scientist, geologist,
hydrogeologist, or geotechnical engineer) shall review aerial photographs,
soils, geologic and other available related data including the local
public water supplier's source water protection study, as the
data relates to the subject property. The qualified professional shall
also conduct a comprehensive field view and inspection of the property,
and consult with the public water supplier's consulting hydrogeologist.
Based on the above information, the professional shall prepare a map
of the site indicating the following:
b)
Based on the map prepared above, and in conjunction
with other mapping and available information, the geologist shall
prepare a report on project suitability and the impact potential on
the groundwater resource of the water quality sensitive areas. The
report shall demonstrate compliance with the requirements of this
chapter, including any recommended mitigating measures designed to
ensure compliance.
c)
In addition, infiltration facilities shall be located
greater than 50 feet from any critical geologic features identified
in the geotechnical report or geophysical testing report unless the
reports provide for approved mitigating measures that will produce
no negative impact on groundwater recharge or quality.
10)
If the preliminary geologic investigation identifies
a significant potential for sinkhole formation, or where soil and
subsurface conditions may not be suitable, additional geologic investigations
may be determined to be necessary. This determination will be at the
discretion of the Township Engineer. The additional investigations
and analysis may include electrical resistivity tomography or other
similar types of geophysical testing. A detailed report of the geophysical
testing along with recommendations for avoidance of sinkhole-prone
areas shall be included in the stormwater management site plan.
(d)
Recommendations for any special design considerations necessary
for the design of recharge systems on the site. For example, required
soil depth over bedrock, appropriate surface grades over recharge
areas, appropriate hydraulic head over recharge areas, alternative
siting, increased use of decentralized and nonstructural SWM techniques
and/or remediation recommendations must be included in the report.
(e)
Justification as to why the site should be developed to a high
impervious density if the site has adverse soil and geotechnical limitations,
which prohibit the ability to induce natural recharge. Explanation
how these limitations will not create the potential for undue harm
to the environment and the Spring Creek Watershed or other any area
in Ferguson Township when the site is developed.
(f)
Where it has been shown that recharge cannot be performed and
a waiver of the recharge requirements is being requested, the Township
shall require that the first one inch of runoff from all new impervious
areas be treated through underdrained facilities. These facilities
may include underdrained basins, rain gardens, and infiltration trenches.
Treatment is to include use of an amended topsoil to provide filtration
of the stormwater. All underdrain outlets are to include a restrictor
plate to prevent the underdrain system from functioning as a primary
outlet.
(13)
The following guidelines apply to the use of subsurface exfiltration
BMPs (often incorrectly referred to as engineered infiltration BMPs):
(a)
Soils shall have minimum infiltration rates as specified in § 26-304, Subsection 3F. If no site soils have percolation rates meeting these standards, subsurface exfiltration BMPs shall not be used.
(b)
A minimum of 30 inches of soil shall be maintained between the
bottom of a subsurface exfiltration BMP and the top of bedrock or
seasonally high groundwater table (the upper surface of a layer of
saturated material in the soil), unless a qualified geotechnical engineer
provides documentation supporting a design depth less than 30 inches
and the minimum depth is approved by the Township Engineer.
(c)
If the minimum percolation rate is not met and/or the minimum
soil depth cannot be maintained on a site, recharge should be accommodated
by directing shallow sheet flow from impervious areas across surface
filter strips and/or undisturbed natural areas, or some other innovative
surface infiltration feature should be used. Limiting subsurface percolation
rates and/or depth to bedrock shall not by themselves warrant a recharge
waiver.
(14)
In addition, since recharge is intended as a volume control,
innovative or new methods that address the significant increase in
the volume of runoff from sites having large impervious areas are
encouraged. These volume control alternatives may be used only if
they can be shown to function with the original intent through sound
engineering and science. The final determination of "original intent"
shall always be the right of the Township Engineer.
D.
Storm Drain Conveyance System Design. Storm drainage conveyance systems
consist of storm sewer pipes, swales, and open channels. Computational
methods for design of storm drain conveyance systems shall be as follows:
(1)
Recommended computational methods (models) for storm drain design
are based on site or watershed drainage area as follows:
|
Size
|
Model
| |
|---|---|---|
|
Up to 200 acres
|
Rational Method
| |
|
Between 200 acres and 1.5 square miles
|
HEC-1
PSRM
TR-20
| |
|
Over 1.5 square miles
|
PSU-IV with the carbonate adjustment factor at the discretion
of the Township Engineer
| |
|
Other methods as approved by the Township Engineer such as SWMM,
SWIRM-ROUTE, etc.
|
(2)
Rational Coefficients used are to be from Rawls et al. (1981),
PADOT Design Manual 2-10 or using the Aron curves (Appendix A, Table
3)[2] to convert CNs to C. For previously disturbed or newly
disturbed pervious areas, runoff coefficients should be appropriately
adjusted to reflect an increase in runoff associated with the loss
of natural soil structure. If the Aron curves are used, CNs must also
be adjusted to reflect the loss of soil structure in disturbed areas
by reducing the HSG as identified by the NRCS by a minimum of 1 HSG
level. For example, if the natural HSG designation is B, an HSG of
C should be used for disturbed soils. These disturbed area adjustments
to HSG levels do not apply to minimally disturbed areas. The design
engineer may choose to use the following Rational C coefficients without
regard to soil HSG for small sites. However, it is recommended that
they be used only for storm drains up to 24 inches in diameter. The
use of these conservative values shall fully be the choice of the
design engineer.
|
All impervious areas
|
C = 0.95
| |
|
All pervious areas
|
C = 0.30
|
[2]
Editor's Note: Appendix A is included as an attachment to this chapter.
(3)
Storm drains shall be designed at a minimum using a ten-year
runoff event without surcharging inlets. Storm drains tributary to
a multiple site stormwater management facility across Township roads
or crossing other properties must convey, at a minimum, a twenty-five-year
runoff event without surcharging inlets. Runoff events in excess of
the indicated design event must be conveyed safely downstream.
(4)
Inlets on grade cannot assume a sumped condition for hydraulic
modeling (i.e., top of inlet casting set below pavement surface in
parking areas).
(5)
The Township Engineer may require the analysis of the one-hundred-year
peak runoff rates for conveyance purposes in some instances where
regional stormwater management facilities are employed.
(6)
Any storm drain within state or federal rights-of-way or one
that falls under the design criteria of any higher authority must
meet the requirements of that agency in addition to the minimum requirements
of this chapter.
(7)
The time of concentration (Tc) may be computed by any method
which best represents the subject watershed. However, the NRCS's
segmental method is not recommended for use with drainage areas that
are predominately undeveloped and are greater than 100 acres in size.
The NRCS Lag Equation or another more appropriate method should be
used under these conditions.
(8)
For any drainage area smaller than five acres in size, a Tc
of five minutes may always be assumed at the discretion of the design
engineer (for the postdevelopment condition), without needing to provide
supporting documentation.
(9)
Precipitation values shall be taken from the current version
of NOAA Atlas 14 at a station closest to the project site and/or most
representative of the project site. The current version of the PennDOT
IDF curves can be used only if required by PennDOT. Appropriate documentation
supporting the selected precipitation values used shall be provided.
(10)
Storm drain conveyance system stability (swales, open channels,
and pipe discharge aprons) shall be computed using a ten-year return
period peak runoff rate.
(11)
Storm sewers, where required by zoning and land use densities,
shall be placed under or immediately adjacent to the roadway side
of the curb, or as directed by the Township, when parallel to the
street within the right-of-way.
(12)
When located in undedicated land, storm sewers shall be placed
within a drainage easement not less than 20 feet wide as approved
by the Township Engineer.
(13)
The use of properly designed, graded and turfed drainage swales
is encouraged in lieu of storm sewers in commercial and industrial
areas and, where approved by the Township Engineer, in residential
areas. Such swales shall be designed not only to carry the required
discharge without excessive erosion, but also to increase the time
of concentration, reduce the peak discharge and velocity, and permit
the water to percolate into the soil, where appropriate.
(14)
Inlets.
(a)
Inlet types and inlet assemblies shall conform to the Pennsylvania
Department of Transportation Standards for Roadway Construction as
approved by the Township Engineer.
1)
Inlets shall, at a minimum, be located at the lowest point of
street intersections to intercept the stormwater before it reaches
pedestrian crossing; or at sag points of vertical curves in the street
alignment which provide a natural point of ponding of surface stormwater.
2)
Where the Township deems it necessary because of special land
requirements, special inlets may be approved.
(b)
In curbed sections, the maximum encroachment of water on the
roadway pavement shall not exceed half of a through traffic lane or
one inch less than the depth of curb during the ten-year design storm
of five-minute duration. Inlets shall be provided to control the encroachment
of water on the pavement. When inlets are used in a storm system within
the right-of-way limits of a street in lieu of manholes, the spacing
of such inlets shall not exceed the maximum distance of 450 feet.
(15)
Accessible drainage structures shall be located on a continuous
storm sewer system at all vertical dislocations, at all locations
where a transition in storm sewer pipe sizing is required, at all
vertical and horizontal angle points exceeding 5°, and at all
points of convergence of two or more influent storm sewer mains. The
construction locations of accessible drainage structures shall be
as indicated on the land development stormwater management plan or
area stormwater management plan approved by the Township.
(16)
When evidence available to the Township indicates that existing
storm sewers have sufficient capacity as determined by hydrograph
summation and are accessible, the subdivider may connect its stormwater
facilities to the existing storm sewers so long as the peak rate of
discharge does not exceed the amount permitted by this chapter.
(17)
All storm drain culvert pipes shall be designed to maintain
a minimum grade of 1/2%. All storm pipes shall have a minimum inside
diameter of 15 inches or a cross-sectional area of 176 square inches,
except that pipes under 20 feet or greater fill shall not be less
than 24 inches or a cross-sectional area of 453 inches and shall consist
of reinforced concrete.
(18)
Computational procedures other than those indicated here should
follow the methods of the Federal Highway Administration's Urban
Drainage Design Manual [Hydraulic Engineering Circular No 22. (HEC-22)].
(19)
Swales provided in cut areas shall be designed such that stormwater
will not encroach upon the shoulder during a twenty-five-year frequency
storm. Flooding of the subbase shall be avoided.
E.
Water Quality Standards.
(1)
Water Quality Performance Standards. To minimize adverse impacts
to stream health resulting from stormwater nonpoint source (NPS) pollution,
standards are provided for the implementation of water quality best
management practices (BMPs) to reduce NPS pollutant loadings resulting
from land development activities. The following performance standards
and guidelines shall be addressed at all sites where stormwater management
is required.
(a)
Site designs shall minimize the generation of stormwater runoff
through the use of low-impact design techniques.
(b)
Stormwater runoff from all land development activities should
be treated through the use of nonstructural and structural BMPs to
effectively treat the adverse impacts of stormwater runoff including
NPS pollutants.
(c)
Water quality BMPs shall be incorporated into site designs to
treat the required water quality volume as defined below.
(d)
The use of nonstructural BMPs shall always take priority over
the use of structural BMPs. The use of innovative BMPs and low-impact
site planning is encouraged to reduce the generation of stormwater
runoff and effectively treat pollutants transported in stormwater
from the site.
(e)
The use of multiple nonstructural water quality techniques along
with new, emerging, and innovative techniques is encouraged to improve
the quality of stormwater runoff to receiving areas and reduce and/or
eliminate the need for structural BMPs. The Township Engineer should
be consulted to clarify the design concept for meeting or exceeding
the intent of this section.
(f)
Where nonstructural BMPs are unable to effectively treat all
of the stormwater runoff generated from land development activities,
structural BMPs shall be designed to capture and treat the computed
water quality volume (WQv).
(g)
The priority pollutant source areas to be treated with BMPs
are streets, parking lots, driveways, and roof areas.
(h)
Due to the karst nature of the watershed, stormwater discharges
from water quality sensitive developments and discharges to sensitive
wellhead protection areas (defined in Appendix B, Exhibit 1)[3] will require special consideration. In these instances,
the applicant shall provide water quality pretreatment (use of a filtering
BMP and/or special structural design features) to prevent the discharge
of stormwater contaminants to groundwater resources. In addition,
preliminary hydrogeologic studies shall be required to document potential
karst-related impacts.
[3]
Editor's Note: Appendix B is included as an attachment to this chapter.
(i)
In water quality sensitive area districts groundwater quality
monitoring wells may be required for development sites greater than
10 acres in size. The need for water quality monitoring wells will
be determined on a case-by-case basis by the Township Engineer based
on the results of the geologic investigations.
(j)
In water quality sensitive area districts where rock excavation
is anticipated as part of the project construction, the following
note shall be added to the plan:
|
"This project is located within a source water protection area
for the (insert name of water provider). Blasting should be used as
a last resort method of rock excavation. If blasting techniques are
to be employed, the contractor shall provide the Township and the
water provider with information supporting the use of blasting techniques
along with copies of blasting permits."
|
(k)
Prior to stormwater management and water quality design, applicants
should consult with the Township Engineer to verify stormwater quality
criteria and present proposed features and concepts for the treatment
of stormwater runoff. Following this meeting, the Township Engineer
shall define any needed support studies or documentation.
(2)
Water Quality Volume (WQv).
(a)
The required water quality volume that must be treated for nonsensitive
areas underlain by carbonate rock (see exhibits in Appendix B)[4] within the Township shall be computed as:
|
WQdepth = 0.25 + (0.012)2.9(0.044(SIA))
| |
|
WQv = WQdepth(A)/12
|
|
Where:
| ||||
|
WQv
|
=
|
water quality volume in acre-feet
| ||
|
WQdepth
|
=
|
depth in inches that must be captured for impervious areas
| ||
|
SIA
|
=
|
percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| ||
|
A
|
=
|
total of all paved areas and asphalt-based roofs on site in
acres
| ||
[4]
Editor's Note: Appendix B is included as an attachment to this chapter.
(b)
The required water quality volume that must be treated for any
WQS development, on sites in sensitive areas underlain by carbonate
rock, and all areas not underlain by carbonate rock is to be computed
within the entire Spring Creek Basin as:
|
WQdepth = the larger of 0.5 inches or
0.25 + (0.012)2.9(0.044(SIA))
| |
|
WQv = WQdepth(A)/12
|
|
Where:
| ||||
|
WQv
|
=
|
water quality volume in acre-feet
| ||
|
WQdepth
|
=
|
depth in inches that must be captured for impervious areas
| ||
|
SIA
|
=
|
percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| ||
|
A
|
=
|
total of all paved areas and asphalt-based roofs on site in
acres
| ||
(c)
For designs in which the final roof material is unknown, the
design engineer must assume an asphalt-based roof.
(d)
The water quality volume must be captured and treated through
a water quality BMP over an extended period of time as per the specific
requirements of each structure. Credits to reduce the effective impervious
area are applicable as presented in this section.
(3)
Water Quality Credits. Due to the karst nature of the Spring
Creek Basin and areas of Ferguson Township, the nonstructural water
quality credits and techniques identified below may be limited for
suitability and use based on development type and location. These
limitations for use are specified in the restrictions section for
each credit. The Township Engineer may require additional documentation
or investigation prior to use of each specific credit to reduce the
risks of sinkhole development or groundwater contamination for sensitive
areas and development types. No area may be double counted for use
with credits. The combined credits of natural area conservation and
vegetated filter strips is limited to 50% of the site's impervious
area. The drainageway credit is limited to 50% of the site's
impervious area. The drainageway protection credit is limited to 50%
of the site's impervious area. The maximum total water quality
credit for any site may therefore be 100% of the site's impervious
area.
|
Nonstructural Technique
|
Water Quality Credit
| |
|---|---|---|
|
Drainageway protection (DWP)
|
Subtract drainageway protection areas from impervious site area
in WQv computation
| |
|
Natural area conservation (NAC)
|
Subtract conserved natural areas from impervious site area in
WQv computation
| |
|
Filter/buffer area
|
Subtract impervious areas discharged over pervious areas from
impervious site area in WQv computation
|
(4)
Drainageway Protection.
(a)
A water quality credit is given for the protection of natural
drainageways on a development site. Natural karst drainageways within
the Spring Creek Watershed and Ferguson Township often do not exhibit
a defined channel bed and banks. More often, these drainageways appear
as wide, shallow parabolic swales. These drainageways are an integral
part of the natural drainage system, and often exhibit significant
infiltration capacity. Protection of these drainageways is critically
important to the health of the watershed.
(b)
The drainageway protection (DWP) area is defined as an area
centered on the drainageway and having a width equivalent to the one-hundred-year
flow spread. The Township Engineer may modify the defined minimum
width in cases where natural landforms define an appropriate alternate
width.
(c)
The impervious area used in the WQv equation
for the development site may be reduced by twice the area of the preserved
drainageway (two to one ratio).
1)
Restrictions on the Credit:
a)
Drainageway protection areas must remain in an undisturbed
condition during and after construction activities. There shall be
no construction activity within these areas including temporary access
roads or storage of equipment or materials. Temporary access for the
construction of utilities crossing this protection area may be permitted
at the Township Engineer's discretion. However, the alignment
of any such crossing must be perpendicular to the drainageway.
b)
These areas should be placed in a conservation easement
or be permanently preserved through a similarly enforceable agreement
with the Township.
c)
The limits of the undisturbed DWP area and conservation
easement must be shown on all construction plans.
d)
The DWP area must be located on the development
site.
e)
The maximum total DWPA credit is 100% of the site
impervious area.
f)
Water quality credits are not permitted for water
quality sensitive (WQS) developments.
(5)
Natural Area Conservation. A water quality credit is given for
natural areas that are conserved at the development site, thereby
maintaining predevelopment water quality characteristics. The impervious
area used in the WQv equation for the development
site may be reduced by the natural area conserved in the water quality
volume computations. Natural area conservation is different than vegetated
filter strip/recharge area and drainageway protection in that in some
cases surface runoff may never be directed over the natural area (i.e.,
if upslope wooded areas are conserved).
(a)
Restrictions on the Credit:
1)
Natural areas must remain in an undisturbed condition during
and after construction activities. Temporary incidental land disturbance
activities associated with utility construction may be permitted within
the conservation area.
2)
These areas should be placed in a conservation easement or similarly
enforceable agreement with the Township.
3)
The limits of the undisturbed area and conservation easement
must be shown on all construction plans.
4)
The area must be located on the development site.
5)
Water quality credits are not permitted for water quality sensitive
(WQS) developments.
6)
The maximum total natural area credit is 50% of the site impervious
area. However, the combination of natural area credit and vegetated
filter strip credit is also 50%.
(6)
Filter/Buffer Area. A water quality credit is given when stormwater
runoff is effectively treated via a filter/buffer area or strip. A
filter/buffer area is a vegetated boundary characterized by uniform
mild slopes. Filter strips may be forested or vegetated with turf
grass. Effective treatment is achieved when impervious area runoff
is directed as sheet flow across vegetative filter or buffer areas
(i.e., concentrated flow discharged to a filter strip does not meet
water quality reduction criteria). The area draining via overland
sheet flow to an undisturbed, natural, vegetated filter strip (natural
unmaintained meadow or forested area) may be subtracted from the site
impervious area (IA) on a 1:1 area ratio in the water quality volume
computation. Impervious areas draining across constructed (disturbed
or regarded) pervious areas may be subtracted from the site impervious
area (IA) on a 1:1/2 area ratio in the water quality volume computation.
(a)
Restrictions on the Credit:
1)
The maximum impervious area that may be included in this credit
shall be computed as follows:
|
IAc = WIA LIA
|
|
Where:
| ||||
|
IAc
|
=
|
Impervious area recharge credit (L2)
| ||
|
LIA
|
=
|
Length of impervious area measured perpendicular to the sheet
flow direction (L)
| ||
|
WIA
|
=
|
Width of impervious area (L). Maximum width permitted for credit
is the smaller of 100 feet or twice the width of the vegetated filter
strip
| ||
2)
To quality for a water quality credit, natural and constructed filter areas or strips must meet the same restrictions identified for natural or constructed recharge areas with regard to width, length, slope, tributary drainage length, and construction. These restrictions are presented in Chapter 3.
3)
Runoff shall enter the filter/buffer strip as overland sheet
flow.
4)
Filter/buffer areas shall remain undisturbed/unmanaged other
than to remove accumulated trash and debris.
5)
Water quality credits are not permitted for water quality sensitive
(WQS) developments.
6)
The maximum total water quality credit for vegetative filter/buffer
areas is 50% of the site impervious area. However, the combination
of NAC and filter/buffer areas is also 50%.
(7)
Comments Related to Water Quality Credits.
(a)
Concurrence of the Township Engineer is required prior to the
use of all water quality credits for the reduction of the water quality
treatment volume. The Township Engineer may approve the use of additional
credits based upon sufficient documentation regarding suitability
for sensitive development types and areas, pollutant removal effectiveness,
and maintenance criteria. Multiple water quality credits cannot be
claimed for the identical area of the site (i.e., a stream buffer
credit and disconnecting roof recharge area cannot both be claimed
for the same area).
(b)
Additional impervious coverage reduction using low-impact development
techniques (development practices which reduce the impact of urban
runoff such as narrower residential road sections, smaller culs-de-sac,
smaller parking stalls, smaller building setbacks to reduce driveway
lengths, etc.) will also reduce the required water quality treatment
volume. Many of these techniques require prior approval by the Township
before implementation into land development design.
F.
Infiltration Test Requirements. Infiltration testing shall be performed
in accordance with the current version of the Pennsylvania Stormwater
Best Management Practices Manual and as outlined here.
(1)
Double ring infiltrometer methods are the preferred method for
establishing design infiltration rates. Borehole tests may be used
in cases where proposed infiltration surfaces are at a depth not reasonably
accessible using infiltrometer methods. Other test methods may be
used with prior approval by the Township Engineer.
(2)
Double ring infiltrometers shall have inner rings six inches
or more in diameter.
(3)
Design infiltration rates shall be based only on infiltration
tests performed less than 15 feet horizontally and one foot vertically
from the infiltration surface. If a more restrictive soil layer is
identified at a depth greater than one foot below the basin, the more
restrictive layer shall be used.
(4)
A minimum of three infiltration tests shall be performed for
infiltration BMPs having footprints less than or equal to 0.5 acre.
For basins larger than 0.5 acre a minimum of six infiltration tests
shall be performed per acre of basin footprint. For borehole tests
using inner casings less than six inches in diameter, the number of
infiltration test locations shall be doubled. In all cases, test locations
shall be evenly distributed across the basin. Infiltration test locations
shall be documented on the stormwater management site plan drawing.
(5)
The infiltration rate shall be consistent with rates recommended
in the current version of the Pennsylvania Stormwater BMP Manual and
as specified in this chapter, including limitations on rates associated
with soils having low cation exchange rates.
[Ord. 1018, 6/6/2016]
1.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Centre County Conservation District and the standards and specifications of the Township.
2.
Additional erosion and sedimentation control design standards and
criteria that must be applied where recharge or water quality BMPs
are proposed include the following:
A.
Areas proposed for these BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity. They shall not be used as sediment
traps unless there are no other options on the site. If used as sediment
traps, they must have two feet of existing soil remain for use during
the sediment trap stage that is excavated just prior to conversion
to an infiltration BMP.
B.
If possible, these BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the BMP has received
final stabilization.
3.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
[Ord. 1018, 6/6/2016]
1.
The use of sinkholes for stormwater management must be carefully
planned, because discharging runoff directly into existing sinkholes
is not an engineered stormwater solution. Aside from potential water
quality effects, cover collapse sinkholes that exist throughout the
watershed can be unstable, and it should be assumed that they could
stop taking water at any time. Numerous sinkholes throughout the region
already flood during larger runoff events. Nonetheless, in the watershed
there are large drainage areas that completely drain to existing sinkholes
and all upslope development tributary to them cannot be realistically
stopped. Therefore, the following sections have been developed.
A.
Stormwater from roadways, parking lots, storm sewers, roof drains, or other concentrated runoff paths shall not be discharged directly into sinkholes without prior filtration in accordance with § 26-306, Subsection 1B, below.
B.
Sinkholes capable of absorbing substantial amounts of stormwater
shall be protected by diverting such runoff around the sinkhole (refer
to Subsection 1F) or, upon recommended approval of the Township Engineer,
by planting and maintaining a dense filter path of suitable vegetative
material in such a manner and location to disperse and slow the runoff
to a sheet flow condition to promote the maximum possible filtration
and sedimentation of impurities.
(1)
The filter path must be at least 100 feet in length and 20 feet
in width. Ten-foot-wide filter paths are acceptable if land slope
is less than 2%.
(2)
Filter paths shall be designed and installed so that they filter
sheet flow rather than concentrated flow. If concentrated flow occurs,
grading and shaping or the use of best management practices such as
grass waterways or drop structures may be required.
(3)
Sedimentation basins designed to DEP Chapter 102 standards or
permanent stormwater storage criteria, whichever is larger, and proposed
vegetative filter paths, in conjunction with temporary stone filter
check dams, shall be installed prior to subdivision or land development
construction activities, where sinkholes are used to accept stormwater
discharges.
C.
If increased runoff is to be discharged into a sinkhole, even in
filtered conditions, a hydrogeologic assessment of the effects of
such runoff on the increased risk of land subsidence and adverse impacts
to existing sinkhole floodplains and groundwater quality shall be
made by a qualified professional and submitted with the stormwater
management plan. Such discharge shall be prohibited if the Township
Engineer determines that such poses a hazard to life, property or
groundwater resources.
D.
Sinkholes that exist in natural drainageways shall be protected and
should be utilized to accept that flow of water currently discharging
to the sinkhole. All sinkholes shall be posted by clearly visible
permanent on-site notices prohibiting any disposal of refuse, rubbish,
hazardous wastes, organic matter or soil into the sinkhole. A uniform
grade select rock fill may be permitted in the sinkhole for the purpose
of preventing dumping of said materials, subject to approval of the
Township Engineer.
E.
To protect sensitive Karst areas, the Township Engineer may require
basins to contain an impervious liner. The liner may be of the impervious
membrane type, placed in accordance with the manufacturer's recommendations,
or may be constructed by mixing Bentonite, or an approved alternative,
with existing soil available at the site as approved by the Township
Engineer.
F.
If it is determined that runoff from upslope developing areas should
be diverted around a sinkhole due to existing problems, the Township
Engineer may require additional upstream volume controls as required
to protect downstream areas.
[Ord. 1018, 6/6/2016]
1.
Materials, Workmanship and Methods: All materials, workmanship and
methods of work shall comply at a minimum with the Pennsylvania Department
of Transportation Form 408 specifications, as accepted and commonly
used by the Township, and shall be considered to be incorporated into
this chapter as if copied in full. In the event a conflict arises
between the requirements of this chapter and the Form 408 Specifications,
the Township Engineer shall resolve the difference, and his opinion
shall be binding.
A.
General.
(1)
Facilities in State Rights-of-Way. Any stormwater facility located
on a state highway right-of-way shall be subject to approval by the
Pennsylvania Department of Transportation (PADOT). Any stormwater
facility that discharges directly onto a state highway right-of-way
shall be subject to review by the PADOT.
(2)
Water Obstructions. Any facilities that constitute water obstructions
(e.g., culverts, bridges, outfalls, or stream enclosures), and any
work involving wetlands as directed in PADEP Chapter 105 regulations
(as amended or replaced from time to time by PADEP), shall be designed
in accordance with Chapter 105 and will require a permit from PADEP.
Any other drainage conveyance facility that does not fall under Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year design storm
with a minimum of 1.0 foot of freeboard measured below the lowest
point along the top of the roadway for local streets, runoff from
the fifty-year design storm measured below the lowest point along
the top of the roadway for collector streets, and runoff from the
one-hundred-year design storm measured below the lowest point along
the top of the roadway for arterial streets. Roadway crossings located
within designated floodplain areas must be able to convey runoff from
a one-hundred-year design storm with a minimum of 1.0 foot of freeboard
measured below the lowest point along the top of roadway. Any facility
that constitutes a dam as defined in PADEP Chapter 105 regulations
may require a permit under dam safety regulations. Any facility located
within a PADOT right-of-way must meet PADOT minimum design standards
and permit submission requirements.
(3)
Conveyance Facilities. Any drainage conveyance facility and/or channel that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the return period design storm as specified in § 26-304, Subsection 3D. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. If the conveyance facility connects to an existing stormpipe or conveyance facility, the design engineer must show that the existing facility has the capacity to accept the additional water flow. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PADOT right-of-way must meet PADOT minimum design standards and permit submission requirements.
B.
Stormwater Basin Design Considerations. Stormwater management basins
for the control of stormwater peak discharges shall meet the following
minimum requirements.
(1)
The design of all facilities over limestone formations shall
include measures to prevent groundwater contamination and, where required,
sinkhole formation. Soils used for the construction of basins shall
have moderate to low erodibility factors (i.e., "K" factors of 0.32
or less). Any basin greater than four feet in height, measured from
the top of berm to the downslope toe of the abutment, must also contain:
(2)
Energy dissipators and/or level spreaders shall be installed
at points where pipes or drainageways discharge to or from basins.
Generally, outlet pipes designed to carry the predevelopment, one-year
storm flow will be permitted to discharge to a stream with only an
energy dissipator. Discharges to drainage swales shall be spread with
a level spreader or piped to an acceptable point.
(3)
Outlet Structures:
(a)
Outlet structures within detention/retention basins shall be
constructed of reinforced concrete or an approved alternate. With
the exception of those openings designed to carry perennial stream
flows, design openings 12 inches or less shall have nonclogging trash
racks and all openings over 12 inches shall have childproof nonclogging
trash racks. Outlet aprons shall be designed and shall extend at a
minimum to the toe of the basin slope. Where spillways will be used
to control peak discharges in excess of the ten-year storm, the control
weirs shall be constructed to withstand the pressures of impounded
waters and convey flows at computed outlet velocities without erosion.
(b)
All metal risers, where approved for use, shall be suitably
coated to prevent corrosion. A trash rack or similar appurtenance
shall be provided to prevent debris from entering the riser. All metal
risers shall have a concrete base attached with a watertight connection.
The base shall be sufficient weight to prevent flotation of the riser.
An antivortex device, consisting of a thin vertical plate normal to
the basin berm, shall be provided on the top of all metal risers.
(4)
Emergency Spillway:
(a)
Any stormwater management facility (i.e., detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year postdevelopment
conditions. The height of embankment must be set as to provide a minimum
0.5 foot of freeboard above the elevation required to safely pass
the one-hundred-year postdevelopment inflow. Should any stormwater
management facility require a dam safety permit under PADEP Chapter
105, the facility shall be designed in accordance with Chapter 105
and meet the regulations of Chapter 105 concerning dam safety which
may be required to pass storms larger than a one-hundred-year event.
1)
Any underground stormwater management facility (pipe storage
systems) must have a method to bypass flows higher than the required
design (up to a one-hundred-year postdevelopment inflow) without structural
failure or causing downstream harm or safety risks.
2)
Any stormwater management facility that has a paved roadway
as the lower berm should safely convey the one-hundred-year storm
under the paved roadway.
(b)
Emergency spillways shall be constructed of reinforced concrete,
vegetated earth, or riprap in accordance with generally accepted engineering
practices. 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 from the one-hundred-year
design storm. The dimensions of the emergency spillways can be determined
from the Centre County Erosion and Sediment Control Handbook. Emergency
spillways shall extend along the upstream and downstream berm embankment
slopes. Protection should be provided on the upstream embankment a
minimum of three feet below the spillway crest elevation. Protection
at the downstream slope of the spillway shall, as a minimum, extend
to the toe of the berm embankment. The emergency spillway shall not
be located on or discharge over uncompacted earthen fill and/or easily
erodible material.
(c)
Rock-filled gabions may be used where combination berm and emergency
spillway structures are required to prevent concentrated flows. The
Township Engineer may require the use of open concrete lattice blocks,
stone riprap, or concrete spillways when slopes would exceed four
feet horizontal to one foot vertical and spillway velocities might
exceed Soil Conservation Service standards for the particular soils
involved.
(5)
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.
(6)
Slope of Detention Basin Embankment. The top or toe of any slope
shall be located a minimum of 10 feet from any property line. Whenever
possible the side slopes and basin shape shall be amenable to the
natural topography. Straight side slopes and rectangular basins shall
be avoided whenever possible.
(a)
Exterior slopes of compacted soil shall not exceed three feet
horizontal to one foot vertical, and may be further reduced if the
soil has unstable characteristics.
(b)
Interior slopes of the basin shall not exceed three feet horizontal
to one foot vertical, except with approval of the Township. Retaining
walls will be required if a stable slope cannot be maintained. 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. Details and calculations prepared and stamped
and signed by a registered professional engineer shall be submitted
to the Township Engineer for any retaining walls greater than 4 1/2
feet in height.
(c)
Where concrete, stone or brick walls are used with side slopes
proposed to be steeper than three feet horizontal to one foot vertical,
the basin shall be fenced by a permanent wire (or other material approved
by the Township) fence 42 inches in height and a ramp constructed
of durable, nonslip material to a grade of less than 10% for maintenance
vehicles shall be provided for access into the basin.
(7)
Width of Berm. The minimum top width of detention basin berms
shall be 10 feet. For shallow rain gardens or basins less than two
feet in depth, the minimum berm width shall be three feet.
(8)
Slope of Basin Bottom. In order to ensure proper drainage of
the detention basin, a minimum grade of 2% shall be maintained for
all basins used exclusively for peak runoff control. Water quality
or recharge basins with filtration systems incorporated into them
may have a minimum grade of 0%.
(9)
The lowest floor elevation of any structure constructed within
50 feet of a detention basin or other stormwater facility shall be
two feet above the detention basin berm. The distance between any
residential structure and any stormwater facility shall be a minimum
of 50 feet. The distance between any nonresidential structure and
any stormwater facility shall be a minimum of 25 feet.
(10)
Landscaping and planting specifications must be provided for
all stormwater management basins and be specific for each type of
basin.
(11)
Liners:
(a)
Basins should be lined with impervious liners only in areas
with a high risk of sinkhole formation or potential groundwater contamination
as determined by a geotechnical engineer. However, where a liner is
deemed necessary or appropriate, the use of controlled, compacted
natural clay liners for stormwater management basins should be considered.
Locally available clay, when properly installed, can provide near
impervious conditions (approximately E-6 cm/s or less). Some of the
advantages of using controlled, compacted, natural clay soil liners
are:
1)
Can offer better long-term solution as a basin liner versus
geosynthetics because of greater thickness and the ability to withstand
settlement;
2)
Can be constructed to allow relatively uniform leakage rates
to facilitate groundwater recharge but not to an excessive degree
that overloads karst bedrock;
3)
When properly constructed in two or more eight-to-ten-inch-thick
lifts, rapid movement of surface water through the clay liner is eliminated
(rapid leaks can occur in geosynthetic-lined basins due to poor seaming,
punctures, or other factors);
4)
Cleaning/maintenance of clay-lined stormwater basins will be
easier/safer versus geosynthetic liners which could easily be damaged
during maintenance operations; and
5)
The abundance of clayey soils (derived from limestone residuum)
within the Spring Creek Watershed can provide adequate, cost effective,
soil resources for construction of clay liner systems at most development
projects.
(b)
However, the installation of any low-permeability clay liner
system needs to be carefully controlled and the designer needs to
ensure that specifications meet standards to ensure integrity. A quality
control plan identifying the specific clay source material and construction
techniques shall be submitted to the Township Engineer for approval
prior to the placement of the clay liner.
C.
Construction of Basins.
(1)
Basins shall be installed prior to or concurrent with any earthmoving
or land disturbances which they will serve. The phasing of their construction
shall be noted in the narrative and on the plan. Basins that include
water quality or recharge components shall have those components installed
in such a manner as to not disturb or diminish their effectiveness.
(2)
Construction specifications in accordance with the minimum criteria
of the Township must be provided for all embankments pursuant to this
chapter.
(3)
Compaction test reports shall be kept on file at the site and
be subject to review at all times with copies being forwarded to the
Township Engineer.
(4)
A quality control program is critical for embankment fills.
Therefore, whenever embankment fill material in excess of three feet
is to be used, each layer of compacted fill shall be tested to determine
its density per ASTM 2922 or ASTM 3017. The density of each layer
shall be 98% of a Standard Proctor Density analysis per ASTM 698.
(5)
When rock is encountered during the excavation of a pond, it
shall be removed to an elevation of at least 12 inches below the proposed
basin floor (or 24 inches to 30 inches for a manufactured liner. All
exposed cracks and fissures are to be structurally filled.
(6)
Temporary and permanent grasses or stabilization measures shall
be established on the sides and base of all earthen basins within
15 days of construction.
(7)
A minimum of six inches of topsoil material shall be placed
on all areas affected by the basin construction. The materials must
meet the requirements of PADOT Form 408 Specifications.
(8)
All areas proposed for recreational use, whether active or passive,
shall be planted to effectively naturalize areas so as to become an
integral and harmonious part of the landscape by contour and type
of plant material employed.
(9)
All disturbed areas shall be planted with grass in accordance
with PennDOT Form 408 Specifications.
(10)
A fence or suitable vegetative screening may be provided, as
required by the Township, around all detention basins. All fencing
shall be at least 42 inches in height and the material type is subject
to approval by the Township. A vegetative screening of suitable landscaping
plant material in or around a detention basin may also be required.
Vegetative screenings should generally provide a barrier to prevent
entrance to, and effectively naturalize the appearance of, the detention
basin area. Combinations of grassed areas and densely planted shrub
areas consisting of species suited to use in the highway environment
are encouraged. Species of raspberry (Rubus spp.) are recommended.
D.
Construction Inspection. Inspections may be conducted by the Township
Engineer during the construction of the stormwater management basin
and facilities. A preconstruction meeting between the Township Engineer,
owner, and owner's contractor is required prior to any construction.
Such inspections do not constitute approval of construction methods
or materials.
E.
Special Use Basins.
(1)
The design and construction of multiple-use stormwater detention
facilities are strongly encouraged. In addition to stormwater management,
facilities should, where appropriate, allow for recreational uses
including: ball fields, play areas, picnic grounds, etc. Provision
for permanent wet ponds with stormwater management capabilities may
also be appropriate. Prior approval and consultation with the Township
are required before design. Multiple use basins should be constructed
so that potentially dangerous conditions are not created. Water quality
basins or recharge basins that are designed for a slow release of
water or other extended detention ponds are not permitted for recreational
uses, unless the ponded areas are clearly separated and secure.
(2)
Multiple Development Basins. Stormwater management facilities
designed to serve more than one property or development in the same
watershed are encouraged. Staged construction of existing or proposed
multiple-use detention facilities by several developers in conjunction
with watershed development is encouraged. Each developer shall be
responsible for the incremental increase in runoff generated by the
respective development and incremental construction improvements necessary
for the overall detention facility. Prior approval and consultation
with the Township is required before design of such facilities.
(3)
Alternative Detention Facilities: Alternative stormwater detention
facilities including rooftop, subsurface basins or tanks and in-pipe
detention storage, or other approved alternative designs are permitted
as determined by the Township Engineer.
[Ord. 1018, 6/6/2016]
1.
Stormwater management facilities located outside of existing or proposed
rights-of-way shall be located within and accessible by easements
as follows:
A.
Drainage Easements. Where a tract is traversed by a watercourse, drainageway, channel or stream, there shall be provided a drainage easement paralleling the line of such watercourse, drainageway, channel or stream. The width of the drainage easement will be adequate to preserve the unimpeded flow of natural drainage in the one-hundred-year floodplain, in accordance with computed top widths for water surface elevations determined under Part 3 of this chapter.
B.
Access Easements. Where proposed stormwater management facilities
are not adjacent to proposed or existing public rights-of-way or are
not accessible due to physical constraints, as determined by the Township
Engineer, a twenty-foot-wide passable access easement specifying rights
of entry shall be provided. Access easements shall provide for vehicle
ingress and egress on grades of less than 10% for carrying out inspection
or maintenance activities.
C.
Maintenance Easements. A maintenance easement shall be provided which
encompasses the stormwater facility and appurtenances and provides
for access for maintenance purposes. The maintenance easement must
be located outside of one-hundred-year surface elevation and the stormwater
facility and appurtenances.
D.
Easements shall stipulate that no trees, shrubs, structures, excavation
or fill be placed and no regrading be performed within the area of
the easement without written approval from the Township upon review
by the Township Engineer. Upon approval of the Township Engineer,
such landscaping may be placed in maintenance easements, provided
it does not impede access.
E.
Whenever practicable, easements shall be parallel with and conjunctive
to property lines of the subdivision.
F.
All easement agreements shall be recorded with a reference to the
recorded easement indicated on the plan. The format and content of
the easement agreement shall be reviewed and approved by the Township
Engineer and Solicitor.
G.
When stormwater conveyance pipes or channels are located in undedicated
land, they shall be placed within a drainage easement not less than
20 feet wide as approved by the Township Engineer.
[Ord. 1018, 6/6/2016]
1.
Nonstormwater discharges are to be regulated as noted below. In general,
nonstormwater discharges are prohibited from entering any portion
of the municipal separate storm sewer system or any waters of the
commonwealth except as noted below:
A.
Prohibited Discharges.
(1)
No person in the Township shall allow, or cause to allow, stormwater discharges into the Township's storm sewer system which are not composed entirely of stormwater, except as provided in Subsection 1A(2) below and discharges allowed under a state or federal permit.
(2)
Discharges that may be allowed, based on a finding by the Township
Engineer, that the discharge(s) do not significantly contribute to
pollution of surface waters of the commonwealth are:
(a)
Discharges from fire-fighting activities.
(b)
Potable water sources including dechlorinated waterline and
fire hydrant flushing.
(c)
Irrigation drainage.
(d)
Routine external building washdown (which does not use detergents
or other compounds).
(e)
Air-conditioning condensate.
(f)
Water from individual residential car washing.
(g)
Springs.
(h)
Water from crawl space pumps.
(i)
Uncontaminated water from foundation or from footing drains.
(j)
Flows from riparian habitats and wetlands.
(k)
Lawn watering.
(l)
Pavement wash waters where spills or leaks of toxic or hazardous
materials have not occurred (unless all spill material has been removed)
and where detergents are not used.
(m)
Dechlorinated swimming pool discharges.
(n)
Uncontaminated groundwater.
(o)
Diverted stream flow.
(p)
Uncontaminated pumped groundwater.
(3)
In the event that the Township determines that any of the discharges identified in Subsection 1A(2) above significantly contribute to pollution of waters of the commonwealth, or is so notified by the Pennsylvania Department of Environmental Protection (PADEP), the Township will notify the responsible person to cease the discharge.
(4)
Upon notice provided by the Township under Subsection 1A(3) above, the discharger will have a reasonable time, as determined by the Township, to cease the discharge consistent with the degree of pollution caused by the discharge.
(5)
Nothing in this section shall affect a discharger's responsibilities
under state law.
B.
Prohibited Connections. The following connections are prohibited, except as provided in Subsection 1A(2) above.
(1)
Any drain or conveyance, whether on the surface or subsurface,
which allows any nonstormwater discharge including sewage, process
wastewater, and wash water to enter the storm sewer system, and any
connections to the storm drain system from indoor drains and sinks.
(2)
Any drain or conveyance connected from a commercial or industrial
land use to the storm sewer system which has not been documented in
plans, maps, or equivalent records, and approved by the Township.
