A.
All regulated activities in the Township which do not fall under the exemption criteria shown in § 13-1.8 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 Pennsylvania Code and the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
B.
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.
(1)
Stormwater management facilities and related installations also shall
be provided:
(a)
To ensure adequate drainage of all low points along the curbline
of streets.
(b)
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 (PA DOT).
(c)
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.
(d)
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.
C.
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.
D.
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, 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, drainage
easements shall be provided conforming to the line of such watercourses.
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.
E.
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 PA DEP through the joint permit application
process, or, where deemed appropriate by PA DEP, through the general
permit process.
F.
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 PA DEP through the joint permit application process,
or, where deemed appropriate by PA DEP, 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 PA DEP.
G.
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
(PA DOT).
H.
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.
I.
To promote overland 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.
J.
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, flood-prone areas, etc.
K.
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 or designated
representative for review and comment.
L.
The applicant shall notify the owners of all adjacent properties.
Notification shall be via United States certified mail with copies
of the return receipts provided to the Township. Notification shall
include the following language: "A land development plan is being
considered by Harris Township for Tax Parcel (insert parcel number),
as shown on the attached exhibit. Public comment for this land development
will be received at a meeting of the Harris Township Planning Commission
on (insert date). If you cannot attend this meeting, comments may
be directed to Harris Township at (814) 466-6228." A letter and location
map exhibit must be approved by the Township prior to distribution
to the owners of the adjacent properties.
M.
Approved stormwater management site plans must be on site at all
times during construction.
N.
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.
Sensitive areas and water quality sensitive developments have
been identified which require special consideration with regard to
stormwater management.
A.
"Sensitive areas" 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 the delineated one-year zone of
contribution and direct upslope areas tributary to the wells (see
Appendix B, Exhibit 1[1]). Municipalities may update the sensitive area 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" are defined as a land
development project that has a high potential to cause catastrophic
loss to local water quality and could potentially threaten groundwater
reservoirs. The following is a provisional list of water quality sensitive
developments. This list may be amended at the discretion of the Township.
(1)
Vehicle fueling stations.
(2)
Industrial manufacturing sites*.
(3)
Salvage yards.
(4)
Recycling centers.
(5)
Hazardous material storage areas*.
(6)
Interstate highways.
|
NOTES:
| ||
|---|---|---|
|
*
|
The Township Engineer will make the determination relative to
what constitutes these classifications 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.
| |
A.
General. Post-development rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff prior to development
for the design storms specified.
B.
Sensitive area district boundaries. The location of sensitive areas
or sensitive area districts (SAD) within the watershed are illustrated
on an Official Map, which is available for inspection at the Township
office. A reduced-scale copy of this map is included as Exhibit 1
in Appendix B of this chapter.[1] The exact location of the boundaries of sensitive areas
are set by drainage areas tributary to each of the points of interest
as illustrated in Appendix B. The exact location of these boundaries
as they apply to a given development site shall be determined using
mapping at a scale which accurately defines the limits of the sensitive
area. If the project site is within the sensitive area (in whole or
in part), two-foot contour interval mapping shall be provided to define
the limits of the sensitive area. If the project site is adjacent
to but within 500 linear feet of a defined sensitive area, a five-foot
contour interval map defining the limits of the sensitive area shall
be included in the stormwater management site plan to document the
site's location relative to the sensitive area.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
C.
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 post-development runoff is directed
towards the sensitive area.
D.
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.
E.
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.
F.
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:
(1)
Natural or man-made channels or swales must be able to convey the
post-development 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.
(2)
Natural or man-made channels or swales must be able to convey the
post-development twenty-five-year return period runoff without creating
any hazard to persons or property.
(3)
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[2] regulations (if applicable) and, at a minimum, pass the
post-development twenty-five-year return period runoff.
[2]
Editor's Note: See 25 Pa. Code Ch. 105.
(4)
It must be demonstrated that the downstream conveyance channel, other
stormwater facilities, roadways, or overland areas must be capable
of safely conveying the one-hundred-year design storm without causing
damage to buildings or other infrastructure.
(5)
Where the downstream conveyance channel or other facility is located
within a special flood hazard area (as documented on the Flood Insurance
Rate Map), it must be demonstrated that the limits of said flood hazard
area are not increased by the proposed activity.
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; and 5) water quality standards. 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 stormwater management
decisions are based more realistically on hydrologic processes. In
addition, common sense should always be used as a controlling criteria.
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 local Township Engineer prior to formal submission
of plans for consideration by the Township. A flow chart documenting
the stormwater management design process is provided as Chart A-9,
Stormwater Procedural Flow Chart, of this chapter.
A.
Peak runoff rate control.
(1)
Exemption.
(a)
Any site where the increase in post-development 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 which 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.
(a)
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.
(b)
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 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 methods for the site
in question. Use of the Modified Rational methodology 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 mode,
their use is not appropriate if the Modified Rational Method is used
for runoff computations.
(c)
The Township Engineer has the right to reject any SWM 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 SWM design that improperly uses the method
for determining runoff volumes or does not properly apply the method.
(d)
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.
(e)
Commercial software packages that use the basic computational
methods of TR-55 or TR-20 are permitted.
(f)
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 post-development 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 post-development
scenarios is acceptable.
(g)
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 post-development 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 post-development 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.
(5)
The twenty-four-hour precipitation depths as obtained from NOAA Atlas
14 shall be used for stormwater management analysis.
(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 post-development 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 percentage of the existing impervious area up to a maximum of 20%
to 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 areas 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,
CN's should realistically be rounded to the nearest whole number.
(11)
The NRCS's method to determine unconnected impervious area
adjustments for CN can 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
post-development conditions, regardless of the NRCS or Soil Survey's
description, except for the following two conditions:
(a)
Compacted structural fill areas shall use a minimum of HSG C
for post-development conditions, regardless of the NRCS or Centre
County Soil Survey's description. For most developments, compacted
structural fill areas are under impervious surfaces, but may include
islands within parking areas, fringe land, etc. A HSG C shall also
be applied to large projects that clear and compact building pad areas
for later phases of development under an initial phase. 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.
(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 for a list of the soils.[1]
[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 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 post-development wooded areas
shall carry the NRCS or 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 post-development conditions, regardless of the NRCS or 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 post-development 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 spill over 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 computational methodologies.
(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]
Post-development; commercial, industrial, or other areas with
large impervious areas (greater than 20% impervious area): NRCS's
Segmental Method.
[3]
Post-development; residential, cluster, or other low-impact
designs less than or equal to 20% impervious area: NRCS'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 affect 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 post-development 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 post-development Tc is greater, it will still be set by default
equal to the predevelopment Tc for modeling purposes.
(19)
Post-development minimum discharges.
(a)
The following post-development minimum discharges are permitted
for use with the NRCS (CN) runoff model:
|
1-year return period
|
Qpmin = 0.018 (DA) + 0.2
| |
|---|---|---|
|
2-year return period
|
Qpmin = 0.03 (DA) + 0.4
| |
|
10-year return period
|
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 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 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.
(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 (WSE). Any embankment emergency spillway can be assumed
to be unclogged. SWM 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 post-development 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 post-development 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) can only be used for preliminary
pond size estimates. 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) can
only be used for preliminary pond size estimates.
(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.
Pond 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 are limited to those delineated as USGS
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. 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)
The 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. Supporting computations that show that 90% of
the capture volume can dewater in a maximum of 72 hours must be provided.
For surface ponds, the maximum depth of ponding for the capture volume
shall be three feet (a health and safety precaution). However, in
areas under karst influence, a limiting maximum ponding depth of 18
inches is recommended. Designers may always increase the capture volume
to a value greater than the identified standard as long as the ponding
depth criteria are met.
(4)
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 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.
(5)
Designs that rely on the natural infiltration of in situ 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.
(6)
The pond capture volume should always be used when upslope areas
are developed where the pond's design creates a point discharge
that did not previously exist.
(7)
Stormwater management detention facilities that connect directly
to storm drain pipe networks that discharge to natural well-defined
channels do not require a capture volume.
C.
Recharge volumes (Rv).
(1)
The purpose of the recharge portion of this chapter is twofold. First,
the recharge requirement is to mitigate the loss of groundwater recharge
associated with the creation of impervious surfaces. In addition,
the recharge criteria 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 Township Engineers and representatives of the Centre County
Conservation District, 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 can 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.
(3)
The following design practices can be used as credits to reduce the
recharge volume requirement:
(a)
Residential roof areas (detached, duplex, and townhome dwellings)
and commercial/industrial buildings with roof areas less than 5,000
square feet can 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 should not exceed 24 inches.
[2]
This roof sump standard shall apply unless the Township has a separate
roof sump standard for water quantity or peak control.
(b)
All or portions of driveways, roadways, and parking areas can
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 1/2 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 1/2 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 compost, lime, animal manures, crop residues, etc.
[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
can 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 can
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 can
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 can
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
can 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, etc.).
(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 can 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, etc.).
(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.
(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 can 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 C(11)(a) 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.
(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, etc.
(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. Explain how
these limitations will not create the potential for undue harm to
the environment and the Spring Creek Watershed 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 are provided relative to the use of
subsurface exfiltration BMPs (often incorrectly referred to as "engineered
infiltration BMPs"):
(a)
Soils should have a minimum percolation rate of 50 min/cm for
effective operation of subsurface exfiltration BMPs. If no site soils
have percolation rates of 50 min/cm, subsurface exfiltration BMPs
should not be used.
(b)
A minimum of 30 inches of soil must be maintained between the
bottom of a subsurface exfiltration BMP and the top of bedrock or
seasonally high groundwater table. This statement is subject to the
recommendation of a qualified geotechnical 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 can 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:
(a)
Up to 200 acres in size: Rational Method.
(b)
Between 200 acres and 1.5 square miles in size: HEC-1; PSRM; TR-20.
(c)
Over 1.5 square miles in size: PSU-IV with the carbonate adjustment
factor at the discretion of the Township Engineer.
(d)
Other methods as approved by the Township Engineer, such as SWMM,
SWIRM-ROUTE, etc.
(2)
Rational coefficients.
(a)
Rational coefficients used are to be from Rawls et al. (1981), PADOT
Design Manual 2-10, or using the Aron curves to convert CNs to C.
If the Aron curves are used, all CNs must be applicable to the HSG,
as identified by the NRCS.
(b)
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.
(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
SWM 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 SWM facilities are employed.
(6)
Any storm drain within state or federal rights-of-way or 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) can 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 post-development condition), without needing to provide supporting
documentation.
(9)
Precipitation values applicable to the entire Spring Creek Drainage
Basin are those reflected in the PA DOT's IDF curves for Region
2, regardless if the area was formerly considered in Region 3.
(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, they 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)
Inlet types and inlet assemblies.
(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 SWM site plan or area SWM site
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 their 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 other storm drain design methods are to be the same as specified
in existing local ordinances.
(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)].
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[2]) 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,
hydrogeologic studies may be required to document potential karst-related
impacts.
[2]
Editor's Note: Appendix B is included as an attachment to this chapter.
(i)
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[3]) within the Spring Creek Basin 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
| ||
[3]
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 inch 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 Chapter 4 of the stormwater management
plan.
(3)
Water quality credits. Due to the karst nature of the Spring Creek
Basin, 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 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 minimum width of 300 feet.
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
can 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 NAC credit is 50% of the site impervious area.
However, the combination of NAC VFRS is also 50%.
(b)
Sensitive area and development restrictions.
(6)
Filter/buffer area.
(a)
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).
(b)
The area draining via overland sheet flow to an undisturbed,
natural, vegetated filter strip (natural unmaintained meadow or forested
area) can 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
can be subtracted from the site impervious area (IA) on a 1:1/2 area
ratio in the water quality volume computation.
[1]
Restrictions on the credit.
[a]
The maximum impervious area that can 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
| ||
[b]
To qualify 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.
[c]
Runoff shall enter the filter/buffer strip as overland
sheet flow.
[d]
Filter/buffer areas shall remain undisturbed/unmanaged
other than to remove accumulated trash and debris.
[e]
Water quality credits are not permitted for water
quality sensitive (WQS) developments.
[f]
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.
A.
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 and Sediment Control," and in accordance with the Centre County Conservation District and the standards and specifications of the appropriate Township government.
B.
Additional erosion and sedimentation control design standards and
criteria that must be or are recommended to be applied where recharge
or water quality BMPs are proposed and include the following:
(1)
Areas proposed for these BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity.
(2)
These BMPs shall not be constructed nor receive runoff until the
entire contributory drainage area to the BMP has received final stabilization.
C.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
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 Subsection B below.
B.
Sinkholes capable of absorbing substantial amounts of stormwater shall be protected by diverting such runoff around the sinkhole (refer to Subsection F) 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[1] 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.
[1]
Editor's Note: See 25 Pa. Code Ch. 102.
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 site plan. Such discharge shall be prohibited if the Township
Engineer determines that such poses a hazard to life, property or
groundwater resources.
D.
All sinkholes shall be posted by permanent on-site notices clearly
visible at the sinkhole prohibiting any disposal of refuse, rubbish,
hazardous wastes, organic matter or soil into the sinkhole. Rock fill
may be permitted in the sinkhole for the purpose of preventing dumping
of said materials.
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.
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 respective 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 Municipal 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 state highway rights-of-way shall be subject to approval by the
Pennsylvania Department of Transportation (PA DOT). Any stormwater
facility that discharges directly onto state highway rights-of-way
shall be subject to review by the PA DOT.
(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 PA DEP Chapter 105 regulations[1] (as amended or replaced from time to time by PA DEP),
shall be designed in accordance with Chapter 105 and will require
a permit from PA DEP. 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. 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
PA DEP Chapter 105 regulations may require a permit under dam safety
regulations. Any facility located within a PA DOT right-of-way must
meet PA DOT minimum design standards and permit submission requirements.
[1]
Editor's Note: See 25 Pa. Code Ch. 105.
(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 § 13-3.4D. 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. 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 PA DOT right-of-way must meet PA DOT 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 dissipaters 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
dissipater. 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 nine 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 anti-vortex 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)
Stormwater management facilities.
[1]
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 post-development
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 post-development inflow. Should any stormwater
management facility require a dam safety permit under PA DEP 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.
[2]
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 post-development inflow) without
structural failure or causing downstream harm or safety risks.
[3]
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 Natural Resources 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.
(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 0.5%.
(9)
The lowest floor elevation of any structure constructed within 50
feet to 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. The Township Engineer may
require fencing at the basin.
(10)
Landscaping and planting specifications must be provided for
all stormwater management basins and be specific for each type of
basin.
(11)
The maximum water depth shall not exceed five feet.
(12)
Basin 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 SWM 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.
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 Subsection B(1) of this section.
(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 upon request.
(4)
When rock is encountered during the excavation of a pond, it shall
be removed to an elevation of at least 12 inches below the proposed
basin floor (for a manufactured liner, 24 to 30 inches). All exposed
cracks and fissures are to be structurally filled.
(5)
Temporary and permanent grasses or stabilization measures shall be
established on the sides and base of all earthen basins within 15
days of construction.
(6)
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.
D.
Construction inspection. Inspections may be conducted by the Township
Engineer during the construction of the stormwater management basin
and facilities. Such inspections do not constitute approval of construction
methods or materials. The design professional shall perform site inspections
as may be necessary to ensure that all stormwater management facilities
are constructed in accordance with the approved plans.
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.
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 § 13-3.4A(2) 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 the 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 site plan. The format and content of the easement agreement shall be reviewed and approved by the Township Engineer and Solicitor. (See § 13-7.4.)
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.
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 A(2) below and discharges allowed under a state or federal permit.
(2)
Discharges that may be allowed, based on a finding by the Township,
as determined by the Township Engineer, that the discharge(s) do not
significantly contribute to pollution of surface waters of the commonwealth,
are:
(a)
Discharges from firefighting activities.
(b)
Potable water sources, including dechlorinated waterline and
fire hydrant flushings.
(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 A(2) above significantly contribute to pollution of waters of the commonwealth, or is so notified by the Pennsylvania Department of Environmental Protection (PA DEP), the Township will notify the responsible person to cease the discharge.
(4)
Upon notice provided by the Township under Subsection A(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 A(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.
