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 contamination
of groundwater reservoirs of a water authority, Penn State University
or regulated water association wellfield. These areas consist of the
delineated one-year zone of contribution and direct upslope areas
tributary to the wells (see Appendix B, Exhibits 1 and 2). Municipalities may update the sensitive area boundaries
based on new research or studies as required.
B. Water quality sensitive (WQS)
development is 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.
(2) Industrial manufacturing
sites.*
(5) Hazardous material storage
areas.*
(6) Interstate highways.
*The Municipal 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.
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Design criteria and calculation methodologies have been classified
by functional group for presentation as follows: A. Peak runoff rate
discharge requirements; B. Stormwater pond capture volumes; C. Recharge
volumes; D. Storm drain design including conveyance, channel protection
and stability; and E. 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.
Prior to stormwater management and water quality design, applicants
should consult with the Municipal Engineer to verify stormwater quality
criteria and present proposed features and concepts for the treatment
of stormwater runoff. Following this meeting, the Municipal Engineer
shall define any needed support studies or documentation. 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 exemptions, alternate standards or procedures under this chapter
must be agreed to in wiring by the Municipal Engineer prior to formal
submission of plans for consideration by the municipality. A flow
chart documenting the stormwater management design process is provided
as Exhibit 4 in Appendix B of this chapter.
A. Peak runoff rate control.
(1) Exemptions.
(a) Any site where the increase
in post-development peak runoff rates is determined to be negligible
by the Municipal Engineer is exempt from the requirement to provide
stormwater detention.
[1] 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 plan
and report documenting these design elements is also required. The
Municipal Engineer shall use a five-percent increase as a general
benchmark for defining "negligible." The final definition of "negligible"
shall be at the Municipal Engineer's discretion.
[2] 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 Municipal
Engineer has the right to reject any plan which uses this assumption
without prior approval of the Municipal 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 Municipal 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.
(2) Stormwater management analysis
must be performed using the following models. The size criteria are
based on drainage area size including site are and all off-site area
draining across the development.
Up to 100 acres in size
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NRCS's TR-55 or TR-20
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Over 100 acres in size
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NRCS's TR-20 or HEC-1 (HEC-HMS)
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(a) The modified rational
method using the Gert Aron Curves may be used for any site less than
or equal to two acres in size without prior authorization from the
Municipal Engineer. The modified rational method may also be used
for sites between two and five acres in size where the Municipal Engineer
has approved the method's use. In this case the design engineer must
make a written request to the Municipal Engineer explaining why the
use of the modified rational method is more appropriate than the NRCS's
methods for the site in question. The design engineer should keep
in mind that the modified rational methodology was not calibrated
to account for the karst nature of the Spring Creek Drainage Basin,
and, therefore, its use should be limited to the special cases identified
above. In addition, since the minimum discharge criteria are based
on a calibration of the NRCS runoff mode, their use is not appropriate
if the modified rational method is used for runoff computations.
(b) The Municipal 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 Municipal 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.
(c) More intensive physically
based models may be used at the discretion of the Municipal Engineer,
but only for sites greater than 100 acres in size.
(d) Commercial software packages
that use the basic computational methods of TR-55 or TR-20 are permitted.
(e) The NRCS models and methods
recommended above are based on data collected from actual watersheds.
In contrast to this, stormwater management analysis for land development
activities is often conducted using property lines to define drainage
boundaries. Drainage areas based on property boundaries are not true
watersheds and are referred to here as "hypothetical" drainage areas.
It is known that these hypothetical drainage areas do not respond
like natural watersheds. Peak runoff rates from hypothetical drainage
areas are much smaller than comparable runoff rates from natural watersheds
of the same size. Therefore, wherever possible, pre- and 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, as such as within residential developments,
is not permitted. Partitioning drainage areas into different subwatersheds
for the post-development scenarios is acceptable.
(f) It is noted that natural
watershed boundaries should not be used where the relative size of
the watershed compared to the site size would inappropriately distort
the pre-to 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) Natural drainage divides
may not be altered without the prior consent of the Municipal Engineer.
(4) Pre- and post-development
stormwater management analysis shall be conducted using the following
design storms: one-year; two-year; ten-year; 100-year.
(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' 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 municipality may require a percentage of the existing
impervious (up to a 20% maximum) 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 non-existing
stormwater management facilities. Developers of sites with existing
impervious are highly encouraged to set up a meeting with the municipality
prior to design so that any additional requirements are identified
prior to plan submission.
(9) The NRCS's curve number (CN)
shall be used as the rainfall to runoff transformation parameter for
all stormwater management analyses.
(10) Curve numbers should be
rounded to tenths for use in prepackaged hydrologic models. It should
be recognized that the CN is only a design tool with a large degree
of statistical variability. For large sites, CNs should 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 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 Municipal 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, Table A-8, for a list of the soils.
(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 lands, etc. A HSG of
C shall also be used for large projects that clear and grade land
for later phases of development. The Municipal 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 Municipal 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 (with approval of the Municipal 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 in-depth
analysis of historical data from the basin.
(18) The following time of concentration
(Tc) computational methodologies shall be used unless another method
is preapproved by the Municipal Engineer.
(a) Predevelopment: NRCS's
Lag Equation.
(b) Post-development; commercial,
industrial, or other areas with large impervious areas: (greater than
20% impervious area): NRCS's Segmental Method.
(c) Post-development; residential,
cluster, or other low-impact designs less than or equal to 20% impervious
area: NCRS's Lag Equation.
The time of concentration is to represent the average condition
that best reflects the hydrologic response of the area. For example,
large impervious areas bordered by small pervious areas may not consider
the effect of the pervious areas in the Tc computation. If the designer
wants to consider the effect of the pervious area, runoff from the
pervious and impervious areas must be computed separately with the
hydrographs being combined to determine the total runoff from the
area.
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-develoment Tc is greater, it will still be set by default
equal to the predevelopment Tc for modeling purposes.
Refer to §
310-16A(28) regarding impervious area flashing (IAF).
(19) The following post-development
minimum discharges are permitted for use with the NRCS (CN) runoff
model:
1-year return period
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Qpmin = 0.018 (DA) + 0.2
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2-year return period
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Qpmin = 0.03 (DA) + 0.4
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10-year return period
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Qpmin = 0.09 (DA) + 1.0
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Where:
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DA = the drainage area in acres
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Qpmin = minimum allowable peak runoff
rate in cfs
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(a) For return periods greater
than 10 years, the minimum discharge shall be equal to the computed
predevelopment peak runoff rate.
(b) 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.
(c) 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 into sinkholes in order to bypass the sinkhole after development.
(d) 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 noncarbonated 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 may
be required 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. Consultation between the Township Engineer and
Design Engineer shall take place to determine the feasibility of the
application.
(22) Stormwater management ponds
that have a capture depth for the purposes of water quality or volume
capture shall assume that the pond water quality or capture volume
is full at the beginning of design event routing.
(23) Stormwater management ponds
must provide safe passage of the 100-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. 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.
(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). Intent:
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 Municipal
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. 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 form 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 chases this may require
cooperation between landowners.
(1) The capture of volume is
defined as a runoff depth of 0.25 inches 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 minimum required as long as the maximum ponding depth criteria
is met.
(2) 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.
(3) 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 Municipal Engineer.
(4) The pond capture volume shall
always be used when upslope areas are developed where the pond's design
creates a point discharge that did not previously existing.
(5) 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). Intent:
The purpose of the recharge portion of the chapter is two-fold. First,
the recharge requirement is to mitigate the loss of groundwater recharge
associated with the creation of impervious surfaces. In addition,
the recharge 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 municipal 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.
(1) Recharge mitigation shall
be provided for runoff from all proposed impervious areas. The required
recharge volume shall be computed as 0.5 inches 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 Municipal Engineer may require submission of supporting documentation
prior to approving structural infiltration areas as pervious areas.
(2) 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 room 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] Additional criteria for residential roof sumps are included in §
310-19G.
(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. If erosion occurs on the buffer/filter area,
velocity control measures shall be installed at the 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
scarring and landscaping is permitted in these areas as long as natural
grades are not altered. In special cases, when approved by the Municipal
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 quality 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 Municipal 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 Municipal Engineer.
[d] The filter strip
surface shall consist of a minimum of six inches of natural or reconstructed
topsoil and with a stable grass surface treatment. Reconstructed topsoil
designs must be approved by the Municipal 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 Municipal 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) Fifty percent of the capture
volume in a pond that includes a subsurface drain may be credited
towards the recharge volume requirement. For sites less than one acre,
a waiver will need to be obtained.
(g) 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.
(3) 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).
(a) 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.
(b) 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).
(c) 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.
(d) 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.
(e) 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.
(f) 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.
(4) The Municipal Engineer may
waive the recharge requirement in the following situations:
(a) In highly developed areas
or areas undergoing redevelopment where the Municipal Engineer has
determined that forced recharge could have adverse impacts on adjacent
landowner structures, property, or municipal 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) 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(4)(a) above.
(c) In areas where recharge
cannot physically occur as documented by a qualified soil scientist,
geologist, or hydrologist. These areas include:
[1] Exfiltration areas
commonly found at the base of wooded hillsides where clay pans or
fragipans exist; and
[2] Saturation areas near
major streams or floodplains.
(5) As identified above, recharge
analysis and/or waiver requests must be supported by a geotechnical
report sealed by a qualified professional (nine 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 Municipal
Engineer prior to initiating the field investigation. At a minimum
this report shall include the following information:
(a) A description of the geotechnical
site investigation performed, including the methods and procedures
used.
(c) Analysis results including
the following 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.
[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.
(6) Where it has been shown that
recharge cannot be performed and a waiver of the recharge requirements
is being requested, the municipality 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 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.
(7) The following guidelines
are provided relative to the use of subsurface exfiltration BMPs (often
referred to as engineered infiltration BMPs):
(a) Soils should have a minimum
percolation rate of 50 min/cm (0.47 in/hr) 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. The
separation requirement shall be increased if recommended by a qualified
professional.
(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.
(8) 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 responsibility of the Municipal
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:
Up to 200 acres in size
|
Rational Method
|
---|
Between 200 acres and 1.5 square miles
|
HEC-1
|
|
PSRM
|
|
TR-20
|
Over 1.5 square miles in size
|
PSU-IV with the carbonate adjustment factor at the discretion
of the Municipal Engineer
|
Other methods as approved by the Municipal Engineer, such as
SWMM, SWIRM-ROUTE, etc.
|
(2) Rational coefficients used
are to be from Rawls et al. (1981), PA DOT Design Manual 2-10 or using
the Aron curves (Appendix A, Table A-3) to convert CNs to C. If the Aron curves are used, all
CNs must be applicable to the HSG as identified by the NRCS.
(a) 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 for storm drains up to 24 inches in diameter. The use of these
conservative values shall fully be the choice of the design engineer.
All impervious areas:
|
C = 0.95
|
All pervious areas:
|
C = 0.30
|
(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 within a public
right-of-way across municipal 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 Municipal Engineer may
require the analysis of the 100-year peak runoff rates for conveyance
purpose 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
systems 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 site of the curb, or as directed by the municipality,
when parallel to the street within the right-of-way. Storm sewers
that cross under roadways should be of the combined storm sewer/underdrain
type to permit drainage to the roadway subbase.
(12) When located outside a
public right-of-way, they shall be placed within a drainage easement
not less than 20 feet wide as approved by the Municipal 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 Municipal
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
shall conform to the Pennsylvania Department of Transportation Standards
for Roadway Construction, Publication 408, as approved by the Municipal
Engineer.
(a) Inlets shall, at a minimum,
be located at the lowest point of street intersections to intercept
the stormwater before it reaches pedestrian crossings or at sag points
of vertical curves in the street alignment which provide a natural
point of ponding of surface stormwater.
(b) Curb inlets on dedicated
bicycle lanes shall use only a throat opening.
(c) Where the municipality
deems it necessary because of special land requirements, special inlets
may be approved.
(d) The interval between
inlets collecting stormwater runoff shall be determined in accordance
with Pa DOT Design Manual (DM-2), Chapter 10, Section 5, "Capacity
of Waterway Areas." 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 drainage plan or area
drainage plan approved by the municipality.
(16) When evidence availability
to the municipality indicates that existing storm sewers have sufficient
capacity as determined by hydrograph summation and are accessible,
the developer 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) Design criteria for swales
and culverts.
(a) Drainage swales.
[1] The maximum velocity
as determined by Manning's equation shall not exceed the allowable
velocity for specific types of vegetative material as specified in
Appendix A, Table A-7. Inlets shall be provided to control the shoulder encroachment
and water velocity.
[2] The side slope for
any vegetated drainage channel requiring mowing of the vegetation
shall have a maximum grade of three horizontal to one vertical on
those areas to be mowed.
[3] Erosion prevention.
All drainage swales shall be designed to prevent the erosion of the
bed and bank areas. Suitable stabilization during vegetative cover
establishment shall be provided to prevent erosion.
[4] All storm sewers or
drainage swales shall discharge to a detention or retention basin
for the control of peak runoff discharge except as provided in the
plan.
[5] A minimum grade of
1% shall be maintained for all swales.
[6] Design standard. Because
of the critical nature of vegetated drainage channels, the design
of all vegetated channels shall, as a minimum, conform to the design
procedures outlined in the Centre-Clinton County Conservation District
Erosion and Sediment Control Handbook.
(b) Culverts.
[1] Culvert design and
construction shall be done in accordance with PA DOT specifications,
except where more stringent requirements are noted within this chapter.
[2] Minimum grade and
size. All storm drain culvert pipes shall be designed to maintain
a minimum grade of 1/2%. All storm pipes shall have a minimum inside
diameter of 15 inches or a cross-sectional area of 176 square inches,
except that pipes under a twenty-five-foot or greater fill shall not
be less than 24 inches or a cross-sectional area of 453 square inches
and shall consist of reinforced concrete.
[3] Corrugated metal pipe
shall be at least sixteen-gauge thickness. Smooth plastic pipe used
for storm sewers shall be SDR-35 or stronger. Corrugated plastic pipe
must meet PA DOT strength specifications.
[4] Where storm sewers
discharge into existing drainage channels at an angle greater than
30° from parallel with the downstream channel flow, the far side
bank shall be stabilized by the use of rip-rap or masonry and/or concrete
walls. The stabilization shall be designed to prevent erosion and
frost heave under and behind the stabilizing media.
(18) Computational procedures
other than those indicated here should follow the methods of the Federal
Highway Administration's Urban Drainage Design Manual [Hydraulic Engineering
Circular No. 22 (HEC-22)].
(19) The minimum diameter for
pipe under driveways is 15 inches.
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 nonpoint source 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 determined 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 Municipal 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
(identified in Appendix B, Exhibit 1) will require special consideration. In these instances,
the developer 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.
(i) Prior to stormwater management
and water quality design, developers should consult with the Municipal
Engineer to verify stormwater quality criteria and present proposed
features and concepts for the treatment of stormwater runoff. Following
this meeting the Municipal 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 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 municipality:
WQdepth = the larger of 0.5 inches or
0.25 + (0.012) 2.9[0.044(SIA)]
|
WQv = WQdepth(A)/12
|
Where:
|
|
WQv = water quality volume in acre-feet
|
|
WQdepth = depth in inches that must be
captured for impervious areas
|
|
SIA = percent of site impervious area (all paved areas and roof
with asphalt-based roofs)
|
|
A = total of all paved areas and asphalt-based roofs on site
in acres
|
(b) The required water quality
volume that must be treated for any non-WQS development in nonsensitive
areas underlain by carbonate rock (see exhibits in Appendix B10) within the municipality 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
|
[1] For designs in which
the final roof material is unknown, the design engineer must assume
an asphalt-based roof.
[2] 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 below.
(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 Municipal 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.
Nonstructural Technique
|
Water Quality Credit
|
---|
Drainage 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.
|
(a) 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 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.
(b) Drainageway protection.
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.
The drainageway protection (DWP) area is defined as an area centered
on the drainageway and having a maximum width of 300 feet. The Municipal
Engineer may modify the defined maximum width in cases where natural
land forms define an appropriate alternate width. The impervious area
used in the WQv equation for the development site may be reduced by
twice the area of the preserved drainageway (2 to 1 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 Municipal 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 municipality.
[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 50% of the site impervious area.
[f] Water quality credits
are not permitted for water quality sensitive (WQS) developments.
[2] Sensitive area and
development restrictions:
[a] DWP areas may not
be counted as a credit in sensitive areas unless the impervious area
actually flows across the area as sheet flow.
[b] Untreated urban
runoff from sensitive development types may not be directed to DWP
areas without pretreatment.
(c) Natural area conservation
(NAC). 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).
[1] Restrictions on the
credit:
[a] 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.
[b] These areas should
be placed in a conservation easement or similarly enforceable agreement
with the municipality.
[c] The limits of the
undisturbed area and conservation easement must be shown on all construction
plans.
[d] The area must be
located on the development site.
[e] Water quality credits
are not permitted for water quality sensitive (WQS) developments.
[f] The maximum total
NAC credit is 50% of the site impervious area. However, the combination
of natural area conservation credits and filter buffer area credits
is also 50%.
[2] Sensitive area and
development restrictions:
[a] NAC areas may not
be counted as a credit in sensitive areas unless the impervious area
actually flows across the area as sheet flow.
[b] Untreated urban
runoff from sensitive development types may not be directed to natural
areas without pretreatment.
(d) Filter/buffer area. A
water quality credit is given when stormwater runoff is effectively
treated via a filter/buffer area or strip. A filter/buffer area is
a vegetated boundary characterized by uniform mild slopes. Filter
strips may be forested or vegetated with turf grass. Effective treatment
is achieved when impervious area runoff is directed as sheet flow
across vegetative filter or buffer areas (i.e., concentrated flow
discharged to a filter strip does not meet water quality reduction
criteria). The area draining via overland sheet flow to an undisturbed,
natural, vegetated filter strip (natural unmaintained meadow or forested
area) 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 §
310-16C.
[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] The maximum total
water quality credit for vegetative filter/buffer areas is 50% of
the site impervious area. However, the maximum credit for the combination
of NAC and filter/buffer areas is also 50%.
[2] Sensitive area and
development restrictions:
[a] Water quality credits
are not permitted for water quality sensitive (WQS) developments.
[b] Untreated urban
runoff from WQS developments may not be directed to filter/buffer
areas without pretreatments.
(4) Comments related to water
quality credits.
(a) Concurrence of the Municipal
Engineer is required prior to the use of all water quality credits
for the reduction of the water quality treatment volume. The Municipal
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 cul-de-sacs, smaller parking stalls,
smaller building setbacks to reduce driveway lengths, etc.) will also
reduce the required water quality treatment volume. These techniques
require prior approval by the municipality before implementation into
land development design.
Intent: 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 below) or, upon approval of the Municipal 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 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. The sedimentation basin shall be designed to
meet the larger volume requirement of DEP Chapter 102 standards or
the permanent stormwater storage criteria.
C. If increased runoff is to be
discharged into a sinkhole, even in filtered conditions, a hydrogeologic
assessment of the effects of such runoff on the increased risk of
land subsidence and adverse impacts to existing sinkhole floodplains
and groundwater quality shall be made by a qualified professional
and submitted with the stormwater management plan. Such discharge
shall be prohibited if the Municipal Engineer determines that such
poses a hazard to life, property, or groundwater resources.
D. All sinkholes shall be posed
by permanent on-site notices clearly visible at the sinkhole prohibiting
any disposal of refuse, rubbish, hazardous wastes, organic matter
of soil into the sinkhole. Rockfill may be permitted in the sinkhole
for the purpose of preventing dumping of said materials.
E. To protect sensitive karst areas,
the Municipal 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 Municipal Engineer.
F. If it is determined that runoff
from upslope developing areas should be diverted around a sinkhole
due to existing problems, the Municipal Engineer may require additional
upstream volume controls as required to protect downstream areas.
G. If a sinkhole that is a component
of an approved stormwater management plan closes or does not accept
water, all owners of the approved stormwater management plans tributary
to the closed sinkhole shall install volume control BMPs or safe conveyance
to a suitable discharge point.
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 100-year floodplain, in accordance with computed top widths for water surface elevations determined under §
310-16D.
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 Municipal 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,
when directed by the Municipal Engineer.
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 100-year surface
elevation and the stormwater facility and appurtenances.
D. Easement shall stipulate that
no trees, shrubs, structures, excavation or fill be placed and no
regarding be performed within the area of the easement without written
approval from the municipality upon review by the Municipal Engineer.
Upon approval of the Municipal 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 relevant subdivision or land development plan. The format and
content of the easement agreement shall be reviewed and approved by
the Municipal Engineer.
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 Municipal Engineer.