*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.
|
(1.a)
|
Any site where the increase in postdevelopment peak runoff rates
is determined to be negligible by the Township Engineer is exempt
from the requirement to provide stormwater detention. In support of
this exemption, it must be shown that the downstream conveyance systems
have adequate capacity to convey the additional discharge without
adversely affecting downstream properties. This does not exempt the
requirement for implementation of designs for water quality, stormwater
conveyance and/or recharge as required. A stormwater management plan
and report documenting these design elements is also required. The
Township Engineer shall use a 5% 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.
| ||
(1.b)
|
Small sites (< five acres) 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.
|
Up to 100 acres in size
|
NCRS's TR-55 or TR-20
| |
Over 100 acres in size
|
NRCS's TR-20 or HEC-1 (HEC-HMS)
|
The Rational Method or Modified Rational Method may be used
for any site less than or equal to two acres in size without prior
authorization from the Township Engineer. The Rational Method or Modified
Rational Method may also be used for sites between two and 20 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 Rational Method
is more appropriate than the NRCS's methods for the site in question.
The design engineer should keep in mind that the Rational Formula
methodology was not calibrated to account for the karst nature of
the Spring Creek drainage basin; and therefore, its use should be
limited to the special cases identified above. In addition, since
the minimum discharge criteria are based on a calibration of the NRCS
runoff mode, their use is not appropriate if the Rational Method is
used for runoff computations.
| |
The Township Engineer has the right to reject any SWM design
that uses hydrograph combinations with the 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.
| |
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.
| |
Commercial software packages that use the basic computational
methods of TR-55 or TR-20 are permitted.
| |
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 sub-watersheds for the postdevelopment
scenarios is acceptable.
| |
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 postdevelopment nuisance flooding
since the models overestimate the predevelopment runoff magnitude.
|
For sites less than one acre in total area that connect directly
to existing storm sewer systems, surface or subsurface (underground)
stormwater detention facilities only need to be designed to control
storm events up to the design return period of the existing pipes
(usually 10 years). However, it must be demonstrated that adequate
conveyance capacity (overland or within the existing storm sewer system)
exists to ensure that flooding or damage from proposed releases will
not exceed the existing potential for the system. If warranted by
historic flooding in the tributary storm sewer system, the Township
may require more stringent criteria.
| |
The Township Engineer may waive the requirement to detain the
one-hundred-year storm as long as the discharge is to a well-defined,
functioning conveyance system that does not currently exhibit flooding
or other conveyance problems. The downstream conveyance system must
be analyzed for the one-hundred-year event to ensure that the proposed
development will not increase flooding or damage to existing buildings
and/or infrastructure.
|
Return Period
|
Precip. Depth
| |
---|---|---|
1 year
|
2.2 inches
| |
2 year
|
2.6 inches
| |
10 year
|
3.5 inches
| |
25 year
|
4.2 inches
| |
50 year
|
4.7 inches
| |
100 year
|
5.3 inches
|
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.
| |
Under no circumstance will the postdevelopment Tc be greater
than the predevelopment Tc for any watershed or sub-watershed modeling
purposes. This includes when the designer has specifically used swales
to reduce flow velocities. In the event that the designer believes
that the postdevelopment Tc is greater, it will still be set by default
equal to the predevelopment Tc for modeling purposes.
| |
Refer to Subsection 3A(28) below regarding impervious area flashing (IAF).
|
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
|
For return periods greater than 10 years, the minimum discharge
shall be equal to the computed predevelopment peak runoff rate.
| |
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.
| |
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.
| |
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.
|
The purpose of the recharge portion of this Part 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 drainage-way 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.
| |||
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 Township Engineer may require submission
of supporting documentation prior to approving structural infiltration
areas as pervious areas.
| |||
The following design practices can be used as credits to reduce
the recharge volume requirement:
| |||
(1)
|
Residential roof areas (detached, duplex and town home 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:
| ||
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.
This roof sump standard shall apply unless the municipality has a
separate roof sump standard for water quantity or peak control.
| |||
(2)
|
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.
| ||
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.
| ||
To qualify for a recharge volume credit, constructed filter
strips shall be designed to the following standards:
| |||
(a)
|
The minimum filter strip width shall be five feet or one half
of the impervious area drainage length immediately tributary to the
constructed filter strip, whichever is greater. This width is measured
parallel to the direction of sheet flow.
| ||
(b)
|
The total impervious area tributary to a constructed filter
strip area cannot exceed twice the constructed filter strip area.
| ||
(c)
|
The surface slope of the area tributary to the constructed filter
strip area, and the surface slope of the constructed filter strip
area itself, may not exceed 5% and 3% respectively. In special cases
steeper slopes may be used if specifically authorized by the Township
Engineer.
| ||
(d)
|
The filter strip surface shall consist of a minimum of six inches
of natural or reconstructed topsoil with a stable grass surface treatment.
Reconstructed topsoil designs must be approved by the Township Engineer
prior to application. Reconstructed topsoil consists of soils augmented
by tillage and the addition of soil amendments such as 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.
| ||
(3)
|
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 5% longitudinal sidewalk slope shall be used as the benchmark
defining steep slopes.
| ||
(4)
|
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.
| ||
(5)
|
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.
| ||
(6)
|
The entire capture volume provided in a pond without a subsurface
drain may be used as a credit towards the recharge volume requirement.
| ||
(7)
|
Fifty percent of the capture volume in a pond that includes
a subsurface drain may be credited towards the recharge volume requirement.
| ||
(8)
|
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.
| ||
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.).
| |||
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.
| |||
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.).
| |||
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.
| |||
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.
| |||
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.
| |||
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.
| |||
The Township Engineer may waive the recharge requirement in
the following situations:
| |||
(1)
|
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 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.
| ||
(2)
|
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 (1) above.
| ||
(3)
|
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:
| ||
(a)
|
Exfiltration areas commonly found at the base of wooded hillsides
where clay pans or fragipans exist.
| ||
(b)
|
Saturation areas near major streams or floodplains.
| ||
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:
| |||
(1)
|
A description of the geotechnical site investigation performed
including the methods and procedures used.
| ||
(2)
|
Data presentation.
| ||
(3)
|
Analysis results including the following minimum information:
| ||
(a)
|
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.).
| ||
(b)
|
Determination of the permeability coefficient for potential
recharge areas.
| ||
(c)
|
Determination of the infiltration capacity of natural site soils.
| ||
(d)
|
Location, depth and permeability coefficient for any restrictive
layers identified.
| ||
(e)
|
Soil uniformity.
| ||
(f)
|
Depth to bedrock in potential recharge areas, and a statement
reflecting the uniformity of the depth to bedrock across the site.
| ||
(g)
|
A statement relating to the site's proximity to fracture
zones within the bedrock.
| ||
(h)
|
Additional information deemed pertinent by the geotechnical
engineer.
| ||
(4)
|
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.
| ||
(5)
|
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.
| ||
The following guidelines are provided relative to the use of
subsurface exfiltration BMPs (often incorrectly referred to as engineered
infiltration BMPs):
| |||
(1)
|
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.
| ||
(2)
|
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 requirement may be modified
subject to the recommendation of a qualified geotechnical professional.
| ||
(3)
|
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.
| ||
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.
|
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 Township Engineer
| |
Other methods as approved by the Township Engineer such as SWMM,
SWIRM-ROUTE, etc.
|
All impervious areas:
|
C = 0.95
| |
All pervious areas:
|
C = 0.30
|
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.
|
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
|
The required water quality volume that must be treated for any
WQS development on sites in sensitive areas underlain by carbonate
rock, and all areas not underlain by carbonate rock, is to be computed
within the entire Spring Creek basin as:
|
WQdepth = the larger of 0.5 inches or
0.25 + (0.012) 2.9 (0.044(SIA))
| |
---|---|
WQv = Wqdepth (A)/12
|
Where:
| ||||
WQv
|
=
|
Water quality volume in acre-feet
| ||
WQdepth
|
=
|
Depth in inches that must be captured for impervious areas
| ||
SIA
|
=
|
Percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| ||
A
|
=
|
Total of all paved areas and asphalt-based roofs on site in
acres
|
For designs in which the final roof material is unknown, the
design engineer must assume an asphalt-based roof.
| |
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.
|
Nonstructural Technique
|
Water Quality Credit
| |
---|---|---|
Drainage-way Protection (DWP)
|
Subtract drainage-way 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
|
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.
|