A.
All regulated activities in the Spring Creek Watershed which do not fall under the exemption criteria shown in § 173-110 of this chapter shall submit a drainage plan to the governing municipality for review. This plan must be consistent with the Spring Creek Watershed Stormwater Management Plan. These criteria shall apply to the total proposed development even if development is to take place in phases. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas, and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the waiver criteria.
B.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
(1)
Stormwater management facilities and related installations also
shall be provided:
(a)
To ensure adequate drainage of all low points along the curb
line of streets.
(b)
To intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained, and to prevent substantial flow of water across intersections or flooded intersections during storms, in accordance with the procedures contained in Design Manual Part 2 (DM-2), Chapter 10, of the Pennsylvania Department of Transportation (PENN DOT).
(c)
To ensure adequate and unimpeded flow of stormwater under driveways
in, near, or across natural watercourses or drainage swales. Suitable
pipes or other waterways shall be provided as necessary.
(d)
To properly drain stormwater runoff from all land development
projects, except as required by recharge criteria. All lot and open
areas shall be designed to drain to the nearest practical street or
drainage system, existing or proposed, as defined by the respective
Township Engineer, with no impact on adjoining properties, unless
an area specifically designed for stormwater detention is provided.
C.
The existing points of concentrated drainage that discharge onto
adjacent property shall not be altered without permission of the altered
property owner(s) and shall be subject to any applicable discharge
criteria specified in this chapter.
D.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If diffused
flow is proposed to be concentrated and discharged onto adjacent property,
the developer must document that adequate downstream conveyance facilities
exist to safely transport the concentrated discharge, or otherwise
prove that no erosion, sedimentation, flooding or other harm will
result from the concentrated discharge.
E.
Where a development site is traversed by watercourses, drainage easements
shall be provided conforming to the line of such watercourses. The
terms of the easement shall prohibit excavation, the placing of fill
or structures, and any alterations that may adversely affect the flow
of stormwater within any portion of the easement. Also, maintenance,
including mowing of vegetation within the easement, shall be required,
except as approved by the appropriate governing authority.
F.
When it can be shown that, due to topographic conditions, natural
drainageways on the site cannot adequately provide for drainage, open
channels may be constructed conforming substantially to the line and
grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by PADEP through the joint permit application
process, or, where deemed appropriate by PADEP, through the general
permit process.
G.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by PADEP through the joint permit application
process, or, where deemed appropriate by PADEP, the general permit
process. When there is a question whether wetlands may be involved,
it is the responsibility of the developer or his agent to show that
the land in question cannot be classified as wetlands, otherwise approval
to work in the area must be obtained from PADEP.
H.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way shall be subject to
approval by the PENNDOT.
I.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, recharge trenches, etc., are encouraged, where soil
conditions permit, to reduce the size or eliminate the need for detention
facilities.
J.
To promote over-land flow and infiltration/percolation of stormwater,
roof drains should not be connected to streets, sanitary or storm
sewers, or roadside ditches unless approved by the Township authority
on a case-by-case basis.
K.
Where deemed necessary by the respective Township Engineer, the applicant
shall submit an analysis of the impacts of detained stormwater flows
on downstream areas within the watershed. These impacts shall be identified
with concurrence from the Township Engineer. The analysis shall include
hydrologic and hydraulic calculations necessary to determine the impact
peak discharge modifications from the proposed development have on
critical locations such as dams, tributaries, existing developments,
undersized culverts, floodprone areas, etc.
L.
When stormwater management facilities are proposed within 1,000 feet
of a downstream municipality, the analysis of downstream impacts shall
be submitted to the downstream municipal engineers for review and
comment.
Sensitive areas and water quality sensitive developments have
been identified which require special consideration with regard to
stormwater management.
A.
"Sensitive areas" are defined as those areas that, if developed,
have the potential to cause catastrophic loss to a water authority
well field. These areas consist of the delineated one-year zone of
contribution and direct upslope areas tributary to the wells (see
Appendix B, Exhibit 1).[1] Municipalities may update the sensitive area boundaries
based on new research or studies as required.
[1]
Editor's Note: Said appendix is included as an attachment
to this chapter.
B.
"Water quality sensitive (WQS) developments" are defined as a land
development project that has a high potential to cause catastrophic
loss to local water quality, and could potentially threaten groundwater
reservoirs. The following is a provisional list of water quality sensitive
developments. This list may be amended at the discretion of the local
municipality.
(1)
Vehicle fueling stations.
(2)
Industrial manufacturing sites.*
(3)
Salvage yards.
(4)
Recycling centers.
(5)
Hazardous material storage areas.*
(6)
Interstate highways.
NOTE: *The Township Engineer will make the determination relative
to what constitutes these classifications on a case-by-case basis.
The PADEP wellhead protection contaminant source list shall be used
as a guide in these determinations. Industrial manufacturing site
and hazardous material storage areas must provide NPDES SIC codes.
A.
General. Post-development rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff prior to development
for the design storms specified.
B.
Sensitive area district boundaries. The location of sensitive areas
or sensitive area districts (SAD) within the watershed are illustrated
on an official map, which is available for inspection at the Township
office. A reduced-scale copy of this map is included as Exhibit 1
in Appendix B of this chapter.[1] The exact location of the boundaries of sensitive areas
are set by drainage areas tributary to each of the points of interest
as illustrated in Appendix B.[2] The exact location of these boundaries as they apply to
a given development site shall be determined using mapping at a scale
which accurately defines the limits of the sensitive area. If the
project site is within the sensitive area (in whole or in part), two-foot
contour interval mapping shall be provided to define the limits of
the sensitive area. If the project site is adjacent to but within
500 linear feet of a defined sensitive area, a five-foot contour interval
map defining the limits of the sensitive area shall be included in
the stormwater management plan to document the site's location
relative to the sensitive area.
C.
Sites located in more than one district. For a proposed development
site which is traversed by a SAD boundary, the design criteria for
sensitive areas must be applied if post-development runoff is directed
towards the sensitive area.
D.
Off-site areas. Off-site areas that drain from sensitive areas through
a proposed development site that is located entirely in a non-sensitive
area are not required to use or apply the sensitive area criteria.
E.
Site areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area shall be subject to the design criteria.
F.
"Downstream hydraulic capacity analysis." Any downstream or off-site
hydraulic capacity analysis conducted in accordance with these standards
shall use the following criteria for determining adequacy for accepting
increased peak flow rates:
(1)
Natural or man-made channels or swales must be able to convey
the post-development runoff associated with a two-year return period
event within their banks at velocities consistent with protection
of the channels from erosion. Acceptable velocities shall be based
upon criteria included in the DEP Erosion and Sediment Pollution Control
Program Manual.
(2)
Natural or man-made channels or swales must be able to convey
the post-development twenty-five-year return period runoff without
creating any hazard to persons or property.
(3)
Culverts, bridges, storm sewers or any other facilities which
must pass or convey flows from the tributary area must be designed
in accordance with PADEP Chapter 105 regulations (if applicable) and,
at a minimum, pass the post-development twenty-five-year return period
runoff.
(4)
It must be demonstrated that the downstream conveyance channel,
other stormwater facilities, roadways, or overland areas must be capable
of safely conveying the one-hundred-year design storm without causing
damage to buildings or other infrastructure.
(5)
Where the downstream conveyance channel or other facility is
located within a special flood hazard area (as documented on the Municipal
Flood Insurance Rate Map), it must be demonstrated that the limits
of said flood hazard area are not increased by the proposed activity.
(6)
Stormwater management ponds that fall under the PADEP Chapter
105 criteria of a "dam" must meet the criteria within PADEP Chapter
105.
A.
Design criteria.
(1)
Design criteria and calculation methodologies have been classified
by functional group for presentation as follows:
(2)
These criteria and calculation methodologies have been developed
to simplify stormwater management designs, unify methods, remove model
parameter subjectivity, remove improperly used methods, and to ensure
stormwater management decisions are based more realistically on hydrologic
processes. In addition, common sense should always be used as a controlling
criteria.
(3)
These standards provide consistent and process-oriented design
procedures for application by land development professionals. It is
recognized that in an attempt to generalize the computational procedures,
assumptions have been made which on some occasions may be violated.
If such a violation is identified, alternate standards and procedures
may be applied. Both the violation and the alternate procedures to
be applied must be documented by a hydrologist or hydrogeologist.
Any request for use of alternate standards or procedures under this
provision must be agreed to by the local Township Engineer prior to
formal submission of plans for consideration by the Township.
B.
Peak runoff rate control.
(1)
Any site where the increase in post-development peak runoff
rates is determined to be negligible by the Township Engineer is exempt
from the requirement to provide stormwater detention. In support of
this exemption, it must be shown that the downstream conveyance systems
have adequate capacity to convey the additional discharge without
adversely affecting downstream properties. This does not exempt the
requirement for implementation of designs for water quality, stormwater
conveyance, and/or recharge as required. A stormwater management plan
and report documenting these design elements is also required. The
final definition of "negligible" shall be at the Township Engineer's
discretion.
(a)
Prior to using this exemption (and prior to any land development
plan submission), the Design Engineer must provide written documentation
and computations as to why no peak runoff control should be required.
The Township Engineer has the right to reject any plan that uses this
assumption without prior approval of the Township Engineer. The intent
of this exemption is to eliminate the need for multiple or "piggyback"
detention facilities as a result of minor changes in imperviousness
or land use upstream of existing stormwater control facilities.
(b)
Small sites (less than 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.
(2)
Stormwater management analysis must be performed using the following
models. The size criteria are based on drainage area size including
site area and all off-site area draining across the development.
|
Size
|
Method
| |
|---|---|---|
|
Up to 100 acres in size
|
NRCS's TR-55 or TR-20
| |
|
Over 100 acres in size
|
NRCS's TR-20 or HEC-1 (HEC-HMS)
|
(a)
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.
(b)
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.
(c)
More intensive physically based models may be used at the discretion
of the Township Engineer, but only for sites greater than 100 acres
in size.
(d)
Commercial software packages that use the basic computational
methods of TR-55 or TR-20 are permitted.
(e)
The NRCS models and methods recommended above are based on data
collected from actual watersheds. In contrast to this, stormwater
management analysis for land development activities is often conducted
using property lines to define drainage boundaries. Drainage areas
based on property boundaries are not true watersheds and are referred
to here as "hypothetical" drainage areas. It is known that these hypothetical
drainage areas do not respond like natural watersheds. Peak runoff
rates from hypothetical drainage areas are much smaller than comparable
runoff rates from natural watersheds of the same size. Therefore,
wherever possible, pre- and 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 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 pre-development 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 over-estimate the pre-development runoff magnitude.
(3)
Major natural drainage divides may not be altered without the
prior consent of the Township Engineer.
(4)
Pre- and post-development stormwater management analysis shall
be conducted using the following design storms: one-year, two-year,
ten-year, and one-hundred-year.
(a)
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.
(5)
The following twenty-four-hour precipitation depths shall be
used for stormwater management analysis for the entire Spring Creek
Basin. These values override the use of TP-40 (the basis of the NRCS
twenty-four-hour precipitation maps).
|
Return Period
|
Precipitation Depth
| |
|---|---|---|
|
One-year
|
2.2 inches
| |
|
Two-year
|
2.6 inches
| |
|
Ten-year
|
3.5 inches
| |
|
Twenty-five-year
|
4.2 inches
| |
|
Fifty-year
|
4.7 inches
| |
|
One-hundred-year
|
5.3 inches
|
(6)
The NRCS's Type II precipitation distribution is required
for all stormwater management analyses.
(7)
The NRCS's dimensionless unit hydrograph "k" factor shall
be 484 for both pre- and post-development stormwater analyses.
(8)
All undeveloped areas are to be modeled as meadow or woods in
good hydrologic condition. Existing impervious areas may be modeled
as being impervious for pre-development conditions. The only exception
is areas that are actively in agricultural production (nonfallow).
The designer may model the watershed using the actual agricultural
land use/cover condition to show that increases from the site as compared
to the all meadow condition are negligible.
(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 pre-packaged
hydrologic models. It should be recognized that the CN is only a design
tool with a large degree of statistical variability. For large sites,
CNs should realistically be rounded to the nearest whole number.
(11)
The NRCS's method to determine unconnected impervious area
adjustments for CN 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 Township Engineer shall make the final
determination as to what areas of a land development site constitute
compacted structural fill. The intent is to account for large compacted
areas, and not minor grading within lawn areas.
(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 pre-development conditions. Disturbed post-development wooded
areas shall carry the NRCS or Soil Survey's defined HSG with
a minimum HSG of B.
(b)
Highly compacted structural fill areas shall use a minimum of
HSG C for post-development conditions regardless of the NRCS or Soil
Survey's description. For most developments these areas are normally
covered with impervious surfaces, but may include islands within parking
areas, fringe land, etc. A HSG of C shall also be used for large projects
that clear and grade land for later phases of development. The Township
Engineer shall make the final determination as to what areas of a
land development site constitute compacted structural fill. The intent
is to account for large compacted areas, and not minor grading within
lawn areas or small areas around buildings, etc.
(14)
Areas draining to closed depressions must be modeled by removing
the storage volume from the pre-development 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 pre-development
peak flows. Assumptions that sinkholes spill-over during some storm
events must be supported by acceptable documentation (as determined
by the Township Engineer). In addition, the design professional must
be aware that bypassing or sealing sinkholes will frequently result
in downstream flooding and should not be done if existing downstream
flooding already occurs. The site designer is responsible to document
downstream impacts if the sinkhole were to stop taking stormwater
runoff.
(16)
Ponds or other permanent pools of water are to be modeled by
the methods established in the NRCS's TR-55 manual (1986). However,
more rigorous documented methods are acceptable (as determined by
the Township Engineer).
(17)
The NRCS antecedent runoff condition II (ARC II, previously
AMC II) must be used for all simulations. The use of continuous simulation
models that vary the ARC are not permitted for stormwater management
purposes. In addition, prior to any continuous simulation model being
used in the Spring Creek Basin for any other purposes, the model unit
hydrograph must be modified for common events in addition to extreme
events based on an in depth analysis of historical data from the basin.
(18)
Time of concentration.
(a)
The following time of concentration (Tc) computational methodologies
shall be used unless another method is pre-approved by the Township
Engineer:
[1]
Pre-development: NRCS's Lag Equation.
[2]
Post-development; commercial, industrial, or other areas with
large impervious areas (greater than 20% impervious area): NRCS's
Segmental Method.
[3]
Post-development; residential, cluster, or other low impact
designs less than or equal to 20% impervious area: NRCS's Lag
Equation.
(b)
The time of concentration is to represent the average condition
that best reflects the hydrologic response of the area. For example,
large impervious areas bordered by small pervious areas may not consider
the effect of the pervious areas in the Tc computation. If the designer
wants to consider the affect of the pervious area, runoff from the
pervious and impervious areas must be computed separately with the
hydrographs being combined to determine the total runoff from the
area.
(c)
Under no circumstance will the post-development Tc be greater
than the pre-development 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 post-development Tc is greater, it will still be set by default
equal to the pre-development Tc for modeling purposes.
(d)
Refer to Subsection A(28) regarding impervious area flashing
(IAF).
(19)
The following post-development minimum discharges are permitted
for use with the NRCS (CN) runoff model:
|
One-year return period
|
Qpmin = 0.018 (DA) + 0.2
| |
|---|---|---|
|
Two-year return period
|
Qpmin = 0.03 (DA) + 0.4
| |
|
Ten-year return period
|
Qpmin = 0.09 (DA) + 1.0
|
|
Where:
| ||||
|---|---|---|---|---|
|
DA
|
=
|
the drainage area in acres
| ||
|
Qpmin
|
=
|
minimum allowable peak runoff rate in cfs
| ||
(a)
For return periods greater than 10 years, the minimum discharge
shall be equal to the computed pre-development 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 pre-development
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.
(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 pre-development 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 pre-development
condition, even if drainage areas are altered following development.
(20)
All lined stormwater management ponds in carbonate and noncarbonate
areas must be considered impervious and may not be used as pervious
areas for stormwater management computations. "Lined" here means lined
with synthetic liners or Bentonite. All other compacted soil liners
will be considered to be HSG D for hydrologic computations.
(21)
Stormwater management ponds that have a capture depth for the
purposes of water quality or volume capture shall assume a negligible
discharge from these structures during design event routing. Only
discharges from the primary principal spillway or emergency spillway
need to be considered. Discharges from subsurface drains that tie
into a principal spillway should not be considered during design event
routing.
(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
one-hundred-year return period peak runoff rate assuming that all
of the principal spillway orifices are fully clogged, and the principal
spillway overflow is fifty-percent clogged. A minimum of a six-inch
freeboard must also be maintained above the resulting "maximum" water
surface elevations (WSE). Any embankment emergency spillway can be
assumed to be unclogged. SWM ponds with embankments completely made
up of natural undisturbed soils (fully in "cut"), or where roadways
act as the emergency spillway, are permitted. However, the Design
Engineer must verify downstream stability and control.
(24)
All pre- and post-development comparisons of peak flows shall
be rounded to tenths of a cfs. The intent here is to recognize the
accuracy and precision limitations of hydrologic modeling procedures.
Again, small differences between pre- and post-development discharge
rates should be permitted when no negative downstream impacts will
result.
(25)
The full Modified Puls routing method must be used for stormwater
management pond analyses. Simplified methods of determining pond size
requirements such as those in TR-55 (1986) can only be used for preliminary
pond size estimates. The full Modified Puls routing method must be
used for stormwater management pond analyses. Simplified methods of
determining pond size requirements such as those in TR-55 (1986) can
only be used for preliminary pond size estimates.
(26)
Pre-packaged 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.
C.
Pond capture volumes (Cv).
(1)
To minimize nuisance flooding from small precipitation events,
a runoff capture volume is required for all stormwater management
ponds that do not discharge directly to natural, well-defined (with
bed and banks) perennial streams. In general, natural well-defined
streams in the Spring Creek Basin are limited to those delineated
as USGS perennial streams. This should be treated as a guideline and
not a steadfast rule. The final determination is at the discretion
of the Township Engineer. The pond capture volume is a volume of runoff
that will be retained in a pond below the elevation of any free surface
principal spillway orifice. No principal spillway orifice (except
those connected to subsurface drains), regardless of how small, shall
be below the pond elevation equivalent to this volume.
(2)
The Centre County Conservation District (CCCD) receives numerous
complaints regarding ponds that are located at the down slope edge
of a property that result in discharging runoff onto downstream properties
in an uncontrolled manner or where no existing defined outlet channel
exists. This is a very common problem in areas underlain by carbonate
rock. These discharges can cause erosion and flooding downstream.
While the pond capture volume is intended to minimize some of these
negative effects, it cannot deter or reduce the impacts from poor
design practices. Therefore, whenever possible, the CCCD recommends
that the designer consider the downstream morphological changes that
may occur and, when possible, consider constructing conveyance systems
to a stable natural channel. In some cases this may require cooperation
between land owners.
(3)
The capture 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 identified standard as long as the ponding
depth criteria are met.
(4)
To simplify computational requirements for design event analysis,
designers do not need to calculate discharges from subsurface drains
related to the capture volumes if the filter media is sand, or smaller
than AASHTO 57 stone. The capture volume is to control runoff rates
from impervious areas and is not related to water quality. However,
pond designs that include a water quality volume that is greater than
the required capture volume, are assumed to have also met the required
capture volume as long as it dewaters as required.
(5)
Designs that rely on the natural infiltration of insitu soils
must provide documentation supporting the infiltration rates used
for analysis. Infiltration rates reported in the Soil Survey of Centre
County or other published rates may be used at the discretion of the
Township Engineer.
(6)
The pond capture volume should always be used when up-slope
areas are developed where the pond's design creates a point discharge
that did not previously exist.
(7)
Stormwater management detention facilities that connect directly
to storm drain pipe networks that discharge to natural well-defined
channels do not require a capture volume.
D.
Recharge volumes (Rv).
(1)
The purpose of the recharge portion of this 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 Township Engineers and representatives of the Centre County
Conservation District, these problems are of significant local concern.
The magnitude of these problems increases with the percentage of impervious
coverage created on a site.
(2)
Recharge mitigation shall be provided for runoff from all proposed
impervious areas. The required recharge volume shall be computed as
0.5 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.
(3)
The following design practices can be used as credits to reduce
the recharge volume requirement:
(a)
Residential roof areas (detached, duplex, and townhome dwellings)
and commercial/industrial buildings with roof areas less than 5,000
square feet can be removed from the computed impervious area when
these roof areas are sumped to dry wells designed in accordance with
the following minimum standard:
[1]
Sump design criteria: To meet the recharge criteria, sump storage
or voids volume shall be equal to 0.04 cubic feet per square foot
of roof area (0.5 inch rainfall depth). If sump stone has a voids
ratio of 40%, the total sump volume will be 0.10 cubic feet per square
foot of roof area. When designed only to meet this recharge criteria,
the maximum size for a single sump is 100 cubic feet, and the minimum
sump surface area (A) to depth (D) ratio (A/D) must be a minimum of
4/1. The sump depth less any freeboard should not exceed 24 inches.
This roof sump standard shall apply unless the Township has a separate
roof sump standard for water quantity or peak control.
(b)
All or portions of driveways, roadways, and parking areas can
be removed from the impervious area calculation when sheet flow from
these areas is directed to undisturbed natural buffer/filter areas
or constructed filter strips. This flow must be dispersed as sheet
flow as it crosses the buffer/filter area. Sheet flow velocities should
be non-erosive as they cross the impervious area/filter interface.
[1]
To ensure proper infiltration characteristics, the natural soil
profile within natural buffer/filter areas cannot be disturbed during
construction. Completely undisturbed natural recharge areas serve
this function best. However, minor surface scarring, seeding, and
landscaping is permitted in these areas as long as natural grades
are not altered. In special cases, when approved by the Township Engineer,
minor grading, combined with soil profile reconstruction, may be permitted
in natural buffer/filter areas. In addition, the following standards
apply to natural filter/buffer areas:
[a]
Natural filter/buffer areas must have a minimum
width of five feet or 1/2 of the impervious area drainage length immediately
tributary to the buffer area, whichever is greater. This width is
measured parallel to the direction of sheet flow.
[b]
To qualify for a recharge volume credit, the surface
slope of natural filter/buffer areas must be conducive to recharge,
and not result in flow concentration or erosion. To meet this intent,
the surface slope of the area tributary to the natural buffer/filter
area, and the surface slope of the natural buffer/filter area itself
may not exceed 5%. In special cases steeper slopes may be used if
specifically authorized by the Township Engineer.
[c]
The total impervious area tributary to a natural
buffer/filter area cannot exceed twice the buffer/filter area.
[2]
To qualify for a recharge volume credit, constructed filter
strips shall be designed to the following standards:
[a]
The minimum filter strip width shall be five feet
or 1/2 of the impervious area drainage length immediately tributary
to the constructed filter strip, whichever is greater. This width
is measured parallel to the direction of sheet flow.
[b]
The total impervious area tributary to a constructed
filter strip area can not exceed twice the constructed filter strip
area.
[c]
The surface slope of the area tributary to the
constructed filter strip area and the surface slope of the constructed
filter strip area itself may not exceed 5% and 3% respectively. In
special cases, steeper slopes may be used if specifically authorized
by the Township Engineer.
[d]
The filter strip surface shall consist of a minimum
of six inches of natural or reconstructed topsoil with a stable grass
surface treatment. Reconstructed topsoil designs must be approved
by the Township Engineer prior to application. Reconstructed topsoil
consists of soils augmented by tillage and the addition of soil amendments
such as compost, lime, animal manures, crop residues, etc.
[e]
To minimize erosion of the topsoil layer during
construction, it is recommended that these areas be sodded. However,
the Township Engineer may permit the use of an acceptable erosion
control seeding application. In this later case, any loss of topsoil
and seed must be replaced until a permanent vegetative stand is achieved.
(c)
Sidewalks separated from roadways and/or other impervious surfaces
by a grass strip of equal or greater width than the sidewalk itself
can be removed from the impervious area calculation when the sidewalks
are graded so that sheet flow from the walk is directed to the grass
strip. Sidewalks with steep longitudinal slopes that themselves would
act as channels during runoff events can not 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 pre-treatment 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)
The entire capture volume provided in a pond without a subsurface
drain may be used as a credit towards the recharge volume requirement.
(g)
Fifty percent of the capture volume in a pond that includes
a subsurface drain may be credited towards the recharge volume requirement.
(h)
Additional credits may apply for undisturbed land areas that
are known to have high infiltration capacity and that are maintained
or enhanced. These areas must be defined and quantified from actual
site data collection.
(4)
After credits, the remaining recharge volume shall be directed
to a recharge BMP such as infiltration trenches, beds, etc. These
facilities can be located in open areas or under pavement structures.
The appropriateness of the particular infiltration practice proposed,
as well as the design parameters used, shall be supported by a geotechnical
report certified by a qualified professional (soil scientist, geologist,
hydrogeologist, geotechnical engineer, etc.).
(5)
Stormwater recharge requirements or credits affect stormwater
management design requirements. For stormwater management computations,
the reduction of site CNs based only on a weighting type analysis,
as is sometimes done for cluster type developments, is not permitted.
However, for stormwater management purposes, the CN for recharged
areas can be computed using the NRCS method for disconnected impervious
areas. The actual hydrologic process that occurs within the basin
must be stressed in all recharge situations.
(6)
These recharge requirements must be met on all sites unless
it can be demonstrated that recharge would be inappropriate. Any request
for such a waiver from these recharge requirements must be accompanied
by a supporting report certified by a qualified professional (soil
scientist, geologist, hydrogeologist, geotechnical engineer, etc.).
(7)
Developers and site design professionals are encouraged to use
a higher standard for recharge volume on sites where local site conditions
do not restrict a higher standard.
(8)
Water quality sensitive (WQS) developments must use an acceptable
pretreatment BMP prior to recharge. Acceptable pre-treated BMPs for
these developments include BMPs that are based on filtering, settling,
or chemical reaction processes such as chemical coagulation.
(9)
Accounting for recharge within lined stormwater management ponds
is not permitted. However, if unlined, uncompacted ponds and/or depressed
lawn areas are used to satisfy water quality or capture volume criteria,
these areas and volumes can also be used to meet recharge requirements
as previously defined. Additional recharge volume may be credited
to these areas as long as it is demonstrated by a qualified professional
that recharge processes can naturally occur in these areas.
(10)
Finally, because this analysis is concerned with trying to adequately
represent real processes that occur within the watershed, there will
be areas that cannot physically recharge stormwater. These areas include
exfiltration areas that are commonly found at the base of wooded hillsides
where clay pans exist, and saturation areas near major streams or
floodplains. These areas may not accept recharge during most runoff
events. These areas are exempt from recharge requirements when these
conditions are documented and certified by a qualified professional
(soil scientist, geologist, hydrogeologist, or geotechnical engineer).
In addition, stormwater management techniques relying on infiltration
techniques are not permitted in these areas.
(11)
The Township Engineer may waive the recharge requirement in
the following situations:
(a)
The Township Engineer may waive the recharge requirement in
highly developed areas or areas undergoing redevelopment where the
Township Engineer has determined that forced recharge could have adverse
impacts on adjacent landowner structures, property, or Township infrastructure.
These waivers should be limited to small land areas (generally less
than five acres in size), where the ability to place recharge beds
may be limited or may hinder redevelopment.
(b)
The Township Engineer may waive the recharge requirement in
areas where a qualified soils scientist or geologist has determined
that none of the site soils are suitable for recharge, or that the
location of the suitable soils is such that harm to adjoining properties
could occur as stated under Subsection C(11)(a) above.
(c)
The Township Engineer may waive the recharge requirement in
areas where recharge cannot physically occur as documented by a qualified
soil scientist, geologist, or hydrologist. These areas include:
(12)
As identified above, recharge analysis and/or waiver requests
must be supported by a geotechnical report sealed by a qualified professional
(soil scientist, geologist, hydrogeologist, or geotechnical engineer).
The intent of this report will be to establish the suitability of
a particular parcel of land or area for recharge, and to identify
areas on a development site appropriate for recharge. It is recommended
that the geotechnical/soils consultant discuss the extent and approach
to the analysis with the Township Engineer prior to initiating the
field investigation. At a minimum this report should include the following
information:
(a)
A description of the geotechnical site investigation performed
including the methods and procedures used;
(b)
Data presentation;
(c)
Analysis results including the following minimum information:
[1]
A map identifying site areas inappropriate for recharge along
with supporting justification. In addition to illustrating topographic
features, significant geologic and hydrologic features should be identified
(rock outcrops, sinkholes, closed depressions, etc.).
[2]
Determination of the permeability coefficient for potential
recharge areas.
[3]
Determination of the infiltration capacity of natural site soils.
[4]
Location, depth, and permeability coefficient for any restrictive
layers identified.
[5]
Soil uniformity.
[6]
Depth to bedrock in potential recharge areas, and a statement
reflecting the uniformity of the depth to bedrock across the site.
[7]
A statement relating to the site's proximity to fracture
zones within the bedrock.
[8]
Additional information deemed pertinent by the geotechnical
engineer.
(d)
Recommendations for any special design considerations necessary
for the design of recharge systems on the site. For example, required
soil depth over bedrock, appropriate surface grades over recharge
areas, appropriate hydraulic head over recharge areas, etc.
(e)
Justification as to why the site should be developed to a high
impervious density if the site has adverse soil and geotechnical limitations,
which prohibit the ability to induce natural recharge. Explain how
these limitations will not create the potential for undue harm to
the environment and the Spring Creek Watershed when the site is developed.
(13)
The following guidelines are provided relative to the use of
subsurface exfiltration BMPs (often incorrectly referred to as engineered
infiltration BMPs):
(a)
Soils should have a minimum percolation rate of 50 minutes per
centimeter for effective operation of subsurface exfiltration BMPs.
If no site soils have percolation rates of 50 minutes per centimeter,
subsurface exfiltration BMPs should not be used.
(b)
A minimum of 30 inches of soil must be maintained between the
bottom of a subsurface exfiltration BMP and the top of bedrock or
seasonally high groundwater table. This statement is subject to the
recommendation of a qualified geotechnical engineer.
(c)
If the minimum percolation rate is not met and/or the minimum
soil depth can not be maintained on a site, recharge should be accommodated
by directing shallow sheet flow from impervious areas across surface
filter strips and/or undisturbed natural areas, or some other innovative
surface infiltration feature should be used. Limiting subsurface percolation
rates and/or depth to bedrock shall not by themselves warrant a recharge
waiver.
(14)
In addition, since recharge is intended as a volume control,
innovative or new methods that address the significant increase in
the volume of runoff from sites having large impervious areas are
encouraged. These volume control alternatives can be used only if
they can be shown to function with the original intent through sound
engineering and science. The final determination of "original intent"
shall always be the right of the Township Engineer.
E.
Storm drain conveyance system design. Storm drainage conveyance systems
consist of storm sewer pipes, swales, and open channels. Computational
methods for design of storm drain conveyance systems shall be as follows:
(1)
Recommended computational methods (models) for storm drain design
are based on site or watershed drainage area as follows:
|
Size
|
Method
| |
|---|---|---|
|
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.
|
(2)
Rational coefficients used are to be from Rawls et al. (1981),
PENN DOT Design Manual 2-10 or using the Aron curves to convert CNs
to C. If the Aron curves are used, all CNs must be applicable to the
HSG as identified by the NRCS.
(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 only for storm drains up to 24
inches in diameter. The use of these conservative values shall fully
be the choice of the Design Engineer.
|
All impervious areas: C = 0.95
| |
|---|---|
|
All pervious areas: C = 0.30
|
(3)
Storm drains shall be designed at a minimum using a ten-year
runoff event without surcharging inlets. Storm drains tributary to
a multiple site SWM facility across Township roads or crossing other
properties must convey, at a minimum, a twenty-five-year runoff event
without surcharging inlets. Runoff events in excess of the indicated
design event must be conveyed safely downstream.
(4)
Inlets on grade cannot assume a sumped condition for hydraulic
modeling (i.e., top of inlet casting set below pavement surface in
parking areas).
(5)
The Township Engineer may require the analysis of the one-hundred-year
peak runoff rates for conveyance purposes in some instances where
regional SWM facilities are employed.
(6)
Any storm drain within state or federal rights-of-way or that
falls under the design criteria of any higher authority must meet
the requirements of that agency in addition to the minimum requirements
of this chapter.
(7)
The time of concentration (Tc) can be computed by any method
which best represents the subject watershed. However, the NRCS's
segmental method is not recommended for use with drainage areas that
are predominately undeveloped and are greater than 100 acres in size.
The NRCS Lag Equation or another more appropriate method should be
used under these conditions.
(8)
For any drainage area smaller than five acres in size, a Tc
of five minutes may always be assumed at the discretion of the Design
Engineer (for the post-development condition), without needing to
provide supporting documentation.
(9)
Precipitation values applicable to the entire Spring Creek Drainage
Basin are those reflected in the PENN DOT's IDF curves for Region
2, regardless if the area was formerly considered in Region 3.
(10)
Storm drain conveyance system stability (swales, open channels,
and pipe discharge aprons) shall be computed using a ten-year return
period peak runoff rate.
(11)
Storm sewers, where required by zoning and land use densities,
shall be placed under or immediately adjacent to the roadway side
of the curb, or as directed by the Township, when parallel to the
street within the right-of-way.
(12)
When located in undedicated land, they shall be placed within
a drainage easement not less than 20 feet wide as approved by the
Township Engineer.
(13)
The use of properly designed, graded and turfed drainage swales
is encouraged in lieu of storm sewers in commercial and industrial
areas and, where approved by the Township Engineer, in residential
areas.
(a)
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 assemblies.
(a)
Inlet types and inlet assemblies shall conform to the Pennsylvania
Department of Transportation Standards for Roadway Construction as
approved by the Township Engineer.
[1]
Inlets shall, at a minimum, be located at the lowest point of
street intersections to intercept the stormwater before it reaches
pedestrian crossing; or at sag points of vertical curves in the street
alignment which provide a natural point of ponding of surface stormwater.
[2]
Where the Township deems it necessary because of special land
requirements, special inlets may be approved.
[3]
The interval between inlets collecting stormwater runoff shall
be determined in accordance with DM-2, Chapter 10, Section 5, "Capacity
of Waterway Areas."
(b)
In curbed sections, the maximum encroachment of water on the
roadway pavement shall not exceed half of a through traffic lane or
one inch less than the depth of curb during the ten-year design storm
of five-minute duration. Inlets shall be provided to control the encroachment
of water on the pavement. When inlets are used in a storm system within
the right-of-way limits of a street in lieu of manholes, the spacing
of such inlets shall not exceed the maximum distance of 450 feet.
(15)
Accessible drainage structures shall be located on a continuous
storm sewer system at all vertical dislocations, at all locations
where a transition in storm sewer pipe sizing is required, at all
vertical and horizontal angle points exceeding 5°, and at all
points of convergence of two or more influent storm sewer mains. The
construction locations of accessible drainage structures shall be
as indicated on the land development drainage plan or area drainage
plan approved by the Township.
(16)
When evidence available to the Township indicates that existing
storm sewers have sufficient capacity as determined by hydrograph
summation and are accessible, the subdivider may connect their stormwater
facilities to the existing storm sewers so long as the peak rate of
discharge does not exceed the amount permitted by this chapter.
(17)
All other storm drain design methods are to be the same as specified
in existing local ordinances.
(18)
Computational procedures other than those indicated here should
follow the methods of the Federal Highway Administration's Urban
Drainage Design Manual [Hydraulic Engineering Circular No 22. (HEC-22)].
F.
Water quality standards.
(1)
Water quality performance standards. To minimize adverse impacts
to stream health resulting from stormwater nonpoint source (NPS) pollution,
standards are provided for the implementation of water quality best
management practices (BMPs) to reduce NPS pollutant loadings resulting
from land development activities. The following performance standards
and guidelines shall be addressed at all sites where stormwater management
is required.
(a)
Site designs shall minimize the generation of stormwater runoff
through the use of low-impact design techniques.
(b)
Stormwater runoff from all land development activities should
be treated through the use of nonstructural and structural BMPs to
effectively treat the adverse impacts of stormwater runoff including
NPS pollutants.
(c)
Water quality BMPs shall be incorporated into site designs to
treat the required water quality volume as defined below.
(d)
The use of nonstructural BMPs shall always take priority over
the use of structural BMPs. The use of innovative BMPs and low-impact
site planning is encouraged to reduce the generation of stormwater
runoff and effectively treat pollutants transported in stormwater
from the site.
(e)
The use of multiple nonstructural water quality techniques along
with new, emerging, and innovative techniques is encouraged to improve
the quality of stormwater runoff to receiving areas and reduce and/or
eliminate the need for structural BMPs. The Township Engineer should
be consulted to clarify the design concept for meeting or exceeding
the intent of this section.
(f)
Where nonstructural BMPs are unable to effectively treat all
of the stormwater runoff generated from land development activities,
structural BMPs shall be designed to capture and treat the computed
water quality volume (WQv).
(g)
The priority pollutant source areas to be treated with BMPs
are streets, parking lots, driveways, and roof areas.
(h)
Due to the karst nature of the watershed, stormwater discharges
from water quality sensitive developments and discharges to sensitive
wellhead protection areas (defined in Appendix B, Exhibit 1[3]) will require special consideration. In these instances
the applicant shall provide water quality pre-treatment (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.
[3]
Editor's Note: Said appendix is included as an attachment
to this chapter.
(i)
Prior to stormwater management and water quality design, applicants
should consult with the Township Engineer to verify stormwater quality
criteria and present proposed features and concepts for the treatment
of stormwater runoff. Following this meeting, the Township Engineer
shall define any needed support studies or documentation.
(2)
Water quality volume (WQv).
(a)
The required water quality volume that must be treated for nonsensitive
areas underlain by carbonate rock (see exhibits in Appendix B[4]) within the Spring Creek Basin shall be computed as:
|
WQdepth = 0.25+(0.012)2.90.044(SIA))
| |
|
WQv = WQdepth(A)/12
|
|
Where:
| ||||
|---|---|---|---|---|
|
WQv
|
=
|
water quality volume in acre-feet
| ||
|
WQdepth
|
=
|
depth in inches that must be captured for impervious areas
| ||
|
SIA
|
=
|
percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| ||
|
A
|
=
|
total of all paved areas and asphalt-based roofs on site in
acres
| ||
[4]
Editor's Note: Said appendix is included as an attachment
to this chapter.
(b)
The required water quality volume that must be treated for any
WQS development, on sites in sensitive areas underlain by carbonate
rock, and all areas not underlain by carbonate rock is to be computed
within the entire Spring Creek Basin as:
|
WQdepth = the larger of 0.5 inches or
0.25+(0.012)2.9(0.044(SIA))
| |
|
WQv = WQdepth(A)/12
|
|
Where:
| ||||
|---|---|---|---|---|
|
WQv
|
=
|
water quality volume in acre-feet
| ||
|
WQdepth
|
=
|
depth in inches that must be captured for impervious areas
| ||
|
SIA
|
=
|
percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| ||
|
A
|
=
|
total of all paved areas and asphalt-based roofs on site in
acres.
| ||
(c)
For designs in which the final roof material is unknown, the
Design Engineer must assume an asphalt-based roof.
(d)
The water quality volume must be captured and treated through
a water quality BMP over an extended period of time as per the specific
requirements of each structure. Credits to reduce the effective impervious
area are applicable as presented in Chapter 4 of the stormwater management
plan.
(3)
Water quality credits.
(a)
Due to the karst nature of the Spring Creek Basin, the nonstructural
water quality credits and techniques identified below may be limited
for suitability and use based on development type and location. These
limitations for use are specified in the restrictions section for
each credit. The Township Engineer may require additional documentation
or investigation prior to use of each specific credit to reduce the
risks of sinkhole development or groundwater contamination for sensitive
areas and development types. No area may be double counted for use
with credits. The combined credits of natural area conservation and
vegetated filter strips is limited to 50% of the site's impervious
area. The drainageway credit is limited to 50% of the site's
impervious area. The drainageway protection credit is limited to 50%
of the site's impervious area. The maximum total water quality
credit for any site may therefore be 100% of the site's impervious
area.
|
Nonstructural technique
|
Water quality credit
| |
|---|---|---|
|
Drainageway protection (DWP)
|
Subtract drainageway protection areas from impervious site area
in WQv computation
| |
|
Natural area conservation (NAC)
|
Subtract conserved natural areas from impervious site area in
WQv computation
| |
|
Filter/buffer area
|
Subtract impervious areas discharged over pervious areas from
impervious site area in WQv computation
|
(4)
Drainageway protection.
(a)
A water quality credit is given for the protection of natural
drainageways on a development site. Natural karst drainageways within
the Spring Creek Watershed often do not exhibit a defined channel
bed and banks. More often, these drainageways appear as wide, shallow
parabolic swales. These drainageways are an integral part of the natural
drainage system, and often exhibit significant infiltration capacity.
Protection of these drainageways is critically important to the health
of the watershed.
(b)
The drainageway protection (DWP) area is defined as an area
centered on the drainageway and having a minimum width of 300 feet.
The Township Engineer may modify the defined minimum width in cases
where natural land forms define an appropriate alternate width.
(c)
The impervious area used in the WQv equation
for the development site may be reduced by twice the area of the preserved
drainageway (2:1 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 Township Engineer's discretion. However,
the alignment of any such crossing must be perpendicular to the drainageway.
[b]
These areas should be placed in a conservation
easement or be permanently preserved through a similarly enforceable
agreement with the Township.
[c]
The limits of the undisturbed DWP area and conservation
easement must be shown on all construction plans.
[d]
The DWP area must be located on the development
site.
[e]
The maximum total DWPA credit is 100% of the site
impervious area.
[f]
Water quality credits are not permitted for water
quality sensitive (WQS) developments.
(5)
Natural area conservation. A water quality credit is given for
natural areas that are conserved at the development site, thereby
maintaining pre-development water quality characteristics. The impervious
area used in the WQv equation for the development
site may be reduced by the natural area conserved in the water quality
volume computations. Natural area conservation is different than vegetated
filter strip/recharge area and drainageway protection in that in some
cases surface runoff may never be directed over the natural area (i.e.,
if upslope wooded areas are conserved).
(a)
Restrictions on the credit:
[1]
Natural areas must remain in an undisturbed condition during
and after construction activities. Temporary incidental land disturbance
activities associated with utility construction may be permitted within
the conservation area.
[2]
These areas should be placed in a conservation easement or similarly
enforceable agreement with the Township.
[3]
The limits of the undisturbed area and conservation easement
must be shown on all construction plans.
[4]
The area must be located on the development site.
[5]
Water quality credits are not permitted for water quality sensitive
(WQS) developments.
[6]
The maximum total NAC credit is 50% of the site impervious area.
However, the combination of NAC VFRS is also 50%.
(b)
Sensitive area and development restrictions:
(6)
Filter/buffer area.
(a)
A water quality credit is given when stormwater runoff is effectively
treated via a filter/buffer area or strip. A filter/buffer area is
a vegetated boundary characterized by uniform mild slopes. Filter
strips may be forested or vegetated with turf grass. Effective treatment
is achieved when impervious area runoff is directed as sheet flow
across vegetative filter or buffer areas (i.e., concentrated flow
discharged to a filter strip does not meet water quality reduction
criteria).
(b)
The area draining via overland sheet flow to an undisturbed,
natural, vegetated filter strip (natural unmaintained meadow or forested
area) can be subtracted from the site impervious area (IA) on a 1:1
area ratio in the water quality volume computation. Impervious areas
draining across constructed (disturbed or regarded) pervious areas
can be subtracted from the site impervious area (IA) on a 1:1/2 area
ratio in the water quality volume computation.
[1]
Restrictions on the credit:
[a]
The maximum impervious area that can be included
in this credit, shall be computed as follows:
|
IAc = WIA
|
|
Where:
| ||||
|---|---|---|---|---|
|
IAc
|
=
|
Impervious area recharge credit (L2).
| ||
|
LIA
|
=
|
Length of impervious area measured perpendicular to the sheet
flow direction (L).
| ||
|
WIA
|
=
|
Width of impervious area (L). Maximum width permitted for credit
is the smaller of 100 feet or twice the width of the vegetated filter
strip.
| ||
[b]
To qualify for a water quality credit, natural
and constructed filter areas or strips must meet the same restrictions
identified for natural or constructed recharge areas with regard to
width, length, slope, tributary drainage length, and construction.
These restrictions are presented in Chapter 3.
[c]
Runoff shall enter the filter/buffer strip as overland
sheet flow.
[d]
Filter/buffer areas shall remain undisturbed/unmanaged
other than to remove accumulated trash and debris.
[e]
Water quality credits are not permitted for water
quality sensitive (WQS) developments.
[f]
The maximum total water quality credit for vegetative
filter/buffer areas is 50% of the site impervious area. However, the
combination of NAC and filter/buffer areas is also 50%.
(7)
Comments related to water quality credits.
(a)
Concurrence of the Township Engineer is required prior to the
use of all water quality credits for the reduction of the water quality
treatment volume. The Township Engineer may approve the use of additional
credits based upon sufficient documentation regarding suitability
for sensitive development types and areas, pollutant removal effectiveness,
and maintenance criteria. Multiple water quality credits cannot be
claimed for the identical area of the site (i.e., a stream buffer
credit and disconnecting roof recharge area cannot both be claimed
for the same area).
(b)
Additional impervious coverage reduction using low-impact development
techniques (development practices which reduce the impact of urban
runoff such as narrower residential road sections, smaller cul-de-sacs,
smaller parking stalls, smaller building set-backs to reduce driveway
lengths, etc.) will also reduce the required water quality treatment
volume. Many of these techniques require prior approval by the Township
before implementation into land development design.
A.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control," and in accordance with the Centre County Conservation District and the standards and specifications of the appropriate Township.
B.
Additional erosion and sedimentation control design standards and
criteria that must be or are recommended to be applied where recharge
or water quality BMPs are proposed and include the following:
(1)
Areas proposed for these BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity.
(2)
These BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the BMP has received final
stabilization.
C.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
The use of sinkholes for stormwater management must be carefully
planned, because discharging runoff directly into existing sinkholes
is not an engineered stormwater solution. Aside from potential water
quality effects, cover collapse sinkholes that exist throughout the
watershed can be unstable, and it should be assumed that they could
stop taking water at any time. Numerous sinkholes throughout the region
already flood during larger runoff events. Nonetheless, in the watershed
there are large drainage areas that completely drain to existing sinkholes
and all upslope development tributary to them cannot be realistically
stopped. Therefore the following sections have been developed.
A.
Stormwater from roadways, parking lots, storm sewers, roof drains, or other concentrated runoff paths shall not be discharged directly into sinkholes without prior filtration in accordance with Subsection B, below.
B.
Sinkholes capable of absorbing substantial amounts of stormwater shall be protected by diverting such runoff around the sinkhole (refer to Subsection F) or, upon recommended approval of the Township Engineer, by planting and maintaining a dense filter path of suitable vegetative material in such a manner and location to disperse and slow the runoff to a sheet flow condition to promote the maximum possible filtration and sedimentation of impurities.
(1)
The filter path must be at least 100 feet in length and 20 feet
in width. Ten-foot-wide filter paths are acceptable if land slope
is less than 2%.
(2)
Filter paths shall be designed and installed so that they filter
sheet flow rather than concentrated flow. If concentrated flow occurs,
grading and shaping or the use of best management practices such as
grass waterways or drop structures may be required.
(3)
Sedimentation basins designed to DEP Chapter 102 standards or permanent stormwater storage criteria, whichever is larger, and proposed vegetative filter paths, in conjunction with temporary stone filter check dams, shall be installed prior to subdivision or land development construction activities, where sinkholes are used to accept stormwater discharges.
C.
If increased runoff is to be discharged into a sinkhole, even in
filtered conditions, a hydrogeologic assessment of the effects of
such runoff on the increased risk of land subsidence and adverse impacts
to existing sinkhole flood plains and groundwater quality shall be
made by a qualified professional and submitted with the stormwater
management plan. Such discharge shall be prohibited if the Township
Engineer determines that such poses a hazard to life, property or
groundwater resources.
D.
All sinkholes shall be posted by permanent on-site notices clearly
visible at the sinkhole prohibiting any disposal of refuse, rubbish,
hazardous wastes, organic matter or soil into the sinkhole. Rockfill
may be permitted in the sinkhole for the purpose of preventing dumping
of said materials.
E.
To protect sensitive Karst areas, the Township Engineer may require
basins to contain an impervious liner. The liner may be of the impervious
membrane type, placed in accordance with the manufacturer's recommendations,
or may be constructed by mixing Bentonite, or an approved alternative,
with existing soil available at the site as approved by the Township
Engineer.
F.
If it is determined that runoff from upslope developing areas should
be diverted around a sinkhole due to existing problems, the Township
Engineer may require additional upstream volume controls as required
to protect downstream areas.
Materials, workmanship and methods: All materials, workmanship
and methods of work shall comply at a minimum with the PENN DOT Form
408 specifications, as accepted and commonly used by the respective
municipality, and shall be considered to be incorporated into this
article as if copied in full. In the event a conflict arises between
the requirements of this article and the PENN DOT Form 408 Specifications,
the Township Engineer shall resolve the difference, and his opinion
shall be binding.
A.
General.
(1)
Facilities in state rights-of-way: Any stormwater facility located
on state highway rights-of-way shall be subject to approval by the
Pennsylvania Department of Transportation (PENN DOT). Any stormwater
facility that discharges directly onto state highway rights-of-way
shall be subject to review by the PENN DOT.
(2)
Water obstructions: Any facilities that constitute water obstructions
(e.g., culverts, bridges, outfalls, or stream enclosures), and any
work involving wetlands as directed in PADEP Chapter 105 regulations
(as amended or replaced from time to time by PADEP), shall be designed
in accordance with Chapter 105 and will require a permit from PADEP.
Any other drainage conveyance facility that does not fall under Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year design storm
with a minimum of 1.0 foot of freeboard measured below the lowest
point along the top of the roadway. Roadway crossings located within
designated floodplain areas must be able to convey runoff from a one-hundred-year
design storm with a minimum of 1.0 foot of freeboard measured below
the lowest point along the top of roadway. Any facility that constitutes
a dam as defined in PADEP Chapter 105 regulations may require a permit
under dam safety regulations. Any facility located within a PENN DOT
right-of-way must meet PENN DOT minimum design standards and permit
submission requirements.
(3)
Conveyance facilities: Any drainage conveyance facility and/or
channel that does not fall under Chapter 105 regulations must be able
to convey, without damage to the drainage structure or roadway, runoff
from the return period design storm as specified in Section 311, A.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from that structure. Roadway crossings located within designated
floodplain areas must be able to convey runoff from a one-hundred-year
design storm. Any facility located within a PENN DOT right-of-way
must meet PENN DOT minimum design standards and permit submission
requirements.
B.
Stormwater basin design considerations. Stormwater management basins
for the control of stormwater peak discharges shall meet the following
minimum requirements.
(1)
The design of all facilities over limestone formations shall
include measures to prevent groundwater contamination and where required,
sinkhole formation. Soils used for the construction of basins shall
have moderate to low erodibility factors (i.e., "K" factors of 0.32
or less). Any basin greater than four feet in height, measured from
the top of berm to the downslope toe of the abutment, must also contain:
(2)
Energy dissipators and/or level spreaders shall be installed
at points where pipes or drainageways discharge to or from basins.
Generally, outlet pipes designed to carry the pre-development, one-year
storm flow will be permitted to discharge to a stream with only an
energy dissipator. Discharges to drainage swales shall be spread with
a level spreader or piped to an acceptable point.
(3)
Outlet structures:
(a)
Outlet structures within detention/retention basins shall be
constructed of reinforced concrete or an approved alternate. With
the exception of those openings designed to carry perennial stream
flows, design openings shall have childproof, nonclogging trash racks
over all openings nine inches or smaller in any dimension. Outlet
aprons shall be designed and shall extend at a minimum to the toe
of the basin slope. Where spillways will be used to control peak discharges
in excess of the ten-year storm, the control weirs shall be constructed
to withstand the pressures of impounded waters and convey flows at
computed outlet velocities without erosion.
(b)
All metal risers, where approved for use, shall be suitably
coated to prevent corrosion. A trash rack or similar appurtenance
shall be provided to prevent debris from entering the riser. All metal
risers shall have a concrete base attached with a watertight connection.
The base shall be sufficient weight to prevent flotation of the riser.
An anti-vortex device, consisting of a thin vertical plate normal
to the basin berm, shall be provided on the top of all metal risers.
(4)
Emergency spillway:
(a)
Any stormwater management facility (i.e., detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year post-development
conditions. The height of embankment must be set as to provide a minimum
0.5 foot of freeboard above the elevation required to safely pass
the one-hundred-year post-development inflow. Should any stormwater
management facility require a dam safety permit under PADEP Chapter
105, the facility shall be designed in accordance with Chapter 105
and meet the regulations of Chapter 105 concerning dam safety which
may be required to pass storms larger than a one-hundred-year event.
[1]
Any underground stormwater management facility (pipe storage
systems) must have a method to bypass flows higher than the required
design (up to a one-hundred-year post-development inflow) without
structural failure or causing downstream harm or safety risks.
[2]
Any stormwater management facility that has a paved roadway
as the lower berm, and therefore cannot provide a traditional berm
emergency spillway, is not required to provide 0.5 feet of freeboard
above the elevation required to safely pass the one-hundred-year post-development
inflow, but is required to show that the design is stable and no significant
undermining, scour or erosion will occur.
(b)
Emergency spillways shall be constructed of reinforced concrete,
vegetated earth, or riprap in accordance with generally accepted engineering
practices. All emergency spillways shall be constructed so that the
detention basin berm is protected against erosion. The minimum capacity
of all emergency spillways shall be the peak flow rate from the one-hundred-year
design storm. The dimensions of the emergency spillways can be determined
from the Centre County Erosion and Sediment Control Handbook. Emergency
spillways shall extend along the upstream and downstream berm embankment
slopes. Protection should be provided on the upstream embankment a
minimum of three feet below the spillway crest elevation. The downstream
slope of the spillway shall, as a minimum, extend to the toe of the
berm embankment. The emergency spillway shall not be located on or
discharge over uncompacted earthen fill and/or easily erodible material.
(c)
Rock-filled gabions may be used where combination berm and emergency
spillway structures are required to prevent concentrated flows. The
Township Engineer may require the use of open concrete lattice blocks,
stone riprap, or concrete spillways when slopes would exceed four
feet horizontal to one foot vertical and spillway velocities might
exceed Soil Conservation Service standards for the particular soils
involved.
(5)
Antiseep collars: Antiseep collars shall be installed around
the principal pipe barrel within the normal saturation zone of the
detention basin berms. The antiseep collars and their connections
to the pipe barrel shall be watertight. The antiseep collars shall
extend a minimum of two feet beyond the outside of the principal pipe
barrel. The maximum spacing between collars shall be 14 times the
minimum projection of the collar measured perpendicular to the pipe.
(6)
Slope of detention basin embankment: The top or toe of any slope
shall be located a minimum of 10 feet from any property line. Whenever
possible the side slopes and basin shape shall be amenable to the
natural topography. Straight side slopes and rectangular basins shall
be avoided whenever possible.
(7)
Width of berm: The minimum top width of detention basin berms
shall be 10 feet.
(8)
Slope of basin bottom: In order to ensure proper drainage of
the detention basin, a minimum grade of 2% shall be maintained for
all basins used exclusively for peak runoff control. Water quality
or recharge basins with filtration systems incorporated into them
may have a minimum grade of 0.5%.
(9)
The lowest floor elevation of any structure constructed adjacent
to a detention basin or other stormwater facility shall be two feet
above the detention basin berm. The distance between any structure
and any stormwater facility shall be a minimum of 25 feet.
(10)
Landscaping and planting specifications must be provided for
all stormwater management basins and be specific for each type of
basin.
(11)
Lining of basins.
(a)
Basins should be lined with impervious liners only in areas
with a high risk of sinkhole formation or potential groundwater contamination
as determined by a geotechnical engineer. However, where a liner is
deemed necessary or appropriate, the use of controlled, compacted
natural clay liners for SWM basins should be considered. Locally available
clay, when properly installed, can provide near impervious conditions
(approximately E-6 cm/s or less). Some of the advantages of using
controlled, compacted, natural clay soil liners are:
[1]
Can offer better long-term solution as a basin liner versus
geosynthetics because of greater thickness and the ability to withstand
settlement;
[2]
Can be constructed to allow relatively uniform leakage rates
to facilitate groundwater recharge but not to an excessive degree
that overloads karst bedrock;
[3]
When properly constructed in two or more eight- to ten-inch-thick
lifts, rapid movement of surface water through the clay liner is eliminated
(rapid leaks can occur in geosynthetic lined basins due to poor seaming,
punctures, or other factors);
[4]
Cleaning/maintenance of clay-lined stormwater basins will be
easier/safer versus geosynthetic liners which could easily be damaged
during maintenance operations; and
[5]
The abundance of clayey soils (derived from limestone residuum)
within the Spring Creek Watershed can provide adequate, cost effective,
soil resources for construction of clay liner systems at most development
projects.
(b)
However, the installation of any low-permeability clay liner
system needs to be carefully controlled and the designer needs to
ensure that specifications meet standards to ensure integrity.
C.
Construction of basins.
(1)
Basins shall be installed prior to or concurrent with any earthmoving
or land disturbances which they will serve. The phasing of their construction
shall be noted in the narrative and on the plan. Basins that include
water quality or recharge components shall have those components installed
in such a manner as to not disturb or diminish their effectiveness.
(2)
Construction specifications in accordance with the minimum criteria of the Township must be provided for all embankments pursuant to § 173-101B of this chapter.
(3)
Compaction test reports shall be kept on file at the site and
be subject to review at all times with copies being forwarded to the
Township Engineer upon request.
(4)
When rock is encountered during the excavation of a pond, it
shall be removed to an elevation of at least 12 inches below the proposed
basin floor (for a manufactured liner, 24 to 30 inches). All exposed
cracks and fissures are to be structurally filled.
(5)
Temporary and permanent grasses or stabilization measures shall
be established on the sides and base of all earthen basins within
15 days of construction.
D.
Construction inspection. Inspections may be conducted by the Township
Engineer during the construction of the stormwater management basin
and facilities. Such inspections do not constitute approval of construction
methods or materials.
E.
Special use basins.
(1)
The design and construction of multiple-use stormwater detention
facilities are strongly encouraged. In addition to stormwater management,
facilities should, where appropriate, allow for recreational uses
including: ball fields, play areas, picnic grounds, etc. Provision
for parking facilities within basins and permanent wet ponds with
stormwater management capabilities may also be appropriate. Prior
approval and consultation with the Township are required before design.
Multiple-use basins should be constructed so that potentially dangerous
conditions are not created.
(a)
Water quality basins or recharge basins that are designed for
a slow release of water or other extended detention ponds are not
permitted for recreational uses, unless the ponded areas are clearly
separated and secure.
(2)
Multiple development basins: Stormwater management facilities
designed to serve more than one property or development in the same
watershed are encouraged. Staged construction of existing or proposed
multiple-use detention facilities by several developers in conjunction
with watershed development is encouraged. Each developer shall be
responsible for the incremental increase in runoff generated by the
respective development and incremental construction improvements necessary
for the overall detention facility. Prior approval and consultation
with the Township is required before design of such facilities.
(3)
Alternative detention facilities: Alternative stormwater detention
facilities including rooftop, subsurface basins or tanks and in-pipe
detention storage, or other approved alternative designs are permitted
as determined by the Township Engineer.
Stormwater management facilities located outside of existing
or proposed rights-of-way shall be located within and accessible by
easements as follows:
A.
Drainage easements: Where a tract is traversed by a watercourse,
drainageway, channel or stream, there shall be provided a drainage
easement paralleling the line of such watercourse, drainageway, channel
or stream. The width of the drainage easement will be adequate to
preserve the unimpeded flow of natural drainage in the one-hundred-year
floodplain, in accordance with computed top widths for water surface
elevations determined under Section 1.1. of this article.
B.
Access easements: Where proposed stormwater management facilities
are not adjacent to proposed or existing public rights-of-way or are
not accessible due to physical constraints, as determined by the Township
Engineer, a twenty-foot-wide passable access easement specifying rights
of entry shall be provided. Access easements shall provide for vehicle
ingress and egress on grades of less than 10% for carrying out inspection
or maintenance activities.
C.
Maintenance easements: A maintenance easement shall be provided which
encompasses the stormwater facility and appurtenances and provides
for access for maintenance purposes. The maintenance easement must
be located outside of one-hundred-year surface elevation and the stormwater
facility and appurtenances.
D.
Easements shall stipulate that no trees, shrubs, structures, excavation
or fill be placed and no regrading shall be performed within the area
of the easement without written approval from the Township upon review
by the Township Engineer. Upon approval of the Township Engineer,
such landscaping may be placed in maintenance easements, provided
it does not impede access.
E.
Whenever practicable, easements shall be parallel with and conjunctive
to property lines of the subdivision.
F.
All easement agreements shall be recorded with a reference to the
recorded easement indicated on the site plan. The format and content
of the easement agreement shall be reviewed and approved by the Township
Engineer and Solicitor.
G.
When stormwater conveyance pipes or channels are located in undedicated
land, they shall be placed within a drainage easement not less than
20 feet wide as approved by the Township Engineer.
