A.
All regulated activities in Delaware River South watershed that do not fall under the exemption criteria shown in § 28B-105 and/or § 28B-402 shall submit a drainage plan consistent with the Delaware River South Watershed Stormwater Management Site Plan to the municipality for review. This criteria shall apply to the total proposed development even if development is to take place in stages. 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 exemption criteria.
B.
Preparation and implementation of an approved SWM site plan is required.
C.
No regulated activities shall commence until the municipality issues
written approval of an SWM site plan, which demonstrates compliance
with the requirements of this chapter.
D.
SWM site plans approved by the municipality, in accordance with Article IV, shall be on site throughout the duration of the regulated activity.
E.
The municipality may, after consultation with DEP, approve measures
for meeting the state water quality requirements other than those
in this chapter, provided that they meet the minimum requirements
of, and do not conflict with, state law including, but not limited
to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
F.
For all regulated earth disturbance activities, erosion and sediment
control BMPs shall be designed, implemented, operated, and maintained
during the regulated earth disturbance activities (e.g., during construction)
to meet the purposes and requirements of this chapter and to meet
all requirements under Title 25 of the Pennsylvania Code and the Clean
Streams Law.[2] Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual[3]), No. 363-2134-008, as amended and updated.
G.
Impervious areas:
(1)
The measurement of impervious areas shall include all of the
impervious areas in the total proposed development even if development
is to take place in stages.
(2)
For development taking place in stages, the entire development
plan must be used in determining conformance with this chapter.
(3)
For projects that add impervious area to a parcel or reconstruct impervious area on a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the water quality (§ 28B-303) and the larger volume requirement of volume control (§ 28B-304) and groundwater recharge (§ 28B-305), and peak rate controls (§ 28B-307) do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
H.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety, and property.
(2)
Meet the water quality goals of this chapter by implementing
measures to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(3)
Incorporate methods described in the Pennsylvania Stormwater
Best Management Practices Manual (BMP Manual[4]). If methods other than green infrastructure and LID methods
are proposed to achieve the volume and rate controls required under
this chapter, the SWM site plan must include a detailed justification
demonstrating that the use of LID and green infrastructure is not
practicable.
I.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
J.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
K.
Normally dry, open top, storage facilities should completely drain
both the volume control and rate control capacities over a period
of time not less than 24 and not more than 72 hours from the end of
the design storm.
L.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the latest version of the Precipitation-Frequency
Atlas of the United States, National Oceanic and Atmospheric Administration
(NOAA), National Weather Service, Hydrometeorological Design Studies
Center, Silver Spring, Maryland. NOAA's Atlas 145 can be accessed
at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
O.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities designed to encourage infiltration, groundwater
recharge, and improved water quality.
P.
The drainage plan must be designed consistent with the sequencing provisions of § 28B-302 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity.
Q.
The existing points of concentrated drainage that discharge onto
adjacent property shall not be altered without permission of the affected
property owner(s) and shall be subject to any applicable discharge
criteria specified in this chapter.
R.
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 applicant 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.
S.
For all subdivision and land development applications, the tributary
area discharging drainage to any location along the site property
boundary shall not increase by more than 25% over the predevelopment
condition without written approval from the adjacent affected property
owner(s).
T.
Whenever a watercourse is located within a development site, it shall
remain open in its natural state and location and should not be piped,
impeded, or altered (except for road crossings). It is the responsibility
of the developer to stabilize existing eroded stream/channel banks.
U.
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.
V.
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 the municipality and the DEP through
the joint permit application process, or, where deemed appropriate
by DEP, through the general permit process.
W.
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 DEP through the joint permit application
process, or, where deemed appropriate by DEP, the general permit process.
When there is a question whether wetlands may be involved, it is the
responsibility of the applicant 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 DEP.
X.
Any stormwater management facilities regulated by this chapter that
would be located on or discharge into state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
Y.
Minimizing site disturbance, impervious surface, and infiltration
of stormwater runoff through seepage beds, infiltration trenches,
etc., are encouraged, where soil conditions permit, to reduce the
size or eliminate the need for detention facilities.
Z.
Roof drains and sump pumps shall discharge to a natural watercourse,
drainage swale, or drainage easements. Roof drains and sump pumps
shall not be connected to a storm sewer or street drainage structure
unless designed as part of a stormwater management facility. In no
case shall roof drains or sump pumps be connected to a sanitary sewer.
BB.
All regulated activities within the municipality shall be designed,
implemented, operated and maintained to meet the purposes of this
chapter, through these two elements:
CC.
No regulated earth disturbance activities within the municipality
shall commence until the requirements of this chapter are met.
EE.
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VIII.
FF.
All best management practices (BMPs) used to meet the requirements
of this chapter shall conform to the state water quality requirements
and any more stringent requirements as set forth by the municipality.
GG.
Techniques described in Appendix E (Low-Impact Development)[8] of this chapter may be considered because they reduce
the costs of complying with the requirements of this chapter and the
state water quality requirements, if approved by the Township.
[8]
Editor's Note: Said appendix is included as an attachment to this chapter.
A.
The design of all regulated activities shall include the following
steps in sequence to minimize stormwater impacts:
(1)
The applicant is required to find practicable alternatives to
the surface discharge of stormwater, the creation of impervious surfaces
and the degradation of waters of the commonwealth, and must maintain
as much as possible the natural hydrologic regime of the site.
(2)
An alternative is practicable if it is available and capable
of being done after taking into consideration cost, existing technology,
and logistics in light of overall project purposes and other municipal
requirements.
(3)
All practicable alternatives to the discharge of stormwater
are presumed to have less adverse impact on quantity and quality of
waters of the commonwealth unless otherwise demonstrated.
B.
The applicant shall demonstrate that they designed the regulated
activities in the following sequence to minimize the increases in
stormwater runoff and impacts to water quality:
(1)
Prepare an Existing Resource and Site Analysis Map (ERSAM), showing environmentally sensitive areas, including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, floodplains, stream buffer zones, hydrologic soil groups A, B, C, and D, any existing recharge areas and any other requirements outlined in Chapter 22, Subdivision and Land Development. Establish stream buffer according to recommended criteria or applicable ordinances.
(2)
Prepare a draft project layout avoiding sensitive areas identified in § 28B-302B(1) and minimizing total site earth disturbance as much as possible. The ratio of disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included in the ERSAM.
(3)
Identify site specific existing conditions, drainage areas,
discharge points, recharge areas, and hydrologic soil groups A and
B.
(8)
Determine what management district the site falls into (Appendix
D)[2] and conduct a predevelopment runoff analysis.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(9)
Prepare final project design to maintain predevelopment drainage
areas and discharge points, to minimize earth disturbance, impervious
surfaces and the use of surface or point discharges, and to reduce
runoff to the maximum extent possible.
(10)
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 28B-306).
(11)
Manage any remaining runoff through treatment prior to discharge,
as part of detention, bioretention, direct discharge or other structural
control.
A.
The SWM Site plan shall specify permanent stormwater BMPs to be implemented,
operated, and maintained to meet water quality requirements. Because
water quality requirements vary depending on the uses of the water
bodies in the watershed, a framework methodology is provided here.
B.
In order to protect and maintain water quality, additional stormwater
runoff created by the development project must be captured, stored,
and treated. In addition, postconstruction stormwater infiltration
of runoff must replicate preconstruction infiltration of runoff to
the maximum extent possible; in high quality and exceptional value
watershed, special requirements may apply.
C.
The volume of additional stormwater runoff to be captured, stored,
and treated is called the "water quality volume" (WQv). The WQv as
calculated below must be addressed in the SWM site plan, unless a
greater volume of stormwater is required to be controlled in accordance
with this chapter.
|
Equation 303.1
|
|
WQv = [(P)(Rv)(A)]/12
|
|
Where:
| ||
|
WQv
|
=
|
Water quality volume (acre-feet)
|
|
P
|
=
|
Rainfall depth in inches, using the 90% storm - the volume of
rainfall for 90% of the storm events which produce runoff in the watershed
annually; for PennDOT Region 5, the current P value is 2.04 inches
|
|
A
|
=
|
Area of the project contributing to the water quality BMP (acres)
|
|
Rv
|
=
|
0.05 + 0.009(1), where I is the percent of the area that is
impervious surface (impervious area/A* 100)
|
(1)
The P-value for the five PennDOT rainfall regions is shown in
Figure A-2 in Appendix A[1] of this chapter.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2)
In special protection watersheds, as described in 25 Pa. Code
Chapter 93, this volume is required to remain on site through infiltration
and other methods to protect water quality. Guidance can be obtained
from PADEP.
D.
Factors.
(1)
The following factors must be considered when evaluating the
suitability of BMPs used to control water quality at a given development
site:
(a)
Total contributing drainage area.
(b)
Permeability and infiltration rate of the site soils.
(c)
Slope and depth to bedrock.
(d)
Seasonal high water table.
(e)
Proximity to building foundations and wellheads.
(f)
Erodibility of soils.
(g)
Land availability and configuration of the topography.
(h)
Peak discharge and required volume control.
(i)
Stream bank erosion.
(j)
Efficiency of the BMPs to mitigate potential water quality problems.
(k)
Volume of runoff that will be effectively treated.
(l)
Nature of the pollutant being removed.
(m)
Maintenance requirements.
(n)
Creation/protection of aquatic and wildlife habitat.
(o)
Recreational value.
(p)
Enhancement of aesthetic and property value.
(2)
To accomplish the above, the applicant shall submit original
and innovative designs for review. Such designs may achieve the water
quality objectives through a combination of BMPs (best management
practices).
E.
Nonstructural stormwater credits.
(1)
The applicant may, subject to approval of Upper Makefield Township,
use any of the following nonstructural stormwater credits, generally
described in the following table, or pursuant to the Pennsylvania
Stormwater Best Management Practices Manual (BMP Manual), latest edition,
in computing the required water quality volume/credits.
|
Stormwater Credit
|
Description
|
|---|---|
|
Natural area conservation
|
Conservation of natural areas, such as forests, wetlands, or
other sensitive areas, in a protected easement, thereby retaining
their predevelopment hydrologic and water quality characteristics.
Using this credit, a designer may subtract conservation areas from
total site area when computing the required water quality volume.
|
|
Vegetated roof
|
Credit may be given for water quality and volume benefits for
vegetated roof covers where vegetation is grown on and completely
covers an otherwise flat or pitched roof (less than or equal to 30°
slope).
|
|
Disconnection of rooftop runoff
|
Credit may be given when rooftop runoff is disconnected and
then directed over a previous area where it may either infiltrate
into the soil or filter over it. Credit is typically obtained by grading
the site to promote overland flow or by providing bioretention on
single-family residential lots. If a rooftop area is adequately disconnected,
the impervious area may be deducted from the total impervious cover.
|
|
Disconnection of nonrooftop runoff
|
Credit may be given for practices that disconnect surface impervious
cover by directing it to pervious areas where it is either infiltrated
or filtered through the soil. As with rooftop runoff, the impervious
area may be deducted from the total impervious cover thereby reducing
the required water quality volume.
|
|
Stream buffer credit
|
Credit may be given when a stream buffer effectively treats
stormwater runoff. Effective treatment constitutes capturing runoff
from pervious and impervious areas adjacent to the buffer and treating
the runoff through overland flow across a grass or forested area.
Areas treated in this manner may be deducted from total site area
when computing the required water quality volume.
|
|
Grass channel (open section roads)
|
Credit may be given when open grass channels are used to reduce
the volume of runoff and pollutants during smaller storms. If designed
according to appropriate criteria, these channels may meet water quality
criteria for certain types of residential development.
|
|
Environmentally sensitive rural development
|
Credit may be given when a group of environmental site design
techniques are applied to low-density or rural residential development.
This credit eliminates the need for structural practices to address
water quality volume.
|
(2)
For design and applicability of nonstructural BMPs, refer to Chapter 5 of the Pennsylvania Storm Water Management Practices Manual, December 2006, as amended. For the nonstructural BMPs proposed, the applicant shall utilize and submit appropriate checklists included in Chapter 8, Section 8.8, of the Pennsylvania Storm Water Best Management Practices Manual, December 2006, as amended (refer to Appendix H[2]) to demonstrate that the BMPs are applicable to the project
and to determine the amount of volume or peak rate credit is applicable.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
F.
The volume and rate of any stormwater discharges allowed under this
chapter must be managed to prevent the physical degradation of receiving
waters, such as by stream bank scour and erosion. If a detention facility
is proposed which is part of the BMPs approved for the project, the
facility(ies) must be designed to provide for a twenty-four-hour extended
detention of the one-year, twenty-four-hour-storm event (i.e., the
stormwater runoff will be released over a minimum 24 hours for the
one-year, twenty-four-hour-storm event from the time of peak inflow
to zero outflow).
The green infrastructure and low-impact development practices
provided in the BMP Manual[1] shall be utilized for all regulated activities wherever
possible. Water volume controls shall be implemented using the Design
Storm Method in Subsection A or the Simplified Method in Subsection
B below. For regulated activity areas equal or less than one acre
that do not require hydrologic routing to design the stormwater facilities,
this chapter establishes no preference for either methodology; therefore,
the applicant may select either methodology on the basis of economic
considerations, the intrinsic limitations on applicability of the
analytical procedures associated with each methodology and other factors.
A.
The Design Storm Method (CG-1 in the BMP Manual[2]) is applicable to any size of regulated activity. This
method requires detailed modeling based on site conditions.
B.
The Simplified Method (CG-2 in the BMP Manual[3]) provided below is independent of site conditions and
should be used if the Design Storm Method is not followed. This method
is not applicable to regulated activities greater than one acre or
for projects that require design of stormwater storage facilities.
For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of this commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
(3)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
A.
Infiltration BMPs shall meet the following minimum requirements: Where site/soil conditions are suitable, regulated activities must recharge (infiltrate) a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. Deep hole testing and infiltration testing shall be performed by a Certified Professional Soil Scientist (CPSS), Certified Professional Soil Classifier (CPSC), or a Sewage Enforcement Officer (SEO). The volume of runoff to be recharged (Rev) shall be determined from § 28B-305A(2)(a) or § 28B-305.A(2)(b). The Rev, as calculated below, must be addressed in the SWM site plan, unless a greater volume of stormwater is required to be controlled in accordance with § 28B-304 of this chapter.
(1)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils and have the following characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
(b)
An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completed as determined by field tests conducted by the applicant's design professional as stated in § 28B-305A.
(c)
The recharge facility shall be capable of completely infiltrating
the recharge volume within four days (96 hours).
(d)
Pretreatment shall be provided prior to infiltration.
(e)
The requirements for recharge are applied to all disturbed areas,
even if they are ultimately to be an undeveloped land use such as
grass, since studies have found that compaction of the soils during
disturbance reduces their infiltrative capacity.
(2)
Recharge volume (Rev) shall be computed by first obtaining the infiltration requirement using methods in either § 28B-305A(2)(a) or § 28B-305A(2)(b) then multiplying by the total proposed impervious area. The overall required recharge volume for a site is computed by multiplying total impervious area by the infiltration requirement.
(a)
NRCS curve number equation.
[1]
The following criteria shall apply.
[2]
The NRCS runoff shall be utilized to calculate infiltration
requirements (P) in inches.
|
Equation 305.1
|
|
For zero runoff: P = I (infiltration) = (200/CN) - 2
|
|
Where:
| ||||
|
P
|
=
|
I
|
=
|
Infiltration requirement (inches)
|
|
CN
|
=
|
SCS (NRCS) curve number of the existing conditions contributing
to the recharge facility
| ||
[3]
This equation can be displayed graphically in, and the infiltration
requirement can also be determined from, Figure 305-1.
[4]
The recharge volume (Rev) required would therefore be computed
as:
|
Equation: 305.2
|
|
Rev = I * impervious area (SF)/12 = cubic feet (CF)
|
(b)
Annual recharge water budget approach.
[1]
It has been determined that infiltrating 0.6 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of § 28B-305A(2)(a) cannot be achieved, then 0.6 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 77. Above a curve number of 77, Equation 305.1 or the curve in Figure 305.1 should be used to determine the infiltration requirement.
|
Where:
| ||
|
I
|
=
|
0.5 inch
|
[2]
The recharge volume (Rev) required would therefore be computed
as:
|
Rev = I * percent impervious area (SF)/12 = (CF)
|
[3]
The recharge values derived from these methods are the minimum
volumes the applicant must control through an infiltration/recharge
BMP facility. However, if a site has areas of soils where additional
volume of infiltration can be achieved, the applicant is encouraged
to recharge as much of the stormwater runoff from the site as possible.
|
Figure 305.1
Figure 305.1. Infiltration Requirement Based Upon NRCS Curve
Number
 |
B.
The general process for designing the infiltration BMP shall be: A detached soils evaluation of the project site shall be required to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified applicant, as stated in § 28B-305A, and, at a minimum, address soil permeability, depth of bedrock, and subgrade stability.
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the watershed to determine general areas of suitability
for infiltration stability.
(2)
Provide field tests, such as double ring infiltration tests,
at the level of the proposed infiltration surface to determine the
appropriate hydraulic conductivity rate.
(3)
Design the infiltration structure for the required storm volume
based on field-determined capacity at the level of the proposed infiltration
surface.
(4)
Where the recharge volume requirement cannot be physically accomplished
due to the results of the field soils testing, supporting documentation,
and justification must be submitted with the drainage plan.
(5)
If on-lot infiltration structures are proposed, it must demonstrate
that the soils are conducive to infiltrate on the lots identified.
C.
Extreme caution shall be exercised where infiltration is proposed
in geologically susceptible limestone areas. Extreme caution shall
also be exercised where salt or chloride would be a pollutant since
soils do little to filter this pollutant and it may contaminate the
groundwater. Extreme caution shall be exercised where infiltration
is proposed in source water protection areas. The qualified design
professional shall evaluate the possibility of groundwater contamination
from the proposed infiltration/recharge facility and perform a hydrogeologic
justification study if necessary. The infiltration requirement in
high quality/exceptional value waters shall be subject to DEP's
Title 25, Chapter 93, antidegradation regulations. The municipality
may require the installation of an impermeable liner in BMP and/or
detention basins where the possibility of groundwater contamination
exists. A detailed hydrogeologic investigation may be required by
the municipality.
D.
The plan must include safeguards against groundwater contamination
for uses which may cause groundwater contamination, should there be
a mishap or spill.
E.
Recharge/infiltration facilities shall be used in conjunction with
other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
A.
To minimize the impact of stormwater runoff on downstream stream
bank erosion, the requirement is to design the BMP to detain the postdevelopment,
two-year, twenty-four-hour design storm to the predevelopment, one-year
flow using the SCS Type II distribution. Additionally, provisions
shall be made (such as adding a small orifice at the bottom of the
outlet structure) so that the postdevelopment one-year storm takes
a minimum of 24 hours to drain from the facility from a point where
the maximum volume of water from the one-year storm is captured (i.e.,
the maximum water surface elevation is achieved in the facility).
B.
Release of water can begin at the start of the storm (i.e., the invert
of the water quality orifice is at the invert of the facility). The
design of the facility shall consider and minimize the chances of
clogging and sedimentation. Orifices smaller than three inches diameter
are not recommended. However, if the design engineer can provide proof
that the smaller orifices are protected from clogging by use of trash
racks, etc., smaller orifices may be permitted.
C.
Whenever a watercourse is located within a development site, it shall
remain open in the natural state and location and shall not be piped,
impeded, or altered (except for road crossings). It is the responsibility
of the developer to restore existing eroded stream/channel banks within
a subdivision or land development site and obtain all permits necessary
from PADEP to do so. The developer must submit pictorial documentation
of existing stream/channel banks to determine whether existing banks
must be stabilized.
A.
Delaware River South watershed has been divided into stormwater management
districts as shown on the Watershed Map in Appendix D.[1] In addition to the requirements specified below, the water quality (§ 28B-303), volume control (§ 28B-304), groundwater recharge (§ 28B-305), and stream bank erosion (§ 28B-306) requirements shall be implemented.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
Standards for managing runoff from each subarea in the Delaware River
South watershed are shown below. Development sites located in each
of the A, B, or C Districts must control postdevelopment runoff rates
to predevelopment runoff rates for the design storms as follows:
|
District
|
Design Storm Postdevelopment
|
Design Storm Predevlopment
|
|---|---|---|
|
A
|
2-year
|
1-year
|
|
5-year
|
5-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
B
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
10-year
|
5-year
| |
|
25-year
|
10-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
C*
| ||
|
2-year
|
1-year
| |
|
5-year
|
2-year
|
|
* In District C, development sites which can discharge directly
to the Delaware River South main channel or major tributaries or indirectly
to the main channel through an existing stormwater drainage system
(i.e., storm sewer or tributary) may do so without control postdevelopment
peak rate of runoff greater than the five-year storm. Sites in District
C will still have to comply with the groundwater recharge criteria,
the water quality criteria, and stream bank erosion criteria. If the
postdevelopment runoff is intended to be conveyed by an existing stormwater
drainage system to the main channel, assurance must be provided that
such system has adequate capacity to convey the flows greater than
the two-year predevelopment peak flow or will be provided with improvements
to furnish the required capacity. When adequate capacity in the downstream
system does not exist and will not be provided through improvements,
the postdevelopment peak rate of runoff must be controlled to the
predevelopment peak rate as required in District A provisions (i.e.,
ten-year postdevelopment flows to ten-year predevelopment flows) for
the specified design storms).
|
A.
General. Postdevelopment peak rates of runoff from any regulated
activity shall meet the peak release rates of runoff prior to development
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix D[1]) and § 28B-307.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
District boundaries. The boundaries of the stormwater management
districts are shown on an official stormwater district map that is
available for inspection at the municipal office. A copy of the map
at a reduced scale is included in the Appendix D[2]. The exact location of the stormwater management district
boundaries, as they apply to a given development site, shall be determined
by mapping the boundaries using the two-foot topographic contours
(or most accurate data required) provided as part of the drainage
plan.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall be the predevelopment peak discharge for that subarea as indicated in § 28B-306. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted if discharges from multiple subareas recombine in proximity to the site. In this case, peak discharge in any direction may be a 100% release rate, provided that the overall site discharge meets the weighted average release rate.
D.
Off-site areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
E.
Site areas. Where the area of a site being impacted by a proposed
development activity differs significantly from the total site area,
only the proposed impact area utilizing stormwater management measures
shall be subject to the management district criteria. Unimpacted or
undisturbed areas that do flow into or are bypassing the stormwater
management facilities would not be subject to the management district
criteria.
F.
"No harm" option. For any proposed development site not located in
a provisional direct discharge district, the applicant has the option
of using a less-restrictive runoff control (including no detention)
if the applicant can prove that "no harm" would be caused by discharging
at a higher runoff rate than that specified by the plan. The no-harm
option is used when an applicant can prove that the postdevelopment
hydrographs can match predevelopment hydrographs, or if it can be
proved that the postdevelopment conditions will not cause increases
in peaks at all points downstream. Proof of "no harm" would have to
be shown based upon the following downstream impact evaluation, which
shall include a downstream hydraulic capacity analysis consistent
with § 28B-307H to determine if adequate hydraulic capacity
exists. The land applicant shall submit to the municipality this evaluation
of the impacts due to increased downstream stormwater flows in the
watershed.
(1)
The downstream impact evaluation shall include hydrologic and
hydraulic calculations necessary to determine the impact of hydrograph
timing modifications due to the proposed development upon a dam, highway,
structure, natural point of restricted stream flow, or any stream
channel section, established with the concurrence of the municipality.
(2)
The evaluation shall continue downstream until the increase
in flow diminishes due to additional flow from tributaries and/or
stream attenuation.
(3)
The peak flow values to be used for downstream areas for the
design return period storms (two-, five-, ten-, twenty-five-, fifty-,
and 100-year) shall be the values from the calibrated model for the
Delaware River South watershed. These flow values can be obtained
from the watershed plan.
(4)
Applicant-proposed runoff controls that would generate increased
peak flow rates at storm drainage problem areas would, by definition,
be precluded from successful attempts to prove "no harm," except in
conjunction with proposed capacity improvements for the problem areas
consistent with § 28B-307H.
(5)
Financial considerations shall not constitute grounds for granting
a no-harm exemption.
(6)
Capacity improvements may be provided as necessary to implement
the "no harm" option which proposes specific capacity improvements
to provide that a less stringent discharge control would not create
any harm downstream.
G.
Stormwater conveyance corridor protection (riparian corridor preservation
and vegetation). Runoff from developed areas of the site, including
but not limited to areas of impervious surface, shall be managed through
a series of riparian corridor vegetation facilities whenever possible.
This will be accomplished in a manner satisfactory to the municipality,
utilizing the "Pennsylvania Handbook of Best Management Practices
for Developing Areas," 1998, riparian forested buffer, and the priority
goal of the riparian vegetation will be the reduction of thermal impacts
on stormwater runoff associated with impervious areas, with a secondary
goal being the protection of capacity of existing stormwater conveyance
channels. These goals will be achieved through the use of design criteria
in § 28B-315D of this chapter, and shall be in addition
to any other municipal ordinance provisions.
H.
Downstream hydraulic capacity analysis. Any downstream capacity hydraulic
analysis conducted in accordance with this chapter 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 increased 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 Department of Environmental Protection's Erosion
and Sediment Pollution Control Program Manual.
(2)
Natural or man-made channels or swales must be able to convey
increased 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 the Department of Environmental Protection's
Chapter 105 regulations (if applicable) and, at minimum, pass the
increased twenty-five-year return period runoff.
I.
Regional detention alternatives. For certain areas within the study
area, it may be more cost-effective to provide one control facility
for more than one development site than to provide an individual control
facility for each development site. The initiative and funding for
any regional runoff control alternatives are the responsibility of
prospective applicants. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined on a case-by-case
basis using the hydrologic model of the watershed consistent with
protection of the downstream watershed areas. "Hydrologic model" refers
to the calibrated model as developed for the stormwater management
plan. It is a requirement that, even if regional basins are proposed
for the water quantity control, that the water quality, stream bank
erosion and recharge criteria be accomplished on site, or as close
to the source of the runoff as possible.
A.
Increased stormwater runoff which may result from regulated activities listed in § 28B-104 shall be controlled by permanent stormwater runoff control measures that will provide the required standards within Article III. The methods of stormwater control or best management practices (BMPs) which may be used to meet the required standards are described in this chapter and are the preferred methods of controlling stormwater runoff. Additional design criteria are included in these descriptions and the Pennsylvania BMP Manual. The choice of BMPs is not limited to the ones appearing in this chapter; however, any selected BMP must meet or exceed the runoff peak rate requirements of this chapter for the applicable hydrologic district.
B.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation.
C.
Collection system standards.
(1)
Curb inlets. Curb inlets shall be located at curb tangents on
the uphill side of street intersections and at intervals along the
curbline to control the maximum amount of encroachment of runoff on
the roadway pavement so that same does not exceed a width of four
feet during the design storm event. Design and location of curb inlets
shall be approved by the Township.
(2)
Pipe materials. All storm sewer piping shall be Class III reinforced
concrete pipe, except when pipe class and strength is required to
be increased in accordance with PennDOT specifications. Piping shall
be saw-cut at ends, as needed, and not hammered or broken. All pipe
joints and lift holes must be mortared except where designed for infiltration.
Smooth lined HDPE, Sch 40 PVC, or approved equivalent pipe material
may be utilized for privately owned BMPs and collection systems located
on individual residential lots and in other public areas, upon approval
by the Township Engineer.
(3)
Minimum pipe size. Minimum pipe diameter shall be 18 inches
(or an equivalent flow area of 1.76 square feet). Smaller pipe size
may be utilized for privately owned BMPs located on individual residential
lots, upon approval by the Township Engineer.
(4)
Inlet and manhole construction.
(a)
Inlet and manhole castings and concrete construction shall be
equivalent to PennDOT design standards. Manholes shall be equipped
with open-grate lids. All inlet grates shall be bicycle safe heavy-duty
structural steel. All storm sewer inlets must be identified with a
storm drain marker. Storm drain markers shall be stainless steel affixed
to the inlet hood with adhesive, rivets or bolts. (Marker may be bolted
to the grate in off-road locations.) Marker shall have a minimum diameter
of 3 1/2 inches and include "No Dumping - Drains to Waterway"
and a fish symbol. Alternate designs/sizes may be used if approved
by the Township.
(b)
Alternate concrete, HDPE, or approved equivalent yard inlets
may be utilized for privately owned BMPs located on individual residential
lots, upon approval by the Township Engineer.
(5)
Open-end pipes must be fitted with concrete endwalls or wing
walls in accordance with PennDOT standards.
(6)
Flow velocity. Stormwater collection systems shall be designed
to produce a minimum velocity of 3.0 feet per second when flowing
full. The maximum permissible velocity shall be 15.0 feet per second.
Pipe slopes shall not be less than 0.50%.
(7)
Inlets and manholes shall be spaced at intervals not exceeding
300 feet, and shall be located wherever branches are connected or
sizes are changed; and wherever there is a change in alignment or
grade. For drainage lines of at least 36 inches diameter, inlets and
manholes may be spaced at intervals not exceeding 400 feet.
(8)
Storm sewer bedding/backfill requirements shall conform to the
construction details in Appendix L.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(9)
Inlets shall be located to intercept concentrated runoff prior
to discharge over public/private rights-of-way, sidewalks, streets,
and driveways.
(10)
The capacity of all Type C inlets shall be based on a maximum
surface flow to the inlets of 4.0 cfs, calculated based on the 100-year-frequency
design storm event. The maximum flow to Type C inlets located in low
points (such as sag vertical curves) shall include the overland flow
directed to the inlet as well as all bypass runoff from upstream inlets.
The bypass flow from upstream inlets shall be calculated using inlet
efficiency curves included in PennDOT Design Manual Part 2, latest
edition. If the surface flow to an inlet exceeds 4.0 cfs, additional
inlets shall be provided upstream of the inlet to intercept the excessive
surface flow. A Type C inlet at a low point of a paved area may be
designed to accept a maximum of six cubic feet per second (cfs). Type
M inlets shall be designed to accept a maximum surface flow of six
cfs based on the 100-year-frequency design storm event, unless otherwise
approved by the Township. Double inlets will not be permitted where
additional pipe and inlets can be placed upstream to intercept excessive
surface flow. A maximum of 12 cfs shall be permitted to be collected
by a Type M inlet located in an isolated pervious area, provided the
designer can verify that such an inlet would not cause stormwater
to accumulate on any adjacent public or private property, outside
of an associated storm sewer easement, and that the depth of the accumulated
stormwater would not exceed 12 inches.
(11)
A minimum drop of two inches shall be provided between the inlet
and outlet pipe invert elevations within all inlets and manholes.
When varying pipe sizes enter an inlet or manhole, the elevation of
crown of all pipes shall be matched.
(12)
Stormwater pipes shall have a minimum depth of cover of 12 inches
(including over the bell) or as designated by the American Concrete
Pipe Association (whichever is greater), and in no case shall any
part of the pipe project into the road subbase or curb. Where cover
is restricted, equivalent pipe arches or elliptical may be specified
in lieu of circular pipe. Where smooth-lined high density polyethylene
pipe is approved in lieu of reinforced concrete pipe, minimum depth
of cover for smooth lined concrete pipe shall be per pipe manufacturer's
specifications, or 12 inches, whichever is greater.
(13)
The capacity of all stormwater pipes shall be calculated utilizing
the Manning Equation for open channel flow as applied to closed conduit
flow. The Manning's roughness coefficient shall be 0.13 for all
concrete pipe (and 0.012 of SLHDPE). In cases where pressure flow
may occur, the hydraulic grade line shall be calculated throughout
the storm sewer system to verify that at least one foot of freeboard
will be provided in all inlets and manholes for the design storm event.
(14)
Culverts shall be designed based on procedures contained in
Hydraulic Design of Highway Culverts, HDS No. 5, U.S. Department of
Transportation, Federal Highway Administration. Where pressure flow
is anticipated in storm sewer pipes (non-open-channel flow), the applicant's
designer shall be required to calculate the elevation of the hydraulic
grade line through the storm sewer system. Wherever the hydraulic
grade line elevation exceeds the pipe crown elevation for the design
flow, pipes with watertight joints must be specified.
(15)
Storm sewer structures (i.e., endwalls, inlets, and sections,
etc.) may not be located on top of or within 10 feet of electric,
communications, water, sanitary sewer, or gas services and/or mains,
unless approval is received from the Township and the authority or
utility having jurisdiction over same.
(16)
Stormwater pipes must be oriented at right angles to electric,
water, sanitary sewer, and gas utilities when crossing above or beneath
same. Crossing angles of less than 90° will only be permitted
at the discretion of the Township. When skewed crossings are permitted,
interior angles between alignment of the storm sewer pipe and utility
shall not be less than 45°. Vertical and horizontal design of
storm sewer must be linear.
(17)
Roadway underdrain is required along both sides of all proposed
roadways, existing roadways proposed to be widened, and within existing
or proposed roadside swales as directed by the Township.
(18)
Where a public storm sewer system is not located within a right-of-way,
or dedicated public property, a twenty-foot-wide easement shall be
established to encompass the storm sewer system. For multiple pipes
or utilities, the width of the easement shall be a minimum of 30 feet.
(19)
A minimum of one foot of freeboard, between the inlet grate
and the design flow elevation, shall be provided in all storm sewer
systems (inlets and manholes) for the 100-year-frequency design storm
event.
D.
Open swales and gutters. Open swales shall be designed on the basis
of Manning's Formula as indicated for collection systems with
the following considerations:
(1)
Roughness coefficient. The roughness coefficient shall be 0.040
for earth swales.
(2)
Bank slopes. Slopes for swale banks shall not be steeper than
one vertical to four horizontal.
(3)
Flow velocity. The maximum velocity of flow as determined by
Manning's Equation shall not exceed the allowable velocities
as shown in the following table for the specific type of material,
unless otherwise approved by the Township and the Bucks County Conservation
District.
Note: Source of the following design criteria is the Pennsylvania
Department of Environmental Protection, Bureau of Soil and Water Conservation
Publication, Erosion and Sediment Control Program Manual.
|
Allowable Velocity
| |
|---|---|
|
Material
|
Velocity in feet per second
(fps)
|
|
Well established grass on good soil
| |
|
Short pliant bladed grass
|
4.0 to 5.0
|
|
Bunch grass - soil exposed
|
2.0 to 3.0
|
|
Stiff stemmed grass
|
3.0 to 4.0
|
|
Earth without vegetation
| |
|
Fine sand or silt
|
1.0
|
|
Ordinary firm loam
|
2.0 to 3.0
|
|
Stiff clay
|
3.0 to 5.0
|
|
Clay and gravel
|
4.0 to 5.0
|
|
Coarse gravel
|
4.0 to 5.0
|
|
Soft shale
|
5.0 to 6.0
|
|
Shoulders
| |
|
Earth
|
(as defined above)
|
|
Stabilized
|
6.0
|
|
Paved
|
10.0 to 15.0
|
(4)
Swales shall be stabilized with biodegradable erosion control
matting to permit establishment of permanent vegetation. Swales shall
be of such shape and size to effectively contain the 100-year, Rational
Method design storm, and to conform to all other specifications of
the Township.
(5)
To minimize sheet flow of stormwater across lots located on
the lower side of roads or streets, and to divert flow away from building
areas, the cross section of the street as constructed shall provide
for parallel ditches or swales or curb on the lower side, which shall
discharge only at drainage easements unless otherwise approved by
the Township.
(6)
Gutters and swales adjacent to road paving shall be permitted
to carry a maximum flow of four cubic feet per second prior to discharge
away from the street surface, unless it is proven to the satisfaction
of the Township by engineering calculations that the road slopes or
other factors would allow higher gutter or swale capacity.
(7)
Flows larger than those permitted in gutters and roadside swales
may be conveyed in swales outside the required road right-of-way in
separate drainage easements, or may be conveyed in pipes or culverts
inside or outside the required road right-of-way.
(8)
Existing and proposed swales shall be provided with underdrains
as deemed necessary by the Township should overland seepage result
in potential maintenance problems. Underdrains must discharge into
a natural drainage channel or stormwater management system.
(9)
Where drainage swales are used to divert surface waters away
from buildings, they shall be sodded, landscaped, or otherwise protected
as required and shall be of a slope, shape, and size conforming with
the requirements of the Township. Concentration of surface water runoff
shall be permitted only in swales, watercourses, retention or detention
basins, bioretention areas, or other areas designed to meet the objectives
of this section.
E.
Bridge and culvert design. Any proposed bridge or culvert to convey
flow within a perennial or intermittent stream shall be designed in
accordance with the following principles:
(1)
Culverts and bridges shall be designed with an open bottom to
maintain natural sediment transport and bed roughness, avoiding acceleration
of water velocity above the natural (preexisting) condition. Rock
(rip rap) lining (native material if possible) shall be installed
within the culvert as needed to prevent erosion within the structure.
Approximate top of rock lining must be at the level of the existing
stream bottom so as to maintain normal water level and unimpeded movement
of native animal species.
(2)
Bottom of opening shall be designed to match the bankfull channel
condition in terms of width and depth. The cross-sectional area of
the bankfull channel (measured at a reference location upstream of
the structure) shall be matched with area in the crossing structure.
(3)
Above the bankfull elevation, the width shall increase a minimum
of 30% to disperse the energy of higher flow volumes and avoid undermining
of the supporting structure by secondary currents.
(4)
The total cross-sectional area of the structure opening must
be equal to or greater than the flood-prone area (cross-sectional
stream area at a depth of twice the maximum bankfull depth, measured
at a reference location upstream of the structure). The flood-prone
area is approximately equal to the area flooded by a fifty-year-return
flood.
(5)
All bridges, culverts, and drainage channels shall be designed
to convey a flow rate equal to a 100-year, twenty-four-hour storm
(refer to Appendix A, Table A-1[2]). All bridges and culverts shall be designed to convey
the 100-year design storm without increasing the extent and depth
of the 100-year floodplain.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
F.
Storm sewer design.
(1)
Design flow rate.
(a)
The storm sewer system shall be designed to carry the 100-year-frequency
design storm peak flow rate. The drainage area and runoff coefficient
to each inlet shall be indicated on the stormwater management plan.
The 100-year flow rate shall be determined by the Rational Method
formula:
|
Q = CIA
| ||
|
Where:
| ||
|
Q
|
=
|
Peak runoff rate measured in cubic feet per second (cfs)
|
|
C
|
=
|
Runoff coefficient - The coefficient of stormwater runoff includes
many variables, such as ground slope, ground cover, shape of drainage
area, etc.
|
|
I
|
=
|
Intensity - Average rainfall intensity in inches per hour for
a time equal to the time of concentration (in/hr)
|
|
A
|
=
|
Area - Drainage area in acres (ac)
|
(b)
Appropriate values for the rainfall intensity can be found in
Appendix A, Figure A-1.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2)
Consideration shall be given to future land use changes in the
drainage area in selecting the Rational (C) coefficient. For drainage
areas containing several different types of ground cover, a weighted
value of C shall be used.
(3)
In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method (as referenced in § 28B-310) shall be used for inlet drainage areas in excess of one acre, unless otherwise approved by the Township. For inlet drainage areas less than one acre, a five-minute time of concentration shall be used unless otherwise approved by the Township.
(4)
In determining the required design flow rate through a storm
sewer piping system, if a five-minute time of concentration (storm
duration) results in a pipe size exceeding a thirty-inch-diameter
pipe (or equivalent flow area of 4.9 square feet), the time of concentration
approach (as defined herein) shall be used in determining storm duration.
(5)
Overflow system. An overflow system shall be provided to carry
all bypass flow and/or flow in excess of storm sewer design capacity
to the detention basin (or other approved outlet point) when the capacity
of the storm sewer system is exceeded. Stormwater runoff will not
be permitted to surcharge from storm sewer structures.
G.
Grading and drainage.
(1)
After completion of rough grading, a minimum of eight inches
of topsoil, or the exiting depth of topsoil encountered on the site
(whichever is greater), shall be returned to remaining disturbed areas
prior to final grading and seeding.
(2)
Lots shall be graded to secure proper drainage away from buildings
and to prevent the collection of stormwater in pools. Minimum 2% slopes
shall be maintained away from and around all structures. Separation
between the top of foundation wall (or slab) shall comply with Township
Construction Code requirements.
(3)
Construction. The developer shall construct and/or install such
drainage structures and/or pipe which are necessary to prevent erosion
damage and to satisfactorily carry off such surface waters to the
nearest practical street, storm drain or natural watercourse.
(4)
Excavation. No excavation shall be made with a cut face steeper
in slope than four horizontal to one vertical (4:1 = 25%), except
under one or more of the following conditions:
(a)
The fill is located so that settlement, sliding, or erosion
will not result in property damage or be a hazard to adjoining property,
streets, alleys or buildings.
(b)
A written statement from a professional civil engineer, licensed
in the commonwealth of Pennsylvania and experienced in erosion control,
certifying the site has been inspected and that the proposed deviation
from the slope specified above will not endanger any property or result
in property damage, is submitted to and approved by the Township Engineer.
(c)
A concrete, segmental block, or stone masonry wall, constructed
in accordance with requirements of the Construction Code, is provided
to support the face of the excavation.
(5)
Fill. No fill shall be made which creates any exposed surface
steeper in slope than four horizontal to one vertical (4:1 = 25%)
except under one or more of the following conditions:
(a)
The fill is located so that settlement, sliding, or erosion
will not result in property damage or be a hazard to adjoining property,
streets, alleys, or buildings.
(b)
A written statement from a professional civil engineer, licensed
in the commonwealth of Pennsylvania and experienced in erosion control,
certifying the site has been inspected and that the proposed deviation
from the slope specified above will not endanger any property or result
in property damage, is submitted to and approved by the Township.
(c)
A concrete, segmental block, or stone masonry wall, constructed
in accordance with Township requirements, is provided to support the
face of the excavation.
(6)
Slopes and fences. The top or bottom edge of slopes shall be
a minimum of five feet from property or right-of-way lines of streets
or alleys, in order to permit the normal rounding of the edge without
encroaching on the abutting property. Where walls or slopes (steeper
than two horizontal to one vertical) are approved under the criteria
in this section, and are five feet or more in height, a protective
fence conforming to Township construction codes shall be required
at the top of the wall (or bank).
(7)
Cleanup. All lots must be kept free of any debris or nuisances
whatsoever during construction.
(8)
Design of erosion and sedimentation control facilities (particularly
stormwater/sediment basins) shall incorporate best management practices
as defined herein.
(9)
Cut-and-fill operations shall be kept to a minimum. Wherever
feasible, natural vegetation shall be retained, protected, and supplemented.
Cut-and-fills shall not endanger or otherwise adversely impact adjoining
property.
(10)
No grading equipment shall be permitted to be loaded and/or
unloaded on a public street, and no grading equipment shall be permitted
to travel on or across a public street unless licensed for operation
on public thoroughfares.
(11)
Grading equipment shall not be permitted in intermittent and
perennial streams. Temporary crossing shall only be permitted where
application is made, and approval is received, from the Pennsylvania
Department of Environmental Protection (where applicable), the Bucks
County Conservation District, and Upper Makefield Township.
(12)
Design of energy dissipation for high volume and/or high velocity
discharge from storm sewer pipes and channels shall be in accordance
with Hydraulic Engineering Circular No. 14, "Hydraulic Design of Energy
Dissipaters for Culverts and Channels" as published by Department
of Transportation, FHA, when deemed necessary by the Township, and
as approved by the Bucks County Conservation District.
(13)
To control the dissemination of mud and dirt onto public roads
and driveways, tire cleaning areas constructed of AASHTO No. 1 stone
(underlain by geotextile structural fabric), at least 50 feet in length
shall be installed at each point of access to the site and individual
lots (upon construction of internal streets in a binder condition).
When deemed necessary by the Township, washing stations shall also
be set up at every construction entrance in order to wash mud and
dirt from exiting vehicles. Appropriate measures must be taken to
control runoff from such locations. The developer shall be responsible
for the placement of appropriate signage identifying construction
entrances and washing stations. Construction entrances shall be maintained
by the developer during construction, as determined by the Township.
(14)
During construction activities, necessary measures for dust
control shall be exercised, including the application of water to
higher traffic areas of the site.
(15)
In the event any mud and/or debris is transported from the site
onto a public roadway, the debris shall be removed immediately and
the roadway swept and/or washed as deemed necessary by the Township
at the owner's expense.
(16)
Adequate provision shall be made to prevent surface water from
damaging the cut face of excavation and the sloping surfaces of fills.
H.
Any stormwater management facility 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 100-year postdevelopment conditions. The height
of embankment must be set as to provide a minimum 1.0 foot of freeboard
above the maximum pool elevation computed when the facility functions
for the 100-year postdevelopment inflow.
I.
Emergency spillways discharging over embankment fill shall be constructed
of reinforced concrete checker blocks to protect the berm against
erosion. The checker block lining shall extend to the toe of the fill
slope on the outside of the berm, and shall extend to an elevation
three feet below the spillway crest on the inside of the berm. Alternate,
permanent erosion control matting, or approved equivalent, may be
utilized for spillways included in privately owned BMPs located on
individual residential lots, upon approval by the Township Engineer.
J.
Vegetated spillways may be utilized for spillways constructed entirely
on undisturbed ground (i.e., not discharging over fill) if the designer
can demonstrate that flow velocities through the spillways will not
cause erosion of the spillway. A dense cover of vegetation shall be
rapidly established in such spillways by sodding or seeding with a
geotextile anchor. Such a vegetated spillway must be stabilized before
runoff is directed to the basin.
K.
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 100-year
event.
L.
Stormwater management facility outlet piping shall be Class III reinforced
O-ring concrete pipe. A minimum of one concrete anti-seep collar shall
be required (except for privately owned BMP with berm height not exceeding
three feet on individual residential property). Precast collars shall
have a minimum thickness of eight inches; field-poured collars shall
have a minimum thickness of 12 inches. Collars may not be installed
within two feet of pipe joints. Collars must be designed to project
a minimum of two feet around the perimeter of the pipe. Maximum collar
spacing is 14 times the design projection around the perimeter. SLHDPE,
Sch 40 PVC, or approved equivalent may be utilized for outlet pipes
included in privately owned BMPs located on individual residential
lots, upon approval by the Township Engineer.
M.
No stone gabion baskets may be used in the construction of stormwater
management facilities.
N.
Retention/detention basins (BMPs):
(1)
Pipe outlet shall permit complete drainage of all detained water,
unless the stormwater management facility is designed as a retention
basin/pond or provides for stormwater renovation with constructed
wetlands.
(2)
When a detention basin is not designed as a stormwater management
constructed wetland, the stormwater management facility shall be planted
with low-maintenance grass/meadow mix or substitute satisfactory to
the Township.
(3)
Detention basin (BMP) bottom slope shall be designed as appropriate
for the intended use/maintenance, and pursuant to Pennsylvania BMP
Manual guidelines.
(4)
Basin (BMP) design.
(a)
To minimize the visual impact of detention basins, the detention
basin shall be designed to avoid the need for safety fencing. To meet
this requirement, basins shall be designed to the following specifications:
[1]
Maximum depth of detained runoff shall be 24 inches for a two-year
or ten-year storm event.
[2]
Maximum depth of detained runoff shall be 36 inches for a 100-year
storm event.
[3]
Interior slopes shall not be steeper than a ratio of 4:1 horizontal
to vertical.
[4]
Ponded water shall never exceed a depth of 24 inches for more
than four hours.
(b)
Depths and slopes may be exceeded by permission of the Township
on a case-by-case basis if lot runoff, topography and/or existing
downstream systems make the required pond area unreasonably large.
In such case, fence and landscape screens will be required.
(5)
An access ramp of 10:1, 10 feet wide, shall be provided to allow
maintenance equipment to reach the basin floor. The ramp shall coincide
with the required gate if fencing is needed.
(6)
When required by the Township, fencing shall provide a suitable
barrier at least four feet in height of material approved by the Township,
such as split rail fencing (vinyl, aluminum, or concrete post w/wood
rails) with wire backing. Access to the basin shall be provided by
a gate or gates having a total opening of at least 10 feet at such
location(s) as to permit ready access to the detention basin with
maintenance equipment.
(7)
Landscaping.
(a)
The perimeter berms and embankments of retention/detention basins,
including wet ponds, and artificial wetland stormwater management
facilities shall be designed to create a natural appearance and reduce
future maintenance requirements. Landscaping shall include a mixture
of native tall grasses and perennial plants, ground cover, shrubs,
and trees to eliminate the necessity of periodic mowing.
(b)
Artificial wetland basins, and other BMPs, shall be designed
pursuant to requirements of the Pennsylvania Handbook of Best Management
Practices for developing areas. Plant material and arrangement shall
be subject to approval of the Township. (Refer to Appendix C,[4] Plant Lists for Wetland Management.)
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
(c)
Retention/detention basin landscaping design.
[1]
The perimeter of the retention/detention basin shall be landscaped
with a mixture of deciduous trees, evergreens, and shrubs arranged
in an informal manner. Retention basin (wet ponds) and artificial
wetland basin landscaping shall be designed to create a "natural"
appearance. Minimum plant material shall include the following per
100 linear feet of basin perimeter measured at the 100-year-reoccurrence
stormwater elevation:
[2]
Retention/detention basin landscaping design is subject to approval
by the municipality. Preservation of existing vegetation surrounding
the facility may be considered in lieu of some or all required landscaping
plantings.
(8)
Retaining walls shall not be specified for use within the 100-year
water surface elevation area of any detention/retention facility or
as part of any embankment or cut slope that is appurtenant to the
construction of a detention/retention facility.
(9)
The developer shall provide written assurance, satisfactory
to the Township, that the retention/detention basin will be properly
maintained. Such assurances shall be in a form to act as a covenant
that will run with the land, and shall provide Township maintenance
at the cost of the landowner in case of default, and further provide
for assessment of costs and penalties in case of default.
(10)
As an alternate to Subsection N(9) above, the Township may, at their own option, assume responsibility of the basin and may accept dedication of the basin by the developer. If the retention/detention basin is dedicated or offered to the Township for long-term maintenance, the following regulations shall apply:
(a)
The dedicated area shall include the entire ponded area for
the 100-year-storm event and the outside slope at the berm.
(c)
If fencing is necessary, the basin design shall provide a level
area (2% slope) eight feet in width on both the inside and outside
of the fence, along the entire length of the fence for proper access
by Township maintenance equipment. The total width of this generally
level area shall be at least 16 feet.
(d)
The developer shall provide for the special financial burden
the Township will be accepting if the Township accepts the detention
basin maintenance. To help mitigate this future financial burden,
the developer shall contribute to the Township a cash payment in the
amount of $15,000 per acre, on a pro rata basis, for any detention/retention
basin site or area dedicated to the Township and being accepted by
the Township. The detention/retention basin site area is measured
to the outside limit of grading necessary to construct the basin and
basin berm. The minimum contribution for any basin, regardless of
size, shall be $7,500. This requirement may be modified by a resolution
of the Township, from time to time, to reflect actual long-term costs
of detention basin maintenance in the Township.
(11)
Basin berm construction requirements:
(a)
Site preparation. Areas under the embankment and any structural
works shall be cleared, grubbed, and the topsoil stripped to remove
the trees, vegetation, roots or other objectionable material. In order
to facilitate cleanout and restoration, the pool area will be cleared
of all brush and excess trees.
(b)
Cut-off trench.
[1]
A cut-off trench will be excavated along the center
line dam on earth fill embankments. The minimum depth shall be two
feet. The cut-off trench shall extend up both abutments to the riser
crest elevation. The minimum bottom width shall be eight feet but
wide enough to permit operation of compaction equipment. The side
slopes shall be no steeper than 1:1. Compaction requirements shall
be the same as those for the embankment.
[2]
The trench shall be kept free from standing water
during the backfilling operations.
(c)
Embankment (the following requirements must be included on the
SWM site plan near the basin berm construction detail).
[1]
The fill material shall be taken from the selected
borrow areas. It shall be free of roots, wood vegetation, oversized
stones, rocks or other objectionable material. Areas on which fill
is to be placed shall be scarified prior to placement of fill.
[2]
The fill material should contain sufficient moisture
so that it can be formed by hand into a ball without crumbling. If
water can be squeezed out of the ball, it is too wet for proper compaction.
[3]
Fill material will be placed in six-to-eight-inch
layers and shall be continuous over the entire length of the fill.
Fill material must be compacted to a minimum of 95% of modified proctor
density as established by ASTM D-1557. Compaction testing by a certified
soils engineer/geologist must be completed as directed by the Township
Engineer to verify adequate compaction has been achieved.
(12)
Any stormwater basin (BMP) intended to maintain a permanent
pool of water shall be designed with a "safety bench" incorporated
in the inside side slope grading. Refer to "wet pond" requirements
below.
O.
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[5] 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 doesn't fall under Chapter 105 regulations must be able
to convey, without damage to the drainage structure or roadway, runoff
from the 100-year design storm with a minimum 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 100-year design storm with a minimum 1.0 foot
of freeboard measured below the lowest point along surface of the
roadway. Any facility that constitutes a dam as defined in PADEP Chapter
105 regulations may require a permit under dam safety regulations.
Any facility located within a PADOT right-of-way must meet PADOT minimum
design standards and permit submission requirements.
[5]
Editor's Note: See 25 Pa. Code Chapter 105.
P.
Any drainage conveyance facility and/or channel that doesn't
fall under Chapter 105[6] regulations must be able to convey, without damage to
the drainage structure or roadway, runoff from the 100-year design
storm. Conveyance facilities to or exiting from stormwater management
facilities 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 100-year design storm.
Any facility located within a PADOT right-of-way must meet PADOT minimum
design standards and permit submission requirements.
[6]
Editor's Note: See 25 Pa. Code Chapter 105.
Q.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
R.
Except for drainage at roadway stream crossings, pipe or artificial
swale discharge shall be set back 75 feet from a receiving waterway,
and the pipe discharge shall be diffused or spread out to reduce and
eliminate high-velocity discharges to the impacted ground surface.
The conveyance mechanism shall minimize disturbance and velocity of
discharge.
S.
All infiltration devices and groundwater recharge facilities shall
be designed to completely drain all water in three days subsequent
to any storm event.
T.
Riparian corridor preservation: Refer Riparian Buffers, § 28B-315D
of this chapter.
U.
All developments which create impervious surface shall provide capacity for and treatment of the "water quality volume," "recharge volume," and "volume control" unless exempt from applicability under § 28B-104.
V.
Special requirements for areas falling within defined exceptional
value and high-quality subwatersheds. The temperature and quality
of water and streams that have been declared as exceptional value
or high quality is to be maintained as defined in Chapter 93, Water
Quality Standards, Title 25 of Pennsylvania Department of Environmental
Protection rules and regulations. Temperature-sensitive BMPs and stormwater
conveyance systems are to be used and designed with storage pool areas
and supply outflow channels, and shaded with trees. This will require
the modification of berms for permanent ponds. At a minimum, the southern
half of pond shorelines shall be planted with shade or canopy trees
within 10 feet of the pond shoreline. In conjunction with this requirement,
the maximum slope allowed on the berm area to be planted is 10 to
1 to lessen the destabilization of berm soils due to root growth.
W.
Developers shall utilize BMPs to provide for additional water quality
improvement and groundwater recharge.
(1)
In evaluating potential stormwater BMPs, the order of preference
is as follows:
(2)
Infiltration BMPs shall be utilized unless the applicant can
demonstrate use of infiltration techniques is not feasible due to
site conditions based upon site-specific soil testing. Vegetated swales,
wetlands or artificial wetlands and bioretention structures shall
be utilized wherever possible if infiltration BMPs are deemed unfeasible.
BMP techniques can and should be used in conjunction with each other
(e.g., vegetated swales with infiltration or retention facilities).
(a)
Infiltration best management practices (BMPs). Infiltration
devices shall be selected based upon suitability of soils and site
conditions. Soil infiltration tests shall be performed on all sites
to determine suitability of the site for infiltration BMPs. Testing
shall include evaluation of selected soil horizons by soil probes,
deep pits and/or percolation measurements. The soil infiltration rate
of discharge from the infiltration area being used in the proposed
design shall be based on these measurements. Deep hole testing and
infiltration testing shall be performed by a Certified Professional
Soil Scientist (CPSS), Certified Professional Soil Classifier (CPSC),
or a Sewage Enforcement Officer (SEO). Infiltration BMPs shall be
designed in accordance with the design criteria and specifications
in Section 5 of the Pennsylvania Handbook of Best Management Practices
for Developing Areas (1998) and shall meet the following minimum requirements:
[1]
Infiltration BMPs shall be constructed on soils with a minimum
depth of 24 inches between the intended bottom of the facility and
the seasonal high water table and/or bedrock (limiting zone).
[2]
Infiltration BMPs intended to receive rooftop runoff shall include
appropriate measures, such as leaf traps and cleanouts, to prevent
clogging by vegetation.
[3]
Where direct discharge is permitted under the requirements of § 28B-307, infiltration BMPs shall be designed to provide adequate storage to accommodate the postdevelopment first flush design storm (one-year, twenty-four-hour storm) volume with outlet and overflow controls to convey runoff larger than the first flush design storm volume safely to a natural outfall.
[4]
In areas where runoff release rates are specified under the requirements of § 28B-307, regardless of the specified release rate percentage, if infiltration BMPs are intended, they shall be designed to, as a minimum:
[a]
Provide adequate storage to accommodate the volume
of runoff calculated as the difference between the predevelopment
runoff volume and postdevelopment runoff volume based on the 100-year
design storm.
[b]
Control the postdevelopment peak rate of runoff to the predevelopment peak rate of runoff for all design storms identified in § 28B-308A of this chapter.
[c]
Provide an overflow or spillway that safely permits
the passing of runoff greater than that occurring during the 100-year
design storm.
(b)
Noninfiltration facilities used as best management practices
(BMPs). All facilities shall be designed in accordance to the design
criteria and specifications in the Pennsylvania Handbook of Best Management
Practices for Developing Areas (1998). This design shall be in particular
coordination with Section 8, Descriptions of Selected Best Management
Practices.
(c)
Artificial wetlands, wet ponds, and bioretention structures.
[1]
Wet pond BMPs shall meet the following requirements:
[a]
Wet ponds shall be constructed on hydric or wet
soils and/or soils which have an infiltration rate of less than 0.2
inch/hour.
[b]
A minimum drainage area of five acres shall be
directed to the pond unless a source of recharge is utilized, such
as a natural spring or well.
[c]
The length of the pond between the inflow and outlet
points shall be maximized. In addition, an irregular shoreline shall
be provided. By maximizing the flow length through the pond and providing
an irregular shoreline, the greatest water quality benefit will be
achieved by minimizing "short circuiting" of runoff flowing through
the pond.
[d]
A shallow forebay shall be provided adjacent to
all inflow areas. The forebay shall be planted as a marsh with emergent
wetland vegetation. The forebay serves to enhance sediment trapping
and pollutant removal, as well as concentrating accumulated sediment
in an area where it can be readily removed.
[e]
All wet ponds shall be designed with public safety
as a primary concern. An aquatic safety bench shall be provided around
the perimeter of the permanent pool. The depth of the bench shall
be a maximum of one foot for a width of at least three feet. A 3:1
slope shall lead from the edge of the safety bench toward the deep
water portion of the pond. At least 15 feet of 3:1 slope shall be
provided from the edge of the safety bench. Slopes in the remainder
of the pond below the permanent pool elevation shall be a maximum
of 2:1.
[f]
The perimeter slope above the permanent pool shall
have a maximum slope of 4:1 for a distance of at least 20 feet. The
remaining areas above the permanent pool shall have a maximum slope
of 3:1.
[g]
Wet ponds shall have a deep water zone to encourage
gravity settling of suspended fines, and prevent stagnation and possible
eutrophication.
[h]
Wet ponds shall be capable of being substantially
drained by gravity flow. Where possible, wet ponds shall be equipped
with a manually operated drain that can be secured against unauthorized
operation.
[i]
A planting plan shall be developed for the wet
pond, showing all proposed aquatic, emergent, and upland plantings.
[j]
Wet ponds shall be designed to discourage use by
Canada geese. Techniques employed shall include the following:
[i]
Elimination of straight shorelines, islands, and peninsulas;
[ii]
Placement of walking paths (where applicable)
along the shoreline;
[iii]
Placement of grassed areas (i.e., playing fields)
at least 450 feet from the water surface;
[iv]
Vegetative barriers;
[v]
Rock barriers;
[vi]
Installation of tall trees within 10 feet of the
water surface;
[vii]
Use of ground covers not palatable to Canada
geese.
[2]
Artificial wetland BMPs shall meet the following requirements:
[a]
Artificial wetlands shall be constructed on hydric
or wet soils and/or soils which have an infiltration rate of less
than 0.2 inch/hour.
[b]
Runoff entering artificial wetlands shall be filtered
through a sediment removal device before entering the wetland.
[c]
A planting plan shall be developed for the artificial
wetland showing all proposed aquatic, emergent, and upland plantings.
The planting plan shall be developed to provide a diversity of species
resulting in a dense stand of wetland vegetation.
[d]
At least 75% of the surface area of the wetland
shall be developed as a shallow water emergent wetland, with a water
depth of less than 12 inches. The reminder shall be constructed as
open water with depths between two feet and four feet.
(d)
Minimum first flush detention/dual purpose BMPs. Minimum first
flush detention/dual purpose detention basin BMPs shall be designed
to meet the following requirements:
[1]
Postdevelopment runoff from a "water quality storm" (a one-year,
twenty-four-hour event) shall be released over a minimum period of
24 hours.
[2]
Two-stage basins shall be utilized where first flush detention
will be employed for water quality and conventional detention used
for peak rate control of storms exceeding the one-year, twenty-four-hour
event.
[3]
Two-stage basins shall be constructed so that the lower part
of the basin is graded to detain stormwater from the "water quality
storm," and the remainder of the basin graded as a flat overbank area
to provide storage only for the larger, less frequent storm events.
The overbank area is encouraged to be developed as an active or passive
recreational area.
[4]
The area inundated by the "water quality storm" is encourage
to be maintained as a wetland environment which will increase the
water quality benefits of the first flush/dual purpose detention basin,
and will prevent the need for mowing of a frequently saturated area.
X.
All stormwater control facility designs shall conform to the applicable
standards and specifications of the following governmental and institutional
agencies:
(1)
American Society of Testing and Materials (ASTM).
(2)
Asphalt Institute (AI).
(3)
Bucks Conservation District (BCD).
(4)
Federal Highway Administration (FHWA).
(5)
National Crushed Stone Association (NCSA).
(6)
National Sand and Gravel Association (NSGA).
(7)
Pennsylvania Department of Environmental Protection (PADEP).
(8)
Pennsylvania Department of Transportation (PADOT).
(9)
U.S. Department of Agriculture, Natural Resources Conservation
Service, Pennsylvania (USDA, NRCS, PA).
Y.
If special geological hazards or soil conditions, such as carbonate-derived
soils, are identified on the site, the developer's professional
engineer shall consider the effect of proposed stormwater management
measures on these conditions. In such cases, the municipality shall
require an in-depth report by a registered professional geologist.
Z.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. Guidelines established
by the Pennsylvania Handbook of Best Management Practices for Developing
Areas (1998), as amended, shall be utilized in determining stormwater
management facility design except where specifically modified by this
chapter or other municipal ordinance. The municipality shall reserve
the right to disapprove any design that would result in the occupancy
or continuation of an adverse hydrologic or hydraulic condition within
the watershed.
Stormwater runoff from all development sites shall be calculated
using either the rational method or a soil cover complex methodology.
A.
Any stormwater runoff calculations shall use a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. Table 310-1 summarizes acceptable computation methods. It
is assumed that all methods will be selected by the applicant based
on the individual limitations and suitability of each method for a
particular site. The municipality may allow the use of the Rational
Method to estimate peak discharges from drainage areas that contain
less than 200 acres. The Rational Method is recommended for drainage
areas under 100 acres.
B.
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms according to National Oceanic and
Atmospheric Administration (NOAA) Atlas 14 rain data corresponding
to the Doylestown rain gauge. The NRCS Type 11 rainfall curve from
NOAA is found on Figure A-1 in Appendix A[1] of this chapter. This data may also be directly retrieved
from the NOAA Atlas 14 website: https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=pa
hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html. If a hydrologic computer
model, such as PSRM or HEC-1/HEC-HMS, is used for stormwater runoff
calculations, the duration of rainfall shall be 24 hours.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
table in Appendix A[2] of this chapter. For the purposes of existing conditions
flow rate determination for applications, undeveloped land and existing
impervious surfaces shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number of
Rational C value (i.e., forest), as listed in table in Appendix A[3] of this chapter. Wooded areas shall use a ground cover
of "woods in good condition." An area shall be considered wooded if
there is a contiguous canopy of trees existing over an area of 1/4
acre or more.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
D.
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from NOAA Atlas 14, Volume 2, Version 2.1 (Table A-1). Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's Equation.
E.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Appendix A[4] of this chapter.
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Appendix A[5] of this chapter. For the purposes of existing conditions
flow rate determination for all subdivision and land development applications,
undeveloped land and existing impervious surfaces shall be considered
as "meadow" in good condition, unless the natural ground cover generates
a lower curve number (CN) or Rational 'C' value (e.g., forest),
as listed in Table A-4 or A-7 in Appendix A of this chapter. Wooded
areas shall use a ground cover of "woods in good condition." An area
shall be considered wooded if there is a contiguous canopy of trees
existing over an area of 1/4 acre or more.
[5]
Editor's Note: Said appendix is included as an attachment to this chapter.
G.
Where uniform flow is anticipated, the Manning Equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's
roughness coefficient (n) shall be consistent with Table A-4 in Appendix
A[6] of this chapter. Outlet structures for stormwater management
facilities shall be designed to meet the performance standards of
this chapter using any generally accepted hydraulic analysis technique
or method.
[6]
Editor's Note: Said appendix is included as an attachment to this chapter.
H.
The design of any stormwater management facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage
Indication Method. For drainage areas greater than 20 acres in size,
the design storm hydrograph shall be computed using a calculation
method that produces a full hydrograph. The municipality may approve
the use of any generally accepted full hydrograph approximation technique
that shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
I.
The municipality has the authority to require that computed existing
runoff rates be reconciled with field observations and conditions.
If the design professional engineer can substantiate through actual
physical calibration that more appropriate runoff and time-of-concentration
values should be utilized at a particular site, then appropriate variations
may be made upon review and recommendations of the Municipal Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question.
J.
All stormwater runoff calculations/reports and design of stormwater
management facilities shall be prepared by a registered professional
engineer licensed in the commonwealth of Pennsylvania.
|
Table 310-1. Acceptable Computation Methodologies For Stormwater
Management Plans
| ||
|---|---|---|
|
Method
|
Method Developed By
|
Applicability
|
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
|
HEC-1, HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simipler than TR-20 or HEC-1
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than 200 acres, or as approved by the municipality
and/or Municipal Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
|
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 Bucks County Conservation District.
B.
Additional erosion and sedimentation control design standards and
criteria that must be applied where infiltration BMPs are proposed
shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase, so as
to maintain their maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has received final stabilization.
C.
No regulated earth disturbance activities within the municipality
shall commence until the municipality approves an erosion and sediment
control plan for construction activities.
D.
All activities or projects involving earth disturbance of 1,000 square
feet or greater require erosion and sediment control plan adequacy
approval from Bucks County Conservation District. It is the applicant's
responsibility to make submission to Bucks County Conservation District
for review and approval of the erosion and sediment control plan.
A copy of erosion and sediment control plan adequacy approval correspondence
from Bucks County Conservation District must be received by the Township
prior to commencing earth disturbance activities, and issuance of
building/zoning permits or approvals by the Township.
E.
In addition, under 25 Pa. Code Chapter 92, a DEP "NPDES Construction
Activities" permit is required for regulated earth disturbance activities.
F.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the municipality. The issuance of an NPDES construction permit or permit coverage under the statewide general permit (PAG-2) satisfies the requirements Subsection C.
G.
A copy of the erosion and sediment control plan and any required
permit, as required by PADEP regulations, shall be available at the
project site at all times.
H.
Additional erosion and sedimentation control design standards and
criteria that must be applied where infiltration BMPs are proposed
include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase, so as
to maintain their maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has received final stabilization.
I.
Peak discharges and discharge volumes from the site shall comply
with the appropriate sections above, with the following additions:
(1)
For purposes of calculating required detention storage during land disturbance, peak discharge volumes shall be calculated based upon the runoff coefficients for bare soils during the maximum period and extent of disturbance which shall be clearing, indicated on the development plan. Controls shall ensure that the difference in volume and rate of peak discharges before disturbance and during construction shall not exceed those peak discharges and discharge volumes required in § 28B-308 of this chapter. Detention storage during the period of land disturbance and prior to establishment of permanent cover may require additional facilities on a temporary basis. Such measures shall be located so as to preserve the natural soil infiltration capacities of the planned infiltration bed areas.
(2)
Wherever soils, topography, cut and fill or grading requirements,
or other conditions suggest substantial erosion potential during land
disturbance, the Township may require that the entire volume of all
storms up to a two-year storm from the disturbed areas be retained
on site and that special sediment trapping facilities (such as check
dams, etc.) be installed.
J.
Areas of the site to remain undisturbed shall be protected from encroachment
by construction equipment/vehicles to maintain the existing infiltration
characteristics of the soil.
K.
A copy of the erosion and sediment control plan and any required
permit, as required by DEP regulations, shall be available on the
project site at all times.
Development on lands adjacent to the Delaware Canal which propose
to discharge stormwater into the canal must obtain special right-of-way
approval from the Department of Conservation and Natural Resources
(DCNR). The DCNR has established a separate policy for granting right-of-way
for stormwater drainage into the Delaware Canal, which shall be used
in conjunction with this chapter.
A.
No regulated earth disturbance activities within the municipality
shall commence until approval by the municipality of a plan which
demonstrates compliance with state water quality requirements after
construction is complete.
B.
The BMPs must be designed, implemented, and maintained to meet state
water quality requirements and any other more stringent requirements
as determined by the municipality.
C.
To control postconstruction stormwater impacts from regulated earth
disturbance activities, state water quality requirements may be met
by BMPs, including site design, which provide for replication of preconstruction
stormwater infiltration and runoff conditions, so that postconstruction
stormwater discharges do not degrade the physical, chemical or biological
characteristics of receiving waters. As described in the PADEP Comprehensive
Stormwater Management Policy (No. 392-0300-002, September 28, 2002),
this may be achieved by the following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions;
(2)
Treatment: use of water quality treatment BMPs to filter out
the chemical and physical pollutants from the stormwater runoff; and
(3)
Stream bank and streambed protection: management of volume and
rate of postconstruction stormwater discharges to prevent physical
degradation of receiving waters (e.g., from scouring).
D.
PADEP has regulations that require municipalities to ensure design,
implementation, and maintenance of best management practices (BMPs)
that control runoff from new development from new development and
redevelopment after regulated earth disturbance activities are complete.
These requirements include the need to implement postconstruction
stormwater BMPs with assurance of long-term operations and maintenance
of those BMPs.
E.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate PADEP regional office must be submitted
to the municipality.
The following permit requirements may apply to certain regulated
earth disturbance activities, and must be met prior to commencement
of regulated earth disturbance activities, as applicable:
A.
All regulated earth disturbance activities subject to permit requirements
by DEP under regulations at 25 Pa. Code Chapter 102.
B.
Work within natural drainageways subject to permit by DEP under 25
Pa. Code Chapter 105.
C.
Any stormwater management facility that would be located in or adjacent
to surface waters of the commonwealth, including wetlands, subject
to permit by DEP under 25 Pa. Code Chapter 105.
D.
Any stormwater management facility that would be located on a state
highway right-of-way, or require access from a state highway, shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
E.
Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by DEP under 25 Pa. Code Chapter
105.
A.
Hot spots.
(1)
Use of infiltration BMPs is prohibited on hot spot land use
areas. Examples of hot spots are listed in Appendix J[1] of this chapter.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2)
Stormwater runoff from hot spot land uses shall be pretreated.
In no case may the same BMP be employed consecutively to meet this
requirement. Guidance regarding acceptable methods of pretreatment
is located in Appendix J.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
West Nile guidance requirements. All wet basin designs shall incorporate
biologic controls consistent with the West Nile guidance found in
Appendix K.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
Riparian buffers.
(1)
In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as part of any subdivision
or land development that encompasses a Riparian Buffer. See JMZO § 903.B.13.
(2)
Except as required by Chapter 102, the riparian buffer easement
shall be measured to be the greater of the limit of the 100-year floodplain
or a minimum of 50 feet from the top of the stream bank (on each side).
(3)
Minimum management requirements for riparian buffers.
(a)
Existing native vegetation shall be protected and maintained
within the riparian buffer easement.
(b)
Whenever practicable invasive vegetation shall be actively removed
and the riparian buffer easement shall be planted with native trees,
shrubs and other vegetation to create a diverse native plant community
appropriate to the intended ecological context of the site.
(4)
The riparian buffer easement shall be enforceable by the municipality
and shall be recorded in the appropriate County Recorder of Deeds
office, so that it shall run with the land and shall limit the use
of the property located therein. The easement shall allow for the
continued private ownership and shall count toward the minimum lot
area required by zoning, unless otherwise specified in the municipal
Zoning Ordinance.[4]
(5)
Any permitted use (JMZO § 903.B.13) within the riparian
buffer easement shall be conducted in a manner that will maintain
the extent of the existing 100-year floodplain, improve or maintain
the stream stability, and preserve and protect the ecological function
of the floodplain.
(7)
Septic drain fields and sewage disposal systems shall not be
permitted within the riparian buffer easement and shall comply with
setback requirements established under 25 Pa. Code Chapter 73.
