A.
Applicants proposing regulated activities in Neshaminy Creek and Delaware River South Watersheds which do not fall under the exemption criteria shown in § 430-302 shall submit a drainage plan consistent with the Neshaminy Creek and Delaware River South Watershed Stormwater Management Plan to the Township for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B.
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.
C.
The drainage plan must be designed consistent with the sequencing provisions of § 430-504 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article IV of this chapter.[1]
D.
Stormwater drainage systems shall be designed in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
E.
Existing points of concentrated drainage that discharge onto adjacent
property shall not be altered in any manner which could cause property
damage without permission of the affected property owner(s) and shall
be subject to any applicable discharge criteria specified in this
chapter.
F.
Areas of existing diffused drainage discharge, whether proposed to
be concentrated or maintained as diffused drainage areas, shall be
subject to any applicable discharge criteria in the general direction
of existing discharge, except as otherwise provided by this chapter.
If diffused drainage discharge 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 impacts will result from the concentrated discharge.
G.
Where a development site is traversed by existing streams, drainage easements shall be provided conforming to the line of such streams. The terms of the easement shall conform to the stream buffer requirements contained in § 430-506G of this chapter.
H.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or delineated
wetlands shall be subject to approval by DEP through the joint permit
application or the environmental assessment approval process or, where
deemed appropriate, by the DEP general permit process. When there
is a question as to 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.
I.
Any proposed stormwater management facilities regulated by this chapter
that would be located on state highway rights-of-way shall be subject
to approval by the Pennsylvania Department of Transportation (PennDOT).
J.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., is encouraged where soil
conditions permit in order to reduce the size or eliminate the need
for detention facilities or other structural BMPs.
K.
All stormwater runoff shall be pretreated for water quality prior
to discharge to surface water or groundwater.
L.
All regulated activities within the Township shall be designed, implemented,
operated and maintained to meet the purposes of this chapter, through
these two elements:
M.
No regulated earth disturbance activities within the Township shall
commence until the requirements of this chapter are met.
O.
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VIII.
P.
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 Township.
Q.
Techniques described in Appendix E to this chapter (Low-Impact Development
Practices)[2] shall be considered because they reduce the costs of complying
with the requirements of this chapter and the state water quality
requirements.
[2]
Editor's Note: Appendix E is included as an attachment
to this chapter.
R.
In selecting the appropriate BMPs or combinations thereof, the applicant
shall consider the following:
(1)
Total contributing area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream-bank erosion.
(10)
Effectiveness of the BMPs to mitigate potential water quality
problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
T.
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 determined by the Township.
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.
No regulated earth disturbance activities within the Township shall
commence until the Township approves an erosion and sediment control
plan for construction activities.
B.
DEP has regulations that require an erosion and sediment control
plan under 25 Pa. Code § 102.4(b).
C.
The Bucks County Conservation District shall review and approve any
earth disturbance activity of 1,000 square feet or more.
D.
In addition, under 25 Pa. Code Chapter 92,[1] a DEP NPDES construction activities permit is required
for regulated earth disturbance activities when there is land disturbance
greater than one acre.
[1]
Editor's Note: Chapter 92 of the Pa. Code was reserved 10-8-2010.
See now 25 Pa. Code Ch. 92a, National Pollutant Discharge Elimination
System (NPDES) Permitting, Monitoring and Compliance.
E.
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 Township.
F.
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.
G.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from
sedimentation and compaction during the construction phase to maintain
maximum infiltration capacity. Barriers such as orange construction
fencing should be used to protect and keep construction traffic in
the area where infiltration BMPs are proposed. Inlet or pipes that
are installed to these systems should be completely sealed or protected
and monitored until the drainage areas are completely stabilized.
(2)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has achieved final stabilization.
A.
The design of all regulated activities shall include the following
to minimize stormwater impacts.
(1)
The applicant shall find practicable alternatives to the surface
discharge of stormwater, such as those listed in Appendix B to this
chapter, Table B-5,[1] 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.
[1]
Editor's Note: Appendix B is included as an attachment
to this chapter.
(2)
An alternative is practicable if it is available and capable
of implementation after taking into consideration existing technology
and logistics in light of overall project purposes and other Township
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 the regulated activities were
designed in the following sequence. The goal of the sequence is to
minimize the increases in stormwater runoff and impacts to water quality
resulting from the proposed regulated activity:
(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 (aka "natural springs"), stream buffers, hydrologic soil groups, land development, any existing recharge areas, potential infiltration areas or permeable soil areas and other requirements outlined in Chapter 440, Subdivision and Land Development, shall also be included.
(3)
Prepare a draft project layout avoiding sensitive areas identified in § 430-504B(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.
(4)
Identify site-specific existing conditions drainage areas, discharge
points, recharge areas and hydrologic soil groups A and B (areas conducive
to infiltration).
(6)
Satisfy groundwater recharge (infiltration) objective (§ 430-505) and provide for stormwater pretreatment prior to infiltration.
(8)
Provide stream-bank erosion protection in accordance with § 430-507, Stream-bank erosion requirements.
(10)
Prepare final project design to maintain existing conditions
drainage areas and discharge points, to minimize earth disturbance
and impervious surfaces and, to the maximum extent possible, ensure
the remaining site development has no surface or point discharge.
(11)
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§ 430-508).
(12)
Manage any remaining runoff prior to discharge, through detention,
bioretention, direct discharge or other structural control.
A.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration facilities shall consider groundwater recharge to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs which may be designed to compensate for the runoff from parking areas or driveways. These measures are required to be consistent with § 430-103 and take advantage of utilizing any existing recharge areas.
B.
Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, then the design professional shall be responsible to show that this cannot be physically accomplished. Soils testing and geotechnical evaluation of the site soils should be performed and submitted to verify that the infiltration requirements cannot be met. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from § 430-505C(2), depending on demonstrated site conditions, and shall be the greater of the two volumes.
C.
Minimum requirements. Infiltration BMPs shall meet the following
minimum requirements:
(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 that have the following characteristics:
(a)
A minimum depth of 24 inches, preferably 36 inches, between
the bottom of the BMP and the top of the limiting zone (e.g., seasonal
high-water table, groundwater, bedrock, etc.).
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's design professional.
(c)
The infiltration facility shall be capable of completely infiltrating
the retention (infiltration) volume (Rev) 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, unless the applicant can demonstrate through construction sequencing
and use of special construction techniques and machinery that this
is not the case. Studies have found that compaction of the soils during
disturbance reduces their infiltrative capacity.
(2)
The recharge volume (Rev) shall be computed by first obtaining the infiltration requirement using methods in either Subsection C(2)(a) or (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. The following criteria shall apply.
[1]
The NRCS runoff shall be utilized to calculate infiltration
requirements (P) in inches.
|
Equation 405.1
| |
|
For zero runoff: P = I (Infiltration) (inch) = (200/CN) - 2
|
|
Where:
| ||||
|
P
|
=
|
I = infiltration requirement (inches)
| ||
|
CN
|
=
|
SCS (NRCS) curve number of the existing conditions contributing
to the recharge facility
| ||
[2]
This equation can be displayed graphically in, and the infiltration
requirement can also be determined from, Figure 405.1.
[3]
The recharge volume (Rev) required would
therefore be computed as:
|
Rev = I (inches) * percent impervious
area (square feet)/12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
|
(b)
Annual recharge water budget approach.
[1]
Within the Neshaminy Creek Watershed it has been determined that infiltrating 0.6 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of Subsection C(2)(a) cannot be achieved, then 0.6 inches 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 405.1 or the curve in Figure 405.1 should be used to determine the infiltration requirement.
|
Where:
| ||||
|---|---|---|---|---|
|
I
|
=
|
0.6 inches
| ||
|
The recharge volume (Rev) required would
therefore be computed as: Rev = I (inches)/12
(inches) * impervious area (square feet) = cubic feet (cf)
|
|
Figure 405.1.
Infiltration Requirement Based upon NRCS Curve Number
for the Neshaminy Creek Watershed:
 |
[2]
Within the Delaware River South Watershed, it has been determined that infiltrating 0.5 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of Subsection C(2)(a) cannot be achieved, then 0.5 inches of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation 405.1 or the curve in Figure 405.2 should be used to determine the infiltration requirement.
|
Where:
| ||||
|---|---|---|---|---|
|
I
|
=
|
0.5 inches
| ||
|
The recharge volume (Rev) required would
therefore be computed as: Rev = I (inches)/12
(inches) * impervious area (square feet) = cubic feet (cf)
|
|
Figure 405.2.
Infiltration Requirement Based upon NRCS Curve Number
for the Delaware River South Watershed:
Required Infiltration (I) in inches by NRCS CN
 |
(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.
D.
Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified design professional and, at a minimum,
address soil permeability, depth to bedrock and subgrade stability.
The general process for designing the infiltration BMP shall be:
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration is not permitted to be ruled out without conducting
these tests.
(2)
Provide field tests such as double-ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(3)
Design the infiltration structure for the required retention
(Rev) 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 shall be supplied to the Township with the drainage
plan.
(5)
If on-lot infiltration structures are proposed by the applicant's
design professional, it must be demonstrated to the Township that
the soils are conducive to infiltrate on the lots identified.
E.
Stormwater hot spots. Following is a list of examples of designated
hot spots. If a site is designated as a hot spot, it has important
implications for how stormwater is managed. First and foremost, untreated
stormwater runoff from hot spots shall not be allowed to recharge
into groundwater where it may contaminate water supplies. Therefore,
the Rev requirement shall not be applied to
development sites that fit into the hot spot category (the entire
WQv must still be treated). Second, a greater
level of stormwater treatment shall be considered at hot spot sites
to prevent pollutant washoff after construction. The EPA's NPDES
stormwater program requires some industrial sites to prepare and implement
a stormwater pollution prevention plan.
(1)
Examples of hot spots:
(a)
Vehicle salvage yards and recycling facilities.
(b)
Vehicle fueling stations.
(c)
Vehicle service and maintenance facilities.
(d)
Vehicle and equipment cleaning facilities.
(e)
Fleet storage areas (bus, truck, etc.).
(f)
Industrial sites (based on Standard Industrial Codes defined
by the United States Department of Labor).
(g)
Marinas (service and maintenance).
(h)
Outdoor liquid container storage.
(i)
Outdoor loading/unloading facilities.
(j)
Public works storage areas.
(k)
Facilities that generate or store hazardous materials.
(l)
Commercial container nursery.
(m)
Other land uses and activities as designated by an appropriate
review authority.
(2)
The following land uses and activities are not normally considered
hot spots:
(3)
While large highways [average daily traffic volume (ADT) greater
than 30,000] are not designated as a stormwater hot spot; however,
it is important to ensure that highway stormwater management plans
adequately protect groundwater.
F.
Extreme caution shall be exercised where infiltration is proposed
in source water protection areas as defined by the local township
or water authority.
G.
Infiltration facilities shall be used in conjunction with other innovative
or traditional BMPs, stormwater control facilities, and nonstructural
stormwater management alternatives.
H.
Extreme caution shall be exercised where salt or chloride (municipal
salt storage) would be a pollutant since soils do little to filter
this pollutant and it may contaminate the groundwater. The qualified
design professional shall evaluate the possibility of groundwater
contamination from the proposed infiltration facility and perform
a hydrogeologic justification study if necessary.
I.
The infiltration requirement in high quality or exceptional value
waters shall be subject to the Department's Chapter 93 antidegradation
regulations.
J.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the Township.
K.
The Township shall require the applicant to provide safeguards against
groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
In addition to all other applicable provisions of this chapter,
an applicant shall also comply with the following water quality requirements
of this article.
A.
No regulated earth disturbance activities within the Township shall
commence until approval by the Township of a plan which demonstrates
compliance with state water quality requirements post construction
is complete.
B.
The BMPs shall be designed, implemented and maintained to meet state
water quality requirements, and any other more stringent requirements
as determined by the Township.
C.
To control post-construction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can be met
by BMPs, including site design, which provide for replication of preconstruction
stormwater infiltration and runoff conditions, so that post-construction
stormwater discharges do not degrade the physical, chemical or biological
characteristics of the receiving waters. As described in the DEP 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 ensure filtering
out of the chemical and physical pollutants from the stormwater runoff;
and
(3)
Stream-bank and streambed protection: management of volume and
rate of post-construction stormwater discharges to prevent physical
degradation of receiving waters (e.g., from scouring and downcutting).
D.
Developed areas shall provide adequate storage and treatment facilities
necessary to capture and treat stormwater runoff.
(1)
The retention volume computed under § 430-505 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the retention volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of
stormwater runoff from the developed areas of the site produced from
90% of the average annual rainfall (P).
(2)
To achieve this goal, the following criterion is established:
(a)
The following calculation formula is to be used to determine
the water quality storage volume, (WQv), in
acre-feet of storage for the Neshaminy Creek, Little Neshaminy Creek,
and Delaware South River watersheds:
|
Equation 406.1
|
|
WQv = [(P)(Rv)(A)]/12
|
|
Where:
| ||||
|
WQv
|
=
|
Water Quality Volume (acre-feet)
| ||
|
P
|
=
|
1.00 inch for Neshaminy Creek Watershed
| ||
|
P
|
=
|
2.04 inches for the Delaware River South Watershed (Region 5)
(obtained from PennDOT Field Manual, May 1986)
| ||
|
P
|
=
|
Rainfall amount equal to 90% of events producing this rainfall
(inches)
| ||
|
A
|
=
|
Area of the project contributing to the water quality BMP (acres)
| ||
|
Rv
|
=
|
0.05 + 0.009(1), where 1 is the percent of the area that is
impervious surface [(impervious area/A) * 100]
| ||
(b)
This volume requirement can be accomplished by the permanent
volume of a wet basin or the detained volume from other BMPs. Where
appropriate, wet basins shall be utilized for water quality control
and shall follow the guidelines of the BMP manuals referenced in Appendix
F to this chapter.[1]
[1]
Editor's Note: Appendix F is included as an attachment
to this chapter.
(c)
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 provide for protection from clogging
and unwanted sedimentation.
E.
For areas within defined special protection subwatersheds which include
exceptional value (EV) and high quality (HQ) waters, the temperature
and quality of water and streams shall be maintained through the use
of temperature-sensitive BMPs and stormwater-conveyance systems. For
other waters, use of temperature-sensitive BMPs and stormwater-conveyance
systems should be considered as temperature can negatively impact
water quality and stream health.
F.
To accomplish the above, the applicant shall submit original and
innovative designs to the Township Engineer for review and approval.
Such designs may achieve the water quality objectives through a combination
of different BMPs.
G.
If a perennial or intermittent stream passes through the site, the
applicant shall create a stream buffer extending a minimum of 50 feet
to either side of the top-of-bank of the channel. The buffer area
shall be maintained with and encouraged to use appropriate native
vegetation. (Refer to Appendix H of Pennsylvania Handbook of Best
Management Practices for Developing Area for plant lists.) If the
applicable rear or side yard setback is less than 50 feet, the buffer
width may be reduced to 25% of the setback to a minimum of 10 feet.
If an existing buffer is legally prescribed (i.e., deed, covenant,
easement, etc.) and it exceeds the requirements of this chapter, the
existing buffer shall be maintained.
H.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office must be provided
to the Township.
I.
Design of BMPs used for water quality control shall be in accordance
with design specifications outlined in the Pennsylvania Stormwater
Best Management Practices Manual (Draft, January 2005) or other applicable
manuals. The following factors shall be considered when evaluating
the suitability of BMPs used to control water quality at a given development
site:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream-bank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property value.
J.
To accomplish the above, the applicant shall submit original and
innovative designs to the Township for review and approval. Such designs
may achieve the water quality objectives through a combination of
BMPs.
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream-bank erosion),
the primary requirement is to design a BMP to detain the proposed
conditions two-year, twenty-four-hour design storm to the existing
conditions 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 proposed conditions 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). 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).
B.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small contributing
drainage areas to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake.
C.
In conditional direct discharge districts (District C) only (see § 430-508), the objective is not to attenuate the storms greater than the two-year recurrence interval. This can be accomplished by configuring the outlet structure not to control the larger storms or by a bypass channel that diverts only the two-year stormwater runoff into the basin or, conversely, diverts flows in excess of the two-year storm away from the basin.
A.
The Neshaminy Creek Watershed peak rate requirements and districts
are as follows:
(1)
In order to implement the provisions of the Neshaminy Creek
Watershed Stormwater Management Plan, Middletown Township is hereby
divided into stormwater runoff districts, whereby specific peak release
rates have been established. The districts are as follows:
|
Stormwater Runoff Peak Rate Districts
| ||
|---|---|---|
|
RR-1
|
100% release rate
| |
|
RR-3
|
75% release rate
| |
|
DD
|
Provisional direct discharge
| |
|
LR
|
Provisional lower reaches 100% release rate
| |
(2)
The exact location of the stormwater runoff peak rate district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot or five-foot topographic
contours provided as part of the stormwater management plan developed
for the site. The district boundaries as originally drawn coincide
with topographic divides or, in certain instances, are drawn from
the intersection of the watercourse and a physical feature such as
the confluence with another watercourse or a potential flow obstruction
(road, culvert, bridge, etc.) to the topographic divide consistent
with topography. The location of the stormwater management district
boundary on a stormwater management plan shall be reviewed and verified
by the Township Engineer.
(3)
Additional stormwater design standards and criteria.
(a)
In addition to the peak runoff rate requirements specified in Subsection A(1) above, the erosion and sedimentation control (§ 430-503), the nonstructural project design (§ 430-504), the groundwater recharge (§ 430-505), the water quality (§ 430-506), and the stream-bank erosion (§ 430-507) requirements shall be implemented. Standards for managing runoff from each subarea in the Neshaminy Creek Watershed for the two-, five-, ten-, twenty-five-, fifty- and one-hundred-year design storms are shown on the map in Appendix D.[1] Development sites located in each of the stormwater runoff
peak rate districts must control proposed conditions runoff rates
to existing conditions runoff rates or percentages thereof.
[1]
Editor's Note: Appendix D is included as an attachment
to this chapter.
(b)
General. Proposed conditions rates of runoff from any regulated
activity shall not exceed the peak release rates of runoff from existing
conditions for the design storms specified on the Stormwater Management
District Watershed Map (Appendix D to this chapter[2] and § 430-508A(1) of this chapter.
[2]
Editor's Note: Appendix D is included as an attachment
to this chapter.
(c)
District boundaries. The boundaries of the stormwater runoff
peak rate districts are shown on an official map that is available
for inspection at the Township office and County Planning offices.
A copy of the official map at a reduced scale is included in Appendix
D to this chapter.[3] The exact location of the stormwater runoff peak rate
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.
[3]
Editor's Note: Appendix D is included as an attachment
to this chapter.
(d)
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 meet the
runoff peak rate/management district criteria for the district in
which the discharge is located. 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 follow runoff peak rate/Management
District A criteria, provided that the overall site discharge meets
the runoff peak rate/management district criteria for which the discharge
is located.
(e)
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.
(f)
Site areas. Where the site area to be impacted by a proposed
development activity differs significantly from the total site area,
only the proposed impact area utilizing stormwater management measures
shall be subject to the management district criteria. In other words,
unimpacted areas bypassing the stormwater management facilities would
not be subject to the runoff peak rate/management district criteria.
(g)
Provisions must also be made for safely passing the runoff greater
than that occurring from the one-hundred-year design storm.
(h)
For proposed development sites located partially within a release rate district and partially within a provisional direct discharge district, in no event shall a significant portion of the site area subject to the release rate control be drained to a discharge point(s) located in the provisional direct discharge district. All runoff from the site will be subject to the water quality requirement as stated in § 430-506 of this article.
(i)
Alternate criteria for redevelopment sites. For redevelopment
sites, one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by Township Engineer;
B.
Delaware River South Watershed has been divided into stormwater management
districts as shown on the Watershed Map in Appendix D.[4] In addition to the requirements specified below, the erosion and sedimentation control (§ 430-503), the nonstructural project design (§ 430-504), the groundwater recharge (§ 430-505), the water quality (§ 430-506), and the stream-bank erosion (§ 430-507) requirements shall be implemented.
(1)
Standards for managing runoff from each subarea in the Delaware
River South Watershed is shown below. Development sites located in
each of the A, B or C, Districts must control post-development runoff
rates to predevelopment runoff rates for the design storms as follows:
|
Table 508-1
| |||
|---|---|---|---|
|
District
|
Design Storm Post Development
|
Design Storm Predevelopment
| |
|
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 of post-development
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
post-development 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 post-development peak rate of runoff must
be controlled to the predevelopment peak rate as required in District
A provisions (i.e., ten-year post-development flows to ten-year predevelopment
flows) for the specified design storms.
|
(2)
Post-development 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[5]) and Subsection B(1).
[5]
Editor's Note: Appendix D is included as an attachment
to this chapter.
(3)
The boundaries of the Delaware River South Stormwater Management
Districts are shown on an official stormwater district map that is
available for inspections at the Township office. A copy of the map
at a reduced scale is included in Appendix D.[6] 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.
[6]
Editor's Note: Appendix D is included as an attachment
to this chapter.
(4)
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 Subsection B(1). 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 one-hundred-percent release rate, provided that the overall site discharge meets the weighted average release rate.
(5)
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.
(6)
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.
(7)
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 a applicant can prove that the post-development hydrographs can match predevelopment hydrographs, or if it can be proved that the post-development 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 Subsection B(8) to determine if adequate hydraulic capacity exists. The land applicant shall submit to the Township this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(a)
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 Township.
(b)
The evaluation shall continue downstream until the increase
in flow diminishes due to additional flow from tributaries and/or
stream attenuation.
(c)
The peak flow values to be used for downstream areas for the
design return period storms (two-, five-, ten-, twenty-five-, fifty-
and one-hundred-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.
(d)
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 Subsection B(9).
(e)
Financial considerations shall not constitute grounds for granting
a no-harm exemption.
(f)
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.
(8)
Any downstream hydraulic capacity analysis conducted in accordance
with this chapter shall use the following criteria for determining
adequacy for accepting increased peak flow rates:
(a)
Natural or man-made channels or swales must be able to convey
the increased runoff associated with a one-and-one-half-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.
(b)
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.
(c)
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.
(9)
Regional detention alternatives.
(a)
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.
(b)
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.
[4]
Editor's Note: Appendix D is included as an attachment
to this chapter.
A.
Stormwater runoff from all development sites with a drainage area
of greater than 200 acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil-Cover-Complex
Method. Table 509-1 summarizes acceptable computation methods, and
the method selected by the design professional shall be based on the
individual limitations and suitability of each method for a particular
site. The Township may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than 200 acres.
The Soil-Complex Method shall be used for drainage areas greater than
200 acres.
|
Table 509-1
| |||
|---|---|---|---|
|
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; simpler 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 Township
and/or Township Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township and/or
Township Engineer
| |
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 the region in which they are located
as presented in Table B-1 in Appendix B-1 to this chapter.[1] If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS
is used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The Alternating Block Method shown in Figure B-1,
or the SCS Type II S Curve, Figure B-3, in Appendix B[2], shall be used for the rainfall distribution.
C.
For the purposes of existing conditions flow rate determination,
the undeveloped portion of the site shall be considered as meadow
in good condition, unless the natural ground cover generates a lower
curve (CN) number or Rational C value (i.e., forest), as listed in
Table B-2 or B-3 in Appendix B to this chapter.[3] For the developed portion of the site, the CN or C value
shall be based upon actual land cover conditions.
[3]
Editor's Note: Appendix B 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 the Region 5 Curves from the Pennsylvania
Department of Transportation Storm-Duration-Frequency Chart (Figure
B-4).[4] 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.
[4]
Editor's Note: See Appendix B which is included as an
attachment to this chapter.
G.
The designer shall consider that the runoff from proposed sites graded
to the subsoil will not have the same runoff conditions as the site
under existing conditions, even after topsoiling or seeding. The designer
shall increase his proposed condition CN or "C" to better reflect
proposed soil conditions.
H.
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 B-4 in Appendix B
to this chapter.[7]
[7]
Editor's Note: Appendix B is included as an attachment
to this chapter.
I.
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.
J.
The design of any stormwater detention 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. The design storm hydrograph shall be computed using a calculation
method that produces a full hydrograph. The Township 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.
A.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
C.
Any stormwater management facility (i.e., detention basin) required
or regulated by this chapter, designed to store runoff and requiring
a berm or earthen embankment, shall be designed to provide an emergency
spillway to handle flow up to and including the one-hundred-year proposed
conditions. The height of embankment must provide a minimum one foot
of freeboard above the maximum pool elevation, computed when the facility
functions for the one-hundred-year proposed conditions inflow. Should
any stormwater management facility require a dam safety permit under
DEP Chapter 105, the facility shall be designed in accordance with
Chapter 105 and meet the regulations of Chapter 105 concerning dam
safety. Chapter 105 may be required to pass storms larger than the
one-hundred-year event.
D.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls or stream enclosures), and any work involving wetlands
governed by DEP Chapter 105 regulations (as amended or replaced from
time to time by DEP), shall be designed in accordance with Chapter
105 and will require a permit from DEP.
E.
Any other drainage conveyance facility that does not fall under Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year design storm
with a minimum one foot of freeboard measured below the lowest point
along the top of the roadway. Any facility that constitutes a dam
as defined in DEP Chapter 105 regulations may require a permit under
dam safety regulations. Any facility located within a PennDOT right-of-way
must meet PennDOT minimum design standards and permit submission requirements.
F.
Any drainage conveyance facility and/or channel not governed by Chapter
105 regulations must be able to convey, without damage to the drainage
structure or roadway, runoff from the one-hundred-year design storm.
Conveyance facilities to or exiting from stormwater management facilities
(i.e., detention basins) shall be designed to convey the design flow
to or from that structure. Roadway crossings located within designated
floodplain areas must be able to convey runoff from a one-hundred-year
design storm. Any facility located within a PennDOT right-of-way must
meet PennDOT minimum design standards and permit submission requirements.
G.
Storm sewers must be able to convey proposed conditions runoff from
a one-hundred-year design storm without surcharging inlets, where
appropriate.
H.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
I.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Township reserves
the right to disapprove any design that would result in construction
in or continuation of a stormwater problem area.
