[Ord. No. 2022-09-03, 9/26/2022]
1.
Applicants proposing regulated activities in the Neshaminy Creek watershed that do not fall under the exemption criteria shown in § 26-106 shall submit a stormwater management (SWM) site plan consistent with the Neshaminy Creek Watershed SWM Plan to the municipality for review. The SWM criteria of this Part shall apply to the total proposed development even if development is to take place in stages. Preparation and implementation of an approved SWM site plan is required. No regulated activities shall commence until the municipality issues written approval of an SWM site plan which demonstrates compliance with the requirements of this Part.
2.
SWM site plans approved by the municipality, in accordance with Subpart
D, shall be on-site throughout the duration of the regulated activity.
3.
The municipality may, after consultation with the Department of Environmental
Protection (PADEP), approve measures for meeting the state water quality
requirements other than those in this Part, 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.
4.
For all regulated earth disturbance activities, erosion and sediment
(E&S) control best management practices (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 Part and to meet all requirements under Title 25 of the Pennsylvania
Code and the Clean Streams Law. Various BMPs and their design standards
are listed in the Erosion and Sediment Pollution Control Program Manual,
No. 363-2134-008 (April 15, 2000), as amended and updated.
5.
For all regulated activities, implementation of the volume controls in § 26-123 of this Part is required.
6.
Impervious areas:
A.
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.
B.
For development taking place in stages, the entire development plan
must be used in determining conformance with this Part.
C.
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this Part; except that the volume controls in § 26-123 and the peak rate controls of § 26-124 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
7.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s). Such stormwater flows shall be
subject to the requirements of this Part.
8.
All regulated activities shall include such measures as necessary
to:
A.
Protect health, safety, and property.
B.
Meet the water quality goals of this Part by implementing measures
to:
(1)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(2)
Create, maintain, repair or extend riparian buffers.
(3)
Avoid erosive flow conditions in natural flow pathways.
(4)
Minimize thermal impacts to waters of this commonwealth.
(5)
Disconnect impervious surfaces [i.e., disconnected impervious
areas (DIAs)] by directing runoff to pervious areas, wherever possible.
C.
To the maximum extent practicable, incorporate the techniques for
low impact development practices (e.g., protecting existing trees,
reducing area of impervious surface cluster development, and protecting
open space) described in the Pennsylvania Stormwater Best Management
Practices Manual; Pennsylvania Department of Environmental Protection
(PADEP) No. 363-0300-002 (2006). If methods other than green infrastructure
and LID methods are proposed to achieve the volume and rate controls
required under this Part, the SWM site plan must include a detailed
justification demonstrating that the use of LID and green infrastructure
is not practicable.
9.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize the use of natural on-site infiltration features
while still meeting the other requirements of this Part.
10.
The design of all facilities over Karst shall include an evaluation
of measures to minimize the risk of adverse effects.
11.
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 unless otherwise approved by the Township Engineer.
12.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
13.
14.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. No. 2022-09-03, 9/26/2022]
Approvals issued and actions taken under this Part do not relieve
the applicant of the responsibility to secure required permits or
approvals for activities regulated by any other code, law, regulation,
or ordinance.
[Ord. No. 2022-09-03, 9/26/2022]
1.
Volume controls will mitigate increased runoff impacts, protect stream
channel morphology, maintain groundwater recharge, and contribute
to water quality improvements. Stormwater runoff volume control methods
are based on the net change in runoff volume for the two-year storm
event.
2.
The green infrastructure and low impact development practices provided
in the BMP manual shall be utilized for all regulated activities wherever
possible. Volume controls shall be implemented using the Design Storm
Method in Subsection A or the Simplified Method in Subsection B below.
For regulated activities equal to or less than one acre, this Part
establishes no preference for either methodology; therefore, the applicant
may select either methodology on the basis of economic considerations,
the intrinsic limitations of the procedures associated with each methodology,
and other factors. All regulated activities greater than one acre
must use the Design Storm Method. For small projects that propose
1,000 square feet or less of impervious surface, Subsection D should
be used.
A.
Design Storm Method (any regulated activity): This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 26-125, Subsection 1.
(1)
Post-development total runoff should not be increased from pre-development
total runoff for all storms equal to or less than the two-year, twenty-four-hour
duration precipitation.
(2)
The following applies to estimate the increased volume of runoff
for the two-year, twenty-four-hour duration precipitation event:
To calculate the runoff volume (cubic feet) for existing site
conditions (pre-development) and for the proposed developed site conditions
(post-development), it is recommended to use the soil cover complex
method as shown on the following page. The calculated volume shall
be either reused, evapotranspired, or infiltrated through structural
or nonstructural means.
Soil Cover Complex Method:
|
Step 1:
|
|
Runoff (inches) = Q = (P - 0.2S)2/(P + 0.8S)
|
|
Where:
| ||
|
P
|
=
|
2-year Rainfall (inches)
|
|
S
|
=
|
(1000/CN) - 10, the potential maximum retention (Including initial
abstraction, Ia)
|
|
Step 2:
| ||
|
Runoff Volume (cubic feet) = Q x Area x 1/12
| ||
|
Where:
| ||
|
Q
|
=
|
Runoff (inches)
|
|
Area
|
=
|
SWM Area (square feet)
|
B.
Simplified Method (regulated activities less than or equal to one
acre):
(1)
Stormwater facilities shall capture the runoff volume from at
least the first two inches of runoff from all new impervious surfaces.
Volume (cubic feet) = (2" runoff/12 inches) * impervious surface
(square feet)
(2)
At least the first inch of runoff volume from the new impervious
surfaces shall be permanently removed from the runoff flow-i.e., it
shall not be released into the surface waters of the commonwealth.
The calculated volume shall be either reused, evapotranspired or infiltrated
through structural or nonstructural means.
Volume (cubic feet) = (1" runoff/12 inches) * impervious surface
(square feet)
(3)
Infiltration facilities should be designed to accommodate the
first 1/2 inch of the permanently removed runoff.
(4)
No more than one inch of runoff volume from impervious surfaces
shall be released from the site. The release time must be over 24
to 72 hours.
C.
Stormwater Control Measures. The applicant must demonstrate how the
required volume is controlled through stormwater best management practices
(BMPs) which shall provide the means necessary to capture, reuse,
evaporate, transpire or infiltrate the total runoff volume.
(1)
If natural resources exist on the site, the applicant is required
to submit an SWM site plan, which shall determine the total acreage
of protected area where no disturbance is proposed. The acreage of
the protected area should be subtracted from the total site area and
not included in the stormwater management site area acreage used in
determining the volume controls.
|
Stormwater Management Site Area =
|
|
{Total Site Area (for both pre- and post-development conditions)
- Protected Area}
|
Natural resource areas should be calculated based upon the municipality's own natural resource protection ordinance. For additional reference see Chapter 5, Section 5.4.1, of the PA BMP manual.
(2)
Calculate the volume controls provided through nonstructural
BMPs.
(3)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
|
Required Volume Control (ft3) - Nonstructural Volume Control
(ft3) = Structural Volume Requirement (ft3)
|
(4)
Calculate the volume controls provided through structural BMPs. See PA BMP manual, Chapter 6, for description of the BMPs.
(5)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on the suitability of soils and site
conditions. Infiltration BMPs shall be constructed on soils that have
the following characteristics:
(a)
A minimum soil depth of 24 inches between the bottom of the
infiltration BMPs and the top of bedrock or seasonally high water
table.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.25 inch/hour should be utilized and for acceptable rates a safety
factor of 50% should be applied for design purposes (e.g., for soil
which measured 0.5 inch/hour, the BMP design should use 0.25 inch/hour
to insure safe infiltration rates after construction).
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate runoff volume within three days (72 hours) from the start
of the design storm.
(6)
Soils. A soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities for all subdivisions
and land developments, any projects that propose 5,000 square feet
or more of new impervious surface or as deemed necessary by the Township
Engineer. A soils evaluation is recommended for all other applications.
All regulated activities are required to perform a detailed soils
evaluation by a qualified design professional which at minimum addresses
soil permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
(a)
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 sub-grade
stability; infiltration may not be ruled out without conducting these
tests.
(b)
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.
(c)
Design the infiltration structure based on field determined
capacity at the level of the proposed infiltration surface and based
on the safety factor of 50%.
(d)
If on-lot infiltration structures are proposed, it must be demonstrated
to the municipality that the soils are conducive to infiltrate on
the lots identified.
(e)
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 municipality.
D.
Small Project Stormwater Management Volume Control Requirements:
This section applies to only small projects (residential and nonresidential)
that propose less than 1,000 square feet of new impervious surface.
(1)
Newly planted deciduous trees can reduce runoff volume by six
cubic feet. Newly planted evergreen trees can reduce runoff volume
by 10 cubic feet.
(2)
Projects that proposed between zero square feet and 500 square
feet of new impervious surface are required to plant trees that would
reduce runoff volume by 10 cubic feet. Projects that proposed between
501 square feet and 1,000 square feet of new impervious surface are
required to plant trees that would reduce runoff volume by 20 cubic
feet.
(4)
If an applicant demonstrates the required number of plantings
cannot be accommodated on the property, the applicant is required
to pay a fee-in-lieu of landscaping in an amount equal to $350 per
tree.
[Ord. No. 2022-09-03, 9/26/2022]
1.
Peak rate controls for large storms, up to the 100-year event, is
essential in order to protect against immediate downstream erosion
and flooding. The following peak rate controls have been determined
through hydrologic modeling of the Neshaminy Creek watershed.
A.
Standards for managing runoff from each subarea in the Neshaminy
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
100-year design storms are shown in Table 26-124.1. Development sites
must control proposed development conditions runoff rates to existing
conditions runoff rates for the design storms in accordance with Table
26-124.1 on the following page.
|
Table 26-124.1
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Neshaminy Creek Watershed
(includes Little Neshaminy Creek)
| ||
|---|---|---|
|
District
|
Design Storm Post-Development
(Proposed Conditions)
|
Design Storm Pre-Development
(Existing Conditions)
|
|
New Britain Township
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
10-year
|
5-year
| |
|
25-year
|
10-year
| |
|
50-year
|
25-year
| |
|
100-year
|
50-year
| |
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 in this section of this Part.
C.
Off-Site Areas. When calculating the allowable peak runoff rates,
developers do not have to account for runoff draining into the subject
development site from an off-site area. On-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
D.
Site Areas. The stormwater management site area is the only area
subject to the management district criteria. The stormwater management
site area includes on-site areas that are not proposed to be disturbed
but drain to a proposed stormwater management facility. Non-impacted
areas or non-regulated activities bypassing the stormwater management
facilities would not be subject to the management district criteria.
[Ord. No. 2022-09-03, 9/26/2022]
1.
The following criteria shall be used for runoff calculations:
A.
For development sites not considered redevelopment, the ground cover
used to determine the existing conditions runoff volume and flow rate
shall be as follows:
(1)
For the wooded portion of sites use a ground cover of "woods
in good condition." An area is classified as wooded if a continuous
canopy of trees exists over a 1/8 acre.
(2)
The undeveloped portion of the site, including agriculture,
bare earth, and fallow ground, shall be considered as "meadow in good
condition," unless the natural ground cover generates a lower curve
number (CN) or rational "c" value (i.e., woods).
B.
For development and redevelopment sites, the ground cover used to
determine the existing conditions runoff volume and flow rate for
the developed portion of the site shall be based upon actual land
cover conditions, except that 50% of the impervious surface area shall
be considered meadow in the model for existing conditions.
2.
Stormwater runoff peak discharges from all development sites with
a drainage area greater than one acre shall be calculated using a
generally accepted calculation technique that is based on the NRCS
Soil Cover Complex Method. Table 26-125.1 summarizes acceptable computation
methods. The method selected by the design professional shall be based
on the individual limitations and suitability of each method for a
particular site. The municipality may allow the use of the Dekalb
Rational Method (Q=CIA) to estimate peak discharges from drainage
areas that contain one acre or less.
Q = Peak flow rate, cubic feet per second (CFS)
C = Runoff coefficient, dependent on land use/cover
I = Design rainfall intensity, inches per hour
A = Drainage Area, acres
3.
All calculations consistent with this Part using the Soil Cover Complex
Method shall use the appropriate design rainfall depths for the various
return period storms according to the National Oceanic and Atmospheric
Administration (NOAA) Atlas 14 rain data corresponding to the Doylestown
rain gage. These rainfall depths are: 2.7 inches (one-year); 3.3 inches
(two-year); 4.1 inches (five-year); 4.8 inches (ten-year); 5.8 inches
(twenty-five-year); 6.7 inches (fifty-year); and 7.6 inches (100-year).
This data may also be directly retrieved from the NOAA Atlas 14 website:
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, then the duration of rainfall shall be 24 hours.
|
Table 26-125.1
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-I
|
|
Dekalb Rational Method (or commercial computer package based
on Rational Method)
|
Emil Kuichling (1889)
|
For sites 1 acre or less, or as approved by the municipality
and/or Municipal Engineer
|
|
Other Methods
|
Varies
|
Other computation methodologies approved by the municipality
|
4.
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. 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 from time to time by NRCS). Times-of-concentration for
channel and pipe flow shall be computed using Manning's equation.
5.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the soil cover complex method shall be based on TR-55.
6.
Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational Method shall be consistent with Table 26-125.2
below.
|
Table 26-125.2
Rational Runoff Coefficients By Hydrologic Soils Group and
Overland Slope (%)
| ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Land Use
|
A
|
B
|
C
|
D
| ||||||||
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
|
0-2%
|
2-6%
|
6%+
| |
|
Residential
| ||||||||||||
|
Lot Size 1/8 Acre
|
0.25
|
0.28
|
0.31
|
0.27
|
0.30
|
0.25
|
0.30
|
0.33
|
0.38
|
0.33
|
0.36
|
0.42
|
|
0.33
|
0.37
|
0.40
|
0.35
|
0.39
|
0.44
|
0.38
|
0.42
|
0.49
|
0.41
|
0.45
|
0.54
| |
|
Lot Size 1/4 Acre
|
0.22
|
0.26
|
0.29
|
0.24
|
0.29
|
0.33
|
0.27
|
0.31
|
0.36
|
0.30
|
0.34
|
0.40
|
|
0.30
|
0.34
|
0.37
|
0.33
|
0.37
|
0.42
|
0.36
|
0.40
|
0.47
|
0.38
|
0.42
|
0.52
| |
|
Lot Size 1/3 Acre
|
0.19
|
0.23
|
0.26
|
0.22
|
0.26
|
0.30
|
0.25
|
0.29
|
0.34
|
0.28
|
0.32
|
0.39
|
|
0.28
|
0.32
|
0.35
|
0.30
|
0.35
|
0.39
|
0.33
|
0.38
|
0.45
|
0.36
|
0.40
|
0.50
| |
|
Lot Size 1/2 Acre
|
0.16
|
0.20
|
0.24
|
0.19
|
0.23
|
0.28
|
0.22
|
0.27
|
0.32
|
0.26
|
0.30
|
0.37
|
|
0.25
|
0.29
|
0.32
|
0.28
|
0.32
|
0.36
|
0.31
|
0.35
|
0.42
|
0.34
|
0.38
|
0.48
| |
|
Lot Size 1 Acre
|
0.14
|
0.19
|
0.22
|
0.17
|
0.21
|
0.26
|
0.20
|
0.25
|
0.31
|
0.24
|
0.29
|
0.35
|
|
0.22
|
0.26
|
0.29
|
0.24
|
0.28
|
0.34
|
0.28
|
0.32
|
0.40
|
0.31
|
0.35
|
0.46
| |
|
Industrial
|
0.67
|
0.68
|
0.68
|
0.68
|
0.68
|
0.69
|
0.68
|
0.69
|
0.69
|
0.69
|
0.69
|
0.70
|
|
0.85
|
0.85
|
0.86
|
0.85
|
0.86
|
0.86
|
0.86
|
0.86
|
0.87
|
0.86
|
0.86
|
0.88
| |
|
Commercial
|
0.71
|
0.71
|
0.72
|
0.71
|
0.72
|
0.72
|
0.72
|
0.72
|
0.72
|
0.72
|
0.72
|
0.72
|
|
0.88
|
0.88
|
0.89
|
0.89
|
0.89
|
0.89
|
0.89
|
0.89
|
0.90
|
0.89
|
0.89
|
0.90
| |
|
Streets
|
0.70
|
0.71
|
0.71
|
0.71
|
0.72
|
0.74
|
0.72
|
0.73
|
0.76
|
0.73
|
0.75
|
0.78
|
|
0.76
|
0.77
|
0.79
|
0.80
|
0.82
|
0.84
|
0.84
|
0.85
|
0.89
|
0.89
|
0.91
|
0.95
| |
|
Open Space
|
0.05
|
0.10
|
0.14
|
0.08
|
0.13
|
0.19
|
0.12
|
0.17
|
0.24
|
0.16
|
0.21
|
0.28
|
|
0.11
|
0.16
|
0.20
|
0.14
|
0.19
|
0.26
|
0.18
|
0.23
|
0.32
|
0.22
|
0.27
|
0.39
| |
|
Parking
|
0.85
|
0.86
|
0.87
|
0.85
|
0.86
|
0.87
|
0.85
|
0.86
|
0.87
|
0.85
|
0.86
|
0.87
|
|
0.95
|
0.96
|
0.97
|
0.95
|
0.96
|
0.97
|
0.95
|
0.96
|
0.97
|
0.95
|
0.96
|
0.97
| |
|
a
|
Runoff coefficients for storm recurrence intervals less than
25 years.
|
|
b
|
Runoff coefficients for storm recurrence intervals 25 years
or more.
|
|
Source: Rawls, W.J., S.L. Wong and R.H. McCuen, 1981, "Comparison
of Urban Flood Frequency Procedures," Preliminary Draft, U.S. Department.
| |
7.
The Manning equation is preferred for one-dimensional, gradually
varied, open channel flow. In other cases, appropriate, applicable
methods should be applied; however, early coordination with the municipality
is necessary.
8.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part using the generally
accepted hydraulic analysis technique or method of the municipality.
9.
The design of any stormwater detention facilities intended to meet
the performance standards of this Part shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than one acre 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.
[Ord. No. 2022-09-03, 9/26/2022]
1.
Hot Spots.
A.
The use of infiltration BMPs is prohibited on hot spot land use areas,
such as vehicle fueling stations, public works storage areas, recycling
facilities, fleet storage areas, facilities that make or store hazardous
materials, etc.
B.
Stormwater runoff from hot spot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
2.
West Nile Guidance Requirements. All wet basin designs shall incorporate
biologic controls to deter the breeding of mosquitoes.
