A.
Applicants proposing regulated activities in the Neshaminy Creek watershed that do not fall under the exemption criteria shown § 288-6 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 chapter 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 chapter.
B.
SWM site plans approved by the municipality, in accordance with Article IV, shall be on site throughout the duration of the regulated activity.
C.
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 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.
D.
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 chapter 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.
E.
For all regulated activities, implementation of the volume controls in § 288-12 of this chapter is required.
F.
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, the total impervious
area on the parcel is subject to the requirements of this chapter.
G.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s) by the developer. Such stormwater
flows shall be subject to the requirements of this chapter.
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:
(3)
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), as amended from time to time. See
Appendix E for a summary description.[2]
[2]
Editor's Note: Appendix E is on file in the Township offices.
I.
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 chapter.
J.
The design of all facilities over karst shall include an evaluation
of measures to minimize the risk of adverse effects.
K.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
hours 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 Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, United States
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/.
M.
N.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (Pennsylvania BMP Manual).
Approvals issued and actions taken under this chapter 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.
A.
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.
B.
Volume controls shall be implemented using the Design Storm Method in Subsection B(1) or the Simplified Method in Subsection B(2) below. For regulated activities equal to or less than one acre, this chapter 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.
(1)
Design Storm Method (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions, refer to § 288-14A.
(a)
Post-development total runoff should not be increased from predevelopment
total runoff for all storms equal to or less than the two-year, twenty-four-hour
duration precipitation.
(b)
The following applies in order to estimate the increased volume
of runoff for the two-year, twenty-four-hour duration precipitation
event:
[1]
To calculate the runoff volume (cubic feet) for existing site
conditions (predevelopment) and for the proposed developed site conditions
(post-development), it is recommended to use the Soil Cover Complex
Method as shown below. Table B-3 in Appendix B is available to guide
a qualified professional and/or an applicant to calculate the stormwater
runoff volume.[1] The calculated volume shall be either reused, evapotranspired,
or infiltrated through structural or nonstructural means.
[1]
Editor's Note: Appendix B is on file in the Township offices.
[2]
Soil cover complex method:
|
Step 1: Runoff (inches) = Q = (P - 0.2S)2/(P + 0.8S)
|
|
Where:
| |||
|---|---|---|---|
|
P
|
=
|
2-year rainfall (in inches).
| |
|
S
|
=
|
(1,000/CN) – 10, the potential maximum retention (including
initial abstraction, Ia).
| |
|
Step 2: Runoff Volume (cubic feet) = Q (inches) x Area (square
feet) x (1 foot/12 inches)
|
|
Where:
| |||
|---|---|---|---|
|
Q
|
=
|
Runoff (in inches).
| |
|
Area
|
=
|
SWM area (in square feet).
| |
(2)
Simplified Method (regulated activities less than or equal to one
acre).
(a)
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 inches runoff) * (1 foot/12 inches)
* Impervious Surface (square feet)
(b)
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 inch runoff) * (1 foot/12 inches) *
Impervious Surface (square feet)
(c)
Infiltration facilities should be designed to accommodate the
first half-inch of the permanently removed runoff.
(d)
No more than one inch of runoff volume from impervious surfaces
shall be released from the site. The release time must be at least
24 hours but not more than 72 hours.
(3)
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.
(a)
If natural resources exist on the site, the applicant who is
required to submit an SWM site plan 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]
|
(c)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
|
Required Volume Control (cubic feet)
|
-
|
Nonstructural Volume Control (cubic feet)
|
=
|
Structural Volume Control (cubic feet)
|
(e)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on the suitability of soils and site
conditions (see Table B-6 in Appendix B[4] for a list of infiltration BMPs). Infiltration BMPs shall
be constructed on soils that have the following characteristics:
[1]
A minimum soil depth of 24 inches between the bottom of the
infiltration BMPs and the top of bedrock or seasonally high water
table.
[2]
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.2 inch per hour (in/hr) should be utilized; and for acceptable
rates a safety factor of 50% should be applied for design purposes
(e.g., for soil which measured 0.4 in/hr, the BMP design should use
0.2 in/hr to ensure safe infiltration rates after construction).
[3]
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.
[4]
Editor's Note: Appendix B is on file in the Township offices.
(f)
Soils. A soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. All regulated
activities are required to perform a detailed soils evaluation by
a qualified design professional which, at a minimum, addresses 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 may not 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 based on field-determined
capacity at the level of the proposed infiltration surface and based
on the safety factor of 50%.
[4]
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.
[5]
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.
Peak rate controls for large storms, up to the 100-year event,
are 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-, fifty-, and 100-year design
storms are shown in Table 124.1. Development sites located in each
of the management districts must control proposed development conditions'
runoff rates to existing conditions' runoff rates for the design storms
in accordance with Table 124.1.
|
Table 124.1
Peak Rate Runoff Control Standards by Stormwater Management
District in the Neshaminy Creek Watershed
(includes Little Neshaminy Creek)
| ||
|---|---|---|
|
District
|
Design Storm Post-Development
(Proposed Conditions)
|
Design Storm Predevelopment
(Existing Conditions)
|
|
A
|
2-year
5-year
10-year
25-year
50-year
100-year
|
1-year
5-year
10-year
25-year
50-year
100-year
|
|
B
|
2-year
5-year
10-year
25-year
50-year
100-year
|
1-year
2-year
5-year
10-year
25-year
50-year
|
|
C
|
2-year
5-year
10-year
25-year
50-year
100-year
|
2-year
5-year
10-year
25-year
50-year
100-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 on the Stormwater Management
District Watershed Map (Appendix D)[1] and in this section of this chapter.
[1]
Editor's Note: Appendix D is on file in the Township offices.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on official maps and are available for inspection
at the municipal office and county planning offices. A copy of the
map is included in Appendix D. 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 SWM site plan.
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
management district criteria for the district in which the discharge
is located.
E.
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.
F.
Site areas. The stormwater management site area is the only area
subject to the management district criteria. Nonimpacted areas or
nonregulated activities bypassing the stormwater management facilities
would not be subject to the management district criteria.
G.
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 Warrington Township:
A.
The following criteria shall be used for runoff calculations:
(1)
For development sites not considered redevelopment, the ground cover
used to determine the existing conditions' runoff volume and flow
rate shall be as follows:
(a)
Wooded sites shall use a ground cover of "woods in good condition."
A site is classified as wooded if a continuous canopy of trees exists
over 1/4 acre.
(b)
All nonforested pervious areas of the site (agriculture, bare
earth, lawn, 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) as listed in
Table B-4 or B-7 in Appendix B.[1]
[1]
Editor's Note: Appendix B is on file in the Township offices.
(c)
Twenty percent of all existing impervious area shall be considered
"meadow in good condition."
(2)
For 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. If the
developed site contains impervious surfaces, 20% of the impervious
surface area shall be considered "meadow in good condition" in the
model for existing conditions.
B.
Stormwater runoff peak discharges from all development sites with
a drainage area equal to or greater than 200 acres shall be calculated
using a generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. Table 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, at its sole discretion,
may allow the use of the Rational Method (Q = CIA) to estimate peak
discharges from drainage areas that contain less than 200 acres.
|
Q = CIA
|
|
Where:
| |||
|
Q
|
=
|
Peak flow rate, in cubic feet per second (cfs).
| |
|
C
|
=
|
Runoff coefficient, dependent on land use/cover.
| |
|
I
|
=
|
Design rainfall intensity, in inches per hour.
| |
|
A
|
=
|
Drainage area, in acres.
| |
C.
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 National Oceanic
and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding
to the Doylestown rain gage, seen in Table B-1 in Appendix B.[2] The SCS Type II rainfall curve from NOAA is found on Figure
B-1 in Appendix B. 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 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 (this is the preferred method for use in Warrington Township)
|
|
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 if approved by the municipality
and/or Municipal Engineer; all applicants must consult with the Municipal
Engineer prior to utilizing this method
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
|
[2]
Editor's Note: Appendix B is on file in the Township offices.
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. 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 based on Table
B-4 in Appendix B.
F.
Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational Method shall be consistent with Table B-7
in Appendix B.
G.
Runoff from proposed sites graded to the subsoil will not have the
same runoff conditions as the site under existing conditions because
of soil compaction, even after topsoiling or seeding. The proposed
conditions' "CN" or "C" shall increase by 5% to better reflect proposed
soil conditions.
H.
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.
I.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using the generally
accepted hydraulic analysis technique or method of the municipality.
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. For drainage areas greater than 200 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.
