A.
Applicants proposing regulated activities in Township that do not fall under the exemption criteria shown in § 206-6 shall submit a stormwater management (SWM) site plan consistent with the watershed SWM plan to the Township 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 Township issues written approval of a SWM site plan, which demonstrates compliance with the requirements of this chapter.
B.
SWM site plans approved by the Township, in accordance with Article IV, shall be on site throughout the duration of the regulated activity.
C.
The Township 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.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
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.[2] 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.
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
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) from 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). See Appendix B[4] for a summary description.
[4]
Editor's Note: Appendix B is included as an attachment
to this chapter.
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
and not more than 72 hours from the end of the design storm.
M.
N.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA 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.
Must comply with Title 25, Chapter 102, of the Pennsylvania Code
and any other applicable state, county and Township codes. PADEP requires
an engineered postconstruction SWM plan with projects proposing earth
disturbance greater than one acre.
B.
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.
C.
Approval of earth disturbance activities is required by the Township
per Table 206-6.2.
D.
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.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
The green infrastructure and low-impact development practices
provided in the BMP Manual shall be utilized for all regulated activities
wherever possible. 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. 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 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.
A.
Design-Storm Method (CG-1 in the BMP Manual) (any regulated activity): This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 206-22A.
(1)
Postdevelopment 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.
(2)
The following applies in order 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 (predevelopment) and for the proposed developed site conditions
(postdevelopment), it is recommended to use the Soil Cover Complex
Method as shown on the following page. Table A-3 in Appendix A[1] is available to guide a qualified professional and/or
an applicant to calculate the stormwater runoff volume. The calculated
volume shall be either reused, evapotranspired, or infiltrated through
structural or nonstructural means.
|
Soil Cover Complex Method:
| ||
|
Step 1: Runoff (in) = Q = (P - 0.2S)2/(P + 0.8S)
| ||
|
Where:
| ||
|
P
|
=
|
2-year Rainfall (in)
|
|
S
|
=
|
The potential maximum retention (including initial abstraction,
la)
|
|
S
|
=
|
(1,000/CN) - 10
|
|
Step 2: Runoff Volume (Cubic Feet) = Q x Area x 1/12
| ||
|
Where:
| ||
|
Q
|
=
|
Runoff (in)
|
|
Area
|
=
|
SWM Area (square feet)
|
[1]
Editor's Note: Appendix A is included as an attachment
to this chapter.
B.
Simplified Method (CG-2 in the BMP Manual) (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
half 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 hours
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 and a SWM site plan submission
is required for the regulated activity, the applicant shall determine
and display the total acreage of protected area where no disturbance
is proposed on the plan. 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 postdevelopment conditions)
|
—
|
Stormwater Management Site Area
|
|
Natural resource areas should be calculated based upon the Township's
own natural resource protection ordinance. If no ordinance exists,
see Table A-2 in Appendix A[2] for guidance to assess the total protected area. For additional
reference, see Chapter 5, Section 5.4.1, of the PA BMP Manual.
| ||||
[2]
Editor's Note: Appendix A is included as an attachment
to this chapter.
(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)
|
(5)
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on the suitability of soils and site conditions
(see Table A-6 in Appendix A[5] for a list of infiltration BMPs). 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 0.2 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 0.4 inch per hour, the BMP design should use 0.2 inch
per 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.
(d)
All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
[5]
Editor's Note: Appendix A is included as an attachment
to this chapter.
(6)
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 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 subgrade
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. Percolation
tests are not recommended for design purposes.
(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 Township 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 Township.
A.
District boundaries. For the purposes of this chapter, the Township
is divided in stormwater management districts as listed in Table 206-18.1.
The approximate boundaries of the stormwater management districts
are shown at a reduced scale on official maps, which are designated
as the Stormwater Management District Map and included in Appendix
A.[1] 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.
[1]
Editor's Note: Appendix A 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 and in this section of this chapter.
C.
Standards for managing runoff from each subarea in the watershed
for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design
storms are shown in Table 206-18.1. 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 206-18.1 on the following page.
|
Table 206-18.1: Peak Rate Runoff Control Standards by Stormwater
Management Districts
| ||
|---|---|---|
|
District
|
Design Storm Postdevelopment (Proposed Conditions)
|
Design Storm Predevelopment (Existing Conditions)
|
|
Neshaminy 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
| |
|
Neshaminy B
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
10-year
|
5-year
| |
|
25-year
|
10-year
| |
|
50-year
|
25-year
| |
|
100-year
|
50-year
| |
|
Neshaminy C
|
2-year
|
2-year
|
|
5-year
|
5-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
Wissahickon
|
2-year
|
1-year
|
|
5-year
|
5-year
| |
|
10-year
|
2-year
| |
|
25-year
|
25-year
| |
|
50-year
|
10-year
| |
|
100-year
|
100-year
| |
|
NOTE:
|
|---|
|
The Neshaminy Creek Watershed includes the Little Neshaminy
Creek.
|
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 the Township:
A.
The design of all regulated activities should include the following
to minimize stormwater impacts:
(1)
The applicant should find practicable alternatives to the surface
discharge of stormwater, the creation of impervious surfaces, and
the degradation of waters of the commonwealth and must maintain as
much as possible the natural hydrologic regime of the site.
(2)
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
A.
If a perennial or intermittent stream passes through the site, the
applicant shall create a riparian buffer extending a minimum of 10
feet to either side of the top-of-bank of the channel. The buffer
area shall be established and maintained in an undisturbed state.
This buffer area may be maintained as a meadow with minimal mowing
of the grassed area, or as a forested buffer, being planted with appropriate
native vegetation (refer to Appendix B of the BMP Manual for plant
lists). 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. This does not include lakes or
wetlands.
B.
Applicants shall adhere to the following stream bank erosion/channel
protection requirements:
(1)
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 storm event 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 or a sand filter) so that the proposed conditions
one-year, twenty-four-hour storm event takes a minimum of 24 hours
to drain from the facility from a point when the maximum volume of
water from the one-year, twenty-four-hour storm event 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 volume control orifice is at the invert of the facility).
(2)
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 drainage areas
contributing 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. It is recommended that the design,
to accommodate maintenance, include a replaceable and/or porous media
filter cartridge.
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)
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) as listed in Tables
A-4 or A-7 in Appendix A.[1]
[1]
Editor's Note: Appendix A is included as an attachment
to this chapter.
(2)
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. If the developed site contains impervious surfaces,
20% of the impervious surface area shall be considered meadow in the
model for existing conditions.
B.
Stormwater runoff peak discharges from all development sites with
a drainage area equal to or greater than two acres shall be calculated
using a generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. Table 206-22.1 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 Township may allow the use of the
Rational Method (Q = CIA) to estimate peak discharges from drainage
areas that contain less than two acres.
|
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
|
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 Table A-1 in Appendix
A.[2] 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 206-22.1: 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 hydrology computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of full hydrology computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
|
Rational Method (or commercial computer package based on the
Rational Method)
|
Emil Kuichling (1889)
|
For sites less than 2 acres, or as approved by the Township
and/or Township Engineer. Not acceptable when a full hydrograph is
required
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township and/or
Township Engineer
|
[2]
Editor's Note: Appendix A 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 Table A-1 in Appendix A.[3] 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.
[3]
Editor's Note: Appendix A is included as an attachment
to this chapter.
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 top-soiling or seeding. The proposed
condition "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 Township
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 Township.
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 two acres in size, 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.
Hotspots.
(2)
Stormwater runoff from hotspot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
Guidance regarding acceptable methods of pretreatment is located in
Appendix D.[2]
[2]
Editor's Note: Appendix D is included as an attachment
to this chapter.
