A.
For all regulated activities, unless preparation of a stormwater
management (SWM) site plan is specifically exempted:
B.
The SWM site plan approved by the municipality shall be on-site throughout
the duration of the regulated activity.
C.
The municipality may, after consultation with the Department of Environmental
Protection (DEP), approve measures for meeting the state water quality
requirements other than those in this chapter, provided that they
meet the minimum requirements of, and do not conflict with, state
law, including but not limited to the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth disturbance activities, 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 (E&S
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.
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). 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:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Maintain or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces by directing runoff to pervious
areas, wherever possible.
(3)
To the maximum extent practicable, incorporate the techniques for
low-impact development practices described in the Pennsylvania Stormwater
Best Management Practices Manual (BMP Manual) or the Philadelphia
Stormwater Management Guidance Manual.
I.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
J.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
K.
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, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland. NOAA's Atlas 145 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
N.
Various BMPs and their design standards are listed in the 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.
Note: Philadelphia County and Montgomery County will follow
different groundwater recharge criteria.
A.
Infiltration best management practices (BMPs) shall meet the following minimum requirements unless the site qualifies for an exemption from the infiltration requirements of this chapter as listed in § 290-6B:
(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 soil depth of 24 inches between the bottom of the
infiltration BMPs.
(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)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) 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 quality
volume within three days (72 hours) from the start of the design storm.
(2)
The size of the infiltration facility shall be based upon the following
volume criteria:
(a)
Montgomery County portion of the watershed.
[1]
Where practicable and appropriate, the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
(1.0) inch of runoff (I) over all proposed impervious surfaces.
[2]
The Rev required shall be computed as:
|
Rev = (P/12) * (I)
| ||||
|
Where:
| ||||
|
Rev
|
=
|
Recharge volume (cubic feet)
| ||
|
P
|
=
|
1.0 inch
| ||
|
I
|
=
|
Impervious area within the limits of earth disturbance (square
feet)
| ||
|
An asterisk (*) in equations denotes multiplication.
| ||||
(b)
Philadelphia County portion of the watershed. The recharge volume
shall be equal to one (1.0) inch of rainfall over all DCIA within
the limits of earth disturbance.
|
Rev = (P/12) * (I)
| ||||
|
Where:
| ||||
|
Rev
|
=
|
Recharge volume (cubic feet)
| ||
|
P
|
=
|
1.0 inch
| ||
|
I
|
=
|
DCIA within the limits of earth disturbance (square feet)
| ||
|
An asterisk (*) in equations denotes multiplication.
| ||||
B.
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
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 for the required retention (Rev) volume based on field-determined capacity at the level
of the proposed infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
design professional, 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.
A.
The applicant shall comply with the following water quality requirements
of this article. Note: Philadelphia County and Montgomery County will
follow different water quality requirements.
B.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The recharge volume computed under § 290-20 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration best management practices (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.
(1)
Montgomery County portion of the watershed.
(a)
The following calculation formula is to be used to determine
the water quality storage volume (WQv) in cubic
feet for the Tookany/Tacony-Frankford Watershed in Montgomery County:
|
Rev = (P/12) * (I)
| ||||
|
Where:
| ||||
|
Rev
|
=
|
Recharge volume (cubic feet)
| ||
|
P
|
=
|
1.0 inch
| ||
|
I
|
=
|
Impervious area within the limits of earth disturbance (square
feet)
| ||
|
An asterisk (*) in equations denotes multiplication. | ||||
(b)
For a noninfiltrating BMP, 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.
(2)
Philadelphia County portion of the watershed. The following calculation
formula is to be used to determine the WQv in
cubic feet for the Tookany/Tacony-Frankford Watershed in Philadelphia
County:
|
Rev = (P/12) * (I)
| ||||
|
Where:
| ||||
|
Rev
|
=
|
Recharge volume (cubic feet)
| ||
|
P
|
=
|
1.0 inch
| ||
|
I
|
=
|
DCIA within the limits of earth disturbance (square feet)
| ||
|
An asterisk (*) in equations denotes multiplication.
| ||||
Note: Philadelphia County and Montgomery County will follow
different stream bank erosion requirements. 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 B of the Pennsylvania
Stormwater Best Management Practices (BMP) Manual 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. [Note: The municipality
may select a smaller buffer width (above) if desired, but the selected
buffer may not be less than 10 feet.]. This does not include lakes
or wetlands.
A.
Montgomery County portion of the watershed. 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 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).
(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 sand or a porous media filter.
B.
Philadelphia County portion of the watershed.
(1)
Redevelopment sites with less than one acre of earth disturbance
or redevelopment sites that demonstrate a twenty-percent reduction
in DCIA from predevelopment conditions are exempt from this requirement.
(2)
Applicants shall adhere to the following stream bank erosion/channel
protection requirements:
(a)
To meet the requirement, stormwater management practices shall
retain or detain the runoff from all DCIA within the limits of earth
disturbance from a one-year, twenty-four-hour Natural Resources Conservation
Service (NRCS) Type II design storm in the proposed site condition
such that the runoff takes a minimum of 24 hours and a maximum of
72 hours from the end of the storm event to drain the management facility.
(b)
The infiltration and water quality volumes may be incorporated
into the channel protection portion of the design, provided the design
meets all requirements concurrently.
(c)
In Conditional Direct Discharge Districts (District C) only (see § 290-25), 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 Tookany/Tacony-Frankford Watershed has been divided into stormwater
management districts as shown on the Management District Map in Model
Ordinance Appendix A.
(1)
In addition to the requirements specified in Table 290-23.1 below, the erosion and sedimentation control (§ 290-20), the nonstructural project design (§ 290-21), the groundwater recharge (§ 290-20), the water quality (§ 290-21), and the stream bank erosion (§ 290-22) requirements shall be implemented.
(2)
Standards for managing runoff from each subarea in the Tookany/Tacony-Frankford
Watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
design storms are shown in Table 290-23.1. Development sites located
in each of the management districts must control proposed conditions
runoff rates to existing conditions runoff rates for the design storms
in accordance with Table 290-23.1.
|
Table 290-23.1
Peak Rate Control Standards by Stormwater Management District
in the Tookany/Tacony-Frankford Watershed
| |||
|---|---|---|---|
|
District
|
Proposed Condition Design Storm
(reduce to)
|
Existing Condition Design Storm
| |
|
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
|
25-year
| ||
|
100-year
|
100-year
| ||
|
C*
|
Conditional Direct Discharge District
| ||
(3)
In District C, development sites that can discharge directly to the
Frankford Creek Main Channel (east of I-95) and to the Delaware River
main channel or Tidal Schuylkill River major tributary without use
of city infrastructure may do so without control of proposed conditions
peak rate of runoff.
(4)
Projects that are required to obtain an NPDES permit for stormwater
discharges associated with construction activities are required to
show no increase in peaks from existing conditions.
(5)
When adequate capacity in the downstream system does not exist and
will not be provided through improvements, the proposed conditions
peak rate of runoff must be controlled to the predevelopment conditions
peak rate as required in District A provisions for the specified design
storms. The predevelopment condition for new development is the existing
condition. For redevelopment purposes, the predevelopment condition
is determined according to the procedures found in the Philadelphia
Stormwater Guidance Manual.
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 (Ordinance Appendix A) and this section of this chapter.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on an official map that is available for inspection
at the municipal and county planning offices. A copy of the official
map at a reduced scale is included in Ordinance Appendix A. 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.
D.
Sites located in more than one district. For a proposed development
site located within two or more stormwater management districts, the
peak discharge rate from any subarea shall meet the management district
criteria in 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 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 Cheltenham Township:
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. The designer must consult with the municipality to gain approval
of design methods prior to design. Table 290-24.1 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 municipality may allow the use of
the Rational Method to estimate peak discharges from drainage areas
that contain fewer than 200 acres. The Soil Complex Method shall be
used for drainage areas greater than 200 acres.
|
Table 290-24.1
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|---|---|---|---|
|
Method
|
Method Developed By
|
Applicability
| |
|
Montgomery County
| |||
|
WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than 200 acres and with time of concentration
less than 60 minutes (tc less than 60 minutes), or as approved by
the municipality and/or municipal engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or municipal engineer
| |
|
Philadelphia County
| |||
|
WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
* Note: Successors to the above methods are also acceptable.
These successors include WinTR55 for TR-55 and WinTR20 for TR-20.
| |||
B.
If a hydrologic computer model such as HydroCAD or HEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference NOAA
Atlas 14.
C.
For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
Rational 'C' value (i.e., forest), as listed in Table E-1 or E-2 in
Appendix E of this chapter.[1]
[1]
Editor's Note: Appendix E is on file in the Township offices.
D.
For Montgomery County only, all calculations using the Rational Method
shall use rainfall intensities from the NOAA 14 Precipitation-Frequency
Atlas of the United States (2004, revised 2006). 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.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table E-2 in
Appendix E of this chapter.
G.
The Manning equation is preferred for 1-D, gradually varied, open
channel flow. In other cases, appropriate, applicable methods should
be applied; however, early coordination with the municipality is necessary.
H.
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.
I.
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.
