A.
For any of the activities regulated by this chapter, unless preparation
of a stormwater management (SWM) site plan is specifically exempted,
the preliminary or final approval of subdivision and/or land development
plans, the issuance of any building or occupancy permit, the commencement
of any earth disturbance activity shall not proceed until the property
owner or applicant or his/her agent has received written approval
from the municipality of a SWM site plan that demonstrates compliance
with the requirements of this chapter, and a written approval of an
adequate erosion and sediment (E&S) control plan from the municipality
or County Conservation District, when and as required.
B.
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 33 U.S.C. § 1251 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.
E.
Impervious areas:
F.
Stormwater discharges onto adjacent property shall not be created,
increased, decreased, relocated, or otherwise altered without written
notification of the adjacent property owner(s) by the applicant. Such
stormwater discharges shall be subject to the requirements of this
chapter.
G.
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 the 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.
H.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
I.
Infiltration BMPs should be dispersed on site, 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.
J.
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.
K.
Design storm volumes and precipitation intensities to be used in
the analysis of discharge or runoff 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 Springs, Maryland. NOAA's Atlas 14 can be accessed
at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
L.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this chapter and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law, and the Storm Water Management Act.[2]
[2]
Editor's Note: See 33 U.S.C. § 1251 et seq. and
32 P.S. § 680.1 et seq., respectively.
M.
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.
A.
Additional erosion and sediment control standards and criteria are
recommended to be applied where infiltration BMPs are proposed. They
shall include the following:
The design of all regulated activities should include the following
to minimize stormwater impacts: (See Appendix C for a Nonstructural
Project Design Checklist.[1])
A.
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.
B.
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.
[1]
Editor's Note: Said appendix is on file in the Township offices.
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 § 291-106:
(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 bottoms of the
infiltration BMPs and bedrock or other limiting zones.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's qualified person.
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end 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.
(2)
The size of the infiltration facility shall be based upon the
following volume criteria: Where practicable and appropriate the recharge
volume shall be infiltrated on site. The recharge volume shall be
equal to 1.0 inch of runoff (I) over all proposed impervious surfaces.
The Rev required shall be computed as:
|
Rev = (1/12)*(I)
|
|
Where:
| ||||
|---|---|---|---|---|
|
Rev
|
=
|
Recharge volume (cubic feet)
| ||
|
I
|
=
|
Impervious area 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 developed
by the applicant to determine the suitability of infiltration facilities.
The evaluation shall be performed by a qualified person, and, at a
minimum, address soil permeability, depth to bedrock, and subgrade
stability. The general process for designing an 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 must be implemented if these tests are not
completed.
(2)
Perform 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 accepted for design purposes.
(3)
Design the infiltration structure for the required recharge
volume (Rev) based on field tests at the elevation
of the proposed infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
qualified person, the applicant must demonstrate to the municipality
that the soils are conducive to infiltrate on the lots identified.
(5)
The applicant must install an impermeable liner in detention
basins where the possibility of groundwater contamination exists.
A detailed hydrogeologic investigation may be required by the municipality.
The low impact development practices provided in the BMP Manual
shall be utilized for all regulated activities to the maximum extent
practicable. Water volume controls shall be implemented using the
Design Storm Method in Subsection A or the Simplified Method in Subsection
B below. For regulated activity areas equal to or less than one acre
that do not require hydrologic routing to design the stormwater facilities,
this chapter establishes no preference for either methodology; therefore,
the applicant may select either methodology on the basis of economic
considerations, the intrinsic limitations on applicability of the
analytical procedures associated with each methodology, and other
factors. All regulated activities greater than one acre must use the
Design Storm Method.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
The post-development total runoff volume for all storms equal
to or less than the two-year, twenty-four-hour storm event shall not
be increased.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre, or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces. (Note: An asterisk (*)
in equations denotes multiplication.)
|
Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
|
(2)
At least the first one inch of runoff from 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. Removal
options include reuse, evaporation, transpiration, and infiltration.
|
Volume (cubic feet) = (1/12) * impervious surfaces (square feet)
|
(3)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
A.
If a perennial or intermittent stream passes through the site, the
applicant shall create a riparian buffer extending a minimum of 50
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 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. 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).
C.
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 Wissahickon Creek Watershed has been divided into stormwater
management districts as shown on the Management District Map (Appendix
A[1]). The peak rate requirements specified in Table 408.1
below shall be implemented in addition to all other applicable requirements.
Standards for managing peak rates of runoff from each subarea in the
Wissahickon Creek Watershed for the two-, five-, ten-, twenty-five-,
fifty-, and one-hundred-year storm events are shown in Table 408.1.
Development sites located in each of the management districts must
control proposed condition runoff rates to existing condition runoff
rates for the design storms in accordance with Table 408.1.
|
TABLE 408.1
| ||||
|---|---|---|---|---|
|
Peak Rate Control Standards by Stormwater Management District
in the Wissahickon Creek Watershed
| ||||
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
A
|
2-year
|
Reduce to
|
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
|
Reduce to
|
1-year
| |
|
5-year
|
2-year
| |||
|
10-year
|
5-year
| |||
|
25-year
|
10-year
| |||
|
50-year
|
25-year
| |||
|
100-year
|
50-year
| |||
|
C*
|
Conditional direct discharge district
| |||
|
NOTES:
| |
|---|---|
|
In District C, development sites that can discharge directly
to the Wissahickon Creek Main Channel and to the Schuylkill River
main channel without use of City infrastructure may do so without
control of proposed conditions peak rate of runoff.
| |
|
Projects that are required to obtain a NPDES permit for stormwater
discharges associated with construction activities are required to
show no increase in peaks from existing conditions.
| |
|
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 in Philadelphia County, the
predevelopment condition shall be determined according to the procedures
found in the Philadelphia Stormwater Guidance Manual.
|
[1]
Editor's Note: Said appendix is on file in the Township offices.
B.
General. Proposed condition 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 A[2]).
[2]
Editor's Note: Said appendix is on file in the Township offices.
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 as Appendix A.[3] 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.
[3]
Editor's Note: Said appendix is on file in the Township offices.
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;
H.
Stormwater control measures which increase storage or infiltration
volume, and which are not associated with new land development or
redevelopment activity that increases runoff volume above existing
levels, are exempt from the peak rate requirements of this chapter,
so long as peak outflow is not increased.
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 qualified person must consult with the municipality to
gain approval of design methods prior to design. Table 409.1 summarizes
acceptable computation methods and the method selected by the qualified
person 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 less than 200 acres. The Soil Cover Complex Method
shall be used for drainage areas greater than 200 acres.
|
TABLE 409.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies For Stormwater Management
Plans
| |||
|
Montgomery County Portion of the Watershed
| |||
|
Method
|
Method Developed By
|
Applicability
| |
|
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
|
US 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 times of concentration
less than 60 minutes (tc < 60 min), or as approved by the municipality
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
| |
|
NOTE:
| ||
|---|---|---|
|
*
|
Successors to the above methods are also acceptable.
| |
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 NRCS
Type II.
C.
For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow," unless the natural
ground cover generates a lower curve number or Rational 'C' value
(i.e., forest).
D.
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 flow velocities as determined by Manning's equation.
E.
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.
F.
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.
G.
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.
