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.
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 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:
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Step 1: Runoff (in) = Q = (P - 0.2S)2/(P + 0.8S)
|
Where:
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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)
|
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 for guidance to assess the total protected area. For additional
reference, see Chapter 5, Section 5.4.1, of the PA BMP Manual.
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(2) Calculate the volume controls provided through nonstructural BMPs.
Table A-5 in Appendix A is recommended as guidance.
(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. Table
A-6 in Appendix A is recommended as guidance. 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
(see Table A-6 in Appendix A 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.
(e)
Pretreatment (See Appendix D) shall be provided prior to infiltration.
(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.