Additional erosion and sediment control standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
A. These areas shall be protected from sedimentation and compaction
during the construction phase.
B. BMPs shall not be constructed, nor the areas receive runoff, until
the entire drainage areas tributary to the infiltration BMPs have
achieved final stabilization.
The design of all regulated activities should include the following
to minimize stormwater impacts (See Subappendix A-3 for a Nonstructural
Project Design Checklist.):
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.
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 Part
3 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.
(2)
For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered "meadow."
(b)
Twenty percent of existing impervious area, when present, shall
be considered "meadow" in the model for existing conditions.
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.]
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Volume (cubic feet) = (2/12) * impervious surfaces (square feet)
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(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.
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Volume (cubic feet) = (1/12) * impervious surfaces (square feet)
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(3)
Wherever possible, infiltration facilities should be designed
to accommodate infiltration of the entire permanently removed runoff;
however, in all cases at least the 0.5 inch of the permanently removed
runoff should be infiltrated.
(4)
This method is exempt from the requirements of §
172-103, Peak rate controls and management districts.
Applicants shall adhere to the following stream bank erosion/channel
protection requirements:
A. 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).
B. 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 sand or porous media
filter cartridge.