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 design of all regulated activities should include the following
to minimize stormwater impacts (See Appendix C 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 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.
(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.]
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 0.5 inch of the permanently removed
runoff should be infiltrated.
(4) This method is exempt from the requirements of §
161-23, Stormwater peak rate control and management districts.