The green infrastructure and low-impact development practices
provided in the BMP Manual shall be utilized for all regulated activities
wherever possible. 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.
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) Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year twenty-four-hour duration
precipitation.
(2) For modeling purposes:
(a)
Existing (pre-development) non-forested pervious areas must
be considered meadow in good condition.
(b)
Twenty percent of existing impervious area, when present, shall
be considered meadow in good condition 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.
(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 this commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
(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.
(4) This method is exempt from the requirements of §
190-16, Rate controls.
Post-development discharge rates shall not exceed the pre-development
discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-,
and one-hundred-year, twenty-four-hour storm events. If it is shown
that the peak rates of discharge indicated by the post-development
analysis are less than or equal to the peak rates of discharge indicated
by the pre-development analysis for one-, two-, five-, ten-, twenty-five-,
fifty-, and one-hundred-year, twenty-four-hour storms, then the requirements
of this section have been met. Otherwise, the applicant shall provide
additional controls as necessary to satisfy the peak rate of discharge
requirement.