Water volume controls shall be implemented using the Design Storm Method in Subsection
A or the Simplified Method in Subsection
B below for all regulated activities not otherwise exempted by §
249-11. For regulated activity areas equal 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 (see Section 8.7 of the most current version
of the SWM 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 (predevelopment) nonforested pervious areas must be
considered meadow or its equivalent.
(b)
Twenty percent of existing impervious area, when present, shall
be considered meadow in the model for existing conditions.
[or]
(c)
Calculate existing (predevelopment) stormwater runoff and infiltration
volumes based on existing site conditions, as verified through design
phase soil infiltration testing.
B. The Simplified Method (see Section 8.7 of the most current version
of the SWM 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 detention or rate control
facilities. For new impervious surfaces:
(1) Stormwater facilities shall be sized to 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) Infiltration facilities should be designed to accommodate infiltration
of the entire permanently removed runoff; however, in all cases, at
least the first 1/2 inch of the permanently removed runoff should
be infiltrated.
(4) The second one inch of runoff from new impervious surfaces should
be detained using structural and nonstructural BMPs (as outlined in
the most current version of the SWM Manual) and released at a controlled
rate.
(5) Regulated activities eligible under this method are exempt from the requirements of §
249-13, Rate controls.
The SWM site plan shall also satisfy the stormwater management and design criteria in Chapter
260, Subdivision and Land Development, for projects meeting the definition of "land development" as defined in that chapter.