The low-impact development practices provided in the BMP Manual [NOTE: See §
42-34A.] 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 all regulated activities defined by §
42-12D, both the Design Storm Method and the Simplified Method shall be calculated; the larger control volume based on the two calculations shall be controlled. Subsection C below provides requirements for mined, karst, or other geologically limiting areas where infiltration shall not occur.
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 postdevelopment 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.
B. When Design Storm Method CG-1 guidelines are not used, the Simplified
Method (CG-2 in the BMP Manual) has been modified to accommodate two
inches of permanently removed runoff volume. This method (provided
below) is independent of site conditions and should be used if the
Design Storm Method is not followed. For new impervious surfaces:
(1) The first two inches of runoff from new impervious surfaces shall
be permanently removed from the runoff flow (i.e., it shall not be
released to the surface waters of this commonwealth). Removal options
include reuse, evaporation, transpiration, and infiltration.
(2) 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.
(3) Facilities shall be designed to drain the permanently removed runoff
volume in a period no less than 24 hours and no greater than 72 hours.
(4) Runoff volume in excess of two inches shall be safely conveyed to
existing stormwater collection systems or streams, in the direction
of the existing drainagecourse.
(5) This method is exempt from the requirements of §
42-14, Rate controls.
C. Before infiltration is proposed on a site, site conditions shall be evaluated by a qualified design professional through subsurface investigation and testing to determine if site conditions are suitable to support proposed infiltration facilities to manage runoff. If it is determined that infiltration is not feasible due to physical constraints of the site, or will adversely impact the environment as demonstrated by the presence of acid mine drainage, sinkhole formation, or other serious environmental issues, then the above volume controls must be achieved through surface BMP mitigation. Reference the BMP Manual [NOTE: See §
42-34A.] for alternative mitigation measures that do not require infiltration.