The low impact development practices provided in the BMP Manual1 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 that require submission
of a formal SWM site plan, 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 (NOTE: See §
444-32A)] 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 (NOTE: See §
444-32A)] 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 into 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, to the greatest extent possible and subject to the Municipal
Engineer's discretion, 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 drainage course.
(5) This method is exempt from the requirements of §
444-13, 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 Manual1 for alternative mitigation measures that do not require
infiltration.