(A) 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 division (B) below or the simplified method in division (C) below. 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.
(B) 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) No plan shall increase the post-development total run-off volume for all storms equal to or less than the two-year 24-hour duration precipitation to more than the pre-development total run-off volume.
(2) For modeling purposes:
(a) Existing (pre-development) non-forested pervious areas must be considered meadow; and
(b) Twenty percent of the existing impervious area of a project site, when present, shall be considered meadow in the model for existing conditions, if the existing impervious area is being altered by the proposed regulated activity.
(C) (1) 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.
(2) For new impervious surfaces:
(a) Stormwater facilities shall capture at least the first two inches of run-off from all new impervious surfaces;
(b) At least the first one inch of run-off from new impervious surfaces shall be permanently removed from the run-off flow, i.e., it shall not be released into the surface waters of this Commonwealth. Removal options for the first on inch of run-off include reuse, evaporation, transpiration and infiltration; and
(c) Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed run-off; however, in all cases at least the first one- half inch of the permanently removed stormwater run-off shall be infiltrated.
(Ord. 2013-2, passed 10-7-2013)