Water quality control shall be implemented using the following methodologies:
A. The simplified method, as detailed below, is independent of site conditions.
(1) Retention and detention facilities shall he sized to capture the first 2 inches of runoff from all impervious surfaces.
(2) The first 1 inch of runoff shall be permanently removed and shall not be released into the surface Waters of this Commonwealth. This is the Permanently Removed Volume (PRV). Removal options include reuse, evaporation, transpiration, and infiltration. A list of the site conditions and BMP's generally suitable for infiltration is provided in Appendix 26- B.
(3) The subsequent 1 inch of runoff shall be detained. This is the Extended Detention Volume (EDV).
(4) Infiltration of the first ½ half inch of the PRV is encouraged. This portion of the PRV is the Groundwater Recharge Volume (GRV). A list of the site conditions and BMP's generally suitable for infiltration is provided in Appendix 26-B.
(5) The Permanently Removed Volume (PRV) requirement fur land areas with existing cover consisting of meadow, brush, wood-grass combina tion, or woods proposed for conversion to any other non-equivalent type of pervious cover shall be ¼ inch of runoff.
(6) Retention and detention facilities should be designed to drain both the PRV and EDV completely within 48 to 96 hours from the start of the storm.
(7) Retention facilities should be designed to accommodate infiltration of the PRV. Infiltration areas should be spread out and located in the sections of the site that are most suitable for infiltration. A list of the site conditions and BMPs generally suitable for infiltration is provided in Appendix 26-B.
B. The Design Storm Method, as detailed below, requires technical modeling based on site conditions.
(1) Do not increase the post-development total runoff volume for all storms equal to or less than the 2-year 24-hour duration rainfall.
(2) Do not increase peak rate of runoff for (1 -, 2-, 10-, 25-, 100-year storms (at minimum), pre-development to post-development; as necessary, provide additional peak rate control for as required by Act 167 planning.
(3) Existing (pre-development) non-forested pervious areas must be considered meadow or its equivalent.
(4) Twenty percent of existing impervious area, when present, shall he considered meadow in the model for existing conditions. Township may require up to 100 percent of the existing area be modeled as meadow for existing conditions for sites with known stormwater concerns.
C. In all cases, retention and detention facilities should be designed to completely drain water quality volumes (in the ease of the Simplified Method this is includes both the PRV and FDV) over a period of time not less than 4S hours and not more than 96 hours from the start of the design storm.
The Pennsylvania Stormwater Best Management Practice Manual (1) provides guidance on selection and application of both water quality control methodologies.
(Ord. 4-09-2007, 4/9/2007, § 303)