The following formula shall be used to calculate the volume of detention required by this chapter:
V = Rd - Rn - Ros - S - GW
V = the change in volume (in cubic feet per second) from the site. This is the base volume of excess storm water flows that would result from the development.
Rd = the volume of storm water runoff (in cubic feet per second) flowing from the site after its development. This determination shall include runoff from pervious and impervious surfaces, changes in areas of forest, changes in soils due to compaction, and changes in the time of concentration, for a 100-year storm of 24-hour duration.
Rn = the volume of storm water runoff (in cubic feet per second) flowing from the site in its natural state. This determination shall include runoff from the site with its natural cover, grassland or woodland for a 100-year storm of 24-hour duration. Farm fields shall be calculated as grassland.
Ros = the volume of storm water runoff (in cubic feet per second) flowing onto the site from upstream properties in their present state of development for a 100-year flood of 24-hour duration.
S = the change in capacity (in cubic feet per second) of the natural on-site detention areas of the site. This indicates any drainageways, small depressional areas or other areas which would naturally retain water during a 100-year storm of 24-hour duration. (This calculation may carry either a plus or minus value.)
GW = the change in subsurface flows due to dewatering techniques which would add to the total surface water runoff during a 100-year storm of 24-hour duration. Included here are dewatering devices such as drain tiles, curtain drains or sump pumps. (This calculation may carry either a plus or minus value.)
('69 Code, § 5-18(b)(4)) (Ord. O-83-1, passed 5-9-83)