I have so many questions concerning storm water management.   My company is a large warehouse.  We have 350 truck parking spots as well as 150 employee parking spots and a 525,000 square foot building.  This all sits on 40+ acres of land.  I have 3 retention ponds onsite, a creek that half of my roof runoff runs into, a retention pond across the street that is fed from our storm water drains.  (Both the creek and pond across the street are not a part of my property.  However, the city when they rebuilt the storm drains drained our storm water to this pond.  In the case of the creek, the Dept of Environmental Management uses our roof runoff to maintain the creek levels.)  The creek is connected to one of our retention ponds, but is not on our property.   The total square footage of hard surfaces is 1,192,212 square feet.  

I guess my first question is, we the stipulations given to me by the city and state, can I even go for the rainwater credit?

Next since I am an O+M project, do I calculate based on just the 25% of the impervious surfaces?  If so, I can capture 130.26% of my runoff in these 3 ponds...would my calculation then be:  runoff cu ft = (1,192,212 * .25) *144 (converting to square inches) * 90th percential  storm event /1 cubic foot in inches

225775.1475 cu ft = ((1,192,212 * .25) * 144 * 9.09)/1728