Storm Interval and Drawdown

Thanks, Michael, I will try to clarify. In case two, you must reduce runoff by 25% as compared to pre-development conditions. From what I understand, the tank doesn't need to be empty. The storage capacity and drawdown rate must be sufficient to reduce post-development runoff by 25%. You have the two year 24 hour storm and, in our case, there is no runoff due to the tank capacity and drawdown rate, however three days later you have the same storm event and this is where you measure post-development runoff? Or due you continue with the deluge every three days? I guess it is the storm interval that has me confused. Thanks again.

Thank you, please bear with me. We are demonstrating a 25% reduction in Post-development runoff. This does not necessarily require capturing and using all of the storm water volume. Our drawdown is achieved through reuse of rainwater to flush toilets and urinals (based on WEp1 performance case flush fixture use). Storm 1 119,000 liters, storm volume 148,000 liters, storage capacity 6,900 liters, drawdown per storm interval (2,300 per day x 3 days) 148,000 + 6,900 - 119,000 = 35,900 storage volume remaining after storm 1. 3 days later, Storm 2 119,000 liters, storm volume 35,900 liters, storage remaining 2,300 drawdown (1 day) Storm Volume - Capacity Remaining - Drawdown = Runoff (in this case) 119,000 - 35,900 - 2,300 = 80,800 liters of runoff after Storm 2. Pre-development runoff 108,000 liters Post-development runoff 80,800 liters Runoff reduction is 25.18% Please let me know if this is correct and, again, thanks for your time.

Michael, can you comment on my last post, please.

I guess not. I wonder, what am I paying for?

Michael, I am probably not describing my doubt clearly and am certainly not asking you to check my math or do any of our work. My doubt concerns what the LEED NC reference guide says on page 97 in the example given. It says that "the captured rainwater must be drained within three days... for the tank to be emptied before the next storm. If the drainage rate is slower, full capacity cannot be assumed to be available during the 2-year 24-hour design storm." My doubt is whether or not the system capacity truly needs to be emptied before the next storm The requirement for case 2 is, as you said, to reduce runoff volume by 25% compared to pre-development conditions and I probably confused things with the example calculations I gave. My question remains if, at the time of the Second Storm Event, the following it true: Storm volume - Storage capacity remaining - Drawdown = Runoff in the case that the system in not completely empty. Thank you

That helps a lot, thanks very much.