Stormwater Harvesting and Runoff Queries

A lot of discussion about time intervals today (see below). I do not necessarily agree with the 72 hour dewatering for a cistern, an above-ground basin, sure (its a mosquito thing), and as far as I know, the ref manual does not require all facilities to drain within 72 hours. Recent research has shown that the chance of back to back 2-year storms within 3 days is less than 1%. If this system is for reuse, can you use all of the water that quickly? If you can, then sure use the 72 hours as your interval. But if you can not, I would make the argument that the interval between 1 and 2 year storms is 1 and 2 years. If the cistern is simply to control the runoff rate, I'd use the 72 hours. Yes, you need to account for the terraces (or the paving below, whichever is greater). Just develop an impervious footprint, if the terraces are stacked above one another, i would not count all of them, just the upper most.

Michael, can you please direct me towards the recent research that you referenced above? I have searched the Villanova Urban Stormwater Partnership for information pertaining to the likelihood (less than 1%) of a 2-year storm occurring twice within a 3 day period. I need an excerpt of the research along with the source. Thank you for your assistance.

I'm not too sure exactly what presentation this came from, but this is simply a math problem. You are seeking the probability that a 2 year storm will occur twice within 3 days. Pn = 1-(1-P)^N P = Chance of 2-year storm occuring any year - 0.50 N = 0.0082 years Pn = 0.006