Rainfall Event Interval

The draw down would be calculated usi g the actual demand on the tank. You rainfall intensity should have nothing to do with this. Your intensity would allow you to calculate the amount of water getting to your tank for reuse.

Can you explain this further, Michael? Because the formula for drawdown rate on the reference guide is tank capacity/ rainfall event interval, and the example used was a "design storm interval" of 3 days. Is this something we can assume, or data we can acquire from a local weather bureau? Thanks

That is a general drawdown equation which illustrates the minimum usage needed between rainfall events to ensure there is storage for the next event. However, Q actual, is how the system actually drawsdown, and must equal or less than the minimum drawdown time.

This is a very generic equation, and I am not fond of it. I could theoretically make an argument that because we are looking at the 1- and 2-year storms, the interval is 1 and 2 years respectively. I usually shoot for a maximum dewatering time of 72-96 hours for a typical stormwater facility.

EX: Say I have 50,000 cf of runoff, I want it drained within 3 days, so:

Qmin = 50,000 cf/259,200 secs = 0.19 cf/sec

What this means is you need to design an orifice in your basin that lets out at least 0.19 cfs in order for the basin to be dry in 3 days. If you do the math using 1- or 2-year intervals, you can see the problem with the language in the reference manual. You would basically poke a pinhole in your outlet structure to meet Qmin.

Your local regs will probably be the driving force here, as they likely have a minimum orifice size of ~3" and a max dewatering time.

Tell me a little more about your design. Is this a cistern, underground basin, or surface basin?