Detain 50% of 1yr 24hr storm for 36 hrs?

[NCSUBOY74]

I have a pond that is detaining almost all of the 1yr 24hr storm in order to provide water quality. The reviewer wants me to show that I am detaining 50% of the storm over 36hr. I routed with Hydro Flow. I am sure this works but how do I show the Calculations?

 

[Twinkie]

(vol cf*.50)/((area sf)(rate/12 ft/hr)) = hours to drawdown

 

[NCSUBOY74]

Any way that I could route it or should it just be a hand calculation?

 

[Twinkie]

Sure you could route it, just give him the volume under the hydrograph from 36 hours on.