OK....we have something we can work with
5 inches = 5/12 ft
The volume of water drained is given by:
pi * (diameter/2)^2 * (height) =
pi * ( 70/2)^2 (5/12) = 6125 * pi / 12 cubic ft of (water??)
If we're talking abbout water...we need to know how many gallons of water are in a cubic ft
1 ft^3 water ≈ 7.48051948 gal of water
So...the total gallons drained ≈ (7.48051948) 6125 * pi / 12 ≈ 11995 gal
Divide this by 15 and we get that about 800 gals are drained per minute