EDIT: DIAMETER WAS USED WHERE RADIUS WAS NEEDED. TITLE IS NOW ACCURATE.

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Riser pipe diameter = 24"OD - 21"ID

The flow speed will be assumed at max speed for a storm drain, 5 meters per second. It could be higher but we will remain at a known feasible speed for the sake of argument. There is more then enough pressure to push 5 meters per second.

Cylinder Area of...
Length of 5 meters = 196.85 inches
21" Diameter x 196.85" Height = 272,834.1 cubic inches

The volume of the 5 meter long pipe is the flow per second since 5 meters will flow per second.

RESULTS

Per Second
272,834 cubic Inches per Second
1,181 US Gallons per Second
28.12 Barrels per Second

Per Hour
4,251,600 US Gallons per Hour
101,232 Barrels per Hour

Per 24 Hours
102,038,400 US Gallons per Day
2,429,568 Barrels per Day


EVEN IF THE FLOW WAS 1 METER per SECOND = 486,000 Bbl/day