EDIT: The figures have been altered due to an oversight in calculation. I had used the diameter where radius was required. This means the previous figures are wrong. I recalculated based on radius now. The figures below are corrected.
______________________________________________________________________​_______

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 = 67,913 cubic inches

The volume of the 5 meter long pipe is the flow per second since 5 meters will flow per second.
2489
RESULTS

Per Second
7 Barrels per Second

Per Hour
25,196 Barrels per Hour

Per 24 Hours
604,719 Barrels per Day


EVEN IF THE FLOW WAS 1 METER per SECOND = 121,000 Bbl/day