The flow though a gravity fed drainage line that discharges to atmospheric pressure can be roughly calculated using the continuity equation as it is a product of flow velocity and cross sectional area.
Q = A * V
Where Q is flow rate, A is cross sectional area of the pipe, and V is velocity.
The units work out this way:
M^3/sec = M^2 * M/Sec
OR
Ft^3/sec = Ft^2 * Ft/Sec
You can almost neglect head pressure as this will be offset by frictional losses in the pipe.
Remember you need to switch your time units from sec to minutes and convert to Gallons to get you gpm rate.
A good article for this can be found here:
http://www.agf.gov.bc.ca/resmgmt/pub...s/590304-5.pdf
Hope this helps.