Thanks for all the replies. Problem was an error in wetted perimeter. Just for info, the pipe is for a single family residence. Actually, it is for part of a yard drainage system. This is for the upper reaches of the system fed by a single 12" x 12" catch basin with a 3"...
In trying to determine maximum gravity flow rates for water in 4" PVC pipe, I'm coming up with some really screwy looking numbers. For years I've use tables from the University of Iowa that are based on flow at 70% of pipe volume. Unfortunately I have misplaced them in an office move...
