Note that the bridge discharge coefficient is defined in Ven Te Chow's, "Open-Channel Hydraulics", on pages 476 to 490. Chow provides figures defining the bridge discharge coefficient for various bridge types. Note that these figures are independent of units--they work for both US and Metric units.
Also note that you need to make certain that you choose a bridge discharge coefficient such that the energy grade line slope through the bridge opening is sufficiently steep to allow the program to compute a flow through the opening. Otherwise the program may have convergence problems.
If you have any additional questions or need any further information, please contact me again.
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com