Dear Carlos :
To model conveyance tubes at a bridge, you need to define flow slices at the bridge and approach cross-sections using the Cross-Section Description dialog box (Input | Cross- Section Description) using the Flow Slices data entry.
If you have any additional questions or need any further information, please contact me again.
Sincerely,
Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com