Jeff -

Try setting NEQUAL=1 on the B2 line. The program then automatically "lengthens" the channel that the program sees while cutting its Manning's n an equivalent amount (see Eq. 2-6 in the Extran manual and accompanying discussion) to comply with the Courant requirement.

Actually, 60 seconds is somewhat on the long side for EXTRAN, in my experience; I usually start out with 5 seconds and NEQUAL=1, and have sometimes had to go to 1 sec. That usually eliminates the Courant problem.

Good luck

Ron Kilmartin
ronkilmartin@ATTBI.COM