Jeff,

I would like to add a bit to Ron's comment. The NEQUAL=1 does work very well to eliminate CFL problems in links but if you use it with a 60 second time step and large natural channels you are probably increasing the volume of your system by a tremendous amount. A general guideline (which is in the SWMM 4 output) is that you should not increase the full volume of your system by more than 5 to 10 percent.

There are other sources of instability in Extran besides the link CFL condition that you might acerbate by using a large time step and the NEQUAL=1 option. The longer natural channels might:

(1) cause instabilities at the nodes as the area of the nodes change during the Extran "HEAD" calculations,

(2) The smaller roughnesses required in the creation of the equivalent conduits may cause instabilities as the HGL moves vertically (Ron has described this as "Area Shock" in past discussions).

(3) the continuity equation for the node may experience problems in the beginning because of the larger node volume.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM