I need to calculate the volume in different sections of a river for several flow values.
HEC-RAS has no problem for low values, but concerning high values (more than the 100-year-flood) there is an error : " Flow optimization failed to converge ".
You can always try a different initial flow distribution. See what RAS is heading towards while going through the optimization. If it looks like the optimization runs are converging, but just run out of iteration attempts before it could get there, then use the flow distribution of the last iteration attempt, and then rerun RAS. If the optimization runs are diverging, or going unstable (i.e. solution errors are oscillating out of control, rather than minimizing for each iteration), then you may just have a geometry that is not conducive to split flow optimization. It could be that one of the channels is so much smaller than the other, or the ground points on one of the splits are so high, that the channel wants to go dry. In that case, perhaps a pilot channel would help.