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Are you performing unsteady computations?
"Warning: The velocity head has changed by more than 0.5 ft (0.15 m). This may indicate the need for additional cross sections.
Warning: The energy loss was greater than 1.0 ft (0.3 m). between the current and previous cross section. This may indicate the need for additional cross sections."
This is a steady flow computation warning. After performing the unsteady computations, RAS will perform a post process [steady flow] analysis on select profiles for more detailed output. So if this is what is going on: if the unsteady flow run goes unstable, the post process [steady] results are meaningless and the steady flow errors should be ignored.
If the unsteady flow model is unstable, you might try a shorter time step and examine the hydrograph output to try and determine why/where the unsteady is going unstable.
2km intervals may or may not be sufficient. If the cross sections are large and/or the cross sections do not change radically from one to the next, 2km may be more than sufficient (sounds like it probably is). If there are large changes in area, than 2km may be too far apart regardless of the ground slope.
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