Lengthening time step

[tmcintosh10]

I understand the lengthening time step feature when using hydrodynamic link routing to increase stability and that the reporting and routing time period also may need to be adjusted.  Are there some rules of thumb or methods that allow for you to set/change these values appropriately as opposed to trial and error? 

For example, I had a model that with the lengthening time step set to zero that had some instability in both pipe velocities and flows.  I initially set the lengthening time step to 30 seconds and my routing and reporting to 30 seconds.  The output report shows that all pipes are stable.  In review of the model, I now have a pipe that was previously not surcharged showing a surge in flow basically at time zero.  There are also some pipes that appear to have velocities that exceed what Mannings would produce.  So out of curiosity I increase the lengthening time step to 60 seconds and the output report obviously shows that all links are stable, but that one pipe is still surcharged.  So I start decreasing the routing and reporting time step.  When I set these to 10 seconds, I get stable links and more normal/expected values pipe velocities and flows and the surcharged pipe goes away.  I ultimately decrease the lenthening time step to 15 seconds and do not have values that appear too high or too low.  Just because I get answers that are more in-line with what I expected, I don't have a ton of confidence in what I did or how I arrived at these values to stabilize my model. 

Thanks,

Toby