I always get a little confused on what flow regime to run for models. (Subcrit. Supercrit, mixed) I know that for most river systems the flow is typically in the subcritical regime. However, I find myself performing more and more small system analysis for the local DOT at bridge crossing and bridge culverts. I currently have a small model that appears to show that the existing ditch system (my reach) is far too small for any of the storm flows generated by the state regression equations. I have ran the model in all three regimes and there is a significant difference in the sub and super regime WS elevations. My question is how do you best decide which is correct? I do not have any historical data to calibrate the model to.
If you're not sure, typically mixed flow will be the best. It properly combines both sub and supercritical profiles that are run and uses the momentum equation through hydraulic jumps. If you are running either super or subcritical only, there will be some errors around flow regime transition areas, or (obviously) anywhere there is not a valid solution for the selected regime.