I am trying to perform sensitivity analysis considering manning’s n and slope for a simplified channel of just one reach. I need to try with both critical depth and normal depth as downstream boundary condition. How can I force the program to consider downstream boundary condition as critical depth?
I take it you are running unsteady flow? (For steady flow, you can choose critical depth.)
Probably the easiest thing to do is go over to steady flow and, for the downstream cross section, compute critical depth for a bunch of different flows. You could then enter this as a rating curve for the unsteady flow model.