This may sound a bit basic, I wonder if a constructed drainage should be designed under subcritical or supercritical condition when carrying the design flood. It is tropical region with heavy run off/flood events.
Sara, that depends on a lot of factors in addition to those you mentioned. If you mean a drainage channel, then it depends a lot on what slopes you are working with. It also depends on your n-value, and your upstream and downstream conditions. Also important are erosive velocity considerations. It is difficult to give you a very meaningful answer without knowing more about those.