Mike-
Conservatively high n values are used when studying flood stages and flood inundation. Conservatively low n values are used when studying velocities and shear stresses. There are obviously many other types of studies, but if you ask the question, “if I increase the Manning’s n value, will it make the thing I’m concerned with better or worse?”, that will usually give you the answer. Sometimes it’s not that clear cut though. For example, increasing n values in the upstream reach may provide higher flood depths, but will also attenuate the flood wave faster. This may result in lower discharges and consequently lower stages further downstream (especially if you have a long reach with a lot of available storage. It’s always good to run three cases in a sensitivity analysis-conservatively high, conservatively low, and best guess n values. Then evaluate each solution. You may eventually combine results of the different sensitivity runs for design conditions.
Chris G.
@RASModel