If the relative variation of the water depth is considerable you should specify Manning numbers. Values in the range 20-40 m1/3/s are normally used with a suggested value of 32 m1/3/s if no other information is available. In an estuary with tidal channels you can use a value of 25 m1/3/s.
If you apply Chezy numbers a model calibration can normally be achieved with values in the range 30 - 50 m1/3/s.
Because of the definition of the resistance numbers the following applies:
· Using a smaller resistance number increases the bed resistance
· Using a greater resistance number decreases the bed resistance