I work with SI units in HEC-RAS and would like to know if HEC-RAS automatically uses the ‘metric’ version of the Manning’s equation
Q = (1/n)AR^(2/3)S^(1/2)
instead of the ‘English’ version
Q = (1,486/n)AR^(2/3)S^(1/2).
In the Reference manual I’ve just found the one based on English units.
If you have selected SI units for your RAS project, then all of your input data should be in SI units and RAS should use the appropriate SI equations for all the computations including Manning’s. I’ve done plenty of SI models and gotten appropriate results.
It is true that the reference manual typically only gives English unit examples and the metric version of new features is sometimes buggier until they get everything figured out.