Flow roughness factor

[Nobody.1990]

Hello everybody.
I have a doubt about how flow roughness factor works.

I was running a model with Manning n=0.05 in every cross section. It worked fine (let’s call it TEST1).
Then (TEST2) I set n=0.025 in very cross section and a flow roughness factor of 2 (for every discharge value). The model went unstable.

Thus my question is. Does’t flow roughnes factor simply multiply Basic Manning Value? Shouldn’t in mu case TEST1 and TEST2 be the same since I set for each Q value a flow roughnes factor of 2 (0.025*2=0.05.

Thanks

[cameron]

from my understanding, that is how the roughness factor is supposed to work.

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