Use consistent units (ft/ft, m/m, etc.) so the discharge matches your inputs.

Open Channel Flow (Manning's n)

Manning's Equation

Q = (1/n) · A · R2/3 · S1/2

Keep the units consistent so the computed discharge matches the units of A and velocity.

Typical Manning's n Values

Surface n
Finished concrete 0.012 – 0.015
Earth, clean & straight 0.016 – 0.025
Earth, winding or weedy 0.030 – 0.050
Natural stream with stones 0.035 – 0.070
Natural stream, heavy brush 0.070 – 0.150