What is Manning's equation for open channel flow?

Manning's equation calculates velocity in open channels: V = (1/n) * R^(2/3) * S^(1/2), where V = velocity (m/s), n = Manning's roughness coefficient, R = hydraulic radius (area/wetted perimeter), S = channel bed slope. Discharge Q = V * A. Manning's n depends on channel surface: 0.010 for smooth concrete, 0.025 for natural streams, 0.05 for heavily vegetated. This empirical equation is fundamental for channel design and flood analysis.