Method: Manning’s equation in SI units is: Q = (1/n) × A × R^2/3 × S^1/2 where Q is discharge in m³/s, n is Manning’s roughness coefficient, A is flow area, R is hydraulic radius and S is energy slope. Hydraulic radius is: R = A ÷ P where P is the wetted perimeter. For a rectangular channel: A = b × y P = b + 2y For a trapezoidal channel with equal side slopes zH:1V: A = y × (b + z × y) P = b + 2y × √(1 + z²) Manning’s equation assumes approximately steady, uniform flow. In a suitable uniform channel reach, the channel or water-surface slope can approximate the energy slope. Roughness selection can significantly affect the result, so actual channel material, vegetation and condition should be considered.