Flow (*Q*) in an open channel, such as a swale, may be calculated using Manning's equation\[Q=VA=\frac{R^{\frac{2}{3}}S^{\frac{1}{2}}}{n}\]
Where\[R=\frac{A}{P}\]

Where:

*Q*= the flow in the swale (m^{3}/s)*A*= the cross sectional area of the swale (m^{2})*P*= the wetted perimeter of the swale (m)*S*= the longitudinal slope (m/m)*n*= Manning's coefficient (dimensionless)