🏞️ Open Channel Flow Calculator
Calculate the flow rate and velocity of water moving through an open channel, culvert, or ditch using Manning’s equation.
What this calculates
Open channel flow calculations describe how water moves through channels, rivers, ditches, and culverts that have a free (atmosphere-exposed) surface, as opposed to a fully enclosed, pressurized pipe. Manning’s equation is the most widely used empirical formula for estimating flow rate in these conditions.
The formula
Manning’s equation (in SI units) is Q = (1/n) × A × R^(2/3) × S^(1/2), where n is the Manning roughness coefficient (depends on the channel material – concrete, earth, grass, etc.), A is the cross-sectional area of flow, R is the hydraulic radius (A divided by wetted perimeter), and S is the channel’s longitudinal slope. Average velocity is then V = Q / A.
Typical Manning’s n values range from about 0.011 for smooth concrete to 0.035 or more for natural channels with vegetation. This calculator is intended for general civil engineering and educational reference.