Open Channel Flow Calculator

Open channel flow has a free surface exposed to atmospheric pressure, as found in rivers, canals, roadside ditches, culverts that are not full, and wastewater channels. Gravity supplies the driving force. Manning's equation is one of the most widely used empirical relationships for estimating steady uniform flow. In SI units, mean velocity equals one divided by the Manning roughness coefficient, multiplied by hydraulic radius raised to the two-thirds power and by the square root of energy slope. Discharge is mean velocity multiplied by flow area. This calculator models a rectangular channel. Its cross-sectional flow area is bottom width times water depth. Its wetted perimeter is the bottom width plus twice the depth, because those are the boundaries in contact with water. Hydraulic radius is area divided by wetted perimeter; it is not simply half the depth or an actual geometric radius. Channel slope is entered as a dimensionless rise-over-run ratio, so a one-percent fall is 0.01. Under uniform-flow assumptions, the energy grade-line slope is approximated by the channel-bed slope. The Manning coefficient represents resistance caused by surface material, vegetation, irregularity, alignment, obstructions, and other conditions. Smooth finished concrete can have a relatively low coefficient, while natural streams with stones and brush have larger values. Because velocity is inversely proportional to this coefficient, selecting an unrealistically small value can substantially overpredict capacity. Published reference ranges help with preliminary work, but field calibration and engineering judgment are important. Seasonal vegetation, sediment, damage, and changing depth can alter effective roughness. Manning's formula assumes steady, uniform, gravity-driven flow and a sufficiently long prismatic reach. It does not capture rapidly varied flow near gates, weirs, drops, bends, transitions, or hydraulic jumps. It also does not test whether the assumed depth is normal depth or whether flow is subcritical or supercritical. Use the calculator for transparent estimates, classroom exercises, drainage screening, and comparisons between rectangular designs. Final flood-control, roadway, and public-safety designs should include surveyed geometry, appropriate design storms and boundary conditions, freeboard, sediment considerations, and review under applicable hydraulic standards.