How to Use Open Channel Flow Calculator
The Open Channel Flow Calculator applies Manning’s equation to compute discharge, velocity, hydraulic radius and cross-section area for rectangular, trapezoidal and circular channel shapes in a single step.
- Select cross-section shape — rectangular (vertical walls, flat bottom), trapezoidal (sloped walls with flat base) or circular pipe flowing partially full.
- Enter geometry — for rectangular: width b and flow depth y; for trapezoidal: base b, depth y and side slope z (horizontal:vertical); for circular: diameter D and depth ratio y/D.
- Enter longitudinal slope S — as a dimensionless fraction (e.g. 0.001 for a 1 m drop per 1 km), not as a percentage.
- Select or enter Manning’s n — choose from preset materials (smooth concrete, rough concrete, earth, grass, gravel, brick) or type a custom value.
- Read velocity V, discharge Q, hydraulic radius R and wetted perimeter P in the result panel, with Q shown in both m³/s and L/s.
Formula & Theory — Open Channel Flow Calculator
The Open Channel Flow Calculator is based on Manning’s equation, the most widely used empirical relationship in open-channel hydraulics:
V = (1/n) · R^(2/3) · S^(1/2)
Q = V · A
R = A / P| Symbol | Meaning | SI Unit |
|---|---|---|
| V | Mean cross-section velocity | m/s |
| n | Manning roughness coefficient | s/m^(1/3) |
| R | Hydraulic radius A/P | m |
| S | Longitudinal channel slope | m/m |
| A | Flow cross-section area | m² |
| P | Wetted perimeter | m |
| Q | Discharge | m³/s |
Manning’s n reference values: smooth formed concrete 0.010–0.013; rough concrete 0.014–0.017; clean earthen channel 0.018–0.025; grass-lined swale 0.025–0.033; gravel bed 0.025–0.035; natural streams 0.025–0.060. The constant 1.0 is for SI units; for US customary (ft, ft³/s) multiply by 1.486.
Use Cases for Open Channel Flow Calculator
- Storm drain and culvert sizing — verify that a concrete-lined section conveys the design flow without surcharging, checking velocity against non-erosive limits.
- Irrigation canal design — select a trapezoidal cross-section that delivers the target discharge at a velocity high enough to prevent silting but below the erosion threshold.
- River and stream discharge estimation — estimate natural channel flow from stage readings and an estimated Manning’s n for ecological surveys and flood studies.
- Sanitary sewer hydraulic checks — confirm that pipes flowing partially full still maintain minimum self-cleansing velocity (≥0.6 m/s at design flow).
- Stormwater management — size open swales, bioretention channels and roadside ditches to meet peak flow reduction targets in urban drainage master plans.
- Educational hydraulics — explore how Q responds to slope, roughness and cross-section shape, and compare the efficiency of rectangular vs trapezoidal sections.