How to use the open channel flow calculator
Prepare the values for this calculation
Calculate uniform open-channel flow with the SI Manning equation. Use the formula below to check that it answers your question. Read the input units before transferring figures from a statement, measurement or dataset.
Enter and check each input
- Flow area A (m²): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Hydraulic radius Rh (m): enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Energy slope S: enter a value at least 0. Use the unit named in the label or selected above. Do not leave this required field blank.
- Manning roughness n: enter a value at least 1e-12. Use the unit named in the label or selected above. Do not leave this required field blank.
The input table gives a complete example. Keep units and time periods consistent across related fields.
Calculate and resolve any input error
Choose Calculate after entering the values. Changes also update the answer after a short pause. If an error appears, follow its message, correct the field and calculate again. Reset restores the example values.
Interpret the answer
Check Open-channel flow first. With the unchanged example inputs, it is 4.199736832983 m³/s. Interpret it using the formula and assumptions on this page.
Compare and keep a useful result
For a controlled comparison, keep every other example input unchanged and change Flow area A (m²) from 2 to 2.2. The resulting Open-channel flow is 4.619710516281 m³/s. This illustrates the equation’s response to that input, rather than a forecast of real events.
Read the chart’s axis labels and data table. A scenario curve changes one stated input while holding the others fixed; it does not add external observations. Use Download graph to save an available chart as a PNG.
Use Copy result or Download CSV to keep the answer. Share tool shares the page address without placing your input values in the URL. Recheck inputs and assumptions before using the result for a decision.
Open Channel Flow Calculator formula
Q=(1/n) A Rh^(2/3) S^(1/2).
Inputs and units
| Input | Format / range | Example |
|---|---|---|
| Flow area A (m²) | minimum 0 | 2 |
| Hydraulic radius Rh (m) | minimum 0 | 0.5 |
| Energy slope S | minimum 0 | 0.01 |
| Manning roughness n | minimum 1e-12 | 0.03 |
Worked example
Using the example inputs above, the open-channel flow is 4.199736832983 m³/s.
- Open-channel flow
- 4.199736832983 m³/s
Results and assumptions
SI Manning equation for uniform open-channel flow.
Fix an input or unexpected result
Flow area A (m²) is required. Enter flow area a (m²) using the format and bounds in the input table. Zero and an empty field are different: use zero only if it is allowed and represents your data.
If your answer differs from the example, choose Reset and compare every field, including selected units and options. Check percentage versus decimal entry and whether values describe the same period. Round only the final displayed answer when reproducing the formula.
If a discrepancy remains, report the calculator issue with the page address, non-sensitive example inputs, actual result, expected result and a reference for your calculation.