Manning Open Channel Flow Calculator
Estimate uniform flow in a rectangular, triangular or symmetric trapezoidal channel using entered SI Manning roughness.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Calculate flow area, wetted perimeter and hydraulic radius from the entered geometry; apply the SI form of Manning equation at uniform slope.
A=y(b+zy); P=b+2y√(1+z²); R=A/P; Q=A R^(2/3)√S/n
Worked example
A rectangular section 2 m wide and 1 m deep has A=2 m², P=4 m and R=0.5 m. With n=0.02 and S=0.01 its uniform discharge is about 6.29961 m³/s.
Assumptions and input details
Steady uniform free-surface flow, symmetric sides and user-entered roughness. Set side slope to zero for a rectangle. Dry depth gives zero discharge; a section with both zero bottom width and zero side slope is rejected. No backwater, hydraulic jumps, unsteady flow, sediment, freeboard or safe-capacity assessment.
References: US National Weather Service: Manning open-channel model
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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