Open Channel Flow Calculator

Inputs

This calculator uses Manning’s equation for uniform open-channel flow:

Q = (k / n) × A × R^2/3 × S^1/2

• Q = discharge (flow rate)
• A = flow area
• R = hydraulic radius = A / P
• P = wetted perimeter
• S = energy slope (use channel slope for steady uniform flow)
• n = Manning roughness coefficient
• k = 1.0 (SI) or 1.486 (US customary)

1. Select the unit system used on your drawings or survey.
2. Choose the channel shape that matches your section.
3. Enter roughness n based on lining and vegetation.
4. Set slope using decimal, percent, or rise-to-run format.
5. Provide the required geometry dimensions, then calculate.
6. Download CSV or PDF to attach to design notes.

1) Construction uses of open-channel sizing

Open channels support temporary bypasses, dewatering conveyance, perimeter swales, and permanent roadside drainage. A quick capacity check helps confirm that the selected section can pass the intended flow without overtopping during critical site activities.

2) Manning framework and reporting expectations

The calculator applies Manning’s equation for steady, uniform, gravity-driven flow. Document the assumed reach length, the controlling grade, and whether the lining is new, aged, or vegetated, because resistance assumptions often drive review comments.

3) Roughness n: typical values and project notes

Common planning values include finished concrete 0.012–0.016, corrugated metal 0.022–0.027, earth ditches 0.022–0.035, and grass-lined channels 0.035–0.060. When in doubt, select the higher n and describe surface condition, debris potential, and the maintenance plan. For temporary measures, add a safety margin because traffic can roughen surfaces.

4) Slope inputs: decimal, percent, and rise-to-run

Field grades may vary along a ditch. Use the controlling reach and enter slope as decimal S, percent grade, or rise-to-run matching your plan set. Because discharge varies with the square root of S, small grade changes can noticeably shift capacity.

5) Geometry and hydraulic radius

Area A and wetted perimeter P depend on the selected shape and dimensions. Hydraulic radius R=A/P summarizes section efficiency; higher R generally increases capacity. Deepening a section increases A and often R, but check stability, access, and right-of-way limits.

6) Selecting a section type for constructability

Rectangular sections suit formed concrete and flumes. Trapezoids are common for excavated ditches due to side-slope stability and easier compaction. Triangular sections approximate shallow swales. Partly full circular geometry illustrates culvert barrels operating as open flow.

7) Velocity, erosion, and lining decisions

Beyond discharge Q, the tool reports velocity V=Q/A. Compare V against allowable limits for soil, turf, riprap, or concrete. If velocity is high, reduce slope where feasible, increase section size, add check structures, or specify a more robust lining. Check outlets because dissipation is often required.

8) QA steps and export-ready submittals

Run sensitivity checks by varying n and slope within plausible bounds and confirm the section still meets demand. Export CSV for calc logs and PDF for submittals. Include units, shape, key dimensions, n source, and the referenced plan/profile stationing for traceability. For peer review, note whether the design flow is peak, average, or pumped.

1) What does Manning’s n represent?

n represents resistance from surface texture, vegetation, and irregularity. A higher n increases energy loss and reduces discharge for the same slope and geometry. Select n from local guidance or proven project experience.

2) Can I use percent grade from drawings?

Yes. Choose the percent option and enter the grade value. The calculator converts percent to decimal slope internally. Record the reach used so reviewers know what controls the calculation.

3) Should energy slope equal bed slope?

For steady uniform flow, energy slope is typically taken as bed slope. If backwater, controls, or rapidly varied flow exist, Manning results become approximate. Use a detailed hydraulic model when controls dominate.

4) Why report both Q and V?

Q shows capacity, while V relates to erosion, lining suitability, and safety. A section may pass the flow but still require protection if velocity is high. Report both to support constructability and durability decisions.

5) How is the circular partially full case handled?

The tool uses standard circular-segment geometry for area and wetted perimeter at depth y. It suits quick open-flow checks. If the barrel runs full or pressurized, switch to pipe-flow methods.

6) What if my channel has different linings?

If roughness varies, compute using a conservative n for the controlling lining, or split the section into subsections and combine conveyance using accepted methods. Document your approach so the review can be reproduced.

7) What should I include in a submittal?

Include units, shape, dimensions, n, slope, A, R, V, and Q, plus a short lining description and drawing references. Attach the PDF export to keep the calculation traceable.