| Scenario | Diameter | Depth Ratio | Slope (%) | Material | Target (m/s) | Typical Use |
|---|---|---|---|---|---|---|
| 1 | 300 mm | 0.60 | 0.50 | Concrete | 0.60 | Sanitary lateral or collector |
| 2 | 450 mm | 0.50 | 0.30 | Vitrified clay | 0.75 | Combined sewer segment |
| 3 | 24 in | 0.40 | 0.20 | Corrugated metal | 0.90 | Storm drainage reach |
- Velocity from Manning: V = (1/n) · R^(2/3) · S^(1/2)
- Discharge: Q = V · A
- Flow-based velocity: V = Q / A
- Required slope for target velocity: S = ( (V · n) / R^(2/3) )^2
- Circular segment area: A = (r²/2) · (θ − sin θ)
- Wetted perimeter: P = r · θ, Hydraulic radius: R = A / P
- Reynolds number: Re = (V · Dₕ) / ν, where Dₕ = 4R
- Select the unit system that matches your inputs.
- Choose a depth mode: enter a depth value, or use depth ratio for part-full flow.
- Enter diameter, slope, and select a material (or enable custom n).
- Pick a target profile, or choose custom and enter your target velocity.
- Select a computation method: Manning for slope-based prediction, or Flow to test measured discharge.
- Click Calculate to view results above the form, including required slope and flow indicators.
- Use the CSV or PDF download buttons to export the calculation record.
Why self cleansing velocity matters in gravity sewers
Self cleansing velocity is a practical threshold that helps limit sediment deposition in gravity pipelines. When velocities stay too low, grit and organics can settle, reducing capacity and increasing odors. Keeping a reasonable transport velocity supports stable conveyance, easier maintenance planning, and fewer blockages. Regular inspection and cleaning intervals can be optimized when velocities are quantified consistently. Design targets vary with solids content, pipe size, and local practice, so velocity is checked alongside slope and depth.
Hydraulic section properties drive the calculation
Part-full flow requires accurate area and wetted perimeter for the observed depth. This tool models a circular segment to compute flow area, wetted perimeter, hydraulic radius, and hydraulic diameter. Those properties influence Manning velocity and discharge directly. Small depth changes can produce large velocity changes, especially near shallow flow conditions.
Roughness and slope should be selected defensibly
Roughness represents pipe material and condition, not just a catalog value. Choose a realistic value for the project stage, then verify sensitivity by adjusting n within a reasonable range. Slope is entered as percent and converted to a dimensionless energy slope for computation. The required slope output helps compare design intent against site grading constraints.
Using measured flow for field verification
When field data is available, the flow-based method computes velocity using V = Q/A at the observed depth. This supports troubleshooting for segments with recurring sediment issues. Reynolds number is included as a quick indicator of turbulent transport potential, using an assumed viscosity. For non-water flows, adjust viscosity to match the expected fluid.
Example data for quick validation
Use these values to confirm your workflow and compare outputs between methods. Scenario A: diameter 300 mm, depth ratio 0.60, slope 0.50%, concrete, target 0.60 m/s. Scenario B: diameter 450 mm, depth ratio 0.50, slope 0.30%, vitrified clay, target 0.75 m/s. Scenario C: diameter 24 in, depth ratio 0.40, slope 0.20%, corrugated metal, target 0.90 m/s. These scenarios illustrate how roughness and slope interact with depth.
1) What is self cleansing velocity?
It is a target minimum velocity that helps keep solids moving, reducing deposition risks in gravity pipes. The value depends on solids load, pipe size, and local criteria.
2) Which method should I use here?
Use the Manning method for design checks when slope and roughness are known. Use the flow-based method when you have measured discharge and observed depth.
3) Why does depth ratio affect the results so much?
Depth changes the flow area and wetted perimeter, which alters hydraulic radius. Because velocity depends on hydraulic radius, small depth shifts can significantly change predicted velocity.
4) How should I choose Manning roughness?
Select roughness based on material and expected condition, then run a sensitivity check. New smooth pipes typically use lower n values than older or roughened surfaces.
5) What does the required slope output mean?
It is the slope needed to meet the selected target velocity at the given depth and roughness. Compare it with your available grade to judge feasibility.
6) Are the target velocities fixed rules?
No. They are typical screening values used for comparison. Always confirm project acceptance criteria from your client standards, local authority guidance, and operating history.
7) Why is Reynolds number included?
It provides a quick check of flow regime. Higher Reynolds numbers usually indicate turbulence, which improves transport. It does not replace local self cleansing criteria.