Flow Velocity
Minimum, maximum and where velocity is actually evaluated.
Velocity is the practical measure of whether a sewer works: too low and solids settle, too high and the pipe, benching and structures are abraded. It is derived from discharge and flow area, and varies with flow depth along the run.
A single velocity figure rarely describes the design condition. Check the low-flow case for self-cleansing and the peak case for scour, and be explicit about which condition each number represents.
Key design considerations
- Check at the flow the authority nominates for self-cleansing, not only at peak flow.
- Maximum velocity is limited by pipe material and by abrasion at bends, junctions and drops.
- Local velocities in maintenance holes are higher than in the pipe and drive turbulence and odour release.
- Flat reaches controlled by downstream tailwater can run much slower than the grade suggests.
- Report velocity at the critical section, and state the flow condition with it.
Equations
V = Q / A
Mean velocity from discharge and flow area.
V = (1/n) · R^(2/3) · S^(1/2)
Normal-depth velocity form of Manning's equation.
Design checks
- Minimum velocity at the self-cleansing check flow.
- Maximum velocity at peak design flow.
- No adverse hydraulic jump or excessive turbulence at junctions.
Standards and authority references
- WSA 02 Gravity Sewerage Code of Australia (current edition), Water Services Association of Australia
- Authority velocity criteria
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Velocity criteria are authority specific.
- Mean velocity does not describe local conditions at bends, drops or junctions.
Related calculators
This reference is general engineering information. Confirm requirements against WSA 02, the current Australian Standards and the local water authority's supplement before use in design, and obtain project-specific advice for the conditions at hand.