Self-Cleansing Requirements
Velocity and shear stress criteria, and which flow condition to check.
A sewer must transport the solids that enter it. Self-cleansing is achieved when flow at an appropriate frequency produces enough velocity or boundary shear stress to move deposited material downstream.
Australian practice uses two measures: a minimum velocity criterion and a boundary shear stress criterion. The applicable values depend on the flow condition being checked, so confirm which condition the authority requires before choosing a criterion.
Key design considerations
- Identify the check condition: daily dry weather flow, peak dry weather flow, or a nominated flushing regime.
- Shear stress depends on hydraulic radius, so diameter affects the result independently of velocity.
- Grease, slime and grit behave differently; a velocity criterion developed for one does not necessarily serve the other.
- Balance against the maximum velocity limit for scour and abrasion.
- Low velocities also drive odour and hydrogen sulfide generation in the head of the system.
- Where a flatter grade is accepted, a maintenance regime is normally part of the agreement.
Equations
V = Q / A
Mean velocity at the flow condition being checked.
τ = ρ g R S
Mean boundary shear stress, with ρ the density of water, R the hydraulic radius and S the friction slope.
Design checks
- Velocity at the check flow compared with the adopted criterion.
- Boundary shear stress compared with the adopted criterion.
- Result recorded at the critical section, usually the flattest low-flow reach.
Standards and authority references
- WSA 02 Gravity Sewerage Code of Australia (current edition), Water Services Association of Australia
- Authority self-cleansing criteria
- Authority sewer maintenance practice
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Criterion values are not reproduced here.
- Self-cleansing checks are indicators; real performance depends on solids, grit and maintenance.
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.