Pipe Sizing and Capacity Criteria
Selecting a diameter against design flow, depth and grade.
Pipe size is selected by checking the design flow against capacity at the adopted grade, then confirming that the resulting depth of flow and velocity are acceptable. Capacity is not simply the full-bore discharge, because sewers are designed to run part-full.
Sizing is iterative with the alignment: diameter, grade, cover and maintenance hole positions are fixed together. A diameter that satisfies one reach may be constrained by the reach downstream.
Key design considerations
- Design flow basis, peaking factors and allowances from the authority.
- Permitted depth of flow at design flow, which sets the usable capacity.
- Downstream constraints: tailwater, existing invert levels, and capacity of the receiving system.
- Standard diameters and local availability, since a non-standard size affects cost and program.
- Minimum diameter requirements for maintenance and property connection practice.
- Allowance for future catchment development over the design horizon.
Equations
Q = (1/n) A R^(2/3) S^(1/2)
Full-bore capacity of a circular pipe, with A = πD²/4 and R = D/4.
d/D from Q/Q_full
Depth ratio from the circular segment geometry for the part-full condition.
Design checks
- Design flow within capacity at the permitted depth of flow.
- Velocity within the minimum and maximum limits.
- Grade and cover satisfied along the same reach.
Standards and authority references
- WSA 02 Gravity Sewerage Code of Australia (current edition), Water Services Association of Australia
- Authority supplement, capacity and flow criteria
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Capacity criteria and design flow bases are authority specific.
- Manning-based capacity assumes uniform flow; backwater and surcharge need a network model.
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.