Calculation Methodologies
The standard sequence for sizing and checking a gravity sewer reach.
Gravity sewer calculations follow a consistent sequence: establish the design flow, fix the alignment and grade, size the pipe, then check depth of flow, velocity, capacity and levels. Each step depends on the one before it, so a change of grade or diameter means repeating the checks.
For a single reach the usual method is a normal-depth solution. For a network, the same checks are applied reach by reach with a backwater or HGL step at each structure, which is what a network model automates.
Key design considerations
- Fix the design flow and its factors before sizing; they are the most influential inputs.
- Work from downstream back to upstream, or from a known control, so tailwater is respected.
- Keep a calculation sheet per reach showing Q, D, S, n, y/D, V and the criteria applied.
- Round displayed values but carry full precision through the calculation.
- Check the controlling reach (flattest, longest, most loaded) first, since it usually governs.
- Have one reach independently hand-checked as a quality check.
Equations
Q = (1/n) A R^(2/3) S^(1/2)
Capacity at the adopted grade and roughness.
iterate y until Q(y) = Q_design
Normal depth for a known discharge; use a depth-discharge table or repeat the Manning calculation.
HGL₂ = HGL₁ − h_f − h_local
Carry levels through the network once discharges and depths are fixed.
Design checks
- Design flow and factors traceable to the authority's requirements.
- Capacity, depth and velocity checked at the design flow and low flow.
- HGL and levels checked against connection and surface levels.
Standards and authority references
- WSA 02 (current edition)
- Authority design criteria
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Normal-depth methods assume uniform flow and do not model backwater, transients or sediment.
- A network model is required for anything beyond simple reach-by-reach checks.
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.