Hydraulic Design Assumptions
The assumptions to record, and how to show sensitivity.
Every hydraulic result rests on assumptions. Writing them down makes the calculation reviewable and shows a reviewer where the answer is sensitive.
The usual set covers the design flow basis and factors, roughness, permitted depth of flow, minimum grades, local losses and downstream control levels. Values that are not sourced should be flagged as provisional.
Key design considerations
- Record each value with its source: standard, authority requirement, or engineering judgement.
- Identify the controlling reach, usually the flattest or most heavily loaded.
- Test sensitivity to roughness and infiltration, since these move the answer most.
- Distinguish design flows from gauged or observed data.
- State what is excluded: major storm inflow, trade waste, future stages.
- State the design horizon and the development scenario assumed.
Design checks
- Assumption register attached to the design report.
- Each criterion referenced to a standard or authority document.
- One reach checked independently by hand.
Standards and authority references
- WSA 02 Gravity Sewerage Code of Australia (current edition), Water Services Association of Australia
- Authority design requirements
- Project planning and land use data
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- An assumption register does not replace verification against current authority requirements.
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.