Design Flow Estimation
Dry weather flow, peaking factors and infiltration allowances.
The design flow is what the sewer must carry. For a gravity sewer it is normally the dry weather flow of the contributing population and businesses, multiplied by a peaking factor, plus allowances for groundwater infiltration and stormwater inflow.
The basis is set by the authority. Every number used should be traceable to a planning assumption or an authority requirement, because the whole hydraulic design inherits it.
Key design considerations
- Per-capita contribution, plus commercial, industrial and institutional loads.
- Peaking factor varies with catchment size, and is higher for small upstream catchments.
- Allowance for future development, rezoning and infill over the design horizon.
- Groundwater infiltration allowance, related to pipe condition and water table.
- Inflow allowance, and the authority's position on illegal stormwater connections.
- Trade waste contributions with different characteristics from domestic flow.
Equations
ADWF = P · q / 86.4
Average dry weather flow in L/s, with P the contributing population and q the contribution in litres per person per day.
PDWF = F · ADWF
Peak dry weather flow, with F the adopted peaking factor.
Q_design = PDWF + infiltration + inflow allowance
Combination defined by the authority; some authorities prescribe a single combined factor instead.
Design checks
- Contributing population and land use reconciled with planning data.
- Factors taken from the authority's current requirements, not from memory.
- Sensitivity check at a higher infiltration or inflow allowance.
Standards and authority references
- WSA 02 Gravity Sewerage Code of Australia (current edition), Water Services Association of Australia
- Authority planning and design flow requirements
Confirm the current edition, amendment and any local supplement before applying any of the above.
Limitations and assumptions
- Peaking factors and allowances are authority specific and are not reproduced here.
- Estimates are only as good as the planning data behind them.
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.