3.1 Stormwater Sizing & Flow Calculations to AS/NZS 3500.3 & E1/AS1

Stormwater sizing and flow calculations

Stormwater design begins with the performance requirement and the approved site information. Clause E1 requires surface water to be disposed of without creating nuisance or damage, but a pipe diameter cannot be selected from that sentence alone. The calculation needs a design event, catchment, runoff response, approved outfall, hydraulic grade, and secondary-flow strategy.

Rainfall and catchment

Use the rainfall source required by the territorial authority or consent. Record the annual exceedance probability, storm duration, location, and any prescribed climate-change allowance. Do not copy a generic city rainfall table into another district. Duration matters because the critical intensity depends on the time of concentration and storage response.

Divide the site into defensible catchments. Roof, paving, compacted yard, and landscaped ground can have different runoff coefficients. In square-metre units, the Rational Method may be written as Q = C I A / 3600, where Q is litres per second, C is dimensionless, I is millimetres per hour, and A is square metres. Check units every time.

For a 420 square metre roof, C = 1.0, and an approved intensity of 120 mm/h, Q = 1.0 × 120 × 420 / 3600 = 14 L/s. That is the catchment peak used for the next design step; it is not proof that a particular pipe will carry it.

E1/AS1 drain gradients

E1/AS1 gives these minimum gradients for drains: 85 mm at 1:90, 100 mm at 1:120, 150 mm at 1:200, and 225 mm at 1:350. The draft value 1:90 for a 100 mm drain confuses the 85 mm row with the 100 mm row. State nominal size and grade together.

After selecting a tentative size and grade, determine capacity using the method and roughness assumptions accepted for the project. Check inlet control, outlet tailwater, junction and bend losses, partial flow, and surcharge where relevant. A table of free-discharge barrel capacities cannot by itself prove a complete network.

Primary and secondary systems

A primary network manages the selected frequent event. A secondary route manages exceedance, blockage, or surcharge. Map where water will go if every inlet is blocked. Preserve overland routes and check finished-floor protection under the site’s approved flood information. E1/AS1 uses 150 mm separation above adjacent ground, road crown, or the lowest site boundary for its applicable housing conditions; it does not create a universal 225 mm unpaved-ground rule or a universal 500 mm flood freeboard.

Inlets and outfalls

E1/AS1 requires a surface-water inlet grating with at least 35 percent open area and a maximum smaller opening dimension of 35 mm. Provide suitable space below the outlet for sediment, but the Acceptable Solution does not prescribe one universal 300 mm-deep sump for every inlet. Size the grate for approach flow and blockage appropriate to the site.

Confirm the outfall is lawful and hydraulically available. A public connection needs network approval; soakage requires field investigation and territorial-authority acceptance; discharge to land or water may involve regional rules. Never assume an invert shown on GIS is construction-grade survey data.

Calculation record

A reviewable record includes the rainfall source and date, AEP and duration, climate factor, catchment plan, coefficients, calculated flows, inlet schedule, pipe sizes and grades, loss assumptions, outfall condition, secondary route, and as-built verification. If the final installed grade differs from the design, recalculate; do not merely relabel the drawing.

Independent source-check exercise

For 3.1 Stormwater Sizing & Flow Calculations to AS/NZS 3500.3 & E1/AS1, practise answering from evidence rather than recall. Write the controlling source, its edition or effective date, its scope, and the exact paragraph, table, figure, or manufacturer instruction that supports the decision. Then state the site inputs that can change the result. In this section those inputs include E1/AS1 minimum gradients are 1:90 for 85 mm, 1:120 for 100 mm, 1:200 for 150 mm, and 1:350 for 225 mm drains.; Use the rainfall intensity, duration, AEP, climate allowance, catchment, and runoff coefficient required by the consent and territorial authority.; The Rational Method in square-metre units is Q = C I A / 3600, producing litres per second when I is millimetres per hour..

Build a four-column check: proposed condition, source requirement, evidence observed, and action. Test the answer with one changed assumption. A different pipe size, discharge-unit load, ground condition, licence status, authority, compliance path, or consent condition may produce a different result. If it does, explain why; if it does not, show which requirement remains controlling.

Finish with a field-verification plan. Name what must be inspected before concealment, what instrument or record demonstrates it, who has authority to accept a change, and what appears on the as-built or completion file. This exercise turns an open-book fact into certifying-level judgement. It also exposes accidental mixing of standards: if the source for a dimension differs from the source for the test or approval, either return to one complete compliance path or document the proposal as an Alternative Solution. Do not promote an example value into a national rule.