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Stormwater pump-out calculator

Enter the catchment, its runoff coefficient, the 10% AEP two-hour intensity and a pump size. The calculator gives the wet well storage the Standard requires for that pump.

NCC 2025 calls up AS/NZS 3500.3 for stormwater drainage (Volume One F1D3 for Class 2 to 9 buildings, Volume Two H2D2(1)(a) for houses and Class 10 buildings), and this calculator sizes a pump-out by its Section 8 and Appendix K.

How it works

It reads from

  • AS/NZS 3500.3:2025 Section 8 (Clauses 8.1 to 8.4) and Appendix K

Combined effective storage

What the pumps move in 30 minutes plus the wet well storage must hold the runoff of a 10% AEP, 120-minute storm (Clause 8.3.6). A bigger pump means a smaller well.

Minimum storage

The wet well between its working levels holds at least 1% of the catchment area, in cubic metres, and never less than 3 m³.

The rest of Section 8

Pumps in duplicate, alternating at each start, each no larger than the receiving system can take (Clause 8.4(a)); a visible alarm if one fails and an audible alarm if both fail; high- and low-level alarms, the high level not higher than 100 mm above the inlet invert; the inlet 100 mm above the design top water level; a gate valve and a non-return valve on each delivery, which the Standard advises be reachable without entering the well.

Appendix K's levels

Figure K.1 (informative) sets the overflow 100 mm above the high-level alarm, the low-level alarm 100 mm below pump stop, and five minutes of one pump's output between pump start and pump stop.

Inflow, band and rising main

Figure K.1 draws five minutes of one pump between pump start and stop inside the wet well storage; the calculator flags a band the storage cannot hold (at 40 L/s the Appendix K example's 10 m³ well would need 12 m³). The pump rate is set from the expected inflow (Clause 8.4(f)): the calculator gives the storm's average inflow and the time one pump takes to empty the well, and compares a peak inflow you enter. AS/NZS 3500.3 gives no head-loss method for the rising main; it is tested at twice the pump's shut-off head for 10 minutes (Clause 9.3.3).

A worked example

The calculator's example inputs, worked through step by step.

Wet well storage

10.0 m³

With 40.0 L/s pumps the wet well must store 10.0 m³ between its working levels; together with 30 minutes of pumping that holds the 79.9 m³ a 10% AEP, 2-hour storm sends from 1 000.0 m². To keep Figure K.1's five-minute start–stop band inside it, make it 12.0 m³.

  1. Run-off in a 10% AEP, 2-hour stormV = C × I × A × 2 h / 1000 = 0.90 × 44.4 × 1 000 × 2 / 1000 = 79.9 m³AS/NZS 3500.3:2025 Clause 8.3.6; Appendix K
  2. Pumped in 30 minutes by one pumpVp = Q × 1800 / 1000 = 40.0 × 1800 / 1000 = 72.0 m³AS/NZS 3500.3:2025 Clause 8.3.6
  3. Minimum wet well storagemax(1% of A in m², 3 m³) = max(10.0, 3) = 10.0 m³AS/NZS 3500.3:2025 Clause 8.3.6
  4. Wet well storage, high to low working levelmax(V − Vp, minimum) = max(79.92 − 72.0, 10.0) = 10.0 m³AS/NZS 3500.3:2025 Clause 8.3.6
  5. Pump start to pump stopPC5 = Q × 300 / 1000 = 40.0 × 300 / 1000 = 12.00 m³AS/NZS 3500.3:2025 Figure K.1 (informative)
  6. Average inflow over the design stormV / 7200 s = 79.92 m³ over 120 min = 11.1 L/sAS/NZS 3500.3:2025 Clause 8.4(f)
  7. Drain-down: one pump empties the working storaget = storage / Q = 10.0 m³ at 40.0 L/s, no further inflow = 4 min

Questions

How big does a stormwater pump-out pit need to be?

Big enough that its storage plus 30 minutes of pumping holds a 10% AEP, 2-hour storm, and never smaller than 1% of the catchment in m³ or 3 m³. For 1 000 m² in Brisbane the Standard's example gives 80 m³ of runoff: 40 L/s pumps need a 10 m³ well.

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The method behind every calculator, in one guide:roof and stormwater drainage to AS/NZS 3500.3:2025.

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