Skip to content

On-site detention calculator

Enter the site, the council's permissible site discharge (in L/s or per hectare) and the IFD for your site, or the council's storage per hectare. The calculator works out the storage, sizes the orifice plate, or a primary and secondary orifice, and lists the construction rules.

NCC 2025 does not set on-site detention: the council sets the permissible site discharge and storage, this calculator estimates them by the modified rational method many councils use, and the construction rules come from AS/NZS 3500.3 Clause 7.10, which NCC 2025 calls up through Volume One F1D3 and Volume Two H2D2(1)(a).

How it works

It reads from

  • Modified rational method storage (Aron and Kibler, 1990); orifice Q = Cd A √(2gh)
  • AS/NZS 3500.3:2025 Clause 7.10 (construction) and Clause 5.4.6 (runoff coefficients)

Storage

The modified rational method, which is not from the Standard: for each duration t, inflow runs at Qi = ΣCA × I(t) / 3600 while outflow rises to the PSD. Storage is Qi × t − ½ PSD × (t + tc); the largest over every minute from the time of concentration to the longest duration entered is the site storage requirement.

Orifice

The orifice passes the PSD at the head to its centre with the storage full: A = PSD / (Cd √(2gh)), with Cd 0.6 for a sharp-edged plate. For an underground tank, Clause 7.10.2(a) requires at least 25 mm in a 3 to 5 mm stainless steel plate machined to 0.5 mm with a sharp edge; its Note advises a screen of at least 50 times the orifice area (20 times above 150 mm), at least 1.5 diameters or 200 mm from the plate, whichever is greater. With two orifices, the primary holds a first-stage PSD until the water reaches the secondary orifice, and the two together pass the PSD at top water level.

Construction

Overflows escape harmlessly and visibly; ponding and overflow levels 300 mm below habitable floors and 150 mm below others (Clause 7.10.1). Underground tanks: floors at 1:140 or steeper to the outlet, an access opening over it (600 × 600 mm up to 800 mm deep, 600 × 900 mm deeper), access about every 10 m where under 1.5 m deep, a ladder over 1.2 m, and AS/NZS 2865 (Confined spaces; called up as AS 2865 by Clause 7.10.2(d)).

Above ground

Appendix M.12 recommends, in landscaping, a slope of 1:60 to the outlet (1:100 at the flattest), ponding of no more than 300 mm and 20% more storage for vegetation, construction inaccuracies and filling; in a driveway or car park, ponding of no more than 200 mm and a transverse slope of 1:140 or steeper.

Council rules first

NCC 2025 does not require on-site detention: the explanatory information to Volume Two H2D2 notes that the NCC does not require drainage systems and that the legal discharge point is generally set by local government. Councils set the PSD and the storage and many prescribe their own method. Use this to size early and to check, and follow the council's method where it has one. Rainwater tank credits and a roof bypass are council methods, not the Standard's: where the council allows them, work them by its method and enter the PSD and storage that result.

A worked example

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

Site storage requirement

26.5 m³

Holding the site to 12.0 L/s needs about 26.5 m³ of storage; the 45-minute storm governs. A 72 mm orifice passes the PSD at 1.20 m of head.

  1. Post-development equivalent impervious areaΣCA = 1.0 Ar + 0.9 Ai + Cp Ap = 320 + 0.9 × 250 + 0.30 × 330 = 644.0 m²AS/NZS 3500.3:2025 Clause 5.4.6
  2. Permissible site discharge = 12.0 L/scouncil requirement, entered
  3. Storage for each storm durationV = Qi × t − ½ PSD × (t + tc) = 5 min: 10.5 m³ · 10 min: 17.0 m³ · 15 min: 20.8 m³ · 20 min: 23.2 m³ · 30 min: 25.4 m³ · 45 min: 26.5 m³ · 60 min: 26.0 m³ · 90 min: 22.9 m³ · 120 min: 18.5 m³ = critical 45 minModified rational method (Aron and Kibler, 1990); not from AS/NZS 3500.3
  4. Site storage requirementSSR = max V over every minute from tc to the longest duration = Qi 16.5 L/s over 45 min (intensity interpolated log–log between the IFD rows) = 26.5 m³
  5. Per hectare of sitePSD and SSR ÷ site area in ha = site 0.090 ha = 133.3 L/s/ha · 294 m³/ha
  6. Orifice diameterQ = Cd A √(2 g h), solved for D = 12.0 L/s, h 1.20 m, Cd 0.6 = 72.4 mm

Questions

How much on-site detention does my site need?

It depends on the council's permissible site discharge and the rainfall. Try each storm duration: storage is the inflow volume less the outflow at the PSD, and the largest value is the requirement. Many councils prescribe their own method or rate per hectare; theirs governs.

What is the minimum orifice size for OSD?

25 mm for an underground tank, in a 3 to 5 mm stainless steel plate machined to 0.5 mm accuracy with a sharp edge (AS/NZS 3500.3:2025 Clause 7.10.2(a)).

More site stormwater calculators

All calculators

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

Ask about your own roof, drain or detention tank

The free plan answers from the National Construction Code, state building and planning legislation, ABCB handbooks, regulator practice notes and codes of practice, with the clause cited. Plus adds the Standards, including AS/NZS 3500.3:2025, from A$79 a month plus GST.

Try it freeSee plans and pricing