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Stormwater management

Stormwater management is the site engineering that handles runoff from roof and paving: detention, soakwells, the point of discharge, and council requirements.

By Chalkline. Last updated 25 May 2026. Verified 25 May 2026.

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Stormwater management is the site engineering that handles rainwater runoff from a property’s impervious surfaces, the roof, paving, and driveway, so it does not overload the public drainage system or flood neighbouring land. Every new build, knockdown-rebuild, or extension that adds hard surface increases runoff, and councils require that increase to be managed on site.

Why it is required

A bare block soaks up most of the rain that falls on it. Cover it with a roof, a driveway, and paving and the same storm now produces a fast, high-volume rush of water with nowhere to go. Left unmanaged, that water overloads the street drainage and pushes onto neighbours. Council development controls require the runoff from new impervious area to be detained, infiltrated, or retained so the peak leaving the site is controlled.

The approaches

A stormwater design uses one or a combination of:

  • Detention (OSD): hold the runoff in a tank or basin and release it slowly at a controlled rate (the permissible site discharge). This is the most common council requirement on urban infill; see on-site detention for the PSD/SSR sizing detail.
  • Infiltration (soakwells, absorption trenches): let the water soak into the ground, where the soil permeability and water table allow. Common in sandy WA soils, unsuitable on reactive clay or high water table.
  • Retention and reuse (rainwater tanks): capture roof water for reuse, which also reduces what has to be discharged.

The point of discharge

Every site has a legal point of discharge, the place the council permits the site to release its stormwater: the street kerb and channel, a council drainage pit, or an inter-allotment drainage easement to a downhill property. Establishing the point of discharge is a first-order question: a site that can gravity-drain to the kerb is simple; one that must pump, or discharge through a neighbour’s easement, is a different cost and design.

Sizing

The system is sized from two inputs:

  • the catchment area (the impervious area draining to it), and
  • the design rainfall intensity for the location (from Bureau of Meteorology IFD data),

worked through AS/NZS 3500.3 for the drainage design. For OSD, the council sets the permissible site discharge (PSD) and the required storage volume (SSR) follows.

For a builder

  • Check the DCP early. The council’s development control plan sets the OSD threshold, the PSD, and the point-of-discharge rules. Confirm them before you design the site, because the underground tank and pipework affect the whole layout.
  • Establish the point of discharge first. It drives whether the system gravity-drains or needs a pump, and whether an easement is involved.
  • Get the sizing done properly. OSD and soakwell sizing is a hydraulic/civil engineering calculation from catchment and rainfall data, not a guess.
  • Coordinate with site coverage. More hard surface means more runoff to manage; the two decisions are linked.

Also known as: stormwater design, site stormwater, runoff management.

See also

Last updated: 2026-05-25. Verified: 2026-05-25. Quarterly review for currency.

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