Floodway
Floodway is the portion of a floodplain where flood water flows fast and deep, actively conveying flow. NSW classifies it as the highest-hazard zone.
Ask Chalkline about this →A floodway is the portion of a floodplain where flood water flows at sufficient velocity and depth that it actively conveys the flow during a design flood event. Floodways are the deepest, fastest, most hydraulically-active part of the flood path. In the NSW Flood Risk Management Manual 2023 (the current state guidance, verified 2026-05-16), the floodway is classified as the highest-hazard zone within a floodplain and development is generally prohibited.
The three floodplain zones:
| Zone | Function during flood | Hazard | Typical control |
|---|---|---|---|
| Floodway (this) | Active conveyance, high velocity x depth product | Highest | Development prohibited or very tightly restricted |
| Flood storage area | Temporary storage of flood volume; lower velocity | High | New floor levels at or above flood planning level; some uses allowed |
| Flood fringe | Inundation only, low velocity, shallow depth | Moderate to low | Floor levels and freeboard; standard residential possible |
The zoning is mapped per catchment by the council using a flood study prepared by a hydraulic engineer (see flood study).
Why floodways are off-limits to development:
- Blocking the floodway raises upstream flood levels. Even a small obstruction (fence, shed) can divert flow and raise the level on neighbouring properties.
- Velocity x depth product: at typical floodway velocities (0.5-2.0 m/s) and depths (0.5-3.0 m), the product exceeds the safe-for-pedestrian threshold (0.4 m²/s). Anyone caught in a floodway during a flood is at serious risk.
- Building survival: even slab-on-pier or piled houses are damaged or destroyed by flood-velocity debris loads in a floodway.
How a floodway is identified:
- Council flood study (the authoritative source per LGA in NSW).
- Flood planning instrument (typically the LEP flood-affected lands clause + DCP overlay) maps the floodway extent.
- Section 10.7 Certificate (NSW): question 7 (flood affectation) reports floodway, flood storage, or flood fringe classification, plus the flood planning level for the design event.
- Risk Information Service (NSW) and ARR2019 (national) guidelines provide methodological frameworks.
Permitted uses (vary by LEP):
Some councils allow limited development in floodways, typically:
- Agriculture (livestock, dryland cropping).
- Recreation (parks, sports fields with no permanent structures).
- Roads and bridges, engineered for flood resilience.
- Small ancillary structures (carports without walls, light-construction sheds), sometimes with conditions.
Distinguishing floodway from other flood terms:
- Floodway (this): the conveyance corridor.
- Flood study: the technical document that maps zones.
- Flood planning level (FPL): the design flood level + freeboard, sets minimum habitable floor.
- Probable maximum flood (PMF): the theoretical worst-case event, used for evacuation planning not floor levels.
- 1% AEP: the 1-in-100 year event, the typical design flood for residential FPL.
Builder takeaway:
If a section 10.7 certificate or a Vic equivalent flags “floodway” or “high-hazard flood prone land”, do not commit to design or contract until the flood study is reviewed by a hydraulic engineer. The council DA path will be slow at best, and the project may not be permittable at all. Owners who buy floodway land for development without checking should be referred to the hydraulic engineer urgently.
Also known as: flood conveyance zone; high-hazard flood land; floodway corridor; primary floodway.
Category: Site & ground.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.