Understanding Flood Classifications

The Bureau of Meteorology, working with state agencies and local knowledge, defines up to three flood classification levels for each flood-relevant river gauge:

Minor flooding causes inconvenience: low-lying areas next to watercourses are inundated, minor roads may close, and low-level bridges may be submerged.

Moderate flooding means the water is inundating larger areas: main traffic routes may be affected, some buildings may take water above floor level, and evacuation of some areas may be needed.

Major flooding means extensive inundation: properties and whole towns can be isolated or flooded, and significant evacuations may be required.

The definitions are national; the heights are local. Minor flooding might start at 2.4 metres on one gauge and 11.2 metres on another, because each set of levels encodes what the water actually does to that specific floodplain.

The classification levels are published by the Bureau of Meteorology in its per-state flood classification tables and Service Level Specification documents. They are reviewed with the states and updated as floodplains and knowledge change. Not every gauge has them: of the thousands of river gauges in Australia, roughly 1,400 are flood-classified. The rest still report useful water levels but have no official flood meaning, which is why an honest flood API must report those gauges as unclassified rather than inventing thresholds.

Gauge readings are heights above that gauge's own zero point, which is usually not sea level. One gauge's zero might sit 216 metres above the Australian Height Datum, another at the riverbed. That makes raw values incomparable across gauges. The only sound cross-gauge comparison is against each gauge's own classification levels: above minor is above minor everywhere, whatever the raw number reads. Where the authority publishes a gauge zero or datum, EmergencyAPI carries it on the station record.

We poll each state's own gauge telemetry, store the readings, and compare the latest value against the published classification levels for that gauge. The result is a floodClass of below_minor, minor, moderate, major, or unclassified, computed at read time and never stored as judgement. Threshold crossings are recorded permanently, so the record of when a gauge crossed into and out of each class survives even after raw readings roll up.

Two honesty rules apply throughout. Stale readings, common where the underlying source is a daily archive rather than telemetry, are flagged and never counted in current flood status. And the classification levels themselves carry provenance: which document they came from and when we fetched them.

The derived status is exactly that: a comparison of published observations against published authority levels. It is not a forecast, and official warnings from your state emergency service always take precedence. Developer detail: the river gauges API guide. Live view: flood status.