The Flood Return Interval is a New Zealand-based measure that estimates how often a flood is likely to occur in a given area. It represents the average time between flood events and is used to assess flood risk, guide land use planning, and inform risk mitigation decisions.
This dataset is rated on a scale from 1 to 6, where:
1 (Nil): No recorded flooding
2 (Slight): Less than 1 in 60 years
3 (Moderate): Between 1 in 20 and 1 in 60 years
4 (Moderately Severe): Between 1 in 10 and 1 in 20 years
5 (Severe): Between 1 in 5 and 1 in 10 years
6 (Very Severe): More frequent than 1 in 5 years
Flooding includes water from streams, slopes, or tides temporarily covering land. Higher ratings mean floods are more frequent and pose greater land use restrictions. This national dataset aligns well with regional flood mapping and supports consistent risk evaluation across New Zealand.
Dataset Characteristics & Limitations
Provides nationwide coverage and generally aligns well with regional flood data.
In areas lacking direct flood data, estimates may be derived from soil profile and vegetation indicators.
If land is protected by flood mitigation works, the return interval used is based on the protection design rather than historical flooding.
Flood classification reliability is indicated by data source flags:
m – Direct measurement or soil-specific analysis
r – Inferred from reliable relationships with similar soils
u – Inferred from unknown or less reliable sources
uf – Broad generalisation from national soil survey data (1:253,440 scale)
Data Update Cycle
This dataset is static, with the last update recorded in November 2024. It is not scheduled for regular updates, and will only be revised if a major flood event or environmental trigger justifies a data refresh to reflect significant landscape changes.
Source: A Manual of Land Characteristics for Evaluation of Rural Land – Landcare Research NZ