Mainstream riverine floods occur when heavy and excessive rainfall causes a river or waterway to overflow its banks onto land that is normally dry. The following individual risk measures are considered when calculating the overall risk rating for a mainstream riverine flood impacting the property:
- Average Recurrence Interval (ARI) of flooding at ground level
- Water depth during an ARI of 100 years.
ARI represents the average time in years between flood events of a given size or greater that could impact a particular land parcel (street address). ARI may also be expressed in percentage: for example, a 100-year ARI means that there is a 1% annual chance of experiencing a flood of a certain level or greater.
A 100-year ARI does not mean floods only occur once every 100 years or at regular intervals of 100 years. It is quite possible to experience a 1-in-100 year flood every few years followed by a long period with no flooding.
Additionally, ARI refers to ground level flooding at the geometric centre of the property land parcel, regardless of its height above the average sea level.
The ARI of riverine flood should not be confused with the ARI of rainfall events which may or may not lead to downstream flooding. The hazard rating in this report does not consider flash flooding or storm water overflow.
A “water depth during an ARI of 100 years” of -1.0 denotes that there is no known flood depth for a 100-year ARI flood. This does not mean there is no flood risk but that there has been no modelling undertaken (or released) for the area.
The 100-year ARI flood height often underpins land-use planning decisions. The water depth provides an indication of potential damage in a 100-year ARI flood but needs to be considered in relation to the height of the floor level of the building. Larger floods, however, are always possible and so water levels may on occasions exceed this depth. As explained above, this information is not available for every address. For properties outside of a floodplain the water depth, in a 100-year ARI flood, is designated as 0 metres.
The Probable Maximum Flood (PMF) is the most extreme flood possible and is considered to have a notional ARI of 10,000 years.
ARI risk ratings are categorised into five levels, as outlined in the table below.
- Very High (5) – ARI less than 20 years
- High (4) – ARI between 20-50 years
- Medium (3) – ARI between 50-100 years
- Low (2) – ARI above 100 years
- Negligible (1) – ARI above the Probable Maximum Flood including within the Flood Exclusion Zones modeled by Risk Frontiers.
- Unknown (-1) – No flood risk information is available for this address