Flood studies and projects

Common terms used in relation to flooding:

• Annual Exceedance Probability (AEP): The probability of a flood event of a given size occurring in any one year, usually expressed as a percentage annual chance.
• Average Recurrence Interval (ARI) Similar to AEP. The long-term average number of years between the occurrence of a flood as big as (or larger than) the selected event. This is an average (a ‘1 in 100 Year’ ARI event last year does not mean a flood that big will not occur for another 99 years).
• Design Flood Level: A hypothetical flood representing a specific likelihood of occurrence. For residential property in Tweed Shire, the peak of the modelled 1% AEP (100 Year ARI) flood is the Design Flood Level.
• Freeboard: A safety factor typically used to set minimum floor levels. In Tweed Shire, the minimum habitable floor level (or Flood Planning Level) is based on the 1% AEP flood event plus 0.5 metres of freeboard.
• Floodplain: Refers to all flood prone areas in the Tweed catchment.
• Metres above Australian Height Datum (m AHD): The reference level for defining ground levels in Australia. The level of 0.0m AHD is approximately mean sea level.
• Minimum Habitable Floor Level: The minimum level in metres AHD at which habitable areas of development (generally including bedrooms, living rooms, kitchen, study, family and rumpus rooms) must be constructed. In Tweed Shire, this is Design Flood Level plus 0.5m of freeboard.
• Probable Maximum Flood (PMF): An extreme flood, considered the largest flood that could occur at a specific location. Generally it’s not physically or economically possible to provide complete protection against this flood event, but it should be considered for emergency response.

The probable maximum flood defines the extent of the Tweed’s flood prone land (floodplain).