Flood Definitions

The following definitions are useful in understanding flood terminology.

Term	Definition
AHD (Australian Height Datum)	The Australian Height Datum (AHD) is the reference level for defining reduced levels adopted by the National Mapping Council of Australia. The level of 0.0 m AHD is approximately mean sea level.
ARI (Average Recurrence Interval)	The Average Recurrence Interval (ARI) is a statistical estimate of the average period in years between the occurrence of a flood of a given size. For example, the 100 year ARI event will occur on average once every 100 years: this is equivalent to a 100 year ARI having a 1% probability of occurring in any given year.
Defined Flood Level / Estimated Flood Level / Estimated Peak Flood Level / Modelled Flood Level	A water level derived through mathematical modelling of the Defined Flood Event.
Defined Flood Event	Terminology consistent with State Planning Policy (SPP, 2003) which states “Defined Flood Event is the flood event adopted by a local government for the management of development in a particular locality”.
Drainage Deficient Area	Areas of land defined in accordance with Regulatory Map 1.5 Flood Prone and Drainage Constraint Areas [PDF 3.72 MB] of the Maroochy Planning Scheme. The Noosa Plan and Caloundra City Plan do not have declared drainage deficient areas.
Flood Regulation Line	A ‘regulation line’ has been set by council to define the limit to which development may encroach onto a floodplain, development is undertaken outside such 'regulation line’. Specific to the Maroochy Planning Scheme only.
Freeboard	A factor of safety usually expressed as a height above the adopted Defined Flood Level. A freeboard may compensate for factors such as wave action and historical and modelling uncertainties.
Highest Historical Flood Level / Nominated Historical Flood Level / Known Flood Level	The highest flood water level from all historic events for which council has reliable records.
Minimum Floor Level / Minimum Floor Height	The minimum floor level calculated in accordance with the applicable planning scheme. Determined through the addition of the appropriate freeboard to the relevant flood level.