The FloodAUS Risk Rating is expressed as either an Annual Exceedance Probability [AEP] or an Average Recurrence Interval [ARI] for flood waters reaching a height equal to ground level at the street address, ground level +1m and ground level + 2m. These three options are produced so that the effects of various floor heights can be assessed.

Vertical Mapper® is used to attach ground elevation and flood heights for all flood surfaces to the street address database. Thus, every street address has an elevation and flood heights for the relevant floods.

An in-house program interpolates between the flood surfaces to estimate the Risk Rating for every street address for the three elevations - ground level, ground level + 1m, and ground level + 2m. The result is a database, exported as a text file and MS Excel® file containing, for each street address, the postcode, three Risk Rating values based on AEP and 3 Risk Rating values based on Average Recurrence Interval. A sample database extract is shown below.

Any elevation below the 5% AEP [ARI 20-year] flood height is entered with a 0.0526315 AEP value and an ARI value of 19 [years]. Information for flood surfaces below 5% AEP is not available as the steep slope of the stage vs. AEP curve makes estimation difficult.

Addresses with elevations above the highest flood surface created are allocated a Risk Rating of that ARI + 1. For example if the highest flood surface is ARI 100-year [0.01 AEP] flood, all addresses above that level are given a Risk Rating of ARI 101 years [AEP 0.0099]. If the PMF has an ARI of 10,000 years then addresses above the PMF are allocated Risk Ratings of 10,001.