Where sufficient flood records do not exist, or a very rare flood needs to be estimated, rainfall based techniques are used. Climate change may also affect how much we can rely on past flood records. However, Australia's flood records do not extend far into the past, and flood events are highly variable, meaning there is still a level of uncertainty in defining such flood estimates. Where a long-term flood record exists, and no significant changes have occurred to the catchment, a statistical technique known as flood frequency analysis can be used to determine the likelihood of floods of different sizes occurring at a specific site in the future (Figure 10).
The best method for calculating the chance of different sized floods occurring is statistical analysis of long-term flood records from stream gauging stations. The more confidence there is in the data the closer the confidence limits (red dashed lines) will be to the estimate (black line). we are 90% sure that it will be in this range - with a 10% chance we will be wrong, and it will be outside this range, higher or lower. This is said to be 'within 90% confidence limits', i.e. For example, the chance of a flood with a stream flow of 2,200 m3/s (as arrowed, left hand axis) in any year is estimated to be between 1 in 50 (2%) and 1 in 10 (10%). There is always a level of uncertainty inherent in such analyses. Estimating the chance of floods of different size based on flood frequency analysis of historical flood records at Bellingen, NSW.
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