Flood frequency
How often a flood occurs reflects its size. Small floods happen frequently, and large floods happen infrequently.
Understanding flood frequencies is important for predicting and preparing for flooding. By studying past rainfall and flow data, we can work with others to reduce the impact of floods on communities and infrastructure.
Describing flood frequency
We use two terms to describe the frequency and size of floods.
Average Recurrence Interval
The Average Recurrence Interval (ARI) is the average time between floods of a certain size.
Large, infrequent floods have higher ARIs than smaller, more frequent floods. For example, a 200 year ARI flood will occur on average once every 200 years. A 50 year ARI flood will occur on average once every 50 years and be a smaller flood than a 200 year ARI.
While a 200 year ARI flood may happen once every 200 years on average, every year there is still a 1-in-200 chance that a flood of this size might occur. A good way to picture this is by rolling a dice. If you roll and get a six, there was a 1-in-6 chance that would happen. If you pick up the dice and roll it again, the chance of you rolling another six is still 1-in-6.
Rangitata River flooding at SH1
Annual Exceedance Probability
Annual Exceedance Probability (AEP) is the probability of a certain sized flood occurring in a single year.
For example, a 0.5 per cent AEP flood has a 0.5 per cent, or 1-in-200 chance of occurring in any year. AEP is the inverse of ARI, where large, infrequent floods have a low AEP and smaller, more frequent floods have a higher AEP.
200-year ARI and 0.5 per cent AEP are different ways to describe the same event.
We review and update river flows for different ARIs every five to 10 years or after major flood events. These can change significantly, especially for rivers where we have a short record of past flows.
Waimakariri River between Baynons and Madeleys Road
Why we use flood frequency
Rainfall patterns and flood flows vary considerably across different catchments and rivers. What may be a large flood event in one river, might be considered a small flood event in another.
In the Waikirikiri/Selwyn River, a 500 cubic meters per second (m3/s) flow would be a large flood event, causing substantial flooding on the floodplain. The same flow in the Waimakariri River would be easily contained within the riverbed and not cause any flooding.
It’s possible to have multiple large floods in Waitaha in a year, as flood events occur within rivers and catchments, not across a whole region.
It’s often thought a 100 year ARI flood is twice as big as a 50 year ARI flood. In reality, there can be little difference in these two flood flows for a given river, and the impact of these two floods once water spreads across the floodplain can be quite similar.
Surface flooding in Otahuna Valley
A good example of this is the Ōrāri River. A 50-year ARI flow in the Ōrāri River is currently estimated to be approximately 920 m3/s, while a 100-year ARI flow is estimated to be approximately 1060 m3/s.
Increased extreme weather events due to climate change will likely alter the frequency of flood flows, which will also change the flows we assign to different ARIs.