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Storm Event Frequencies
What are the Storm Frequencies?
What are the Storm Frequencies?
Storm frequencies are statistical measures used in hydrology and flooding to describe how often rainfall or flood events of a certain magnitude are expected to occur or be exceeded. These frequencies are typically denoted as AEP ARI and EY.
AEP – Annual Exceedance Probability
AEP – Annual Exceedance Probability
Annual Exceedance Probability (AEP) describes the likelihood of a rainfall or flood event being equalled or exceeded in any given year. It is expressed as a percentage, such as 1%, 5% or 20% AEP. A lower AEP means a rarer and more intense event. Engineers use AEP values to design drainage systems, stormwater infrastructure and flood mitigation works that meet required safety and performance standards.
ARI – Average Recurrence Interval
ARI – Average Recurrence Interval
Average Recurrence Interval (ARI) represents the long‑term average time between events of a particular size. For example, a 100‑year ARI event is expected, on average, once every 100 years. ARI is a statistical measure, not a prediction, and multiple events of the same magnitude can occur within a short period. Although many standards now prefer AEP, ARI is still widely referenced in engineering design and historical datasets.
EY – Exceedances per Year
EY – Exceedances per Year
Exceedances per Year (EY) indicates how many times an event of a certain magnitude is expected to occur within a single year. For example, an EY of 0.5 means the event is expected once every two years on average. EY is particularly useful for analysing frequent, low‑intensity rainfall events that influence stormwater quality, erosion control and minor drainage design.
“1 in X” Terminology
“1 in X” Terminology
The phrase “1 in X” (such as “1 in 100 year event”) is a common way to describe the rarity of a rainfall or flood event. It means the event has a 1 in X chance of occurring in any given year. For example, a “1 in 100” event corresponds to a 1% AEP. While this terminology is familiar to the public, it can be misleading because it suggests a fixed cycle. In reality, these events are probabilistic and can occur more than once within the same period.
EY AEP ARI Table
EY AEP ARI Table
How are EY AEP and ARI calculated?
How are EY AEP and ARI calculated?
They are related by the following equations
EY = -ln(1 - AEP)
AEP = 1 - exp(-EY)
ARI = 1 / EY
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