The size of flood events

When we talk about the size of flood events, we refer to the frequency that an event of that size would occur at. “That was a 1 in 100 year flood”, someone might say.

While these numbers may be statistically correct, it’s worth remembering two things:

  1. Flood events can occur at any time. It’s entirely possible for two (or more!) “100 year” floods to occur in one year. When we discuss the frequency that an event occurs at, we’re really referring to its average frequency over a long period of time.
  2. The frequency of an event is only an estimate. It’s based on rainfall or river flow measurements made over a period of time, under the assumption that those measurements are representative of long-term climate. Consider the following:
    • Many rainfall or river flow records are short (e.g. 20 to 50 years), and may well have been from a particularly dry period of time, or from a particularly wet period! Hydrologists rarely have “enough” data to work from.
    • Climate change means that floods (and droughts) are beginning to happen more often. An event that used to occur once every 100 years might now be occur once every 90 years, for example.

Hydrologists represent frequency in two ways. The first is average recurrence interval (ARI), which is the average time period between events of that size or greater, and is also known as the return period. The second is annual exceedance probability (AEP), which is the probability that an event of that size or greater will occur in a given year. ARI is easier for the general public to understand, but is also easy to misinterpret (see above!). AEP is a better technical description but can be harder to conceptualize.

When designing things that could be affected by flooding, such as buildings or infrastructure, it’s helpful to consider how likely that flooding is to occur during its expected lifespan.