Hazard Frequency and Recurrence Interval


One threat of a natural hazard is that it’s either constantly present or it’s subject to fluctuations. Many hazards are cyclical; for example, earthquakes occur with a definable time interval because of the gradual build-up of strain on the fault. Other hazards, especially meteorological ones, may be seasonal. However, with human population increasing at an exponential rate, hazard vulnerability increases simply because there are more people to be affected.

Recurrence Interval

Washington Flood Frequency Map[4]
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It is important to understand the relationship between frequency of an hazard event and the size of a hazard event. For just about any hazard event, statistical analysis reveals that larger events occur less frequently than small events.

Statistical frequency analysis is used to estimate the probability of the occurrence of a given precipitation event. The recurrence interval is based on the probability that the given event will be equalled or exceeded in any given year and is usually used for risk analysis (e.g. to decide whether a project should be allowed to go forward in a zone of a certain risk, or to design structures to withstand an event with a certain return period.[1]

Recurrence Interval Equation

Recurrence interval (T) =

  • n is number of years on record;
  • m is the number of recorded occurrences of the event being considered.

In paleoseismology (the study of the timing, location, and size of prehistoric earthquakes), recurrence interval refers to the time between ground-rupturing events at a point on a fault.

When applied to hydrological events, the recurrence interval refers to the average period between floods of a given size or greater. For floods, the event may be measured in terms of m³/s or height; for storm surges, in terms of the height of the surge, and similarly for other events.[2]

Recurrence Interval and Probability of Occurrence

The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year. Likewise, the term “100-year storm” is used to define a rainfall event that statistically has this same 1-percent chance of occurring. Below is a chart demonstrating recurrence interval, probability and percent chance of occurrence.[3]

Recurrence interval
Probability occurrence
Percent occurrence
100 years
1 in 100 / year
1% / year
50 years
1 in 50 / year
2% / year
25 years
1 in 25 / year
4% / year
10 years
1 in 10 / year
10% / year
5 years
1 in 5 / year
20% / year
2 years
1 in 2 / year
50% / year



  1. USGS – Floods: Recurrence intervals and 100-year floods: http://ga.water.usgs.gov/edu/100yearflood.html
  2. USGS – CE 04026 Engineering Hydrology – Chapter 7 – Floods p.83 – http://www.most.gov.mm/techuni/media/CE_04026_chap789.pdf
  3. USGS – Floods: Recurrence intervals and 100-year floods: http://ga.water.usgs.gov/edu/100yearflood.html
  4. Image Source: http://www.ecy.wa.gov/programs/sea/floods/riskmap_images/frequency_flooding_emd.jpg [Accessed: May 15, 2013]