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 buildup 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
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 groundrupturing 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 “100year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1percent chance of occurring in any given year. Likewise, the term “100year storm” is used to define a rainfall event that statistically has this same 1percent 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

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