Divide the number of years on record (N) by the number of events (n), when there is no magnitude associated with the event. The years on record are the years of available data. For example, a city may have records only for the past 200 years. Public agencies such as the U.S. Geological Survey collect this information. The number of events is the number of floods during the time on record. Express the formula algebraically as N/n.
As an example, suppose that 200 years of data are available, and during that time there were only two floods. Divide 200 years by two floods to get a recurrence rate of 100 years per flood.
Add one and the number of years on record (n) when there is a magnitude associated with the event. Then divide by the magnitude (m). For a flood, the magnitude is equal to the size of the discharge of the flood. Express the formula algebraically as (n+1)/m. For example, suppose there are 200 years of data and in that time, flooding took place four times and two of the floods were considered major. The equation would be (200+1)/2= 100.5 years per major flood.
Divide one by the flood recurrence interval (T) to forecast the probability the event will happen in the future. Express the formula algebraically as 1/T.
In an area with a 100-year flood recurrence interval, the probability a flood could occur in any year equals 1/100, or 1 percent. Therefore, during any year in the area there is a 1 percent chance the area will flood.
