(The instructions here are
simply a "review" of those of the previous page.
If you know how to determine the probability of, for example
a 50 year flood during a 10 year period, skip to the bottom
of the page and complete the table.)

Keep in mind that every
year the probability **(P)** of a Maximum Annual Peak
Discharge (we'll call this a flood) with a given recurrence
interval (RI) is **1 divided by the Recurrence Interval**

**P **** = 1****
/ ****RI**

From that it follows that
the probability of there **NOT** being a flood within
one year is

**P**_{(NOT) }
**=** **(1 - ** **1****
/ ****RI****)**

Over a period of ** X**
years, the probability of there NOT being a flood with
a certain recurrence interval is,

**P**_{(NOT in X years)}_{
}**=** **P**_{(NOT)}
^{X} =
**(1 - ** **1**** /
****RI)** ^{X}

And finally, the probability
of there bing a certain size flood in **X **years is

**P**_{(Within X years)}_{
}** ****=****
1 - ****P**_{(NOT in X years) =
}**1 - (1 - ** **1****
/ ****RI****)**
^{X}

As an example, let's answer
the question: What's the probability a flood with a recurrence
interval of 25 years, during a 10 year period?

**P**_{(Within 10 years)}_{ }_{
}**= 1 - (1 - 1/25)**^{10}
= 1 - 0.66 = 0.33 or 33%