Question

1. Suppose that a 100-year flood (flood that has a 1% chance of occuring in one year) occurs in your location this year. What are the odds (expressed as a percentage) that your location experiences another flood of the same magnitude in the next 12 months?
2. What are the odds (expressed as a percentage) that your location experiences two 25-year floods in the year 2020?

Answer 1

We have

The probability of flood occurrence of a 100-year flood =1/100 =0.01

Now the chance of occurrence in next year = 0.01 and the probability of not happening =1-0.01 =0.99

Thus,we have 1 chance of occurring in 99 odds in the next 12 months.

Odds =1/99 = 0.0101 =0.101%

b.Probability of 25 year flood =100/25 =4%

Now probability of two 25 year floods in 2020 =0.04*0.04 = 0.0016

The chance of occuring is 16 in 10000.

The chance of not occurring is 10000-16=9984

Odds =16/9984 = 0.16%.