Shortcut P= 1/t where t is the time period (i.e. 2yr, 10yr, 25yr, 50yr, and 100yr), p is the probability of an accident to be occurred.
take a look on this http://www.oas.org/cdmp/document/kma/mobay/mobay.htm
Or wikipedia http://en.wikipedia.org/wiki/Return_period
let me know if you need more info for developing IDF equation or flood scenarios.