I recently read an engineer's report that mentioned "a 100 year event" relating to rainfall. Is there a set definition of this? How can I find out exactly what this is?
Yes, there are several definitions of the 100-year event, all related to one another:
(1) generally assumed to be an event with a 1% chance of happening in any given year, or
(2) more theoretically, an event that is expected to occur, "on average," once every 100 years.
Note that both definitions do not rule out the possibility - although very remote - that the 100-year "event" could occur two (or even more) times in the same year. Also, for any fixed 100-year period, the chance that the of the 100-year event occurring is actually LESS than 100% -- the chance is actually only about 63% that a 100-year event will be present within any pre-defined 100-year period, again a function of statistics.
Now one other MAJOR detail: defining the "event": what is the time frame of the event? Do you want to know the 100-year event for a 1-hour rainfall, a 6-hour rainfall, or a 12-hour rainfall? Obviosuly the 100-year event differs depending on the duration (length of time) of the event in question. Typically we define rain events in terms of both "frequency" (100-year, 50-year, 5-year, etc.) and "duration" (1-hour, 6-hours, 1-day, 3-days, etc.).
Last, there is the "where" of the event. Frequency/duration numbers for Baton Rouge differ from those in other parts of the state and nation.
So, as you can see, your question is FAR more complicated than you might first imagine!
The answer(s) to your question can be derived from several sources, depending on "WHAT" it is that you are after. The results for Louisiana are derived from a couple of different publications.