When the Army Corp of Engineers and NFIP came up with the 100-Year Flood and 500-Year Flood designations, it’s almost as though they wanted to confuse the public. With Hurricane Harvey, much has been written on the meaning of the terms 100-year flood (it means a 1% chance of flooding in a single year), and the term 500-year flood (a 0.2% chance of flooding in a single year). While these basic definitions are correct, they don’t really help homeowners, whose question is: what’s the chance that my house will flood while I own it?
In the case of the 100-year flood zone, this means that the chance of flooding is at least 1% in a single year. But what if you plan to own your home for 30 years? In this case, you have a 99% of NOT flooding each year, but you’ve got to NOT flood for all 30 years. The probability of NOT flooding over 2 years is 0.99 * 0.99, and thus the probability of not flooding over 30 years is 0.99^30, or 74%. This means that the home has a 26% of flooding over the 30 years in question.
Of course, that doesn’t take into account the change in probabilities resulting from a combination of climate change and reckless development in most American cities. According to Kenneth Trenbeth, a scientist at the National Center for Atmospheric Research, “What used to be a 500-year event has become a 50- or 100-year event.” With this in mind, we can
— source truecostblog.com | Aug 30, 2017