The So-Called 100-Year Flood
Flood probability is based on a number of factors, and scientists and engineers use a specific set of criteria to determine the likelihood that rivers may flood in heavy storms. Flooding can happen wherever it rains, and it can cause troubles like 2019’s sudden troubles in Conyers; but there’s a certain term on which much of our flood conversation hinges: the “100-year flood.”
It’s a rather foreboding term, one that definitely makes the listener aware that it’s something to be concerned about. But what exactly is it? Is it a general term, or are there actually 100-year floods we can measure and predict?
First, a 100-year flood is a very real, scientific measurement, as opposed to a general declaration like, say, a gullywasher or deluge. Without getting overly technical, a 100-year flood is, in theory anyway, a flood that statistically should only occur about once every 100 years.
In other words, based on data from nearby rivers, bodies of water, elevation levels and other factors, a 100-year flood is a flood that has a 1% chance of occurring in any given year.
This kind of flood is also known as a 1% Annual Exceedance Probability (AEP) flood.
So, does a 100-year flood only occur every hundred years? Well, yes and no. While the average space between them typically does meet that criteria, that doesn’t preclude the possibility of two 1% floods happening in back-to-back decades, years, or even months.
Flooding is unpredictable, but statistically speaking, the 100-year or 1% designation gives us a baseline for estimating the frequency and severity of a certain kind of flood.
Why does all this matter? Well, for one thing, aside from giving us a sense of how bad a flood was or could be, and how likely it may be to occur again, the 1% AEP flood is the basis for the National Flood Insurance Program, which, among other things, requires homeowners within high-risk flood zones to carry flood insurance.
So you can see why it’s important that we have both accurate flooding data and analysis as well as risk prediction. The accuracy of the portrait and prediction of 100-year floods could impact whether your home sits in a high-risk zone or not.
The 1% analysis also helps us plan things like infrastructure, drainage and building needs for flood plain areas.
The last 100-year flooding in our area came in the form of massive floods in September 2009, which resulted in half a billion dollars in damages, and actually reached the 500-year flood designation in some areas, meaning there was only a 0.2% chance of that flood occurring that year in this area.
Flooding isn’t inevitable, but it’s always a possibility when heavy storms overwhelm bodies of water or drainage systems. If flooding or other water damage cause trouble for your home or business, contact SERVPRO for fast, complete cleanup and recovery.