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Floodplains Explained

You may have heard references to some area storm events as being a “100-year flood,” or perhaps being even greater than a “500-year flood,” as was the case with Tropical Storm Allison in June 2001. Does a flood of that magnitude really happen once in a hundred years... or, once in 500 years? Not really. Also, different areas of Harris County can experience different flood events from the same storm due to the size of the storm relative to the size of the county.

These flood events are often described by their anticipated rate of recurrence. Hence, the so-called “100-year flood.” But, this can be confusing terminology relating to the likelihood of a flood of a specific size.

So, let’s take it ONE YEAR AT A TIME.

A more straightforward way to describe a flood’s magnitude is with its likelihood of happening within ANY SINGLE YEAR. For example, a flood that is referred to as a “100-year flood” would have a 1% chance of being equaled or exceeded in a given year at a given location. Over thousands of years, such an event would occur, on average, about once every 100 years, which gives us the term “100-year flood.” In other words, a 1% flood and a 100-year flood are the same thing. But 1% flood makes better sense for the course of our lifetime, and is a more straightforward term. Similarly, a so-called “500-year flood” has a 0.2% chance of being equaled or exceeded in any given year at a given location, or on average about once every 500 years. Let’s stick with “0.2% chance.”

The risk of experiencing any of these rare events increases when periods of more than one year are considered. Remember, it’s not necessarily once in a hundred years for that 100-year, or 1% event. It’s at least a 1% chance EVERY year. As the years stack up, so do the chances. For a homeowner located on the very edge of the 1% floodplain (like House “B” above), the increased likelihood of experiencing a 1% flood over a number of years becomes quite a serious consideration. For example, over a 30-year mortgage, this house has a 26% chance that it will experience at least one storm that is equal to or greater than the 1% event. But, these odds are only true for houses on the very edge of the 1% floodplain. If a house is located somewhere between the creek or bayou and the edge of the floodplain, it would have a much higher chance of experiencing a flood (like House “A” above). This house is not only in the 1% floodplain, but is also in a higher frequency floodplain, which greatly increases its chances of flooding. With odds like that – and the consequence of risking everything you own – what better reason is needed to buy flood insurance?