Louisiana floods are “historic” but not surprising

By Gabriel Lotto, Temblor

This weekend, record-breaking rainfall in Louisiana and Mississippi caused extreme flooding in the region. As of this posting, eight people have died and over 30,000 have been rescued from their homes and cars during what state officials have called “historic” flooding. But just because a flood is large or historically significant doesn’t mean it’s unexpected.

Temblor just added flood risk maps in all U.S. states and territories to its web app, app.temblor.net

In most US counties, FEMA publishes maps of where flooding is likeliest. Where should we expect floods to happen in Louisiana? Almost everywhere, according to this Temblor map:

Much of the Baton Rouge, LA, area is located in a FEMA-designated flood zone.
Much of the Baton Rouge, LA, area is located in a FEMA-designated flood zone. Darker blues represent zones in which there is a 1% or greater chance of a flood per year (equivalent to a 10% or greater chance of a flood in a decade). These are often, erroneously, called ‘100-year flood zones.’

Louisiana and Mississippi are especially prone to flooding because they are largely built on low-lying marshes and alluvial floodplain (places where rivers deposit their sediments when they overtop their banks). Additionally, the Gulf Coast’s moist climate means that the soil saturates quickly during large rainfall events. Under normal conditions, soil absorbs rainwater like a sponge. But when the ground is fully saturated with water, excess rainwater stays on the surface and pools at the lowest elevations.

How well did FEMA’s maps predict this week’s Louisiana floods?

Let’s take a look at some examples. In Gonzales, LA, the East Ascension High School baseball field flooded while the track was largely unscathed. Indeed, the flood maps did a pretty good job of predicting where flooding would occur.

“There used to be a ballpark.”
“There used to be a ballpark.” At East Ascension High School, FEMA’s flood map did a pretty good job of predicting which fields would get hit the hardest. Photo: Bill Feig, The Advocate.

Another place the flood map was mostly accurate was in Denham Springs, at the Juban Crossing over the Interstate-12 highway. Here, the highway separates a flood zone from dry land, both in FEMA’s predictions and in reality. However, you can see that the 1% per year flood zone missed some of the areas that were inundated this week.

At the Juban Crossing, I-12 marked the line between flood and dry land.
At the Juban Crossing, I-12 marked the line between flood and dry land. Photo: Bill Feig, The Advocate.

One final example of extreme flooding comes from Baton Rouge, where Airline Highway was hit with major floods just north of Greenwell Springs Road. Here, even the 0.2% per year flood zones were exceeded by the this week’s flood. This comparison makes the strongest case that the recent Louisiana floods are in fact historically significant.

The Louisiana floods were truly historic.
Here at Airline Highway in Baton Rouge, the flood surpassed even the 0.2% per year (2% per decade) flood zone as designated by FEMA. This was truly historic. Photo: Bill Feig, The Advocate.

Do you live in a flood zone?

Devastating floods like this week’s events in Louisiana are rare, but their occurrence shouldn’t be surprising. If you have a mortgage on a home located within the 1% per year flood zone, you are required to carry flood insurance. But historic flooding like this demonstrates that even if you live within the 0.2% per year flood zone or outside a flood zone altogether, your risk is not negligible and flood insurance may be a wise investment.

Temblor’s free web app allows you to view your flood risk anywhere in the United States. Simply visit app.temblor.net, enter in your location, click the layers button in the top-right corner, and select “Flood.” If you see no flood zones anywhere on the map, it may be because FEMA has not yet mapped that region. We will soon release our flood maps in the Temblor iPhone and Android apps as well.

Sources for Louisiana Floods


Bill Feig Photography

“Interactive map: See Baton Rouge areas hit hardest by historic flooding over the weekend.” Advocate Staff Report, 12 Aug 2016. The Advocate.

Flood Insights

US Flood Maps: Do you live in a flood zone?

Click to read previous Temblor blog posts

  • Thorify

    Sorry but the area that was flooded had never ever flooded before, and places that are expected to have a high risk of flooding require you to own flood insurance. 90% of the people in that area had no flood insurance because none of that area is a flood zone, so to say it is to be expected is kind of stupid. By your same logic, all of Florida should be under water.

    • Anthony Carl Martin Edd

      sounds like you are the one that does not understand. No one can accurately predict exactly when or where a flood will occur. They put the odds of a flood and each property owner either pays for insurance or gambles that they won’t experience a flood. If they gamble and lose….it’s their own responsibility. I lived in an area where flood insurance was not required, twice the sacramento river rose and was undermining the levies near my home….soooo either get insurance or move…..I moved.

      • Thorify

        No I meant that it wasn’t “to be expected” at all. That wording is a fallacy and that was the purpose of my comment.

        • Anthony Carl Martin Edd

          I know what you mean from my own experience, but we each are responsible for our own decisions, always expecting the government to bail us out from the results of nature, or our own mistake is “entitlement” in my humble opinion. Although, I wouldn’t mind being bailed out, just don’t expect someone to do it…

      • bruce jones

        smart, common since good move

    • yodecat

      Whelp, Thorify, Louisiana gonna be flooded by the Gulf before it’s over.

    • gyptzy

      Thank you Throrify, “The only one with sense in the room.”

      • wally

        We know this area was created by silt deposits from the MS River. Before that time, this area was covered by water. Add to this the fact that these areas have not been occupied by record-keeping people until the last few hundred years, and you cannot possibly say with any authority or validity that the area had “never ever flooded before.” I also have doubts about your understanding of Florida geology and geography.

    • Anthony Carl Martin Edd

      name calling proves a person doesn’t have a good argument……

  • TeetotheMac

    Gabe is the best!