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<title data-rh="true">A ‘500-Year Flood’ Could Happen Again Sooner Than You Think. 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<div class="g-graphic g-graphic-freebird" data-preview-slug="2017-08-28-500-year-storm">

  

    

    
      <div class="g-item g-text ">
    

      <p class="g-body"><blockquote class="twitter-tweet" data-lang="en"><p lang="en" dir="ltr">Wow - Now experts are calling <a href="https://twitter.com/hashtag/Harvey?src=hash">#Harvey</a> a once in 500 year flood! We have an all out effort going, and going well!</p>&mdash; Donald J. Trump (@realDonaldTrump) <a href="https://twitter.com/realDonaldTrump/status/901797906046439426">August 27, 2017</a></blockquote></p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body"><script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script></p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">In the aftermath of Hurricane Harvey, President Trump tweeted that the storm had brought a “once in 500 year flood” to Houston, and expressed support for relief efforts. His words suggested the kind of catastrophic event that hadn’t been seen in the area for five centuries. But parts of Houston saw “500-year flooding” <a href="https://twitter.com/JeffLindner1/status/722070493499490304">just last year</a>. And in 2001, <a href="http://www.chron.com/news/houston-texas/article/Progress-and-lessons-10-years-after-Tropical-1688752.php">Tropical Storm Allison</a> also delivered severe flooding to the area.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">Harvey has been even more devastating, and officials say the flooding is the worst in state history. But is it really a once-in-500-years event?</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">Terms like “500-year flood” and “100-year flood” are used as shorthand by government officials and actuaries, but they are misnomers that can confuse the public, said Sandra Knight, a senior research engineer at the University of Maryland and a former official at the Federal Emergency Management Agency.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">Rather than predicting the <em>timing</em> of a flood, these terms refer to the <em>chance </em>of it occurring at all.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body"><i>A <strong>500-year event</strong> has a<strong> 1 in 500 chance </strong>of<strong> </strong>occurring in a single year.</i></p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body"><i>A <strong>100-year event</strong> has a<strong> 1 in 100 chance </strong>of occurring in a single year.</i></p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body"><i>A <strong>10-year event </strong>has a<strong> 1 in 10 chance </strong>of occurring in a single year.</i></p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">You get the idea.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">“People think: ‘Well, it’s a one-in-100-year flood. We had one last year, so it won’t happen again,’” Dr. Knight said. But really, “they can happen back to back.”</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">A 100-year flood means there is a 1 percent chance — or 1 in 100 — that a flood of that magnitude will occur in that given location in any one year. In 500-year flood zones, the probability goes down to 0.2 percent. In other words: not very likely to happen — or, at least, not likely based on historical data, which helps scientists to determine these probabilities.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">Here is a map of Houston’s flood zones as defined by FEMA. It shows the areas that have a <span class='g-highlight dark-orange'>1-in-500 chance of flooding</span>, and a <span class='g-highlight light-orange'>1-in-100 chance of flooding.</span> (FEMA uses various scales for 100-year floods, which we’ve rolled into one below.)</p>


    </div>

    

    
      <div class="g-item g-graphic ">
    

      <div class="g-item-ai2html" style="max-width:1050px;">


  <p class="g-asset-hed">Harris County flood zones in 2017</p>




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<!-- ai file: map.ai -->
<!-- preview: 2017-08-28-500-year-storm -->
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			<p class="g-pstyle2">Reservoir</p>
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      <div class="g-item g-text ">
    

      <p class="g-body">But changing landscapes — and a changing climate — could alter the probability of flooding.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">“We’re looking at historical data when really we have something that is called non-stationarity,” Dr. Knight said. “The world isn’t stationary anymore and the hydrology isn’t. The landscape isn’t. So why are we still presuming the future will look like the past?”</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">In Houston, factors like rapid urbanization over the past few decades have already affected how and where water flows, she said, and climate change will strengthen the <a href="https://www.nytimes.com/2016/09/08/science/global-warming-louisiana-flooding.html">likelihood</a> and <a href="https://www.facebook.com/MichaelMannScientist/posts/1515449771844553?smid=tw-nytclimate&smtyp=cur">severity</a> of future extreme events.</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">Devastating storms like Harvey are still relatively rare, “but the probability of them occurring has increased substantially because of climate change,” said Kevin Trenberth, a distinguished scientist at the National Center for Atmospheric Research in Boulder, Colo. “What used to be a 500-year event has become a 50- or 100-year event.”</p>


    </div>

    

    
      <div class="g-item g-text ">
    

      <p class="g-body">The National Oceanic and Atmospheric Administration has not yet classified the deluge caused by Hurricane Harvey as a 500-year flood, but some scientists say it may ultimately exceed the mark.  Now you know what that means: Such flooding was unlikely, but it could happen again before the distant year 2517.</p>


    </div>

    

  

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          <div class="story-info interactive-source"><p>Source: FEMA; Houston-Galveston Area Council</p></div>
          
          <div class="story-info interactive-credit"><p>Additional reporting by Lisa Friedman.</p></div>
          
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