NOAA rainfall data redefines 100-year-storm totals for Houston

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The National Oceanic and Atmospheric Administration has confirmed what many Houstonians may have already had a hunch about: a 100-year storm event is not necessarily what it was in the past.

By compiling rainfall data over the past several decades, NOAA has redefined some instances of the 100-year rainfall total to be a 25-year event for Houston, and it increased the rainfall amount attributed to a 100-year event.

The data is reflected in an update released Thursday to the agency’s Atlas 14 study, a comprehensive analysis of probabilities of rainfall events for various durations, from 5 minutes to 60 days, across the state of Texas.

“The 100-year estimates around Houston increased from 13 inches to 18 inches—almost 35 percent in value. Previously classified as 100 year events are now classified as much more frequent 25-year events,” said Mark Glaudemans, an official with the agency’s water prediction office. Those inch totals are for a 24-hour rain event.

What is called a 100-year event is a reference to the statistical likelihood of a rain event occurring in any given year in any given location. For example, a 100-year event has a 1 percent chance of happening every year. A 25-year event has a 4 percent chance.

Previous estimates that the agency had been using for the state were almost 60 years old in some cases, Glaudemans said. The new data is far more accurate and draws on many more sources of rainfall information, in some cases going back as far as 100 years, he said.

Hurricane Harvey dropped 6.8 inches over Harris County in the first hour and almost four feet of water over four days, according to data compiled by the Harris County Flood Control District. Using the new Atlas 14 standard, that would make Harvey’s first hour somewhere between a 500 and 1,000-year event, and its four-day total beyond a 1,000 year event.

The new NOAA standards override all other existing rainfall data, he said, and have implications for drainage systems, development guidelines, flood maps and flood insurance.

“I think with everything the Houston-Harris County area has been through, three major events in four years, I think that has a lot to do with these new numbers,” said Mark Hanna, a spokesman for the Insurance Council of Texas. “Anyone living anywhere with a even a hint that their home could flood, they should purchase flood insurance.”

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  1. The Stats here don’t make sense to me. Why would 6.8 inches in the first hour be classified at 500 to 1000 year event when the new standard for a 100 year event be 18 inches in the first hour.

    • Thanks for the comment, Chuck. We had an error in the text that has now been corrected. A 100-year event for Houston would have about 18 inches in a 24-hour period.

  2. So with this new data is any work being done to reconfigure what is necessary for new development on projects that have already been approved? Major examples would be the shopping center at the end of BlackHawk at Dixie Farm Road and the lakes at Riverstone Ranch Subdivision which barely held during Harvey?

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Matt Dulin
Matt joined Community Impact Newspaper in January 2018 and is the City Editor for Houston's Inner Loop editions.
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