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The 100-year-storm isn’t what it used to be

They’re bigger.

More than three months after Hurricane Harvey battered southeast Texas with unprecedented and costly flooding, an analysis of rainfall trends across Texas suggests the standards used to develop floodplain regulations, map flood zones and design flood control projects routinely underestimate the severity of the Houston area’s downpours.

That analysis by the National Oceanic and Atmospheric Association, which looked at rainfall data stretching back decades, up to and including Harvey, shows the amount of rain that defines a “100-year storm” – one that has a 1 percent chance of occurring in any given year – has risen by 3 to 5 inches in Harris County since the last estimates were put in place in 2002.

Instead of expecting 12 to 14 inches in a day during a 100-year storm, the data shows the county should expect 15 to 18 inches.

A higher rainfall estimate for a 100-year storm means developers would need to design subdivisions and strip malls to compensate for higher runoff, and more existing residential and business properties would be included in new floodplain maps that drive insurance costs and development regulations.

“We design our infrastructure and our society and homes to be resilient to a certain level of risk,” said John Nielsen-Gammon, the Texas state climatologist and professor of atmospheric science at Texas A&M University. “Having updated, more accurate numbers means that we’re better able to do that, and the risk we’re undertaking matches the risk we’re designing for.”

[…]

The preliminary data released last week includes estimates across Texas. A broad swath of Texas, stretching from Beaumont and Port Arthur, across Austin and the middle of the state, and all the way to Del Rio, indicates rainfall greater than that modeled during the 1961 study. In some parts of west Texas, the data shows the amount of rainfall indicating a 100-year storm should actually be lower than current standards.

St. Laurent said the increase in the 100-year storm severity for the Houston area could be attributed in part to the several severe storms the region had experienced since 2002. NOAA included data from Harvey in its analysis.

“The additional decade or two of data have some significantly high rainfall events that definitely contribute to higher precipitation estimates,” he said.

Nielsen-Gammon, the state climatologist, said any time officials have more data with which to work, estimates change, particularly when including events like the Tax Day flood and Harvey. The new estimates, he added, also reflect a changing climate.

“You wouldn’t want to look at an individual location and say that all of the change there is due to climate change,” Nielsen-Gammon said. “But if you look at the state as a whole, it seems that the increases have outnumbered the decreases, at least in the current draft. So, that’s some combination of longer-term natural variability and climate change that’s doing that.”

I don’t have much to add to this. As a region, we seem to be internalizing the notion that we can’t keep doing what we’ve always done, with regard to development and flood mitigation. That’s good and necessary and long overdue. The much bigger question is whether we are internalizing the fact that climate change is a big part of the reason why we can’t keep doing what we’ve always done, and that means we can’t keep polluting and emitting carbon as before. That’s a question for more than just the region, or for the state. I think – I hope – our region is up to our part of the task. Whether our state and our country are remains to be seen, but the track record of the powers that are now in charge is not encouraging. That needs to be part of the discussion, not just in the 2018 campaign but in every campaign after 2018 as well.

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One Comment

  1. Jason Hochman says:

    I am having a bit of trouble understanding this. The designation of a 100 year storm sounds a bit arbitrary. So, is this saying that every year has a 1% chance of a storm dropping 12-14 inches of rain, but now, there is a 1% chance of a storm dropping 15-18 inches of rain? Does this mean that, for example, there is now a 2% chance every year of a 12-14 inch rain storm?

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