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The Problems With “100-Year Floods”

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texas 100 year flood

Above, the aftermath of Hurricane Allison. Pictured below, Houston last month. Two 100-year floods less than two decades apart.

It’s time to ditch the whole concept of 100-year storms and rain events and come up with a new term.

I think it was in the aftermath of Tropical Storm Allison that I first started hearing these phrases on the news, along with the even more dire “500-year flood.”

Then, as now, most people erroneously take that to mean that such devastating deluges take place once a century or half-millennium, as the case may be. And, gee-willikers, I wonder why they would do that? I mean, what kind of dum-dum would actually think that something called a “100-year-flood” would happen once every 100 years?

But even then, in June 2001, the concept was a head-scratcher to me, as a similarly disastrous “rain event” took place in Houston in December of 1935. If my math was correct, I thought to myself, that was less than a hundred years before Allison. And the flood of ’35 came only six years after the flood of ’29. Ike brought more flooding seven years after Allison, and I could remember other flash floods wrecking homes, drowning motorists, and paralyzing the city on several occasions in my youth in the seventies and eighties. That’s a lot of 100-year floods for an 85-year period.

Yeah, yeah, I know. A 100-year-flood does not mean that these floods happen once a century. Per the United States Geological Survey:

The term “100-year flood” is used in an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year.  

This attempt to simplify does so at the expense of offering any meaning whatsoever. You could have 100-year-floods three years running. You could have two 100-year floods in a single year. Or you might go 300 years or more without a single one. Probability is weird like that. Also of note from a probability standpoint: if you gamble with a 1 percent chance of losing once a year for thirty years, you end up with a 25 percent chance of crapping out at least once. Think about those odds the next time you sign a mortgage for a house in what is reassuringly described to you as located in a 100-year floodplain. (Across thirty years in a 500-year floodplain, you run a six percent chance of losing.)

And there is such a thing as official 100- and 500-year floodplains. They exist to determine who must buy flood insurance and how much they must pay, and whether or not developers can or should be allowed to build in certain low-lying areas.

Much of Houston’s choicest real estate lies in these areas, as this map attests. Over the past century, Houston developers built willy-nilly alongside Houston’s bayous, gullies, creeks, and ditches, either unaware or not caring that these homes and businesses were in floodplains. Today much of Rice University, the Texas Medical Center, West University Place, and the city of Bellaire are in floodplains described as 100-year. So are the suburbs of Meyerland and Braeswood Place, the former of which bore the local brunt of these latest storms and the latter of which saw a quarter of its housing stock destroyed by Allison.

And with two concepts as fluid as the climate and the ever-evolving beast that is the city region Leviathan of Houston, past cannot predict future.

Yes, we probably have decent weather data for downtown, the Heights, the East End, and Montrose for the past 100 years, but how many people were scientifically monitoring the Sims, Greens, and Brays Bayou watersheds in 1915? How do we know what a 100-year flood even means when we haven’t been measuring the bayous for 100 years? Especially since those areas were raw prairie back then—and now are not.

Zooming out from the Inner Loop, how much former prairie in the Houston area has been paved over just since Allison, much less in the last century? How many McMansions have sprouted up and enveloped whole lots, rendering how much more total land area impermeable? Since 2001, the region has been working fitfully on large-scale flood control projects, but those are now running seven years behind schedule, and even if they were finished on time, could they have mitigated this storm’s damages?

It’s a good thing those safeguards are in place, one expert says, but they lag behind the whims of Houston’s developers.

“Houston may be doing things to try to improve … but there’s a long history of pre-existing stuff that is still there,” said Walter Peacock, an urban planning professor at Texas A&M and director of the school’s Hazard Reduction and Recovery Center.

“Think about every time you put in a road, a mall and you add concrete, you’ve lost the ability of rain to get into the soil and you’ve lost that permeability,” Peacock said. “It’s now impermeable. And therefore you get more runoff.”

Or as A&M’s Sam Brody, director of that school’s Center for Beaches and Shores put it in the same article:

“Houston is so vulnerable. There’s very little topography. They’ve added hundreds of miles of pavement and can’t keep up with all the positive initiatives. … So we get these floods.”

And also this:

 “Houston is the No. 1 city in America to be injured and die in a flood.”

In the grand scheme of Houston rainstorms, this one was significant, but no monster. Compared to Allison, it was an April shower:

FEMA reported Allison dumped 32 trillion gallons of water. Early estimates for this week’s storm are about 162 billion gallons, with about 4,000 homes reporting damage. In Allison, 73,000 homes were damaged, plus 95,000 cars and trucks. Thirty people died in the Houston area, including 22 in Harris County. The death count Wednesday here was seven. About 2,500 cars were abandoned.

Thirty-two trillion. Thirty-two trillion! Scientists estimate that there are 300 billion stars in the Milky Way, so multiply that times a hundred to get how many gallons of water gushed forth from Allison. That is only five trillion fewer gallons than the amount that fell during this year’s Memorial Day weekend storms, across all of Texas.

And compared to 1979’s Tropical Storm Claudette, Allison was just another thunderstorm. (Forgive us for sounding like one of Monty Python’s Four Yorkshiremen here.)

Nobody talks much about Claudette today mainly because that storm spent its fury on an area that was then sparsely populated. That would be Nolan Ryan’s hometown of Alvin, and Alvin still owns the U.S. record for rainfall in a day. Forty-three inches fell in 24 hours. That’s a shade under four feet in one day, people, and it happened in the Houston area only 22 years before Allison, and 36 years before this one. (Also of interest: Houston can flood any time of year. It’s not all about tropical storms. For every Ike, Claudette, or Allison, there are events like this one or that of ’35, which occurred in December.) Yet another dagger in the heart of the concept of the 100-year flood.

Houston attorney Chris Odell has litigated floodplain cases and thinks that the concept could be skewed by another factor.  “What is the process for reviewing and revising the one-hundred-year-flood designations? Now you’ve got to wonder whether with climate change, whatever the policy is now, does it need to be updated itself, so we can now track if the original definitions are still effective,” he tells the Daily Post. “I think you are going to see more and more flood events that reach farther and farther than what we expect, and those hundred-year floodplain markers are going to have to move.”

So, given that they are based on faulty or nonexistent data, seem to have been generally ignored here as a matter of course, and might not be reliable going forward, why exactly do we still speak of 100-year floods, in Houston, or anywhere else for that matter?

(Photos: AP/Houston Chronicle)

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  • Jed

    your basic point still holds, even with the clarification.

    if all these floods are being called floods that “only had a one percent chance of happening,” somebody out there doesn’t understand what one percent chance means.

    the problem, of course, is what may have been a “one-percent” flood fifty years ago is a whole lot more likely now (odell’s point).