Is Houston Going to Flood Again

Information technology's difficult to comprehend the calibration of the flooding and devastation that Hurricane Harvey and its aftermath are wreaking on the Houston expanse. Conditions experts call the storm unprecedented, and note that it'southward gone beyond even the most pessimistic forecasts. In the terminal reckoning, it's certain that Harvey will be classified a 500-year overflowing — and maybe even a 1,000-twelvemonth inundation.

Just those terms can be a scrap misleading — especially when high-profile people, like the president of the United States, confuse the consequence by calling Harvey "a once in 500 yr flood."

In theory, a 500-year alluvion is something that has a 1-in-500 shot of happening in any given year — in other words, the sort of upshot that's so rare that it might non make sense to programme around the possibility of it happening. The trouble is that 500-yr floods are happening more often than probability predicts — particularly in Houston. And, especially in Houston, prevention planning hasn't evolved to acknowledge that a "500-year" flood isn't actually a 1-in-500 chance anymore.

"500-year" floods are based non on history, but on probability

The severity of floods tends to get put in terms of years: a 100-year flood, a 500-twelvemonth alluvion, a one,000-year flood. Only this isn't an assessment of "the worst alluvion in" that time — places like Houston don't actually take detailed weather condition records going back to 1017 Ad, later all.

The lack of hundreds of years' worth of flood data is actually the reason we have the term "100-twelvemonth" alluvion to begin with. When the government decided to map flood-prone areas to improve the National Flood Insurance Plan in the early 1970s, the maps couldn't just use the worst flood ever recorded in a given area to judge what a "bad alluvion" would wait like — because some areas had more records than others, and besides, just because a bad flood hadn't happened yet didn't mean it couldn't.

Instead, the standard fix for mapping flood-prone areas was a compromise between the existing Army Corps of Engineers standards for dams and levees, and the (much more small) standards that most communities had prepare for overflowing prevention. The areas deemed at take a chance of a bad flood were the areas that had about a 1 per centum chance of flooding in any given year: in other words, the areas that would flood approximately one year out of every 100.

A 500-year flood is based on the same principle: Experts gauge that in any given yr, there's a 1-in-500 (0.ii per centum) chance a flood this bad will strike a detail expanse. In theory, that means that over 500 years, that will happen once: so there will be 1 alluvion that bad over a 500-twelvemonth period.

Of form, different areas flood at unlike frequencies. So how bad a alluvion has to get to authorize as a 1-in-500-year flood is going to vary depending on where you're judging information technology — and a alluvion will probably qualify as a "500-year" occurrence in some locations but not others. This table, for case, shows the flooding in various locations in the Houston area in Apr 2016. You can see that the flood reached 500-year levels in sure places, but in others it was the sort of event meteorologists would predict to happen every 10 years.

Harris Canton Alluvion Control Commune

When information technology comes to planning for future floods, you have to become a little more than abstruse. Then FEMA maps out 100- and 500-year "floodplains" — the places that would get flooded past the kind of rainfall that has a 1 percentage risk (or 0.ii percent chance) of falling on an area in any given year.

A 2013 map of the estimated 100-year and 500-twelvemonth floodplains for Harris County, where Houston is located.

Army Corps of Engineers, "National Economic Development: Flood Take chances Direction," 2013.

But probability never works out perfectly in practice (as you know if you've always flipped a coin twice and gotten heads or tails both times). And it's especially hard to go the probability of an event perfect when the circumstances keep irresolute — as they practice when we're talking nearly atmospheric condition events, which become more or less common depending on the underlying climate. And electric current climate change trends could hands increase the take chances of bad flooding — there'due south more water vapor in the atmosphere now (fix to condense into storm clouds and atmospheric precipitation), for example, than there was 70 years ago.

Every bit a consequence, FEMA has to keep updating its assessments of the floodplains — i.e., which locations should think of flooding as a i percentage possibility, and which should think of it as something with a 0.2 percent chance of happening in any given year. The FEMA maps for Harris County had just been updated in late 2016. Nonetheless, the city of Houston itself is working off a Hazard Mitigation Programme information technology developed in 2012 — based on where FEMA was so proverb the city's 100-year and 500-yr floodplains were located.

In the meantime, the reality of Houston'south flooding has already shown the old models to be out of date. An surface area of West Houston called Memorial City, for case, was outside Houston's 500-year floodplain but flooded three times in the past decade: in 2009, 2015, and 2016.

Information technology'south hard to design policy around preventing impairment for something with a ane-in-500 adventure of occurring...

Anti-alluvion policy tends to focus on 100-twelvemonth floods, not 500-yr ones. Many mortgage lenders require whatever homeowner living in a 100-year floodplain to buy flood insurance (homeowners with federally backed mortgages have to take alluvion insurance, as well). Only homeowners living in a 500-yr floodplain don't typically have to purchase overflowing insurance — after all, 500-to-ane odds are pretty long odds.

Similarly, it's hard to make city policies to prepare for remote events. In Houston, for example, the metropolis's current Gamble Mitigation Plan calls to discourage people from building homes in the 100-twelvemonth floodplain, just it doesn't say anything nearly the 500-year floodplain.

There'south a elementary, probability-based reason for this. Everything that a person (or city) tin practise carries some amount of adventure, and if a metropolis prevents any activity that might exist risky, it won't be able to allow people to build new houses at all. And then if something really but has a 1-in-500 adventure of occurring, information technology'southward not necessarily logical to forcefulness people to alter their plans considering of information technology.

...only certain areas are seeing "500-twelvemonth" floods pretty often these days

What if storms happen more often than probability suggests they would, though? What if a place gets hit with more than one 100-year tempest over a 10-yr period — or even in consecutive years?

The U.s.a. appears to be getting hit with major storms with unusual frequency. From Baronial 2015 to August 2016, there were eight 500-year flood events recorded by the National Weather Service. In that location were six "1,000-year" floods in the US over the five years from 2010 to 2014; in 2015 and 2016, though, in that location were at to the lowest degree three each year.

Looking at the US as a whole can be a flake misleading — it's a big state, and probability suggests that if there are a few k flood-prone locations effectually the country, a few places a twelvemonth volition feel 1-in-1,000 events. But what's more suggestive are the places that keep getting hit with the kind of severe flooding that's supposed to be uncommon — places like Houston.

Tomball, Texas, Public Works managing director David Esquivel told a local paper there this year that the Houston area had "two 500-twelvemonth storms back to dorsum": over Memorial Day weekend of 2015 and early April 2016. That ways that Hurricane Harvey constitutes the third "500-yr" flood in three years.

Flooding in Richmond, Texas, a metropolis in the Houston area, in 2016.

Photo past Bob Levey/Getty Images

Theoretically, the odds of a i-in-500 event occurring 3 straight times are i in 125 meg. Because Houston is a big city and the aforementioned spots aren't necessarily reaching 500-twelvemonth levels each time, those odds don't quite apply — but we're still, as the Memorial City instance shows, talking most events that FEMA estimates to be vanishingly unlikely.

Either Houston is incredibly unlucky or the chance of severe flooding is a lot more serious than the FEMA modeling has predicted — and the odds of a inundation as bad as the ones Houston has seen for the by few years are actually much higher than 1 in 500.

Houston isn't the only city to get striking by "historic" floods with rapid frequency. Cedar Rapids, Iowa, got striking with a 500-year flood in 2008, followed by a 100-year flood in 2016.

Afterward the 2008 alluvion, the urban center worked to develop a fundamental greenway that could be used to protect residents from future floods, and a plan to put upward temporary barriers. That plan worked to mitigate the damage done past the 100-twelvemonth flood of 2016. Just city officials acknowledged that if they'd been striking with a 2d 500-year flood in nine years, their planning wouldn't have been sufficient to protect them.

Houston wasn't even prepared for a 15-year inundation

Houston is not Cedar Rapids. For nearly a decade — ever since Hurricane Ike narrowly missed Houston in 2008 — urban center and country officials and experts have been alarm that Houston wasn't even prepared for a 15-yr tempest.

In 2016, ProPublica and the Texas Tribune collaborated on a project detailing how bad the damage from an Ike-caliber storm would be — and how little Houston had done to mitigate it. "We've done zippo to shore up the coastline, to add resiliency ... to practice annihilation," Phil Bedient, a Rice University professor who co-directs the Storm Surge Prediction, Education, and Evacuation from Disasters (SSPEED) Center, told ProPublica.

Fifty-fifty the things the city of Houston was trying to do weren't working — the mitigation plan chosen to discourage building inside the 100-year floodplain, but 7,000 homes were built since 2010 in depression-lying areas of the city. And while the Army Corps of Engineers and Harris Canton had launched a partnership to widen channels and build bridges in the Brays Bayou area to reduce the impact of flooding, the city was unable to muster the resources to build new seawall or floodgates — which Bedient and other experts agreed would exist needed to mitigate the impairment of a 100-year or 500-year tempest.

In April 2017, George P. Bush, the commissioner of the Texas General Land Office, wrote to the White House asking President Trump to allocate $15 billion for a "coastal bulwark system" to protect Houston and Galveston — what the press called a "hurricane wall."

The proposed system was designed to a "100-year-flood" standard, not a 500-yr-flood one. But information technology still likely would have been better than what the city currently has.

In 2015, Bedient told the Atlantic's David Graham that "I'yard really fairly optimistic that something will be done, because it needs to be done." But when Graham interviewed him the week earlier Harvey hit, Bedient conceded that "something" hadn't turned into much. "I don't think much has been done to mitigate flooding in the past 2 years," he told Graham.

In other words, for two years in a row, Houston saw 500-twelvemonth floods and didn't practise enough to prepare for the next 1. Now it's dealing with what could be a 1,000-yr flood — the sort of storm that no city could ever prepare for, but that could absolutely cripple a urban center that wasn't even prepared for a much smaller and more mutual disaster.

If 500-twelvemonth floods actually came only once every 500 years, Houston's reaction would exist understandable. But it'southward clear that something is securely wrong with how the Us judges flooding gamble. And when floods are hitting a metropolis 500 times more oft than probability predicts, it'southward reasonable to look that local and federal officials will start taking the possibility of the next "500-year" storm equally something that could come next year.

Description: This article has been updated to more accurately reflect the chronology of the National Flood Insurance Program; the NFIP itself was created in 1968, only overflowing maps weren't adult until the 1970s.

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Source: https://www.vox.com/science-and-health/2017/8/28/16211392/100-500-year-flood-meaning

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