It’s the city’s third “500-year” flood in the past three years.
It’s difficult to comprehend the scale of the flooding and devastation that Hurricane Harvey and its aftermath are wreaking on the Houston area. Weather experts call the storm unprecedented, and note that it’s gone beyond even the most pessimistic forecasts. In the final reckoning, it’s certain that Harvey will be classified a 500-year flood — and maybe even a 1,000-year flood.
But those terms can be a bit misleading — especially when high-profile people, like the president of the United States, confuse the issue by calling Harvey “a once in 500 year flood.”
In theory, a 500-year flood is something that has a 1-in-500 shot of happening in any given year — in other words, the sort of event that’s so rare that it might not make sense to plan around the possibility of it happening. The problem is that 500-year floods are happening more often than probability predicts — especially in Houston. And, especially in Houston, prevention planning hasn’ t evolved to acknowledge that a “500-year” flood isn’ t really a 1-in-500 chance anymore.
The severity of floods tends to get put in terms of years: a 100-year flood, a 500-year flood, a 1,000-year flood. But this isn’ t an assessment of “the worst flood in” that time — places like Houston don’ t actually have detailed weather records going back to 1017 AD, after all.
The lack of hundreds of years’ worth of flood data is actually the reason we have the term “100-year” flood to begin with. When the National Flood Insurance Program was created in 1968, and the government was tasked with mapping flood-prone areas, the maps couldn’ t just use the worst flood ever recorded in a given area to judge what a “bad flood” would look 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 set for mapping flood-prone areas was a compromise between the existing Army Corps of Engineers standards for dams and levees, and the (much more modest) standards that most communities had set for flood prevention. The areas deemed at risk of a bad flood were the areas that had about a 1 percent 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 estimate that in any given year, there’s a 1-in-500 (0.2 percent) chance a flood this bad will strike a particular area. In theory, that means that over 500 years, that will happen once: so there will be one flood that bad over a 500-year period.
Of course, different areas flood at different frequencies. So how bad a flood has to get to qualify as a 1-in-500-year flood is going to vary depending on where you’ re judging it — and a flood will probably qualify as a “500-year” occurrence in some locations but not others. This table, for example, shows the flooding in various locations in the Houston area in April 2016. You can see that the flood reached 500-year levels in certain places, but in others it was the sort of event meteorologists would predict to happen every 10 years.
When it comes to planning for future floods, you have to get a little more abstract. So FEMA maps out 100- and 500-year “floodplains” — the places that would get flooded by the kind of rainfall that has a 1 percent chance (or 0.2 percent chance) of falling on an area in any given year.
But probability never works out perfectly in practice (as you know if you’ ve ever flipped a coin twice and gotten heads or tails both times) . And it’s especially hard to get the probability of an event perfect when the circumstances keep changing — as they do when we’ re talking about weather events, which become more or less common depending on the underlying climate. And current climate change trends could easily increase the chance of bad flooding — there’s more water vapor in the atmosphere now (ready to condense into storm clouds and precipitation) , for example, than there was 70 years ago.
As a result, FEMA has to keep updating its assessments of the floodplains — i.e., which locations should think of flooding as a 1 percent 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. However, the city of Houston itself is working off a Hazard Mitigation Plan it developed in 2012 — based on where FEMA was then saying the city’s 100-year and 500-year floodplains were located.
In the meantime, the reality of Houston’s flooding has already shown the old models to be out of date. An area of West Houston called Memorial City, for example, was outside Houston’s 500-year floodplain but flooded three times in the past decade: in 2009,2015, and 2016.
Anti-flood policy tends to focus on 100-year floods, not 500-year ones. Many mortgage lenders require any homeowner living in a 100-year floodplain to buy flood insurance (homeowners with federally backed mortgages have to have flood insurance, as well) . But homeowners living in a 500-year floodplain don’ t typically have to purchase flood insurance — after all, 500-to-1 odds are pretty long odds.
Similarly, it’s hard to make city policies to prepare for remote events. In Houston, for example, the city’s current Hazard Mitigation Plan calls to discourage people from building homes in the 100-year floodplain, but it doesn’ t say anything about the 500-year floodplain.
There’s a simple, probability-based reason for this. Everything that a person (or city) can do carries some amount of risk, and if a city prevents any activity that might be risky, it won’ t be able to allow people to build new houses at all. So if something really only has a 1-in-500 chance of occurring, it’s not necessarily logical to force people to change their plans because of it.
What if storms happen more often than probability suggests they would, though? What if a place gets hit with more than one 100-year storm over a 10-year period — or even in consecutive years?
The US appears to be getting hit with major storms with unusual frequency. From August 2015 to August 2016, there were eight 500-year flood events recorded by the National Weather Service. There were six “1,000-year” floods in the US over the five years from 2010 to 2014; in 2015 and 2016, though, there were at least three each year.
Looking at the US as a whole can be a bit misleading — it’s a big country, and probability suggests that if there are a few thousand flood-prone locations around the country, a few places a year will experience 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 director David Esquivel told a local paper there this year that the Houston area had “two 500-year storms back to back”: over Memorial Day weekend of 2015 and early April 2016. That means that Hurricane Harvey constitutes the third “500-year” flood in three years.
Theoretically, the odds of a 1-in-500 event occurring three straight times are one in 125 million. Because Houston is a big city and the same spots aren’ t necessarily reaching 500-year levels each time, those odds don’ t quite apply — but we’ re still, as the Memorial City example shows, talking about events that FEMA estimates to be vanishingly unlikely.
Either Houston is incredibly unlucky or the risk of severe flooding is a lot more serious than the FEMA modeling has predicted — and the odds of a flood as bad as the ones Houston has seen for the past few years are actually much higher than 1 in 500.
Houston isn’ t the only city to get hit by “historic” floods with rapid frequency.
