100-Year Flood, 500-Year Flood: Real Risk Probabilities

When the Army Corp of Engineers and NFIP came up with the 100-Year Flood and 500-Year Flood designations, it’s almost as though they wanted to confuse the public. With Hurricane Harvey, much has been written on the meaning of the terms 100-year flood (it means a 1% chance of flooding in a single year), and the term 500-year flood (a 0.2% chance of flooding in a single year). While these basic definitions are correct, they don’t really help homeowners, whose question is: what’s the chance that my house will flood while I own it?

In the case of the 100-year flood zone, this means that the chance of flooding is at least 1% in a single year. But what if you plan to own your home for 30 years? In this case, you have a 99% of NOT flooding each year, but you’ve got to NOT flood for all 30 years. The probability of NOT flooding over 2 years is 0.99 * 0.99, and thus the probability of not flooding over 30 years is 0.99^30, or 74%. This means that the home has a 26% of flooding over the 30 years in question.

Of course, that doesn’t take into account the change in probabilities resulting from a combination of climate change and reckless development in most American cities. According to Kenneth Trenbeth, a scientist at the National Center for Atmospheric Research, “What used to be a 500-year event has become a 50- or 100-year event.” With this in mind, we can lay out the following table of homeowners’ flood risks:

Flood Probability Over 10 Years Flood Probability Over 30 Years
100-year flood zone  10% 26%
100-year flood zone, climate-change adjusted (to 10-year flood) 65% 96%
500-year flood zone 2% 6%
500-year flood zone, climate-change adjusted (to 50-year flood) 18% 45%

The flood risks over 30 years likely exceed many homeowners’ assumptions even before accounting for climate change – the climate-change adjusted risks make flooding within flood zones a virtual certainty! Homeowners in these areas are well advised to buy flood insurance, which is an incredible value as it is priced by the government below fair value. Homeowners not in, but simply near the 100-year floodplain, should realize that THEY are now likely in the true 100-year flood risk area, while their neighbors in the floodplain are likely at much greater risk.

Think that the probabilities can’t possibly have shifted that much? Consider that parts of Houston have had 3 500-year flood events since 2001 – Harvey, the 2016 Tax Day Flood, and Hurricane Allison. The chance of having one such “500-year” flood in 16 years is around 3.2%, but three such events? It’s less than 0.15% if these are really 500 year floods! [1] The reality is that 500-year floods are likely more than 10 times more common than the old statistics indicate, and homeowners should plan accordingly.


[1] The chance of 3 or more 500 year floods is equal to 100% minus the chance of 0, 1, or 2 such floods. The chance of 0 floods is 96.85%, and the chance of exactly 1 flood is roughly 3%, leaving less than 0.15% for the other potential outcomes (which drop off very rapidly because of the 0.2% chance happening multiple times).

Global warming is real; should you care?

There is little scientific dispute at this point that global warming is occurring, and that humans are causing some part of it, as even the Competitive Enterprise Institute (a conservative free enterprise think tank) is now willing to admit. The remaining question: just how big a problem is global warming, and what are the consequences if we do nothing? An Inconvenient Truth makes it appear that Florida will be underwater sometime soon if we sit idly by. Reality, at least as scientists currently understand it, probably lies somewhere between Al Gore’s doom-saying and CEI’s laughable slogan, “CO2: We Call It Life.”

Of all the potential effects of global warming, rising sea levels are thought to have the most catastrophic consequences. If the Greenland ice sheet or a large part of Antarctica really do melt, the resulting 20 foot rise in sea levels would destroy the majority of the world’s great cities and displace billions of people. But how long will a rise of 20 feet, or even two feet, take at current rates of warming and ice melt? Realclimate.org summarizes recent research here, wherein the most aggressive estimates indicate that Greenland’s ice sheet melting is increasing sea levels by up to 0.57mm per year. But if Greenland continues melting at that rate, it will take one thousand years to raise sea levels one foot!

Even an order of magnitude increase in ice melting would only cause sea levels to rise a foot by 2100. The Intergovernmental Panel on Climate Change Report’s most aggressive estimate calls for a three foot sea level rise by 2100; this estimate includes significant ice melt. While this scenario has significant implications for coastal cities, it is not apocalyptic, and it also represents an outlier prediction compared to most climate models. It seems then that we should care about global warming in the very long term, but it is unlikely to have a significant impact during our lifetime. There are dozens of environmental and social issues that deserve greater present concern, including the AIDS pandemic and infectious diseases like tuberculosis and malaria, which continue to kill tens of millions annually.

At the same time, it wouldn’t hurt to take some simple steps to curb CO2 emissions growth. The CEI and others complain that it is impossible to curb CO2 emissions growth without hurting the economy. On the contrary, prudent shifts in government policy can reduce emissions while increasing growth. If the United States were to fund all highway construction with gasoline taxes, for instance, this would pass the costs of car travel directly on to the end consumer – which increases economic efficiency while decreasing emissions. I’ve written previously about applying the gas guzzler tax fairly, so that consumers are not rewarded for buying large SUVs instead of large cars. Finally, ending the huge subsidies to the oil, gas, and coal industries would make alternative energy more competitive, while saving taxpayers billions.