Rainfall Rates, Durations and Frequencies for This Area

The upper Texas Coast is famous for intense, frequent rainfall. Sometimes, like during Harvey, rainfall can last for days. So how do you know when you’re experiencing something truly out of the ordinary? Consult the table below. This table relates three factors: rainfall total, rainfall duration, and rainfall classification. From this chart, you can see that all it will take for us to have our fourth five-hundred year storm in four years is about an inch an hour for 18 hours, or about two inches per hour for six hours.

Rainfall Rates, Intensities and Frequencies for The Woodlands Area on the West Fork, near Humble and Kingwood, Texas

What are the odds of getting hit with three 500-year storms in three years (which we did in 2015, 2016 and 2017)? One might think they are 1 in 125 million which was computed by multiplying 1/500 * 1/500 * 1/500.

The odds of getting four 500-year storms in four years would then SEEM astronomical. Using a similar formula, you would arrive at 1 in 62.5 billion!

But that is not necessarily correct because with that calculation you are inferring that the rainfall events are connected. But they actually are not connected. Just because we had a 500-year rainfall event last year, does not mean we may not see another 500- year rainfall event this year.

EVERY year we have a 0.2% chance or 1 in 500 chances of seeing a 500-year flood for a specific location.

This assumes that the odds are no greater in one year than any other year, and that each event is independent of the others.

How do mathematicians compute the probabilities of these rare events? Obviously, it isn’t through observation. The earth is only about 4.5 billion years old. Humans have only walked the earth for about 200,000 years. And reliable rainfall records in this part of the world only go back a little more than a 100 years.

Probabilities for rare events, such as hundred- and five-hundred year storms are based on a branch of statistics called EVA, extreme value analysis. EVA tries to calculate the probabilities of unobserved events by looking at the distribution of observed events.

But all this technical brilliance is based on one particularly flawed assumption that never gets communicated to the public. The assumption is that for the period under examination, nothing changes. Mathematicians even have a word for it: stationarity. It means underlying factors can neither increase, nor decrease.

Duh! Nothing changes in 500 years? In Houston?

Obviously, those folks never rode around for a day in a Ford F350 with a Houston developer.

In 1900, Houston had a population of 44,000 and was the 85th largest city in the U.S.

Today, the Houston region has a population of more than 6.9 million. That’s growth of 157X in a little more than a century. And that’s a lot more concrete than even Bubba and Jim Bob together  could spit on in a lifetime.

Diane Cooper, a Kingwood resident with more than 20 years of forecasting experience for the National Weather Service points out a couple other problems with these projections. First, the data is very, very, very thin and rarely updated.

Second, the probabilities are computed for a specific point, not a city, county, region or country. Storms know no geographic boundaries.

In fact, she says, it’s a little bit misleading to say that Houston got hit by three 500-year storms in three years. That’s because any given storm may not have equal intensity over all parts of the city. A storm may have had 500-year intensity on the north side. but only 100-year intensity on the south. Following the same line of logic, but in a different direction, if you expanded the boundaries out to the entire U.S., we might have multiple 500-year storms in one year (each in different places).

Cooper also points out that 500-year storms do not necessarily produce 500-year floods. They are two different beasts.

If the ground is dry, say from a drought, a large percentage of a heavy rain might be absorbed, yielding less than a 500-year flood. Conversely, if the ground is saturated and we get a 100-year rain, get out the oars and inner tubes.

Even though charts like the one above have more uncertainty than a dart player who just downed a fifth of Jack Daniels, they do put big storms in perspective.

By the way, the term “500-year flood” originated in the 1960s when the National Flood Insurance Program was being developed. At the time, people intended it to mean “a storm with a .002% chance of happening in any given year.” However, over the years, the meaning became distorted. Because it had a 1 in 500 chance of occurring each year, insurers started calling it a 500-year storm. People mistook that to mean “the interval between intense storms.”

More on that in a future post and how to calculate the chances of getting hit by a monster storm during the life of your 30-year mortgage. Hint: call your insurance agent now!

Posted May 23, 2018 by Bob Rehak

267 days since Hurricane Harvey