In the School of Hard Knocks, there’s an introductory course called, “Money Has a Short Memory.” Most students fail this free course and, as a consequence, are still paying “tuition” years later. The irony was never more visible than last week. As I reviewed a Houston Public Media Story about how the City of Houston was not attempting to curb development in the 100-year flood plain – despite everything we learned from Harvey – I had a presentation about the 1994 flood waiting for review on my desktop.
1994 Flood Should Have Taught Us Lessons We Still Haven’t Learned
The presentation, “Rain by the Cubit: The Great Southeast Texas Flood of 1994,” brought back memories. That was the year I started my company. I was supposed to move into my first commercial office space when this flood hit.
Kingwood received 29″ of rain that week. Rainfall averaged 19.5 inches over the entire 2,880-square mile San Jacinto River watershed. The event lasted four days. It started on Saturday, October 15, 1994 when Pacific Hurricane Rosa met a gulf coast warm front over Texas. It affected 38 Texas Counties, an area as large as Maine.
1.9 million acre-feet of runoff passed through Lake Houston: almost 12 times the volume of the entire lake! The lake crested 8.3 feet above the 3,160-foot spillway.
Stunning Photos of 1994 Flood
The presentation contains photos of flooding:
- On Atascocita point, where new construction was just beginning at the time.
- In Forest Cove townhomes that would flood at least four more times before buyouts
- In Banana Bend below Lake Houston, which is also just now being bought out
- Around Toys ‘R Us on 59 – before an entire strip center of big box stores surrounded it
- That collapsed the 59 bridge
- That downed power lines over Lake Houston
- That went up to the roofline of what was then Reeves Furniture on the southbound 59 feeder just north of the West Fork
- That ruptured pipelines across the San Jacinto and started a toxic blaze
- That buried downstream areas in sand and gravel.
Sound familiar? It should. Virtually all those things happened during Harvey, with the exception of the pipeline fire. However, toxic waste pits were involved during Harvey.
What are the Chances?
At the time, experts opined about how rainfall exceeded the expected 100-year levels. But the new Atlas-14 data released by NOAA, now advises that a four-day flood averaging 19.5 inches would have an average recurrence interval of 50 years.
After Harvey, people dazed by the devastation, solemnly concluded that the storm must have been a 500-year, a 1,000-year, or even a greater storm. They had absolute faith in the numbers that developers, engineers, bankers, insurers, and government agencies certified. They assumed storm intensity had to be greater than expected. It never occurred to them that perhaps the numbers could be off…in the other direction.
How Average Recurrence Interval is Determined
All these numbers (500-year, etc.) are based on extremely small data sets. Forecasters use a branch of mathematics called Extreme Value Analysis (EVA). With EVA, they try to forecast the probability of unobserved future events based on the frequency of somewhat smaller past events. EVA may produce the best numbers possible, but predicting 500-years into the future based on 100 years of data takes a lot of guess-work.
Limitations of Numbers
Complicating things, most people are oblivious to the nuances of probabilities. The naming convention (100-year storm) misleads them into thinking that if we had a 100-year storm last year, “we must be good for another 99 years.” Wrong. Theoretically, if you tossed a coin and it came up heads 99 times in a row, you have a 50:50 chance of getting heads on the hundredth toss, too.
How many people read…or understand…the fine print in tables like the one above? Did you read the footnotes? If not, please go back and read them now. It’s important for your own safety and the safety of your investment.
They’re trying to say, “We can’t predict extremes with accuracy.”
Conclusions of 1994 Flood Presentation
Yung and Barrett conclude with several warnings. They include.
- Extreme rainfall events will continue to occur.
- The adoption of criteria that exceed FEMA minimum requirements should be considered by communities to guard against severe events.
So until the City learns this lesson, what’s someone without a PhD in math supposed to do when buying a home? Forego the river or lake view and buy on the highest ground you can find. Buyer beware! There are huge markups on floodplain property. And money has a short memory.
Posted by Bob Rehak on 6/9/2019, based on a presentation by Andy Yung and Duange Barrett
649 Days since Hurricane Harvey