Las Vegas Flooding on 1.24 inches of rain

Giving Thanks to the Women and Men of Harris County Flood Control

A reader who visited a trade show recently in Las Vegas sent me several links to news stories about flash flooding there. 1.24 inches of rain caused widespread flooding, killed at least two people, and resulted in dozens of high water rescues!

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His comment: “Imagine if those types of videos were in Houston – for less than two inches of rainfall. You can’t, because it doesn’t happen.”

Why Houston Doesn’t Flood On Two Inches

While HCFCD employees take a lot of heat every time someone floods, we should remember that it takes far more rain for people to flood here. There are several reasons for that.

First, Harris County formed a flood control district in 1937. Clark County, Nevada (Las Vegas) didn’t start its until 1985. So, we had an almost 50-year head start on them.

But sadly, some fast-growing counties around Harris County STILL don’t have flood control districts! (We’ll save that discussion for the next legislative session.)

Second, our topography is different. Because Harris County is so flat, rainfall spreads out and starts soaking into the ground before flooding starts. Rainfall in Las Vegas is funneled by the rugged landscape. It picks up velocity, so it doesn’t have time to soak in. Concentrated rainfall turns into flash flooding. The Las Vegas Wash funnels a 1,879 square mile watershed toward a metro area of 2.29 million people.

I was almost killed by a flash flood in Tucson once. While hiking along a stream bed with friends in the desert, we saw rain in the distant mountains. They immediately suggested moving to higher ground. Minutes later, a wall of water 6-8 feet high came boiling down that stream bed!

Third, we build to different rainfall standards. Las Vegas averages 4.18 inches of rain each year. Harris County averages 51.84 inches.

In Harris County, new building codes and flood-mitigation standards currently use the 24-hour, 100-year amount shown in the Atlas-14 table below – 17.3 inches.

atlas 14 rainfall probabilities
Atlas-14 rainfall probabilities for northern Harris County now form the basis for building codes and flood-mitigation projects.

In other words, we build things to withstand more rain in a day than Las Vegas receives in FOUR YEARS.

That takes some talent. Especially when surrounding areas send ever-increasing amounts of floodwater downstream because of lack of comparable controls upstream. And that could be why flood control districts around the country try to recruit talented HCFCD employees.

Despite our occasional frustrations, we should never forget: They stand between us and disaster.

Posted by Bob Rehak on 8/13/2022

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