What would Isaac have done to Houston?

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by Doug Miller / KHOU 11 News

khou.com

Posted on August 30, 2012 at 7:07 PM

Updated Thursday, Aug 30 at 7:20 PM

HOUSTON—Every time a hurricane spins into the Gulf of Mexico, we watch and wonder whether it will come ashore close us.

Usually, the storm veers away from Galveston Bay, striking somewhere else along the coastlines curving around the gulf. We see a minute or two of television news coverage and we forget about it.

Sometimes, though, a storm smashing into another city captures our attention. We pity and pray for the people caught in its path. And maybe we wonder whether the same sort of destruction could happen here.

That’s what’s happened with Hurricane Isaac. After the destructive Diaspora of Katrina, many Houstonians have a visceral reaction to hurricanes hitting New Orleans, but the dynamics of Isaac struck an even more ominous tone with people who lived through the stunning destruction of Tropical Storm Allison.

All of which raises an interesting question: What would’ve happened if Hurricane Isaac had hit Houston?

We took that question to the Rice University office of the city’s leading expert on flooding, Dr. Phil Bedient. His answer was not reassuring.

"Oh my gosh, it would’ve been massive flooding," Bedient said. "And it would’ve been widespread because of the sheer size and magnitude of the rains that hit."

On a good day, Bedient explains, Braes Bayou can handle maybe 6 ½ inches of rain before flowing over its banks. Isaac dumped a foot of rain on New Orleans in a single day, he says.

"Louisiana got really hammered because, again, it’s a slow-moving system," he explained.

Notice the word "again." Bedient makes the inevitable comparison to Allison, another slow-moving rain dumper that wrought massive flood damage in Houston. 

Allison formed quickly in the gulf, slammed ashore as a tropical storm, then stalled and spun in an arc around the Houston area. Over five days in July 2001, it struck Houston in three successive waves of heavy rainfall. The last wave slammed in on a fateful Friday night that caught most Houstonians by surprise, dumping 26 inches of rain on the city in just eight hours, according to KHOU’s Dr. Neil Frank, who followed the storm’s progress. 

"What happens when theses storms stall," Bedient explained, "it basically acts like a gigantic rainfall pump that just pumps those bands of rainfall in for long periods of time."

Now, imagine a storm hitting Houston with Allison’s rainfall and Isaac’s hurricane force winds. 

"This is not out of the realm of possibility," Bedient said. "A slight kink in the driving mechanism of a storm like this could bring it right into Houston."

Indeed, a decade ago, Houston city officials publicly said they shuddered at what might have happened if they’d had to orchestrate clean-up after an Allison-style food and a hurricane force windstorm. 

Houston does have one major advantage over the areas of Louisiana struck by Isaac: Unlike New Orleans, the Bayou City sits about fifty feet above sea level.          

"And we do not have huge areas that are below sea level and that reside behind dikes and dams and levees that are frail," Bedient says.

Still, he warns, no matter how much money we spend on flood control, Houston will always be vulnerable to slow-moving storms dumping deluges of rain and flooding our streets and homes.

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