Setting the Record Straight — or Getting Scared Straight — on Our Recent Flood
New Orleans 17th Street Canal, after Katrina breached its floodwall.
Maybe flooding is always a glass half-empty. Who wants it full? Monday, some of the 200-plus evacuees in the area were still out of their homes; some houses were still inundated and two deaths had occurred, but The Dallas Morning News editorial page crowed that Dallas had “tamed the great floodwaters.”
Really? Tamed them? I guess we didn’t have to get into an ark and float around with a bunch of goats for 150 days, but for many people the experience was close enough. For the dead it was much worse.
And then there is this: The Trinity River is still at flood levels and the reservoirs upstream, especially Lakes Lewisville and Ray Roberts, are still, at this writing, 14 and 11 feet above their maximum safe water levels almost a week after the rains stopped.
What that means is that floods in Dallas have precious little to do with nature. Since the mid-1980s, the North Texas region has pursued an almost entirely mechanical means of flood control, man-made and run by human beings, rather than adopting the state-of-the-art flood control policies in effect in places such as the Netherlands.
Called “Living with Water,” the Dutch system is a paradigm shift from early 20th century flood control methods that just didn’t get the job done. The central scientific and political insight at the core of the Dutch system is the realization that channels and reservoirs, whether natural or man-made, cannot and will never be capable of containing all the water that falls from the sky.
Our system here is based entirely on beating Mother Nature by catching her deluges in basins and behind dams and levees. But Mother Nature intends for most of her rain to fall on open soil and keep right on raining down through the earth into the water table.
Not only do we defy Mother Nature here, we also kick her in the ass by utterly failing to control the spread of concrete and rooftops across our watersheds. So all of that water courses quickly across impervious surfaces and winds up overflowing the reservoirs. And we have a bunch of engineers working 24/7 opening and shutting valves trying to spill it around evenly so nobody gets wiped out.
The people all around Lake Lewisville took it on the chin in this one so that the levees would not breach in downtown Dallas. That’s a simple fact, and people need to know it. It is remarkable and reprehensible that our officials pursue things like plans to build an entire new expressway right in the middle of the most crucial and sensitive portion of the entire flood control system, when they have never convened an effort to know exactly how the system works and what the risks are.
We have a special City Council committee devoted to pushing the toll road project. But we don’t have a committee devoted to understanding basic flood control.
That’s upside down. It’s crazy. It’s dangerous and fantastically irresponsible.
A week ago James Frisinger, spokesman for the Fort Worth District of the U.S. Army Corps of Engineers, gave me good information about flood levels on the Trinity River, some of which I bollixed up at the time, my excuse being that his numbers were not compatible with numbers I found in Dallas City Council briefing materials. By now the city and the Corps have coordinated to make their numbers match, more or less, so I can state a few facts about the recent flood based on their shared measurements.
When the river hit its highest crest of 42 feet on the levee system downtown — about 410.5 feet above sea level — it would have been a foot and a half above the planned toll road, had the toll road been in place already. So this recent flood, which was nowhere near the major flood against which government officials have repeatedly promised us the road would be protected, would have reached a height higher than the toll road.
The so-called “100-year flood” would push the river up to 48 feet on the levees or 416.3 feet above sea level, Frisinger told me and the city now agrees. The earthen bench on which the toll road is to be built would raise the road to only 41 feet on the levee or 409 feet above sea level.
So the 100-year flood — the kind of event from which the road is supposed to be safe — would fill the levee system with water more than 7 feet higher than the road. But, wait.
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The city and the North Texas Tollway Authority (NTTA), which is designing the road, assures us that neither the current flooding conditions nor even the 100-year flood would wet the road, because the road would still be protected by a “floodwall.”
Pause with me for one second. Reflect on this. If you thought the road was going to be higher than the water, you were mistaken. It will be way lower — more than seven feet — than the big “100-year” flood. It will be lower by a foot and a half than the flood we just had.
But it’s worse than that, because those numbers reflect the toll road at its greatest height. The toll road will have to go up and down in order to swoop under a series of bridges. When the toll road dips down to go underneath bridges like the one at Commerce Street, it will fall to about 403 feet above sea level according to NTTA renderings — more than 7 feet beneath the surface of the flood we just had and more than 13 feet beneath the level of the big one, according to the NTTA’s renderings.
So what does that mean? Glug-glug-glug, three men in a tub? According to the city and the NTTA, no. We will be safe. But not because the toll road will be above the water and not because it will be protected by the 50-feet thick levees built to protect the rest of downtown from the surging force of a massive flood. We will be safe, they say, because we will be separated from the flood waters by a flood wall.
What is a flood wall? Good question. The NTTA renderings say, “The specific wall type for the flood separation walls will be determined with future geotech investigation during future PS&E (plans, specifications and engineering) design.” A crude translation of that would be, “Dunno. But get back to us.”
As for PS&E, you know, that’s not a process with an especially good odor right now, not since the 2005 flood wall failures that devastated New Orleans in the Katrina event. After Katrina, the Corps and local flood control officials in New Orleans insisted that the sheer mass and volume of Katrina had overwhelmed the New Orleans flood wall system, pouring over the tops of the flood walls and basically pushing them down.
But subsequent research and computer modeling by scientists at Louisiana State University’s Hurricane Center found that storm waters had not come close to over-topping the flood walls; instead, researchers found that the walls had collapsed because they were too weak. They were badly designed or under-designed to do the job for which they were intended. In other words, the PS&E that went into those walls wasn’t any good.
So here we are. The average height of a full-size passenger sedan, according to my researches, is just under 5 feet. According to the NTTA’s renderings, the toll road will drop to a height above sea level of about 403 feet where it dips to go under the Commerce Street bridge, one of many bridges it must negotiate on its way down the river bank.
The recent flood hit a crest of 410.5 above sea level. So the toll road, had it been there, would have been more than 7 feet lower than the recent flood, protected from it by the flood wall. The 100-year flood, at a height above sea level of 416.3 feet, would put the water level more than 13 feet higher than the road. The entire safety factor, then, for the road, would be the flood wall. (Or closing the road for long stretches.)
I’m not saying engineers can’t learn from Katrina and design something better. I assume they can. Of course, the people of New Orleans assumed they already had when those walls went into place, and thousands of people died when those walls failed anyway.
Here is what I am saying. In the wake of this recent emergency, it’s damn near criminal for the mayor and the City Council not to convene some kind of serious study process to determine the following: 1) What kind of flood control system do we have in this area? 2) How does it work? 3) How is it similar to or dissimilar from flood control systems elsewhere deemed by experts to be state of the art? 4) What are its limits? 4) Are we improving our safety levels or reducing them with current development patterns?
We should know all of that before a single stake goes in the ground for that toll road.
I wanted to show you an illustration that would convey the role of the flood wall in terms of the level of the proposed road and the level of the 100-year flood. I couldn’t find one done by a real engineer or graphics person, so I just made my own. And I never said I was Leonardo Da Vinci, did I? (The Deep Sea Diver is in the background, so he looks kind of short next to the car. That's called perspective.)
See the car on the left. Now look at the bumpy line at the top on the right. Those are waves. That's the level of the 100 year flood. The thing on the far left is a scale, like on a graph.
All rights reserved to Jim Schutze. May not be cobbed by upscale publications unless Mr. Schutze is paid.
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