Tamping Down Fears, Officials Contradict Each Other on Lewisville Dam Danger. Feel Calm Now?
A parade of media vehicles long enough for a state funeral drove out onto Lewisville Dam under police escort yesterday for a de-panicking by the Corps of Engineers.
Spent the morning yesterday out on Lewisville Dam at the foot of Lake Lewisville with an entire caravan of other mediatoids getting de-panicked by the U.S. Army Corps of Engineers, which seems to be in sort of a public relations panic.
Small wonder. Publication last weekend by The Dallas Morning News of a splendid piece — maybe a masterpiece — on weaknesses in the six-mile dam has everybody wondering if we may one day witness the local version of the end times. The story by University of North Texas journalism professor George Getschow said the troubled Lewisville Dam holds back 125 times the volume of water that flooded Johnstown, Pennsylvania, in 1889, the worst dam disaster in American history, which claimed 2,209 lives.
In the business of being de-panicked on technical issues, we people of the media are always at a huge disadvantage because we are not technical, so we all have our own tricks for evaluating what we’re being told. My own is to look for the small contradictions that sometimes go to the basic integrity of the information being purveyed.
I had a modest headstart on myself yesterday because I had noticed a detail in Getschow’s story that reminded me of something I had read. Most of the media attention yesterday was to an area on the top of the dam, called a slide, where the dam had partially eroded and sloughed off into the lake. I wasn’t uninterested in that, but I was much more interested in what was going on underneath the dam because of the mention in Getschow’s story of a “sand boil.”
Getschow reported that Corps of Engineers personnel inspecting the area just below the dam last May, out looking for soggy ground, had “spotted something far more disturbing: water and sand bubbling up from a tiny hole in the ground.”
A sand boil.
Getschow wrote: “Such a ‘sand boil’ indicates that increasing seepage has created a passage under the base of the dam. If not stopped, it could lead to a rupture of the dam.”
While any erosion on top of an earthen dam is bad, the good thing about surface damage is that you can see it. A sand boil happens when powerful forces push water from the lake through or underneath the dam, finding and tunneling its way through soft sand until liquefied sand, earth and water boils up on the other side. Unattended, a sand boil can create a “pipe” or subterranean channel that can expand suddenly and pull down the dam. The problem is that the tunneling process is invisible until it gets to the other side.
I had read about sand boils in John M. Barry’s wonderful book, Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. Barry explained sand boils in a PBS interview about the book. He was speaking about sand boils in river levees, not lake dams, but the basic principle is the same:
“A sand boil is really like a miniature volcano,” he said. “It looks just like a volcano, and it'll spout water in a sort of a gusher. If the water is clear, then it's safe. But if the water is muddy, that means that it's eroding the levee. The water, as it runs through the levee, is taking the earth of the levee with it.”
Corps of Engineers Fort Worth District Operations Chief Tim McAllister was among a dozen top Corps officials on Lewisville Dam yesterday for the official de-panicking.
Sand boil, then, is an important, well-known term of art and red flag in the science of dams and levees. If you’re really interested, you can watch a good sand boil lecture here.
Erosion at the surface of the dam can be the result of wind and weather. That could happen to anything made of dirt. Sand boils are urgent warning signs of things going wrong deeper in the structure of a dam or levee. The threat they pose is both more insidious and more explosive than surface erosion. So that was what I wanted to know about when I presented myself at the dam yesterday to be de-panicked.
It was the first thing I asked Colonel Calvin C. Hudson II. He is commander of the Fort Worth District of the U.S. Army Corps of Engineers, which has ownership and oversight of the dam. I said, “Have there been any other sand boils of the type that was described in the Morning News story?”
“First of all,” he said, “it is not a sand boil. It is a seepage, just to make sure we have the correct terminology. And, no, sir.”
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Twenty-five minutes later in the tour, I found myself standing next to what looked very much like the sand boil that Gestschow had described in his story, ringed with a single tier of sandbags. That, by the way, is the state of the art way to stop a sand boil from creating a pipe: The sandbags create a little elevated pool of water, and that elevation is enough to balance the water pressure and stop the seepage.
Or, you know: sand boil.
This little mud hole is the "sand boil" in question at Lewisville Dam, no longer boiling.
Sarwenaj Ashraf, who is the dam safety program manager for the Corps Fort Worth District, was explaining to a couple of other reporters that the object we were looking at was what she called a sand boil.
“We came out and looked at it,” she explained, “and we put some sandbags there, and that is what you see over here. The sandbags were there to add some weight on the sand boil [and] equalize the pressure.”
She said that a sand boil is, “kind of like a very weak spring.”
I interjected: “It tunnels, and if you don’t stop it, it will form a pipe, is that correct?”
She said it was, “but that was not the case here.” They got to this one in time.
I ran this apparent contradiction — no sand boil versus yes, a sand boil — by the Corps media relations people later, and they suggested I had misunderstood the colonel. What he meant to say, they said, was that, “There is no current sand boil at the seepage area. There was one observed during the spring [rain] event, and that is where the sandbags were placed.”
Listen. I’m not trying to make a federal case out of this. The colonel did not deny that the event in question occurred. He did not lie to me. He just didn’t like my term. Sand boil.
The engineer did like the term. Sand boil. She used it readily.
What this is really about is the fact that flood safety in this country is in the hands of the military, and their first impulse is always going to be to get everybody under control — de-panicked. Then journalism professors, engineers and reporters can talk about what’s true later on.
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