Big Bend and Trump’s Wall

For our spring vacation, I took Alysia to Big Bend National Park in southwest Texas where I first worked for the National Park Service forty years ago. The most unexpected change is that the Chisos Mountains now have 15-20 black bears. Bears weren’t there when I was there. They had all been killed during the ranching era. But a few found their way across the desert from the Mexican mountains in the 1990’s and now there is a breeding population within the park.

We attended an evening program at the amphitheater where I had given my first evening programs as a young ranger. My first program had been on the geology of the park in which I told the geological history of the park in 40 minutes using slides. It was well-received and I was quite proud of it at the time but now I see it as falsely empowering the audience. They might think they now know the geology of the park but they only know it as words with very little application to world around them. I found myself now beginning to design an evening program that would use slides to teach people how to notice certain revealing details. The black streaks on the desert mountain faces, for example, indicate where water flows during a storm. Knowing this pattern helps your eyes start seeing the drainage patterns within the mountains. Lots of small, illustrated specific examples like that helping people start noticing more on their own as they drove within the park – and beyond. Give them the power to start seeing stories on their own.

As we drove around the rugged desert landscape, we began talking about the specific difficulties of Trump building his Wall. I’ve written before that we need, as a species, to learn to not cheer or applaud when a politician says any variation of “All shall” or “none shall” (“100%”, “0%”, “every child”, “none”, or “no child”). Trump’s Wall is an example of this because it promises that no person shall get across. Most of the commentary against his wall has been directed at the politics of it. The few engineering-type questions have focused on “how high?” and therefore “how much concrete and how much water is required?” Good questions. As far as I can tell, Trump answers with images of precast concrete slabs lined up across the land. But in Big Bend, we found ourselves asking, “How do you build a wall across an arroyo?”

The Rio Grande forms almost two-thirds of the U.S.-Mexican border. What this means is that every tributary has to flow through this wall. Or, put another way, the wall will have to cross every single drainage along more than 1200 miles of the river. We were counting drainage crossings as we drove along various stretches of Texas desert and estimated them at about three per mile. At that rate, there would be about 3500 drainage crossings. More than half of these would be small. A few would be full-size rivers. But a lot of them would look like the picture below.

This fairly typical arroyo within the park flows towards the Rio Grande. The cutbanks on the right (along the outside bend where the water erodes the strongest) are about twelve feet tall. The main thing to note is that both the stream bottom and the stream banks are alluvium – unconsolidated stream deposits of sand, silt, gravel, and cobbles. Within the arroyo are found many plants including a dead cottonwood tree in the background. These arroyos form natural trails through the desert so the wall has to block them. How does it do this? This is desert and the arroyo is usually dry but if there is a thunderstorm up in its watershed, runoff will flow. It hopefully won’t be a dramatic flashflood but it will flow for several hours.

The wall must stop anybody wanting to walk along the arroyo but it can’t stop water. If it tries to stop water, the wall becomes a dam. As the runoff backs up, the water pressure increases with depth and can buckle a dam. Dams handle this by being much broader at the base than most people realize – which makes a dam much more expensive than a wall. Also, because of this pressure, dams must be anchored in place to solid bedrock so that the water pressure won’t push them out of place. But that is not what we have here. Bedrock might be fifty feet beneath the arroyo. This alluvium is unconsolidated which means that if water backs up behind the wall, the water pressure will force the water through the alluvium, the silt and sand grains will start to flow and water pressure will eventually blow a hole underneath the wall. To prevent this, one would need to excavate down to bedrock and build upwards from there. But if the alluvium is tens of feet thick, grounding the “wall” to bedrock becomes very expensive.

One runs into a similar issue with the banks of the arroyo on either side. You can’t simply abut the wall up against the arroyo bank. Water pressure will force the water through the alluvium around the edge of the wall and blow a hole on the side. Therefore, one would have to trench back into the bank a long way to anchor it in place. In other words, each arroyo crossing is an expensive proposition.

The solution might seem obvious. Let the water pass through a culvert or overflow channel. But overflow channels have to slope to prevent the water from hitting the bottom of the dam so hard that it plucks out a plunge pool that undermines the dam and this slope can be climbed by people. Similarly, culverts can be crawled through. Again, the solution might seem obvious, weld grates across the culverts or overflow channels with openings smaller than people can crawl.

This is where we run into the other problem about building a wall across an arroyo. Water is not the only thing flowing. The runoff carries debris: leaves, dead bushes, broken branches, and, eventually, the trunks from those dead cottonwood trees. Each item stuck against the grate divides the openings into smaller-sized openings which allows smaller things to get stuck which divides the openings into even smaller size openings. The floating things pile up, pinned against the grate with increasing hydraulic force as the runoff backs up. Within a few hours, this wall with grated culverts turns into an under-designed dam and it fails.

This picture conveys a sense of this. The bridge crossing that arroyo has channels twelve feet wide and ten feet high to let the debris safely pass and even still, debris is piled three feet high and six feet wide against one of its abutments. Imagine what would happen against grating that has just one-foot gaps.

Culverts need to be maintained, often during the height of the storm when tree trunks are washing down and everything is backing up. If the wall is wide enough for a backhoe to drive onto, this work can be done during the storm. But a wall wide enough for a backhoe is expensive and there will be many hundreds of these arroyo crossings that will need to be patrolled. That creates a permanent maintenance cost that needs to be factored in. You can clear the debris out by hand and chainsaw after the flood subsides but if the flood overtops the wall, the wall might wash out before then.

I once had a ride on a highway that the Mexican government had built, I’m sure with much fanfare, to connect the small villages south of Big Bend to the interior of Mexico. The highway had several concrete bridges crossing the arroyos. But the truck kept turning off the highway and dropping down into the arroyo and up the other side back onto the road. As I looked up, I saw that the bridges had not been anchored into the banks and erosion had worn the alluvium away from one or both ends of the bridge so that there was now a five or ten foot gap between the highway and one end of the bridge. The bridges had made the government look good for a year and had enriched some contractor but now they were just expensive landmarks to incompetence.

We really have to learn to hold our applause whenever politicians promise “all” and “none” and ask serious questions. “Mr. Trump, what is your design for building your wall across the many hundreds of arroyos that your wall must go across? How will your wall let the water and all that it carries through without letting the people through, year after year? Ranchers have struggled for over a century with how to maintain a simple barbed wire fence across a stream. Please share your design, if for no other reason, to help them out.”

I wanted to end this article with a picture of a barbed wire fence that crosses a “dry” streambed in our neighborhood. It survives by having an easily crawled under three-foot gap beneath the lowest wire. However, when I walked there to take a picture, I found that it was down.

You can see a fencepost standing on the left. You might see the strands of wire lying on the ground across the 10 foot wide streambed. On the right hand side of the streambed is a small log. It hooked onto the wires during high water and the force of the water against the log was sufficient to pull the fence down.