Fixing the Most Dangerous Dam in the World — Practical Engineering

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[Note that this article is a transcript of the video embedded above.]<br>Mosul Dam rises 370 feet or 113 meters above the Tigris River in northern Iraq as one of the tallest dams in the Middle East. The dam was built in the 1980s, but, in a way, construction never really stopped. That’s because ever since the reservoir filled behind Mosul Dam, the ground has literally been dissolving, nonstop, below the structure. Almost immediately on filling, water started flowing through the foundation of the dam and back out on the downstream side. Just a year later, the volume of seepage was measured at 800 liters or about 200 gallons per second.<br>I usually hate to use the olympic-sized swimming pool equivalent, but in this case it makes sense because it was enough to fill one every hour of every day. And the issue is that, once a process like this gets started, it’s pretty hard to stop. So, for the past 40 years or so, the problem at Mosul Dam has been ongoing, scrutinized by some of the most preeminent engineers across the world and complicated by politics, bureaucracy, and, of course, armed conflict. Failure of a structure this large would be catastrophic; towns along the Tigris River would be fully wiped off the map, and some estimate that the breach wave would be so massive that even major parts of Baghdad, hundreds of miles downstream, would be submerged. In 2006, the US Army Corps of Engineers called it, unequivocally, “the most dangerous dam in the world.” That was 20 years ago, and Mosul Dam is still standing, in better shape than ever. And the story of how it got there is fascinating. I’m Grady, and this is Practical Engineering.<br>Mosul Dam is an earthen embankment dam not far from the City of Mosul in Iraq, built to generate hydropower and store water for irrigation and drinking. The hydro plant is on the west side of the dam with four turbine generators. You can see the massive surge tanks sticking up from the plant that absorb changes in pressure when the units are started and stopped. The dam has an outlet structure through the embankment here. It has a service spillway with radial gates here. And an auxiliary spillway with earthen fuse plugs here. Check out my videos on spillway gates and fuse plugs if you want to learn more about those types of structures after this.<br>The dam itself is impressive, but the rock that serves as its foundation is extremely complex, and in many ways, far from ideal. The geology of northern Iraq includes a lot of gypsum, a sedimentary rock that is widely used for things like fertilizer, plaster, and drywall. What it’s not widely used for is the foundations of dams. In fact, the consensus of experts involved on Mosul Dam throughout the years is that it was, all around, a terrible idea. One consulting group said that, quote, “the decision to locate such a major and important dam on the foundation rock mass which exists at the Mosul Dam site was fundamentally flawed.” That’s because of a critical property of gypsum, one that it doesn’t share with many other types of rock formations: it dissolves in water.<br>You might be familiar with limestone caves and karst geology, where water creates voids in the subsurface. Some of these can be quite dramatic like Carlsbad Caverns in New Mexico or Mammoth Cave in Kentucky. They’re formed because the limestone is just a tiny bit soluble in water, as long as it’s a bit acidic, which rainwater usually is. So over the course of millions of years, that water kind of carves away the earth from the inside. Gypsum, on the other hand, is roughly 200 times more soluble in water than limestone. It’s not quite like a spoonful of sugar or salt that dissolves almost instantly, but processes that usually take centuries in limestone are accelerated to human timescales in gypsum. And that’s especially true in the subsurface, because dissolution isn’t a linear process. More dissolving means more space for water which means more dissolving and so on. It’s a positive feedback loop.<br>Many dam failures have resulted from internal erosion, where water seeping through the soil or rock carries away particles, leaving voids. This process is what led to the demise of Teton Dam, which I covered in an earlier video. But where internal erosion can be combatted by designing filtration systems that catch waterborne particles before they escape the subsurface, you can’t easily filter dissolved gypsum out of seepage.<br>The designers of the dam knew the gypsum was going to be an issue, and they had a few ideas to address it. One was to install a blanket of bentonite clay lining the bottom of part of the reservoir. This would block seepage from flowing into the subsurface, at least in the dam’s immediate vicinity, lengthening the flow paths and thus reducing the total volume of the flow. However, the volume of material would be enormous, and...