Limestone dissolves in water. The effect is generally very slow, but most of the world’s great caverns were formed by the action of water eating away at limestone over thousands of years. Gypsum dissolves much more readily in water. So readily that only in the driest regions do you find caves formed in gypsum. Where the climate brings much in the way of moisture, exposed gypsum simply crumbles away.
The strata near Mosul in Iraq is composed of alternating thin bands of limestone and thicker beds of gypsum. Which is a serious problem. Because just outside Mosul is the Mosul Dam. Behind the 370 foot tall, two mile wide dam is Lake Dahuk, holding about 2,500,000,000,000 gallons of water.
In front of the Mosul Dam are the largest cities in Iraq.
If the dam ruptured, it would likely cause a catastrophe of Biblical proportions, loosing a wave as high as a hundred feet that would roll down the Tigris, swallowing everything in its path for more than a hundred miles. Large parts of Mosul would be submerged in less than three hours. Along the riverbanks, towns and cities containing the heart of Iraq’s population would be flooded; in four days, a wave as high as sixteen feet would crash into Baghdad, a city of six million people. “If there is a breach in the dam, there will be no warning,” Alwash said. “It’s a nuclear bomb with an unpredictable fuse.”
Because the dam was constructed on such soluble material, it has a special system shared with no other dam in the world: A “grouting gallery” that allows teams of workers to pour hydraulic cement into breeches as they develop. That system of addressing faults requires not just the handful of people that work at most dams. It requires 1,500 workers constantly focused on the task of keeping the dam safe.
In the last decade, Iraq has not exactly been the sort of place where people can stay on task.
Iraq’s leaders, apparently fearful of public reaction, have refused to acknowledge the extent of the danger. But Alwash told me that nearly everyone outside the Iraqi government who has examined the dam believes that time is running out: in the spring, snow melt flows into the Tigris, putting immense pressure on the retaining wall.
The Iraqi government wasn’t ignorant of the risk when they built the dam. Five separate geological surveys were conducted over a span of thirty years, looking for the safest place to dam the sprawling Tigris River. British, French, Yugoslavian, Finish, and Swiss teams all combed the area. There was no good spot. The dam is built near Mosul because that’s where the water, and the power from the dam’s hydroelectric plant, was most needed.
The biggest mistake was that the dam was built in a hurry. The design firm had recommended the whole area be grouted in advance, which would have greatly reduced the need for ongoing maintenance, but would have greatly increased initial cost and delayed construction. Saddam Hussein had taken power only two years before construction began, and was in the middle of a war with Iran that had turned out to have a fantastic cost in both dollars and lives. He was anxious to show that he could accomplish something “great.” Also the government was worried by news that Turkey was going to dam the river north of the border, drying out Iraq’s best farmlands. So, for a number of reasons, they hurried.
In 2014, ISIS dislodged a force of Kurdish Peshmerga and captured the dam as they swept into the area. They only held the dam for a matter of weeks, but the disruption was enough to not only break the routine of constant maintenance, it also scattered the workforce.
When isis fighters took the dam, in 2014, they drove away the overwhelming majority of the dam’s workers, and also captured the main grout-manufacturing plant in Mosul. Much of the dam’s equipment was destroyed, some by isis and some by American air strikes. The grouting came to a standstill—but the passage of water underneath the dam did not.
There is a solution, but not one that seems to be on the brink of implementation.
... to erect a “permanent” seal of the existing dam wall—a mile-long concrete curtain dropped eight hundred feet into the earth. This would cost an estimated three billion dollars. The Iraqi government—nearly paralyzed by internal conflicts—seems unlikely to impose a solution anytime soon.
As spring approaches, the snow in the mountains far to the north begins to melt. The river rises, and the dam will be tested again.