When collapse is only a matter of time

Oct 21, 2016Dams, Featured

As we speak, a battle is taking place in the towns near Mosul Dam to free the region from ISIS control. This would not normally be notable, except for one thing: the dam is slowly dissolving from the inside out, and the fighting is keeping engineers from mitigating the problem.

Mosul Dam was built on a foundation of Karst Limestone, which is widely known to dissolve under pressure. The underside of the dam is slowly being eroded away by water piping through the foundation rock layers, which means its only a matter of time before the Tigris River takes back the valley. Incremental piping like this is possible to slow and even stop by injecting the rock with quick-drying cement (a process known as “grouting”), but its a race against the clock and has to be monitored constantly. There comes a point where more material is being eroded away than can be filled in with cement, and the battle is over.

Hales Bar Dam

This exact same scenario happened in the 1960s at the Hales Bar Dam on the Tennessee River. The 30 year old structure had long been susceptible to leaks, and over the time the problem got worse and worse. The Hales Bar site would never be considered for a dam today – the limestone rock layers are guaranteed to erode over time. Just like at Mosul Dam in Iraq.

In 1968, the Tennessee Valley Authority constructed Nickajack dam a few miles downstream of Hales Bar, inundating the old dam site and effectively replacing it altogether. Saddam Hussein had the same idea for the Mosul Dam site, but construction of the replacement structure (downstream of the old) was hampered by finances and the occasional war. Today, Mosul Dam is leaking like a sieve sitting on a block of Swiss Cheese. But nobody can do much about it because…ISIS.

Hales Bar Dam

If the rate of erosion gets high enough, the dam will be compromised from the inside out, leading to a total failure of the earthfill embankment, and release of the water behind it. Without dramatic intervention (lots of grout), it’s only a matter of time.

Today, you can visit the old Hales Bar Dam site in Southern Tennessee and see its powerhouse still sticking above the water of the replacement reservoir. The structure is used for indoor boat storage by a local marina operator. River traffic glides effortlessly over the old spillway, no longer a threat now that the water pressure has been neutralized. Sometimes you remodel an old house, and sometimes it makes sense to tear the whole thing down and start over. In the case of Hales Bar, the choice was clear.

We will see what the government of Iraq decides about their own fixer-upper.

For an in-depth look at the geology of the Hales Bar site, check out Missouri S&T’s Dr. J. David Roger’s presentation: View PDF of Presentation