If you’ve ever ridden a boat through a lock, you probably remember the gates, the noise and the slightly odd feeling of a whole river rising or falling around you. You probably don’t remember the floor. That’s because you can’t see it, and on most days no one can.<\/p>\n
But for the last few weeks, the Army Corps of Engineers has been staring at a lot of floor in Natrona, Pennsylvania. Its Pittsburgh District drained Allegheny River Lock and Dam 4’s chamber and shut the river to traffic there for months, and the whole thing comes down to one piece of oak.<\/p>\n
In the release it posted on Nov. 6<\/a>, the Corps describes the chamber walls as 99 years old and says it’s the first time in three decades the chamber has been completely drained. The Pittsburgh Post-Gazette<\/a> puts the last time it was emptied at 1996.<\/p>\n You don’t get a look like this very often. We got one when a South Dakota dam’s generator was taken apart for the first time in 70 years<\/a>, and another this week when a Seattle lock was pumped dry<\/a>.<\/p>\n If you’ve seen a river lock, you’ve probably seen miter gates. Basically, they’re two big steel doors that swing out from the chamber walls and meet in the middle at an angle, so the weight of the water pushes them shut even harder. The miter sill is the ledge on the chamber floor that the bottom of those doors presses against to make the seal.<\/p>\n On Lock 4, that ledge is oak. Timber sills go back to the 1800s on American locks, since oak is tough and makes a tight seal, and the Corps says this one follows a design picked back in Warren G. Harding’s presidency. It sits under the downstream gates, which hold the water in when the chamber’s full.<\/p>\n Then winter happened. The Corps says ice coming down the Allegheny during last winter’s severe freeze damaged the sill and opened gaps that let water leak past the gates. According to the Post-Gazette, the culprit was a late-January storm that sent sheets of ice more than 10 feet wide and 8 inches thick down the river, and the leaks showed up in early spring. The lock kept working, sort of. The Corps says its aging filling valves were working overtime to replace the water leaking out through the bad seal, and a few more equipment failures could have left the chamber unable to fill to the upstream level. And a lock that can’t fill doesn’t let anything through.<\/p>\n That’s where Lock 4’s design hurts. Plenty of bigger river locks have two chambers side by side, so one can close for repairs while traffic squeezes through the other. Lock 4 has exactly one, 56 feet wide and 360 feet long, next to a fixed-crest dam (a concrete wall the river simply pours over). So when that chamber is out of action, the Allegheny is closed at Natrona, and there’s no detour for a barge.<\/p>\n You can’t just pull a plug. The chamber’s got river on both ends, and the moment you start pumping, the Allegheny wants to pour right back in. The Medium Capacity Fleet, a Corps maintenance team run out of its Huntington District, took the job, and it had to wall the chamber off first.<\/p>\n Crews set 16 bulkheads, which are basically heavy panels dropped in to block the water, plus two needle-dam systems to seal off the chamber. Then they braced it with 13 structural struts. An empty chamber has the river and the ground around it pretty much leaning on its walls, and the Corps says the struts are there to support them while the water’s gone. After that, the pumps went to work on millions of gallons.<\/p>\n Getting those bulkheads in took divers, and not in a nice spot. Some of the work happened near the crest of the dam, where the Corps says the water’s rough enough to be a real safety concern. Chris Smidl, a Pittsburgh District project engineer, described the bulkheads for the valves as high-risk work. To make conditions a little friendlier, the Corps’ water managers juggled releases at eight flood-control reservoirs upstream to dial the river down for a while.<\/p>\nSo what’s a miter sill, and why does it matter?<\/h2>\n
Draining a lock takes a lot more than a big pump<\/h2>\n
The oak is on its way out<\/h2>\n