Lock Dewatering: Why It's Winter Work
By Ray Whitfield, ADCI-certified commercial dive supervisor. Reviewed by Amanda McCallister, editor.
Underwater Inspection | Commercial Diving Contractors Alabama Last January I stood on the dry floor of the Marmet Lock on the Kanawha River. It was 18°F and the steel skin plates were rimed with frost. A year's worth of silt and gravel lay at the toe of the miter gate, and walking on it crunched like old snow. The water had been pumped down a week earlier, and the gate seals were already dried out and cracking. That's dewatering. It's a brutal, cold, precise job, and it almost always happens in winter. Why? Because the river is closed, that's why. Navigation shuts down on the upper Ohio and its tributaries from about December through late February, give or take a couple of weeks depending on the plant. The Corps of Engineers runs the lock and dam complexes, and their navigation season ends with the winter flotilla of towboats laid up. So if you're going to pull a 110-foot miter gate or rebuild a culvert valve, you have a window. Miss that window and you're fighting traffic and water levels for months.
The River's Slow Season
The Corps' lock dewatering schedule is published in the agency's annual navigation notice. For each lock, they list a proposed closure, usually from early December to mid-March. On the Monongahela, some of the older locks have to close every two years for inspection. On the Ohio, the newer 1200-foot chambers can go longer, but the miter gates still wear out. Winter is also when water levels are most predictable. Spring rains and snowmelt push the rivers up, and the Corps can't let the gate areas flood. So the schedule has to ride that line between low flow and cold. And believe me, the cold is part of the deal. You're standing on a dewatered floor that was under 40 feet of water the week before, and the seepage from the culverts sends up a spray that freezes on your suit.
What Dewatering Takes
We set our dive barge above the upstream miter gate and use a pair of 10-inch Flygt submersible pumps to draw the chamber down. On an 84-foot wide lock, that's maybe 600 acre-feet of water to move, and it takes the better part of two days if everything's working. The upstream seal is a rubber-and-steel arrangement, and you run the pumps through the night, checking the bypass valves. When the sill starts to show, you send a diver down in a Kirby Morgan 37 to set the limber-pipe lines and make sure nothing's blocking the sump. Once it's dry, the real work begins. Mud, sediment, rocks, sometimes a car tire. The crew goes in with muck buckets, a rope trolley, and a lot of swearing. Then the welders come down with Miller Trailblazers, and we start cutting out the old stainless steel gate seals and fitting new ones. All in a 14-foot-thick concrete cave with the outside temperature below freezing and a wind that funnels down the chamber. It's not for everyone. But that's the job.
Scheduling and the Corps
The lock dewatering schedule is a year-long chess game. The Corps publishes a draft in August, takes comments from towboat operators and the American Waterways Operators, and issues a final in November. But the actual shutdown date moves based on river conditions. I've seen a start date pushed two weeks because a towboat was late, and the closure got shortened to a month. That puts pressure on everyone. Contractors like American Bridge and Kokosing have crews on standby. You can't call a dive team a week before and ask them to clear their calendar. We on the diving side are part of the pre-outage planning. We know months out what they'll need. Still, it's a dance. One thing people don't think about: You can't dewater a lock mid-winter if the inlet valves leak. There's always a fair amount of water coming through even with the gates closed. Our pumps have to keep up with that seepage 24/7. If a pump jams on a suction line at 2 a.m., you run the risk of the whole chamber reflooding. That's why we stack two sets of pumps, and we've got a diver on call the whole time.
The Cold and the Concrete
Cold weather helps the concrete, in a way. If you're chipping out deteriorated concrete, you want it cold enough that the epoxy doesn't kick too fast. But we also have to heat the work area with space heaters to keep the steel from cracking. Used to have a real problem with freeze-thaw damage on the concrete surface. The Corps specs say dewatering shouldn't start if the ambient temp is below 10°F. We've worked at 5°F, but that's when the welders get really unruly. Digression: On one dam job on the Monongahela, we had a diver down on a backup valve inspection and the tenders spotted a beaver swimming in the stilling basin. A beaver, in December, behind a dewatered gate. We watched it for ten minutes. The diver surfaced and said, 'There's a beaver on my hose.' We think it had been living in the culvert. That's the kind of thing that makes this work interesting. Anyway. The reason we do it in winter is simple. The river belongs to the shipping industry from spring through fall, and it belongs to maintenance in winter. If you want to keep a lock running for the rest of the year, you accept the cold, the ice, the short mornings. It's a fair trade.
Frequently Asked Questions
When is the typical lock dewatering schedule? On the Ohio River system, most dewatering closures run from early December through March. Each lock has its own published closure, set by the Corps of Engineers, and it can shift by a week or two due to river conditions. Why do locks require dewatering so often? Concrete and steel wear out. Miter gates, culvert valves, and sealing surfaces need inspection and replacement. A dewatered chamber lets workers see and fix what you can't access underwater. How long does a lock dewatering last? It depends on the lock and the work. Simple inspection can take two to four weeks. Gate replacement can take three to five months. The Corps plans closures to fit the work. Can locks be dewatered in summer if needed? It happens only for emergencies. The towboat traffic is too heavy, and water temperature affects concrete curing. Summer dewatering is expensive and disruptive, so the Corps tries to avoid it.