ALMA, Wis. -- It takes 6.2 million gallons of water to raise a barge from the lower level of U.S. Lock & Dam No. 4 to the upper level. That happens thousands of times each year, but right now, there are no barges and no water in the lock chamber.
The locks and dams that make commercial navigation on the Mississippi River possible are approaching 90 years old, and like an automobile or other mechanical device, they need periodic maintenance.
“If you don’t change the oil, check the fluids, and get new tires, you are going to have problems with your car,” said Joe Schroetter, project manager for the dewatering and maintenance work on the lock chamber at Lock 4, one of 13 lock and dam units supervised by the U.S. Army Corps of Engineers, St. Paul District.
Schroetter said that as part of the Army Corps of Engineers’ responsibility to maintain a 9-foot deep navigation channel, each lock and dam is scheduled for rehabilitative work every 15-20 years. The process last happened at Lock 4 in 2001. He added that the work this year is a $4.5 million project.
Dewatering the lock
Releasing the water in the lock to lower a boat is simply a matter of opening the emptying valve in the large pipes below the lock chamber. Gravity takes over and enough water flows out to lower the boat to the level of the downstream part of the river. There is still enough water inside the lock chamber to float the boat or barge out into the river.
During major maintenance, the entire lock chamber must be dewatered, which means the workers are performing much of their work at levels lower than the water on both ends of the lock. Sump pumps remove the water at a prescribed rate as workers monitor the lock chamber and gates for any problems.
“We put in bulkheads at both ends of the lock,” Schroetter said. “They block the water on both ends so we can pump the water out of the chamber and work on the miter gates.”
The miter gates are the large gates that allow a barge or boat to enter and exit the lock chamber. Schroetter said that the gates, having been underwater for 20 years, need to be thoroughly cleaned and inspected for damage or corrosion. Work crews repair or replace parts as needed. They will also update the bubbler system, which prevents ice from building up inside the lock chamber.
Timing the project
“We are working 24 hours a day,” Schroetter said. “We have sandblasters that come in and work through the night so that the painting crew can come in during the day. We have to use special paint, because we are painting in the winter.”
The project is scheduled to finish in mid-March, so it will be done before the start of the 2021 commercial navigation season.
“We have a luxury here in the St. Paul District,” Schroetter said. “The river closes, so we can do these projects without interrupting shipping. Farther south, they can’t do that, because it’s always moving.”
Repairing the lock
In addition to updating the bubbler system and cleaning and maintaining the miter gates, work crews will inspect and repair the lock chamber. Some sections of the concrete walls have deteriorated, and a backhoe with a grinder attached to the arm can efficiently remove damaged layers of concrete. Crews can then install frames and pour new concrete in to rebuild the walls as needed.
The backhoe, along with several other pieces of large equipment, are floated into the lock chamber on a barge. As the sump pumps remove the water from the lock chamber, the barge settles onto the floor and the machines can be driven down a ramp off the barge and onto the lock chamber floor for their work.
When rehabilitation work is complete, the machines will be driven back onto the barge, water will be returned to the lock chamber, and the machines can be floated out of the lock.
“The reason that we do this maintenance work is that barge traffic is so important. A tow with 15 barges equals 1,050 semi-trucks,” Schroetter said. “Ten million tons of commodities came through Lock and Dam 4 last year. Keeping these locks and dams maintained is an important part of what we do.”