Photo by Emily Buss
A barge used to lower equipment rests on the river floor inside the drained lock. It will float to the top when the project is completed.
From the control tower, a glance down a dewatered 600-foot-long lock is breathtaking. Climbing down a steep set of temporary metal stairs and actually standing at the bottom of the river, however, presents an entirely different view. The U.S. Army Corps of Engineers, St. Paul District, pumped more than 10 million gallons of Mississippi River water out of the enormous Lock and Dam No. 6 in Trempealeau, Wis., in early December to perform routine maintenance.
Despite the bitterly cold wind coming off the river last Wednesday morning, 45 skilled welders and metal workers fired up industrial blow torches and put sledge hammer to metal to repair the 76-year-old lock.
“Over a 20-year cycle, we perform routine maintenance on the 13 locks and dams in our district to help them perform better,” said Deputy District Engineer LTC Kendall Bergmann. The St. Paul District operates and maintains each lock and dam from Upper St. Anthony Falls in downtown Minneapolis to Guttenberg, Iowa. “This lock and dam was last rehabilitated in 1994. We moved the maintenance up a year because of some damage to the doors that the engineers found during inspection.”
The upper and lower miter gates that help guide large commercial vessels, and also pleasure craft, through the lock were showing signs of wear, Bergmann said. During a recent inspection, experienced divers discovered the many crossbars that keep the miter gates straight had begun to twist and warp.
To prepare the lock for the necessary repair work, in early December engineers spent 27 hours draining more than 10 million gallons of water from the lock pool, enough water to fill 16.5 Olympic-sized swimming pools. Crane operators then lifted in large metal bulkheads, each more than 100 feet long, designed to keep the freezing river water at bay. However, before work could begin, workers shoveled out four barge loads of what construction supervisor Scott Uhl called “river gunk.”
“There was about 400 yards of material on the chamber floor,” Uhl said. “We saw a large number of zebra mussels the last few years during dewaterings but we didn’t really find any here this year. It’s just another one of those life cycles. But, we did find shells, silt, and a lot of rocks. Nothing out of the ordinary.”
When pressed about his most unusual find during a lock and dam dewatering, Uhl said an intern once found an old silver dollar.
Once the lock was empty, repairs on the upper miter gates started. Workers spent hours hammering and torching off each crossbar. The gates were then sandblasted, primed and given a new coat of battleship gray paint, a process now more than halfway complete.
On Wednesday, employees were beginning work on the lower miter gates by stripping the gates of each mangled crossbar. Once each bar is removed, the lower miter gates will also be sandblasted, primed and painted.
On either end of the lock, workers were tirelessly hammering each crossbar back into its straight shape.
“This is portion of the rehabilitation is probably the biggest,” Uhl said.
Concrete work on parts of the lock wall will also be repaired and any necessary cleaning will be completed inside the now empty concrete lock chambers. The lock chambers are small three-foot tall by three-foot wide square openings carved into both sides of the bottom of the lock pool. The chambers, which possess a distinct odor of decaying zebra mussels, lead into a large, cylindrical chamber that helps distribute the minimum of 3.9 million gallons of water that is needed to maintain the lowering pool levels. On either end of the cylindrical chamber is a main bulkhead that allows water to enter the chamber to raise or lower the vessels locking through.
Another part of the project will be the replacement of the old bubbler system. During the last 20 years, high- and low-flow bubbler systems have helped several northern locks and dams take a more proactive role against damaging ice. Replacing the system is a relatively low-cost repair that is becoming widely accepted at locks and dams that experience colder temperatures. The U.S. Army Corps of Engineers say that locks on the upper Mississippi River use low-flow blower systems that continuously supply air to one-inch diffuser lines near the base of the miter gates to bring warm water up from the bottom. The bubbler systems help deflect ice pushed ahead of barges, reducing the amount of ice that enters a lock.
The $3.7 million rehabilitation project was budgeted years in advance, Bergmann said, and is paid for by the Corps as part of general operations and maintenance costs.
Work on Lock and Dam No. 6 is to be complete by March 11 and will not affect any river business up stream.