As the Mississippi River roared nearby, two huge cranes swung a 32-ton steel gate onto rollers that slid it into place at the Coon Rapids Dam on Monday.
The 67.5-foot-long gate, the second of nine to be installed, is part of a two-year, $16 million upgrade intended to prevent voracious Asian carp from leaping over the dam and reaching the upper Mississippi watershed. A silver carp, a variety of Asian carp known for its leaping ability, was found dead near Winona, Minn., in August, the farthest north that species has been detected.
The nine gates, including six 97-footers, will be massaged into position to replace aging rubber tube gates, which are less effective against fish passage that is possible during serious flooding.
A few white flakes fluttered down Monday morning as about two dozen workers in hard hats and safety vests worked under the gray sky. Several men bolted the eight-foot-tall gate into place under the bridge walkway.
The crew is racing the clock, trying to get five gates installed this year on the Coon Rapids side before conditions get too icy and cold to work.
“We’re on a tight schedule,” said David Kost, project superintendent for contractor Edward Kraemer & Sons, of Burnsville.
The five gates and a new stilling basin below the dam should be done by Dec. 1, said Dennis Jonjak, Kraemer’s project engineer. At that point, two coffer dams that divert water around the work area will be pulled out for the winter, and the regular dam will operate as usual until the project resumes in the spring.
Jonjak said the 450-foot-long stilling basin at the base of the dam will replace an underwater, concrete apron that is pocked with scour holes. The new basin will consist of 2,000 cubic yards of concrete laced with 450,000 pounds of steel rebar. It will sit on one-foot-deep layers of sand and rock and will be anchored by more than 230 H-beams pounded into the river bed.
Crane engines droned Monday as the gate was lowered onto steel rollers and slid into place. About 10 feet above the work area, the Mississippi was held at bay by a wall of steel sheeting that forms the coffer dam. A few pumps sucked up leaking water and dumped it back into the river. A bigger pump siphoned water from the work area below the dam, shielded by the lower coffer dam.
Kost said his main safety concern is the weight of loads swung overhead by the tall cranes, one of which can carry up to 150 tons. Hoisting barge work platforms or gates into place can get precarious on windy days. He said the cranes are stopped when winds gust to 20 or 25 miles an hour.
“Wind is always a factor,” said veteran crane operator John Sinna. “Wind is nobody’s friend.”