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Crats Island Project Involved Letting Carriage Water Percolate into Containment Site Groundwater

The dredge Michael working in the sand pile at Crats Island. This pile was originally 100 feet high and contained
over 800,000 cubic yards of sand.

The dredge Michael working in the sand pile at Crats Island. This pile was originally 100 feet high and contained over 800,000 cubic yards of sand.

By Judith Powers
This August, J.F. Brennan Company completed a two-year project to transfer 830,000 cubic yards of stockpile sand from Crats Island in Wisconsin across the Mississippi River to the Wabasha Gravel Pit in Minnesota.  
Jeff Abraham was superintendent and Travis Sperberg was the project manager. 
The two-year project involved dredging the material with the 14-inch by 12-inch dredge Michael, and pumping the slurry through 26,500 feet (five miles) of 14-inch pipeline using the dredge pump and six booster pumps.
The booster pump installations were built at Brennan headquarters in La Crosse, Wisconsin, and placed at strategic points along the pipeline route.  They were operated by trained personnel from the International Union of Operating Engineers (IUOE) Local 139. 

A dozer works in the containment site, with the sump in the background. Brennan created the sump by deepening a section of an existing depression, and digging channels
to allow water to run into it, while the dredge pumped 8,000 GPM of slurry into the site.


The Corps of Engineers plans specified that all the carriage water was to remain in the gravel pit and allowed to percolate into the ground water.  To accomplish this while adding 8,000 gallons of water per minute to the site, Brennan created a sump by deepening a section of an existing depression, and digging a ditch from some of the low points to allow water to flow into the sump.
As the containment area filled and the sump became smaller. Brennan created a large percolation pond at the top of the pile and pumped water from the sump into it, which allowed the water to drain through the previously-placed sand.

   

A view of the sump where the carriage water from the slurry could percolate into the water table. The sump
became smaller as the job progressed, and in the second year, Brennan began pumping water out of the sump
into a percolation pond at the top of the pile.


During the 2016 dredging season, the dredge Michael, a DSC custom-built 12-inch Barracuda dredge, moved about half of the material.  The Michael is a hull pump, swinging ladder dredge that was added to the Brennan fleet in 1994.  It has a dredging depth of 27 feet and is transportable by truck, enabling use on inland lakes as well as on the river.  This project is 62.4 miles upriver from Brennan’s headquarters in La Crosse, and the Michael was towed to the site, along with a support barge with excavator and a small push boat.  The equipment was returned to the Brennan yard for maintenance at the end of each dredging season.  
Brennan began the 2017 season in April with 415,822 cubic yards of sand remaining on Crats Island.  A long reach excavator for changing out wear parts was added to the equipment in 2017.  Other equipment included two dozers for managing the fill. 
Crats Island is on the Wisconsin side of the Mississippi River in Buffalo County, near Upper Mississippi RM 759.  The placement site in the Wabasha Gravel Pit is on the Minnesota side in Wabasha County near RM 761.  Crats Island is without road access, and has historically been used for the placement of material dredged from the Mississippi River navigation channel.  When the island is at capacity, the St. Paul Engineer District contracts the removal of the stockpiled material to a permanent location, leaving the island free for ongoing placement of maintenance material from the river. 
The pipeline was routed along the Wisconsin side of the river, and was mostly floating, with three locations where it was submerged to allow passage of recreational vessels, and at the location where it crossed the navigation channel to the Minnesota side of the river.  The placement site was inland from the river, and Brennan crew routed it through culverts under the Canadian Pacific Railroad tracks and under a road to access the Wabasha Gravel Pit. 
This project was titled Crats Island Dredge Unloading, Contract W912ES-15-C-0007 awarded on September 25, 2015.

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