How Dredging Works

Hydraulic dredging is the spectacular feature of a restoration project. To be effective, dredging requires a lake loaded with silt, adequate machinery to remove it (the dredge), plastic pipe to deliver the slurry of silt, and a deposit site to receive the slurry. The pictures demonstrate these procedures--how earth moving machinery constructs a dike around a farm field where the silt that is pumped by the dredge will be received.

A dredge is a large, heavy, flat bottom boat. A cutter head extends on an arm from the front of the boat while a discharge pipe protrudes from the rear. The function of the large diesel motor on the boat is to lower the arm and place the cutter head under the water and into the silt bed. There it bites into the silt bed, acting much like an egg beater. The motor also is the power for activating a pump, which sucks up the slurry of silt and water in front of the boat. It then propels it through the discharge pipe at the rear. Plastic pipe, strong but flexible and 14 inches in diameter, is attached to the discharge pipe and carries the slurry of silt to a silt deposit site.

The silt deposit site is usually farm land which has been prepared in advance by creating a dike or wall around the site. This is constructed by pushing the soil from the center of the deposit area to its periphery. The result is a bowl shaped basin into which the slurry of silt is pumped from the lake by the dredge. The site itself usually covers from 40 to 80 acres.

Small lakes can be emptied if water pumped into the deposit site is not returned to the lake through an outlet pipe. The slurry of silt, which is usually 18% mud, is allowed to settle for several days and then the supernatant returns to the lake through its own discharge pipe.

Watershed management, both urban and rural, requires continuous long term education and management. It is less spectacular than dredging but a necessary ingredient for permanent improvement.

Water quality monitoring is a scientific gauge of how we are doing and this will continue for several years after dredging ceases.

The Equipment
Our dredge was manufactured in Louisiana, then broken down into three parts, and transported here on three large low-boys. Two cranes unloaded the sections and assembled them on shore before the craft was launched into Five Island lake.

A dredge is a very large floating vessel containing a large motor and pump. The dredge purchased for the Five Island Lake project is 80 feet long, 16 feet wide, and weighs 125,000 pounds. The 750 horse power V-12 diesel motor pumps out 250 to 325 cubic yards of actual silt per hour (This is at least 12 transport truck loads each hour.). It functions much like a huge vacuum cleaner. The motor operates a cutter head suspended on a large arm in the front of the boat stirring up a slurry of silt and water. The pump sucks up the slurry and discharges it into a pipe at the rear of the boat. Additional pipe is attached to carry the slurry to a deposit site which may be a mile or several miles away.

The top photo shows a bulldozer constructing a wall of dirt around a deposit site. The second photo shows a front-end loader which is pushing rocks against the shoreline to reinforce it and prevent shoreline caving. These rocks had been gathered previously from farm fields.

Plastic pipe is the conduit that carries the slurry of silt, created by the dredge, to its destination at a deposit site some distance away. This pipe has a 14-inch inner diameter and is strong but pliable. When the pipe is full of the silt slurry, it lies on the bottom of the water; when full only of lake water, it floats to the top where it can be easily repaired. The pipe for our project--over 5000 feet--was obtained in Hibbing, Minnesota.