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Dredging 101

What Is Dredging: Dredging is the removal of bottom material from lakes, rivers, harbors, and water treatment settling ponds.  Dredging is done to remove sediments and undisturbed bottom materials to increase water depth to permit the passage of ships and barges, to increase the capacity of water storage reservoirs, for recreation boating, to rebuild beaches, and to remove sludge from water treatment ponds.

There are two basic types of dredging.

1.  Hydraulic Dredging:  Dredging Specialists has completed close to 150 hydraulic dredging projects in 22 states. Since 1979, all contract work has been done with dredges we designed and built.  
A hydraulic dredge consists of a floating barge.  There is boom, called a ladder that is lowered into the mud.  The ladder has a device called a cutterhead, on the end.  The cutterhead rotates and excavates the material.  There is a large pump in the hull of the dredge.  The excavated material is sucked from inside the cutterhead through a pipe to the dredge pump.  The pump then pumps the slurry to a spoil area.  The pipeline contains about 20% mud and 80% additional water.

The spoil area has levees around it to contain the mud and water and also provide the means for the mud to settle and the water to become clear, so it can be returned to the lake.  The spoil area should be 1.3 times the volume of material to be dredged, plus at least 1 foot of freeboard.  A weir must be installed in the spoil area to remove the clear water so it can be returned to the lake.  The clear water is returned by pumping in a pipeline or by gravity in a ditch.

Hydraulic dredges come in many sizes and production capacities.  They weigh from 10 tons to 50 tons and more.  To transport the dredge, pipeline and support equipment, will require 3 to 10 truck loads.  A large crane is needed to assemble and launch the dredge.  The pipeline from the dredge to the spoil area must be assembled.  The dredge and pipeline must be disassembled and hauled from the site when the work is completed.  This is called mobilization and de-mobilization.

2.  Mechanical Dredging: Mechanical dredging is generally done with a barge mounted crane using a clam bucket or dragline bucket.  A hydraulic excavator can also be used.  The material is excavated and placed in a barge.  A boat, [towboat / tugboat], moves the barge to the disposal area.  On inland lakes and rivers, the barge is generally moved to the shore and the mud is unloaded with a crane using a clam bucket.  The mud is placed in dump trucks, and hauled to a disposal site. 
On coastal dredging, the material is placed in a barge.  The barge is then towed to an approved area that is generally far out in the ocean and the mud is bottom dumped from the barge.  The barge can also be unloaded by pumping, for beach repair or the mud can be pumped to a spoil area.
The Dredging Specialists exclusive Drag Scraper Dredging System is another form of mechanical dredging.  Click on this link to learn more about it.

How Dredging Work Is Priced: Dredging work is priced by the cubic yard of material that is dredged.  To determine the cubic yards to be dredged, take the acres to be dredged X 1,600 cubic yards per foot of material depth.  Note: There are 43,560 square feet in one acre.  Example: If you have a 10 acre lake and a 5 foot depth of material to be dredged.  10 X 5 X 1,600 = 80,000 cubic yards.  Another method is to take the
[length x width x depth of material to dredge] / 27 = cubic yards to be dredged. 

Hydraulic Dredging Costs.

Cost to design and build the spoil area, and dredge the material: $4.00 to $8.00 per cubic yard.

Combined charge for mobilization and de-mobilization: $20,000 to $50,000.

For preliminary cost estimates, use the average of the above costs.  

If there are words herein that you do not understand, you can look them up in an online dictionary or search the Internet for that word.

Keep in mind that Dredging Specialists provides professional consulting services based on over 30 years of hands on dredging experience.

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