Types of Dredging

SS Badger Passing Lighthouse Outbound Lake Michigan
SS Badger Steaming Out of Harbor. Lake Michigan Carferry

Dredging is a recurring task that must be addressed on a regular basis. It is like your lawn, the problem will continue to grow whether you pay attention or not. Eventually the task will need to be completed in order to keep maritime traffic lanes and facilities functional.

Much of Europe dredges their infrastructure regularly while in other regions like the United States some areas must become nearly unusable before action is taken.

To be fair there are U.S. ports that take great care to keep up with dredging but many areas have not had any dredging in decades.

The Great Lakes and Mississippi Rivers are sediment rich systems that carry a large amount of freight but loads have been reduced by up to forty percent to contend with limited draft conditions.

These are the main types of equipment used for sediment removal. They can be divided into two types; the first type is a scoop type dredge which lifts a scoop at a time, the second type is a suction dredge that moves material at a constant volume.

Scoop Type Dredges

Bucket Dredge – This dredge has many buckets that travel along a boom on cables. The boom is maneuvered underwater so the buckets take a scoop of sediment and travel back to the vessel or barge where they deposit their load. Bucket dredges can cope with loose bottoms from large rocks to sand. Teeth are placed at the opening of the buckets to dig deeply into the material.

This type of dredging is best for longer distances like harbor approaches.

Clamshell – These are what many people think of when picturing a dredge. These are often used in harbor construction when boulders or other difficult materials cannot be removed with one of the more efficient designs. The clamshell has two sides that close to scoop a large amount of sediment but the method is slow and used only when necessary.

Much of the Panama Canal was dredged using huge clamshell buckets that were high technology at the time.

Dipper – The same idea as the clamshell but with only one side and connected to a hydraulic arm. Excavators on large barges are often used as dipper dredges. It is a slower process than a clamshell but can fit into the tightest spaces.

Suction Type Dredges

These dredges all use suction to draw water and included sediment into a pipe that empties into storage compartments fitted with dewatering devices like seeps and overflows. The drained sediment is then deposited in a designated area.

Basic Suction dredges are big vacuum cleaners for sediment. Sorry that everyone uses that example but it is the most accurate. Some suction dredges come with some attachments that pull sediment into the suction pipe others just use the pipe alone.

These are some specialized types of suction dredges.

Cutter Head Suction Dredge – The business end of this dredge has a large rotating collar fitted with cutters or teeth to break off and pulverize material before it is drawn into the suction pipe.

This type of dredging can remove soft rock and coral. It is also used to clear rubble after blasting and clamshell dredging harder rock.

There are many different designs of cutter teeth engineered for specific conditions. Cutter teeth must be very durable but also impervious to jams and easy to replace. These criteria come together to make some very exotic new shapes that are displayed at trade shows each year.

Auger Head Suction Dredge – The head of this dredge is outfitted with one or two large augers that are attached at a right angles to the suction pipe. This is the dredge that can cut a wide, shallow swath with each pass. The augers rotate and pull material from the sides of the cut to the center where it is taken up by the suction pipe. The augers can also be fitted with teeth for more aggressive removal of bottom material.