Dredging Options

Dredges available in the Pacific Northwest today include pipeline dredges, sea-going hopper dredges, clamshell and barge dredges, and land-based equipment such as drag lines and back hoes. Pipeline and hopper dredges are commonly used for port development and maintenance and in larger dredging projects such as navigation channel maintenance by the U.S. Army Corps of Engineers. Clamshell and barge equipment is typically involved in smaller port and private dredging jobs but is sometimes used in larger navigation channel maintenance jobs. Land-based equipment is usually used for very small projects such as maintenance dredging at boat ramps.

Pipeline Dredges
Pipeline dredges are commonly used in larger dredging projects such as navigation channel maintenance by the U.S. Army Corps of Engineers or port development and maintenance. Generally, pipeline dredges are used for either large cutline shoals or areas with multiple sandwave shoals.

Pipeline dredges usually consist of a large centrifugal pump mounted on a non-propelled, specially designed barge. The bottom materials are then pumped up through a large diameter suction pipe to the barge, and then to the disposal area through a pipeline. The dredging end of the suction pipe is equipped with a revolving cutter-head that breaks up the bottom for easier transport. The pipeline is floated on pontoons, extending as far as 4,000 to 5,000 feet to the disposal site. Greater piping distances can be attained through the use of booster pumps.

The major limitation of pipeline dredging is that the disposal areas must be relatively close to the dredging site. The main advantage is the ability to dredge a large volume of material in a short period of time.

Pipeline dredges are typically classified by the diameter of their discharge pipeline. Dredges available in the Pacific Northwest range from a minimum size of 8 inches to a maximum of 30 inches. Pumping distance and production capability of the pipeline dredges is directly related to pipeline diameter; larger diameter yields greater discharge distances and higher production capability.

Figure A-1: Pipeline Dredge

Diagram courtesy of www.mgs.md.gov/coastal/dredge/sedstudy.html

Hopper Dredges
A hopper dredge is a type of hydraulic dredging consisting of a propelled floating plant which is capable of dredging material, storing it onboard, transporting it to the disposal area, and dumping it. Material from hopper dredges is disposed in deep water in or alongside the navigation channel, generally described as flow lane disposal, or in approved/designated disposal sites. Hopper dredges are commonly used on sandwave shoals and in river entrances and ocean environments.

The advantage of hopper dredges is that they are flexible, they can operate anywhere on the river and are mobile. Hopper dredges reduce the generation of suspended sediment but, as a result, produce large volumes of dredged material that must be dewatered. However, dewatering of sand takes place in a short period of time.

A limitation of the hopper dredges is that they will not load as well with finer sediments because the overflow will carry fines, that have settling rates slower than sand, out of the hopper and back into the river or harbor.

Figure A-2: Dredge Essayons

Photo Courtesy of U.S. Army Corps of Engineers

Figure A-3: Hopper Dredge

Clamshell Dredges
A clamshell dredge is a mechanical cable excavator dredge that uses a single bucket attached to the dredge crane with cables. The dredge operates by lifting the bucket (the clamshell), dropping it into the bottom sediments, lifting the bucket and dredged material to the surface, and emptying the dredged material into a nearby disposal facility or barges for transportation to either an upland or in-water site disposal facility.

The advantage of bucket dredges is that they can be used in tight quarters (e.g. around dock or piers). They can also be used in shallow areas where draft restrictions limit other methods. Furthermore, the dredge material comes up virtually in situ, making the clamshell particularly effective in silts or contaminated material.

Figure A-4: Clamshell Dredge