Why do we need dredging?
Cargo ships move a lot of goods around the world every day and require deep water to travel in shipping channels. Over time, sedimentation - the accumulation of silt, sand, rocks, dirt, and other debris - can reduce that depth. Compounding the problem, most rivers and harbors are also naturally shallow and wouldn’t naturally be deep enough for many larger vessels. Periodically, the material needs to be removed to accommodate commercial shipping requirements and waterway health. Dredging is the underwater removal of this sediment, which is usually done by a specialized vessel. Regular dredging keeps shipping lanes clear and can even produce new shipping lanes for larger vessels.
In some cases, the sediment can also contaminate plants and wildlife or erode beaches and coastal areas. Environmental dredging is done to remove contaminants, which sometimes come from cities and industrial areas. According to NOAA, “These pollutants are introduced to waterways from point sources such as sewer overflows, municipal and industrial discharges, and spills; or may be introduced from nonpoint sources such as surface runoff and atmospheric deposition.” Removal of material is common for both waterway maintenance and environmental concerns. According to Dredging Contractors of America, “More than 400 ports and 25,000 miles of navigation channels are dredged throughout the United States to keep traffic operating efficiently.”
Dredger spraying sand. Image from Wärtsilä.
Uses of dredging
Dredging is more than deepening water for shipping lanes. Here are some of the many uses for dredgers:
Waterway maintenance: as listed above, this is the most common use of dredging to ensure safe water depths for vessels.
Creating new waterways: building a new waterway can increase speed of trade and make transportation easier.
Excavation: This is sediment removal for construction like a bridge or a dock.
Reclamation: cleanup of contaminated sediment, buildup, or runoff.
Increasing depth of a waterway: removing accumulated build-up keeps waterways safe for large vessels.
Expansion for larger vessels: digging a deeper and wider channel can improve the ability of larger ships, creating more economic value. A good example of this is the Suez Canal, which after the Ever Given ran aground, has plans to increase the size of the waterway to prevent future accidents.
Shore replenishment: Storms, mining, and disasters can cause beachfront erosion, which can be restored with dredging.
Trash pickup: dredging can be used to pick up accumulated trash in the bottom of a waterway.
Construction materials: dredging can be done to get needed construction materials for a project.
Pond or lagoon cleaning: removing some of the sediment from a stagnant water body can reduce the odor and improve water health.
Mining: sometimes gold and even diamonds are present in sediment, so the sediment is removed to capture these precious minerals.
Wildlife preservation and ecosystem maintenance: dredging can help to remove many kinds of debris to keep water clean. According to GeoForm: “It also remediates eutrophication, which is an excess of nutrients in the water due to runoff. By solving eutrophication, you stop the excess growth of plant life, which can cause oxygen deprivation.”
Backhoe dredger. Image from Marine Insight.
What are the benefits?
In addition to the above benefits, dredged material can be used in land-based construction. The material is removed, dried, and then can be used where additional land is needed. It can restore shorelines, improve the health of aquatic ecosystems, and remove pollutants. The most common benefit is maintaining waterways for trade.
How does dredging actually work?
Modern dredgers have a dredge that removes the material from the bottom or the shore by the water. The boom is lowered into the water or to the shore, where a rotating cutter-bar may help by chewing through the material while a pump moves the material to the ship. The material is later deposited somewhere else it might be needed, or where it will be out of the way. In the past, dredgers struggled to keep waterways clear and there weren’t good methods for underwater surveys or station keeping. Dredging was a combination of art and science. Modern dredgers have detailed topological survey data and use dynamic positioning to keep position while dredging.
Infographic image from GeoForm.
Types of dredgers
There are a few different types of dredgers, some overlapping as hopper and cutter suction are both considered hydraulic dredges.
Mechanical dredge: these are the simplest type of dredgers. Mechanical dredging is manually removing material with a scooping bucket such as a backhoe, dipper, or clamshell. Mechanical dredges are robust and work in tight areas, with minimal moving parts. Usually, a disposal barge, or scow, is required to hold the material. Clamshells are used for loose material while backhoe and dippers are used for denser material.
Bucket-chain: this is another type of mechanical dredger that uses a continuous chain of buckets that scoops material at the bottom and dumps it as the buckets tip over at the top. This is an old design that is no longer commonly used.
Plain suction: these function like a big vacuum cleaner and just rely on suction alone to remove loose sediment.
Cutter-suction dredges use a cutting tool that loosens material before removing it the same as a plain suction dredge. This works for most materials because it breaks up the material and feeds it to the suction system. They can also deposit material directly to where it is needed, or store it on the vessel.
Auger-suction dredges are similar, except that they burrow a hole into the surface. It works well for heavy-duty removal applications and deep removal.
Jet-lift uses a high-volume stream of water to pull in material.
Hopper dredge is a vessel that can work in all types of sediments, is very nimble, and can transport the material to another location. They work well in rough seas, deep water, and in high-traffic areas.
Dredger with a suction arm. Image from Wärtsilä.
Dredging vessel
Check out this video of Boskalis trailing suction hopper dredgers.
Video from IADC Dredging (International Association of Dredging Companies).
Hopper dredgers like the one above use dynamic positioning to stay on course for precise dredging. They lower a suction arm that uses water jets to loosen sediment, which is then sucked into the hopper to be stored. There are three options for the vessel to discharge the material: through rainbowing, or shooting the material out from a nozzle at the bow of the vessel, which works well when the intended target is land or just below water. The sand can also be pumped to shore via pipelines for land reclamation or coastal projects. There are even doors in the bottom of the ship that can be opened to release the material to a specific location underwater.
Where does all the dredged material go?
If material is contaminated, it has to go to a confined disposal facility. Most material, however is reused, such as using sand to replenish a beach being washed away by erosion. Fine clay and dirt are used as construction materials for new land in harbors and ports, while fine dirt can also be mixed with other materials for topsoil. Dredging can be used to harvest peat moss, which can then be used to enhance soil or to create fuel. Even golf courses sometimes dredge their ponds, with one golf course recovering five 55-gallon drums of golf balls! Mining can also be done to recover precious minerals like gold and diamonds.
Most people probably don’t realize how common dredging is, or how many ports and harbors are created and kept usable through dredging maintenance. According to the Dredging Contractors of America: “Billions of cubic yards of material are removed from sites around the globe annually in an effort to keep the big ships and their cargo moving.” It’s a very interesting process that keeps ships transporting goods around the world, saves wildlife, and helps with erosion.
Happy Friday!
Sources: