Dredging is the process of excavating material from the bottom of a water body, such as a seabed, riverbed, or lake bottom. This underwater excavation is necessary because natural sedimentation, where silt, sand, and debris accumulate, gradually fills and shallows waterways over time. Specialized floating plants, known as dredgers, remove these sediments and transport them elsewhere. The entire operation involves four steps: loosening the material, bringing it to the surface, transporting it, and finally disposing of it or placing it for reuse.
Why Dredging is Necessary
Dredging is performed to maintain or improve the functionality of waterways and coastlines. The primary driver is the need to maintain or increase the depth of navigation channels, ports, and berths for the safe passage of modern, deep-draft vessels. This routine necessity is called maintenance dredging, which counteracts the natural siltation process.
Another major application is coastal protection and environmental restoration, where suitable dredged material is used to combat erosion and reinforce shorelines. This includes beach nourishment, where sand is placed back on eroded beaches. Dredging also facilitates land reclamation, creates new waterways, and can recover valuable mineral deposits. Furthermore, environmental dredging removes contaminated sediments from industrialized areas to reduce pollutant exposure.
Mechanical Dredging Methods
Mechanical dredging involves physically scooping or lifting material directly from the water bottom using buckets or grabs. These methods are preferred for their precision and ability to handle consolidated, heavy, or cohesive materials, such as stiff clay, fractured rock, or debris. The operation typically involves a stationary floating platform or barge secured by spuds or anchors.
Backhoe Dredgers
The backhoe dredger operates much like a land-based excavator, using a rigid arm with a single bucket that digs by drawing the material backward. This type is highly versatile for working in confined spaces, such as alongside wharves, or for precision work in shallow waters.
Clamshell Dredgers
Clamshell, or grab, dredgers use a hinged bucket that is lowered open to the seabed, closes to capture the material, and is then hoisted back up. They are effective for deep-depth dredging and for removing material from congested areas where a backhoe’s swing radius is limited. Material excavated by these mechanical methods is brought to the surface near its in-situ density and deposited directly into barges for transport.
Hydraulic Dredging Methods
Hydraulic dredging relies on a powerful centrifugal pump system to mix the excavated material with water, creating a fluid slurry transported through a pipeline. This approach is best suited for moving large volumes of soft material, including loose sand, silt, and soft clay. The most common hydraulic dredgers are the Cutterhead Suction Dredger (CSD) and the Trailing Suction Hopper Dredger (TSHD).
Cutterhead Suction Dredgers (CSD)
The Cutterhead Suction Dredger is a stationary vessel that uses a rotating mechanical cutter at the end of its suction pipe to loosen compacted material before suction. Because the CSD can cut through resistant soils like consolidated clay and rock, it is used for capital projects requiring a deep, permanent cut. The slurry is pumped continuously through a pipeline directly to the placement site, making this method efficient for projects requiring direct upland reclamation.
Trailing Suction Hopper Dredgers (TSHD)
The Trailing Suction Hopper Dredger is a self-propelled, seagoing vessel that dredges while slowly sailing. This allows it to operate effectively in busy shipping lanes or rough open-sea conditions. The TSHD lowers one or two suction pipes with drag heads that trail along the seabed, collecting the material. The slurry is pumped into a large hold within the vessel’s hull, called a hopper. Here, the solid material settles out and excess water overflows back into the sea, allowing the TSHD to transport the dredged material over long distances before discharge.
Handling and Placement of Dredged Material
Once excavated, the final stage is material management, involving transport and placement. Material removed by mechanical dredgers is loaded onto barges, while hydraulic dredgers pump slurry through pipelines or store it in the vessel’s hopper. Placement location depends on the material’s composition and contamination level. Options include open-water disposal, placement in a Confined Disposal Facility (CDF), or beneficial reuse.
A Confined Disposal Facility is a diked area used to hold dredged material and allow it to dewater over time. Beneficial reuse prioritizes using the dredged material as a resource instead of treating it as waste. Clean, sandy material is commonly used for beach nourishment or land reclamation. Fine-grained material can be used for habitat creation, such as wetland restoration. Approximately 85% of dredged material is available for some form of beneficial use, providing environmental and economic benefits.