During the second phase of a dredging cycle, the dislodged material is raised. This can be done either mechanically or hydraulically. Using the mechanical alternative, the material is raised in a grab or bucket (e.g. backhoe or bucket ladder dredgers). In the second case, hydraulic dredgers (e.g. suction, cutter suction, trailing suction hopper dredgers) use a suction pipe. The dislodged material is then sucked into the suction mouth by means of a centrifugal pump. The material is further raised through the suction pipe towards the pump and, from there, through the discharge line to the deck of the dredger.

The third phase of the dredging cycle is the horizontal transport of the excavated and raised material from the dredging area to the site for further treatment or final relocation. This can be achieved mainly by one of three methods: hydraulic pipeline transport, transport by hopper dredgers or transport by barges.

Each method is linked primarily to a certain type of dredger. Barge transport is generally selected for mechanical excavation, while pipeline transport is used mainly with hydraulic dredgers, with the exception of hopper dredgers which are designed to load hydraulically. Of course, other types of transport exist, such as truck or conveyor belt transport, but to date their application in the dredging industry has been limited.

The final phase of a dredging sequence is the relocation of the excavated material to its final destination or to an intermediate site for further treatment. There are numerous options at this phase: reclamation of a site, beach nourishment, wetland creation, relocation on land, relocation in a pit, relocation at sea (underwater) and relocation to a contained site.

The TSHD is a normal inland-waters- or sea-going ship equipped with a suction pipe. At the end of the suction pipe is a draghead, which can be lowered onto the seabed while the TSHD navigates at a reduced speed. Most TSHDs have twin-screw propulsion and powerful bow thrusters, which provide a high degree of manoeuvrability. During the forward movement of the TSHD, the draghead agitates a thin layer of the seabed. The loosened material, together with some water as transport medium, is sucked into the suction pipe by means of a centrifugal pump, which is installed in the vessel’s hull or on the suction pipe.

The BHD is essentially a hydraulic excavator mounted on a pontoon equipped with a spud carriage system. Whereas the conventional land-based hydraulic excavator is normally mounted on a tracked or wheeled undercarriage, the dedicated dredging machine usually is mounted on a fabricated pedestal at one extremity of a spud-rigged pontoon.

Sometimes called clamshells, these dredgers can exist in pontoon or self-propelled forms. The GD is basically a conventional cable crane mounted on a vessel. The bed material is excavated by the grab of the crane and raised by the hoisting movement of the cable. Once above water, the crane arm swings and the material is loaded into its own hopper for the grab hopper dredger, or in a transport barge for the grab pontoon dredger. 

At the relocation site the material is discharged, either through the bottom doors or through the split hull of the dredger or transport barge, mechanically unloaded, or pumped ashore by means of a barge-unloading system.

Most CSDs do not have an optimal combination of cutting capacity and suction capacity for all types of soil. The type of cutterhead may make a significant difference in the amount of dislodged residue left. Depending on the cutting performances required, the optimal cutterhead is usually selected, which, if properly operated, also produces the least residue material.

Good accuracy can be obtained because the movement of the dredging head is controlled from a fixed point (the working spud). Vertical accuracies down to 10-15 cm are feasible, although, at full productivity, the accuracy level will depend on the soil characteristics and will be approximately 25 cm or more.

For optimal use of the CSD, the complete height of the cutter should be utilised for cutting purposes. This means that the minimum layer thickness (1 to 3 m depending on the size of the cutterhead) is often greater than the layer thickness which needs to be removed, especially in the case of selective dredging.

Owing to the hydraulic character of the transport, water is added to the soil for transportation purposes. Depending on the soil type and the attainable layer thickness, the amount of added water varies significantly. It should be noted that dilution can be reduced by an under-dimensioned cutting power, compared to the pumping power, but this will increase the dislodged residue layer effect. An optimum has to be found for each individual project.

Generally, the CSD has a powerful engine, which generates a high sound level in air. Given that the CSD is a stationary vessel, which often works in populated areas, the dredger can be a continuous source of significant sound levels. Precautions, such as low-sound engines, sound tempering covers and procedures to keep the engine room closed under any circumstance, are possible.

The emissions of a CSD are a function of the energy consumption, which is strongly influenced by resistance the cutter encounters and by the transport distance of the soils dredged, whether it be transport due to pumping or transport by barges. The main to mitigate emissions is through the use of newer LNG-powered or hybrid vessels, though these remain relatively uncommon.

Because the cutting process is basically a scratching action, only limited amounts of soil are loosened. Consequently, most of the material is picked up by the suction process without leaving a dislodged residue layer. Larger residue layers can be generated by the settlement of large quantities of overflowing fine-grained material that settles in a fluid mud layer on the bottom after a certain time.

The accuracy of the dredging depth is low compared with the CSD, owing to the fact that the position of the suction pipe is flexible and more difficult to control. At best a vertical accuracy of approximately 15-25cm can be obtained, provided sophisticated monitoring and steering equipment and automation and control systems are used.

The cutting process is strictly horizontal. As such, the mixing of soil layers can be controlled fairly accurately. However, taking into account the lower accuracy level compared with the CSD, the TSHD is not especially suited to accurate removal of thin layers of (contaminated) material.

Significant amounts of water are added during the suction process. With modern monitoring and control equipment, this amount can be limited. Where the pump-ashore facility is used, an additional volume of water has to be mixed with the dredged material to facilitate pipeline transport.

The TSHD is equipped with powerful engines generating significant sound levels, although the sound level is reduced to acceptable levels at a distance of a few hundred metres. As the TSHD generally operates in more distant areas sound generation is less of an issue. 

 As is the case with all traditionally-fuelled vessels, emissions are associated with energy consumption. In addition to the energy spent on actual dredging (strongly influenced by soil type and dredging depth), the total time needed to load a full hopper, and the time spent on sailing from dredging area to the placement location, and back, dominate the total exhaust emission. When pumping ashore has to be made, this also contributes to the total releases. Contrary to stationary dredgers, emissions are made at various locations, and along the sailing routes.

The cutting face of the backhoe is the edge the bucket. Almost all the soil loosened by the bucket is carried away, leaving a clean surface. A minor risk of a dislodged residue layer remains if there is excessive spillage while the material is being raised.

The accuracy of a traditional backhoe is relatively low, as the excavating bucket has to be repositioned at every cycle. Without sophisticated monitoring equipment, an accurate dredging depth is impossible. However, such monitoring systems exist and are now implemented on a routine basis for larger backhoes. Accuracy down to 10cm is attainable, albeit with reduced productivity.

Thin layers can be excavated selectively, provided a good monitoring and control system is available.

Backhoes require the use of transportation barges, which come with their own sound generation. Production of underwater sounds by these mechanical dredges depends on the dredging cycle, including the availability of barges. In general, they produce relatively low frequency sounds. Tugboats, or other equipment that move the barges, produce underwater sound when manoeuvring and sailing.

Emissions by backhoe dredging are determined by the power needed to actually dredge the material (influenced by hardness of the soil, excavation profile and water depth), plus the emissions by sailing of the transportation barges (influenced by distance and barge load efficiency)and those generated by the barge unloading spread.

Because the GD uses a mechanical cutting process to scrape the material from the bed, only a thin spill layer is left.

The accuracy of the GD is limited because the excavating bucket has to be repositioned at every cycle. Without sophisticated monitoring equipment, precise positioning is impossible. Monitoring systems and special-purpose grabs allow vertical accuracies of around 0.35 to 0.50 m. Horizontal accuracy is poor, especially in deep waters and in water currents where a pendulum effect occurs, except for the grabs equipped with an active ROV steering system.

With a traditional GD it is difficult to achieve a horizontal cut as the excavation depth of each cycle cannot be kept under control. Therefore, mixing different layers cannot be avoided. Recently, new bucket types and monitoring and control systems have been developed with improved characteristics in this respect.

The situation here is again similar to that with the BHD. Where hydraulic unloading of the transport barges takes place, there is a need for transport water.

Production of underwater sounds by these mechanical dredges depends on the dredging cycle, including the availability of barges. In general, they are deemed to produce relatively low frequency sounds. However, tugboats, or other equipment that move the barges, produce underwater sound when manoeuvring and sailing.

Emissions by GD dredging are determined by the power needed to actually dredge the material (influenced by hardness of the soil, excavation profile and water depth), plus the emissions by sailing of the transportation barges (influenced by distance and barge load efficiency) and those generated by the barge unloading spread.