Grab dredgers are a relatively simple method of dredging which involves the collection of sediments in a crane mounted bucket, the jaws of which are opened and closed (rope operated or hydraulically) like a clamshell trapping sediments. There are various grab buckets designed for different types of material, such as mud grab, sand grab and the heavy digging grab. The upper structures of conventional grab buckets are open, and if they are overfilled, which is a common occurrence, sediments spill out of the bucket as it is raised through the water column. Suspended sediments are also released from the impact of the grab on the bottom, pulling the grab out of the sediments, seepage from grabs due to poor closure and from overflow of barges or hoppers.

Specially designed grabs are available with a closed plated upper structure which reduces spillage by over-topping, however whilst they appear to reduce suspended sediment levels in the upper water column, there is evidence to suggest that they may increase levels near the bottom. When dredging thin layers of sediment, for example to remove a thin layer of contaminated sediments, the closing arc of a conventional grab may cause over-dredging which can be avoided by using a ‘cable arm grab’ which closes horizontally.

Backhoes are shore-based or pontoon mounted ‘diggers’ which can be used in marine environments. Especially where ground conditions are difficult, such as shallow waters and confined spaces. Suspended solids can be released into the water column during excavation of the sediments, as the bucket is raised and lowered to the seabed, and from the overflow of barges. Suspended solid levels generated during this activity are likely to be similar to those generated using grab dredgers. This method of dredging is highly accurate and which may be of particular benefit when working in environmentally sensitive areas or contaminated sediments.

The trailing suction hopper dredger or ‘trailer’ is commonly used for maintenance dredging in coastal areas. As the ship moves slowly ahead sediments from the seabed are pumped through trailing dragheads into a hopper (reception tank). Suspended sediments can be generated as the draghead moves over the seabed, and from various other operating activities, although the largest contribution to increased concentration arises from overflow during loading. Trailer dredgers can be used for maintenance dredging in environmentally sensitive area if special care is taken. For example, they were successfully used for the deepening of the navigation channel in Lough Foyle, Northern Ireland without adverse affects to important shell fisheries in close proximity (Bates 1998). Certain modifications can be made to equipment to minimise the release of suspended solids including:

use of special dragheads which minimise sediment suspension,

reduced trailing speed,

increased under keel clearance to minimise propeller scour,

use of degassing to maximise pump performance in organic materials,

use of underwater pumps to maximise solid concentration, and

avoid using draghead water jets.

Water injection ‘jetsed’ is relatively new method of dredging which operates by injecting water into certain fine-grained sea bed materials, reducing their density to the point where they act as a fluid and flow over the bed through the action of gravity to lower levels. The aim of this type of dredging is not to raise sediments into the water column, and where properly applied environmental affects due to suspended solids are restricted to the vicinity of the seabed and are minimised greatly. However, some resuspension of sediments can occur using this equipment, intentionally or otherwise. At present this practise is exempt from FEPA licensing, as the sediments are not raised from the surface of the water and therefore no disposal takes place. However, this situation may be subject to review.

Another technique without the requirement for FEPA licensing because it there is no disposal is seabed levelling whereby a plough or seabed leveller is towed behind a suitable boat to flatten areas without lifting material from the seabed and dumping it elsewhere. There are three main types of plough; agitators, levellers and material movers. There is no available information on the potential for this dredging technique to increase levels of suspended sediments, although this is likely to occur during sediment movements.

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