Dredging is the operation of removing material from one part of the water environment and relocating it to another. In all but a few situations the excavation is undertaken by specialist floating plant, known as a dredger. Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material which has some value or use, or to create a greater depth of water. The latter is often associated with navigation, and it is the dredging of ports and harbours that is the most common form of dredging and the one with which most engineers are familiar. But dredging can form part of many other construction activities and it is important to be able to recognise both the capabilities of modern dredgers and the problems associated with their use.
Dredging has grown into a very specialised activity. The plant and equipment involved may have a capital cost of many millions of euro's; the largest of the modern dredgers found operating in the world today are capable of moving hundreds of thousands of cubic metres of material each week. Dredgers can bring great benefits. They allow new ports and harbours to be developed safely. They create the conditions for pipelines to be buried in the seabed, for sandy beaches to be maintained and protected from erosion, for oil to be extracted from deep sea wells and for maritime navigation to take place under safe conditions.
Dredging can be carried out wherever there is sufficient water depth to allow a dredger to operate. Thus dredgers can be found around the coasts, in rivers and in canals. They can be found in lakes and ponds far from the sea and they can be found in exposed offshore locations far from land. Dredgers operate in every ocean of the world, and they can also be found working reservoirs high up in the mountains of the Andes. Dredgers exist in a wide variety of shapes and sizes, ranging in complexity from a simple grab crane on a floating pontoon to some of the most sophisticated and technologically advanced vessels afloat. Some dredgers can be moved from site to site by road transport; others are capable of sailing half way around the world under their own power.
Dredging involves many different skills. Although essentially a civil engineering activity, the work requires knowledge of mechanical and electrical engineering, electronics, naval architecture, the marine environment and many other disciplines. Above all it is a practical subject. Many of the activities are computerised and highly automated, but there is still great reliance on the experience of those involved in both the planning and the execution of the different types of projects.
The operation of dredging involves four distinct processes. The in-situ material must first be disturbed and loosened from its natural state. It then has to be moved from that position to the water surface. These two phases are generally considered to be the extraction operation. Following on are the equally important transport and relocation phases which complete the dredging cycle. Transport may take place in a barge or hopper, or as a suspension in pipeline flow. The cost of transport may be a high proportion of the overall dredging cost, especially if the relocation site is at some considerable distance from the dredging site.
Dredging is recognised as having the potential for major environmental impact. Whether in the extraction operation or in the relocation stage, care must be taken to minimise disturbance to marine life. Also, the dredged material should not be regarded simply as waste. Consideration has to be given to the potential for some form of beneficial use of the material. With increasing environmental awareness and tightening legislative control, finding a suitable site to relocate the dredged material can be a major constraint on the implementation of a dredging project.