Reasons for Dredging
The first major objective of dredging is the recovery from under water of material which has some value or use. In this instance dredging can be regarded as a form of mining. The dredging of tin ore was an early example of mineral dredging, but gold, diamonds, coal, rare earths and phosphates are among other materials which are won by dredging. The dredgers involved in this type of mining are highly specialised and custom built for a particular project. They may include some form of on-board processing of the recovered material and are usually owned by the mining company.
A much more common material won by dredging is sand and gravel for use in concrete manufacture. Sea-dredged aggregate is a valuable alternative to land-based sources of these construction materials. The UK's marine-aggregate dredging industry is one of the largest in the world and lands many millions of tonnes of sand and gravel each year. Aggregate dredging also takes place in many inland waters, including rivers, lakes and ponds. Aggregate dredgers are usually specially designed and constructed for the particular operation and are owned and operated by companies whose main activity is construction material supply.
The creation of new land by hydraulic fill is an important use of dredged material. Around 80% of the largest population centres in the world are found in coastal areas. As a consequence, in many coastal situations there is a great shortage of land suitable for development. One way of providing additional space is to raise the existing sea-bed levels by the placement of suitable material recovered from another location. The dredging site may be one where there is a need for deeper water associated with the new land, but more usually is some natural deposit of sediment which can be dredged easily and quickly. After transport to the area to be reclaimed, the material is pumped ashore as a suspension. The sand quickly settles to leave a compact base for such projects as new industry, housing, tourism, transport infrastructure and port development. The emphasis is very much on moving large volumes of material as quickly and as economically as possible.
Beach nourishment is another aspect of dredging where the prime objective is achieved by the recovery of suitable material. Where coastlines erode and degrade an alternative to the construction of such hard forms of protection as rock armour and concrete walls is the placement on the shore of natural sands and gravels, perhaps recovered from where the eroded material has deposited. By nourishing or replenishing the beach the natural balance is maintained. This type of work requires dredgers able to place the sand on what is often a shallow and exposed coastline. The creation or enhancement of wetlands by using finer sized dredged material is another potential beneficial use, as is the construction of offshore berms and islands.
Deepening of Water
The second main objective of dredging is the creation of deeper water. If the natural depths in an area are increased for the first time the activity is known as ‘capital dredging’, ‘development dredging’ or ‘new works dredging’. With capital dredging the full range of geotechnical materials may be encountered. ‘Soft’ material, such as sand, silt and clay, may well be mixed with much stiffer clays, boulders and in some cases rock. This activity covers the construction of new harbours, ports, basins, canals and waterways.
Deepening below the pre-existing bed levels can result in sediment being moved into the deepened area by the actions of water currents and waves. The siltation then has to be removed to maintain the required depth. This type of dredging is known as ‘maintenance dredging’. In some situations maintenance dredging may be required only once every few years. In others it may be needed two or three times each year. And in others it may be a continuous operation throughout the year.
Dredging for deeper water or greater cross-sectional area may be undertaken for many different reasons. The dredging may be part of a water supply or flood relief project. Dredgers can be used to construct reservoirs, deepen flood-prone rivers and form irrigation channels. The storage capacity of reservoirs subject to siltation can be maintained by dredging, and hydroelectric power projects can be assisted with their water supplies.
The more common reason for seeking deeper water is to improve navigation. This applies to ships and structures of all types, in the sea, in estuaries and in inland waterways. Navigational dredging is the most common form of dredging activity and is undertaken in ports, harbours and shipping channels throughout the world. In some locations the dredging may be for vessels with deep draughts, such as large oil tankers, bulk carriers and container ships. In others it may be for coasters or inland waterway barges, fishing vessels, naval vessels, ferries or leisure craft. Some of the work may involve increasing the natural depths as ships become larger or new ports are developed. Much navigational dredging is the periodic removal of sediment deposited in the deepened channels. Maintenance dredging is a necessity for almost every navigable waterway and port in the world.
An expanding area of dredging activity is that termed remedial or ‘environmental’ dredging. There is increasing recognition of the amenity value of waterways, especially in urban areas. With many rivers, canals, drains and lakes being choked with all forms of rubbish, their clearance has presented new challenges to the dredging industry. Sometimes the sediment is contaminated with pollutants that require special handling to minimise risk to the environment. This is particularly the case in heavily industrialised ports or in river mouths with industry upstream. The clearance of industrial sludge lagoons and settlement ponds is another highly specialised area of dredging activity, which may even involve the use of a remote-controlled dredger.
Civil engineering construction work is another activity that can create a demand for dredging. Here the nature of the work is not dissimilar to that undertaken on land. The creation and backfilling of trenches for pipelines and tunnels, and the forming of foundations for structures, are all dredging activities which have their counterparts in the dry. The dredging is generally a specialist subsidiary activity to the main construction with the work often being sub-contracted to the specialist dredging contractor.