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Dieren en planten

Water en land

Mens en Milieu

Engineering at sea   Dredging works   
Sand dregder, Foto Fitis, www.fotofitis.nl

Dredging works

Sand and mud are transported by the rivers and have the opportunity to subside by the mouths of a river because the velocity of the water currents decline strongly. In order to maintain the depth of the navigational channels for shipping, this sand and mud must be regularly dredged from the bottom. This kind of work activity is called dredging.

  • Pollution

    Fifty million cubic meters of mud are dredged every year in the Netherlands from channels and waterways. Because the dredged material is usually polluted with oil, heavy metals and PAHs, strict demands have been made when processing this mud. Consequently, less dredging occurs than necessary, whereby rivers and canals become very shallow: inland ships can only carry half the amount of cargo at times without grounding. To get rid of the dredged material, two-thirds are brought to the North Sea by ship.
    The dredged material dumped on land is well ventilated and stirred so that bacteria can degrade the molecules. The oil and PAHs can be degraded naturally in this way. However, some of the dangerous materials remains stuck to soil particles and is only available when water is present which diffused the materials. A solution to this problem was found by an environmental chemist from Wageningen: plant trees in the mud and the roots of the trees will ventilate the wet mud. The degradation process takes six to seven years instead of thirty years.

  • Maintaining shipping lanes in the Netherlands

    The shipping lanes must be dredged in order to maintain their depths. Every year, around 30 million cubic meters of dredging material must be removed in order to keep the entry channels to Rotterdam and Amsterdam harbours at depth. Around 2 million cubic meters of dredging material must be dumped in a depot; the rest is clean enough to be distributed around the marine environment.
    The necessary dredging activities in these channels are usually combined with extracting clean sand: the IJ Channel produces 3 to 4.5 million tons of industrial sand every year, and the Euro and Maas Channels together produce 5 to 7 million tons of sand. The amount of maintenance necessary on the sailing lanes therefore varies from year to year.
    Dredged materials are often dumped in open sea. One problem is that the harbour mud can contain tributyltin, heavy metals and other pollutants attached to the mud particles. By dumping the mud, these materials end up in the sea.

  • Hoppers
    Hopper, Foto Fitis, www.fotofitis.nl

    Hoppers reclaim sand by sucking up the sea bottom with large 'vacuum cleaners'. The technique was originally developed for removing superficial layers of silt, however it is being applied more and more for sand exploitation. This form of sand exploitation has the advantage that instead of causing deep pits in the sea bottom, superficial channels are made which quickly fill in. A disadvantage is that the life within a larger surface area of the sea bottom is damaged. It takes a sandy bottom two to six years for the benthic fauna to recover from dredging tracks.
    Hoppers are also used to dig grooves, for example when placing pipelines. Some ships can work down to 120 meters. One dredger has developed a special installation which can get down to depths of 500 meters.
    During forebank nourishments, the hoppers release the sand by spouting it in an arch together with seawater onto the bank. During a beach nourishment, the discharge pipe is connected to a floating pipeline which transport the sand-water misture to the beach.