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Dredging is an excavation activity usually carried out underwater, in shallow seas or freshwater areas with the purpose of gathering up bottom sediments and disposing of them at a different location. This technique is often used to keep waterways navigable. It is also used as a way to replenish sand on some public beaches, where sand has been lost because of coastal erosion. Dredging is also used as a technique for fishing for certain species of edible clams and crabs (see fishing dredge). ==Uses== * Capital: dredging carried out to create a new harbor, berth or waterway, or to deepen existing facilities in order to allow larger ships access. Because capital works usually involve hard material or high-volume works, the work is usually done using a cutter suction dredge or large trailing suction hopper dredge; but for rock works, drilling and blasting along with mechanical excavation may be used. * Preparatory: work and excavation for future bridges, piers or docks/wharves, often connected with foundation work. * Maintenance: dredging to deepen or maintain navigable waterways or channels which are threatened to become silted with the passage of time, due to sedimented sand and mud, possibly making them too shallow for navigation. This is often carried out with a trailing suction hopper dredge. Most dredging is for this purpose, and it may also be done to maintain the holding capacity of reservoirs or lakes. * Land reclamation: dredging to mine sand, clay or rock from the seabed and using it to construct new land elsewhere. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge. The material may also be used for flood or erosion control. * Beach nourishment: mining sand offshore and placing on a beach to replace sand eroded by storms or wave action. This is done to enhance the recreational and protective function of the beaches, which can be eroded by human activity or by storms. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge. * Harvesting materials: dredging sediment for elements like gold, diamonds or other valuable trace substances. * Seabed mining: a possible future use, recovering natural metal ore nodules from the sea's abyssal plains. * Construction materials: dredging sand and gravels from offshore licensed areas for use in construction industry, principally for use in concrete. Very specialist industry focused in NW Europe using specialized trailing suction hopper dredgers self discharging dry cargo ashore. * Anti-eutrophication: Dredging is an expensive option for the remediation of eutrophied (or de-oxygenated) water bodies. However, as artificially elevated phosphorus levels in the sediment aggravate the eutrophication process, controlled sediment removal is occasionally the only option for the reclamation of still waters. * Contaminant remediation: to reclaim areas affected by chemical spills, storm water surges (with urban runoff), and other soil contaminations. Disposal becomes a proportionally large factor in these operations. * Removing trash and debris: often done in combination with maintenance dredging, this process removes non-natural matter from the bottoms of rivers and canals and harbors. * Flood prevention: this can help to increase channel depth and therefore increase a channel's capacity for carrying water. * Peat extraction: in former times, so-called ''dredging poles'' or ''dredge hauls'' were used on the back of small boats to manually dredge the beds of peat-moor waterways before extracting the peat for use as a fuel. This tradition has now become more or less obsolete and the tools used to do this have also changed significantly. * Oyster dredging or harvesting: in Louisiana and other states with salt water estuaries that can sustain bottom oyster beds. A heavy metal rectangular scoop device is towed astern of a moving boat with a chain bridle attached to a cable and winch which scoops up oysters as it drags along the bottom. The device is periodically hauled aboard and the oysters in it are sorted and bagged for shipment to an oyster processing facility. * As a hobby: hobbyists examine their dredged matter to pick out items of potential value, similar to the hobby of metal detecting, or the hobby form of dumpster diving, on land. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Dredging」の詳細全文を読む スポンサード リンク
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