Dredging

Dredging, a process of removal of matter from the bottom of rivers, lakes, or seas, for scientific or commercial purposes. The instrument required for gathering up specimens, etc., is known as a dredye. It consists essentially of a strong square netted bag lined with canvas, with two parallel metal scrapers fixed at a distance apart of about 8 inches on each side of the mouth of the bag. The depth of the bag is about two feet, but varies according to the nature of the work to be done with it. A curved iron handle with ends fixed to each side of the mouth serves as a holder for the dredge, which is raised or lowered by a well-tested rope attached to the middle of the handle. A canvas flap is sometimes used as a lid to the case, to protect delicate specimens within, though not preventing the entrance of fresh objects. Commercial dredging has received a great deal of attention in recent years. So much depends on the clearance of silting or deposition of mud and gravel at the bottom of canals, docks, etc., that it is necessary to devise efficient methods of removal, if the plans for its complete prevention manifest themselves imperfect. Scouring-basins have been introduced in tide harbours, in which the strong currents due to the tides are made to preserve a clearance in the right localities. Of small dredging-machines the spoon apparatus is an example. It consists of a. tough bag of hide or leather attached to a stout iron ring and dragged along the bottom of the waterway at the end of a long pole. The dragging is done by a winch; the loaded bag is lifted out of the water by a crane and emptied into the barge which carries the apparatus. In large continuous dredging operations some form of steam-dredger is used. The bucket-dredger was first employed by Rennie at Hull. The principle is that of an endless chain passing round two fixed barrels, provided with iron buckets at regular intervals. The chain is erected vertically in a steamboat, with one end passing through a long opening at the base of the vessel and reaching down to the level of the cutting. When the chain is rotated, the buckets pass down with one side of the chain to the mud or silt at the bottom of the channel. They scoop up a certain quantity and then pass up on the other side of the chain till in a position to drop their contents into a barge or other near receptacle.