A Treatise On The Principles And Practice Of Harbour Engineering

ENTRÄNGE CHANNELS. 235 rock, and from the most friable sand to the stickiest clay, but vicissitudes of climate, weather, tide, current, and wind, have also to be reckoncd with, and operations must generally be conducted in such a way as to cause the least possible disturbance to the existing conditions of navigation. All these matters cause frequent and expensive stoppages and delays. In some cases, the actual useful working time only amounts to one-fourth of the whole year, and it is never safe, under any circumstances, to reckon upon more than 200 working days per annum. A very large proportion of time is taken up with repairs; breakdowns are a common occurrence, and the expense arising from this cause is no inconsiderable sum.1 Yet, in spite of all these drawbacks, dredging is an institution of untold value. By its means ports are brought into commercial prominence and saved from extinction. No other system can vie with it. The principle of dredging, originally that of digging and draggirig, has been extended to include pumping, so that modern dredgers are divisible into two types: first, those in which the action is mechanical erosion, and secondly, those in which it is hydraulic suction. In the most recent machines, both actions are combined. Mechanical eroders comprise scrapers, cutters, picks, buckets, and grabs, s>ngly or 111 combination. Scraping implements, apart from suctional adjuncts, have only a very restricted application. They are intended to disturb and comminute material to such an extent as to render it readily removable by the force of the current. But the power of a current to maintain material in suspension is strictly limited, and it soon becomes laden to its fullest capacity. When this point has been reached, it can absorb no increment without an increase in velocity, and at the first diminution in its speed it deposits a portion of its load. Hence, mechanical scouring rarely produces more than a slight displacement, and it certainly is not capable of sustaining operations on a scale of any magnitude. Combined with a suction tube and pump, however, it is a most useful agency. Experiments have demonstrated that, with the aid of suitable cutters and scrapers, marl, stiff clay, and adhesive material generally, may be separated and dissected to a degree compatible with its removal by pumping. The cutters employed are, generally speaking, cylindrical in shape, with straight or spiral blades mounted concentrically round the extremity of the suction tube. The efficiency of a cutter dépends very largely on its design, on the size, number, and shape of the blades and their positions relatively to one 1 Twelve montiis’ record of U.S. dredger “ Gedney,” working at entrance of New York Harbour :— Actual working time, parts of 112 days, equivalent to . Work prevented by weather (fog, storm, etc.) Occupied in general repairs during winter Occupied in minor repairs ...... Lost from other causes . ... . . Sundays and holidays..................................... 92f days 29J 154 21 10 59 366