A Treatise On The Principles And Practice Of Dock Engineering

CURRENT. 23( It has been stated that 2 feet is the greatest height of waves consistent with the safe working of dock gates.* The writer’s experience convinces him that this estimate is too low, for he is acquainted with instances in which the gates of exposed entrances have been worked without difficulty in waves of at least twice that height. Furthermore, vessels have safely weathered the pierhead of an entrance look with a rise and fall, due to surging, of 7 or 8 feet in the level of their decks. It may be said that while no definite limit can be fixed as the point at which the working of an entrance becomes dangerous, the practicability, or otherwise, of docking operations will largely depend on the tug and capstan power available, on the strength of the ropes and hawsers employed, and, above all, on the skill and capability of those who superintend and carry out the necessary manœuvres. For the record (Table xx.) of noteworthy conditions during a period of four months, at the Canada Basin entrances, Liverpool, the writer is indebted to the Dockmaster, Captain Parkes. Current. —In contradistinction to the intermittent character of the previous agencies, the third is continuous and cumulative in action. To the influence of the littoral current is due the maintenance or closing of the fairway of an entrance. Currents arise from several causes and their work- ings are often complex and conflicting. At one period of the day the tidal current will predominate in a river and cause an in ward flow, at another it will reverse its direction, augmented by the fluvial current. At different stages of the tide there will be zones of slack water, counter-currents, and eddies. It is no uncommon feature for the tide to be flowing into the mouth of an estuary at one side while it is ebbing on the opposite shore. The course of a river is never straight, and the current is greater at the concave side of each bend than at the convex side. Hence it is that currents are perhaps the most erratic and least understood of all aqueous agencies.I In tidal estuaries, just about the time at which the tide reaches its highest and lowest levels, there are periods of slack or still water, in whicli matter, hitherto kept in suspension by the movement of the current, is deposited. If allowed to accumulate in the vicinity of an entrance, the silt thus formed becomes a danger to navigation. It may, possibly, be removed by a succeeding current; if not, it will be necessary to remove it either by dredging, scouring, or sluicing. * Encyclopædia Britannica, 9th ed., Art. “ Harbours.” It is not quite clear whether the measurement is from trough to crest or merely above mean water level. The author assumes the former. + Lord Kelvin is reported to have said to a Parliamentary Committee, in reply to an enquiry respecting his investigation into the probable effect of certain works upon tidal currents, that he had considéred the question seriously, had made many calcula- tions, and was quite unable to arrive at any satisfactory result. Vide Farren on the “ Silting of Small Harbours,” Min. Proc. Liverpool Engineering Society, vol. xviii., p. 226.