A Treatise On The Principles And Practice Of Dock Engineering

MAINTENANCE OF FAIRWAY. 237 direction, and an 80-foot entrance pointing down-stream. Ships docked before high water, anchor above the upper entrance, and, when the gates can be opened, are breasted in alongside the jetty-head. The lower entrance is intended for the use of vessels which cannot arrive before high water; it is also required during freshets in the rainy season, when the current in the river is always down-stream. In connection with parallel entrances it has been noted, in the Mersey, that during the time in which they are open, a circulating current has been set up, the water entering through one passage and making its exit by the other, and this quite regardless of any change in the tide. At the entrance to the Manchester Ship Canal there are three parallel looks—30 feet by 150 feet, 50 feet by 350 feet, and 80 feet by 600 feet respectively. (3) Half-tide basins, which are practically locks on a very large scale, are said to be due to the initiative of the late Mr. Jesse Hartley. They differ only from locks in regard to their irregular shape and great size. The gates of the dock proper are closed at, or soon after, high water, whereas the gates of the half-tide basin are kept open, as the name implies, for several hours afterwards, so that belated vessels can enter as long as there is sufficient depth of water over the outer sill which, of course, is necessarily lower than that of the inner dock. Vessels may remain in the half-tide dock until the ensuing flood tide and discharge part of their cargo there, or, if it be desirable to establish immediate communication with the inner dock, this can be done by pumping water into the half-tide dock from some external supply, usually the river itself. To equalise the level by running down the water in the inner dock would generally prove to be too wasteful of water, unless the latter were relatively much larger than the half-tide basin. This last condition may, of course, be fulfilled by grouping several inner docks together. The Sandon half-tide dock at Liverpool has an area of 14 acres, and is in direct communication with the Sandon Dock (10 acres), the Huskisson Dock and branches (36 acres), the Wellington Dock (8 acres), and the Bramley Moore Dock (10 acres)—64 acres in all.* The North Dock (13 acres) at Swansea is approached by two half-tide basins, one at each end, with areas of 2J and 1^ acres, respectively. At Sunderland there is a half-tide basin of 2| acres, acting as a vestibule to the Hudson Docks, of over 40 acres in extent. Maintenance of Fairway.—The absolute necessity for a sufficient and continuons depth of water in the channel leading to a dock entrance is self- evident. The tendency, which the channel has, to become silted up, must be checked by some corrective agency, either natural or artificial. The natural means would be the utilisation of some beneficial current. Where this is im- practicable, recourse must be had to sluicing, scouring, scraping, or dredging. Sluicing. —This method consists in forming an aqueduct or culvert in the side walls of an entrance, communicating with the dock at its inner end,. * And indireetly with others, the total impounded area being over 100 acres.