A Treatise On The Principles And Practice Of Dock Engineering

ARRANGEMENT AND TYPES OE ENTRANCES. 235 Commenting on and emphasising this statement in 1898, Dr. Elmer Oorthell* added the following rider : — “It may be stated as a fact, palpable and undoubted, that no port of the world will, in the near future, be classed or used as a first-class port which will not readily admit steamers drawing at least 30 feet of water. This means 35 feet in the entrance channels through sea-bars, 32 feet in river channels and other entrance approaches, and 31 feet in harbours, basins, and along the quays and wharves.” The length to be given to an entrance will depend upon its arrangement, either as a lock with two pairs of gates, or as a simple entrance with one pair. In the latter case, apart from the wing walls adopted for entrances pointing amid stream, the length need not be more than will accommodate the gates and their side recesses. The length to be given to a lock entrance will, of course, be governed by the length of vessel which the lock is intended to receive. The largest lock on the Thames is the Tilbury entrance look, 700 feet long, followed by the northern entrance lock of the Albert Dock,. 550 feet long. The largest lock at Liverpool is 602 feet long ; at Barry there is a lock 647 feet in length; at Barrow, 700 feet; and at Cardiff, 800 feet. This last represents the maximum length yet obtained. The new lock at Bremerhaven is 705 feet long. Swansea has an 800-feet lock in hånd. Arrangement and Types of Entrances.—Following local dispositions and requirements, there åre three varieties of dock entrance, which are used either singly or in combination, viz : — (1) A simple entrance, provided with one pair of ebb-gates. (2) A lock, with at least two pairs of ebb-gates. (3) A half-tide basin, intervening between the river and the dock and separated from each by a pair of gates. Referring to these seriatim, it may be remarked that (1) a simple entrance is only available for navigation at or about the time of high water. Where the rise and fall in the tide is sufficient to necessitate the use of gates, the- working period will generally be confined to a period of three hours, or less, in each tide. Furthermore, a single pair of gates is but inadequate pro- vision against contingencies. Should an accident by any means happen to the gates so that they could not be closed, a very grave risk would be ineurred. The unexpected running dry of a dock would probably cause irreparable damage to the shipping berthed within it. (2) A lock offers additional facilities for the docking of vessels, since it can discharge its fonctions for some time after the water within the dock has been impounded ; to be precise, as long as there is sufficient depth of water on the outer sill to admit of boats entering the lock. It is a particu- larly useful arrangement when the dock is frequented by barges, lighters, and other small craft ; and its value is enhanced by dividing the lock, by * Corthell on “ Maritime Commerce,” Min. Proc. American Association foi -the A Avancement of Science, vol. xlvii.