A Treatise On The Principles And Practice Of Dock Engineering

DIMENSIONS. 233 retain in suspension and transport material, the current will have to exceed this limit, and, in some cases, to be very much greater. The figures in Table xxii. relate to the bottom or bed velocity, which, according to Professor Rankine, varies between | and | of the surface velocity. A moderate current in the fairway of an entrance is a desideratum from more points of view than one. It prevents silting and it assists in the manæuvring of vessels. For this reason it will be advisable to locate an entrance in the vicinity of a concavity in a river’s bank rather than at a convexity. But the question is somewhat too complicated for generalities, and the engineer will have to rely largely upon his own judgment, aided by such local information as he is able to procure. Direction.—Having determined the site, the next point to be settled is the direction of the entrance. There are three main directions in which an entrance may point—viz. (a) down-stream, (6) up-stream, and (c) amid- stream, or at right angles to the direction of flow. (a) A down-stream entrance is not convenient for vessels entering on a flood tide. The way on a ship is maintained or increased by the tidal flow, and effective control is more difficult. It is better, for purposes of naviga- tion, to dock or undock a ship against the tide or current. Hence such an entrance would only be suitable for vessels docking alter high water or undocking before high water. In non-tidal rivers, or those portions unaffected by the tide, the circumstances are in favour of a down-stream entrance, especially if the current is at all strong. (6) The advantages and disadvantages of an up-stream entrance are the converse of those appertaining to a down-stream entrance. There is the additional consideration that an up-stream entrance is more likely to be silted up by detritus brought down by the river and deposited in the mouth of the entrance. (c) An entrance pointing amidstream is at once the least convenient and the most convenient form for general purposes. In itself it offers grave drawbacks to navigation, for the moment a vessel’s bow cornes within its shelter, the unprotected stern will be swung round by the force of the current, unless it exceptionally happens to be dead high water at the moment; but if it be provided with a bell-mouth, or with trumpet-shaped wing walls, this drawback is overcome and the entrance becomes availible for both ebb and flood tides, since a vessel may thus gain the leeward of either of the wing walls for her entire length before engaging in the entrance proper. Dimensions.—The dimensions to be assigned to an entrance will «bviously be regulated by the size of the largest ship frequenting the port, with a due allowance for future incrément. Halfa century ago, under the régime of paddle steamers, entrances and locks had to be constructed of very considerable widths. When, in process of time, screws and propellers displaced paddles, the necossity for a great width of waterway temporarily disappeared, but with the growth and