A Treatise On The Principles And Practice Of Dock Engineering

WIND. 227 generally representative of practice in this respect :—Barry has an entrance channel, between breakwaters, 470 yards long. At Leith, a similar channel extends to 660 yards. Sunderland has an enclosed outer harbour affording a run of 900 yards. At Dover the present protected length is 750 yards, but when the new works, now in progress, are com- pleted there will be a sheltered reach of at least 1,100 lineal yards within the breakwaters. The objections attending a sea-exposed entrance are, of course, equally potent in the case of ports situated on broad river estuaries, flanked by low-lying country. Though the river inouth may be, to a certain extent, considered as supplying the functions of an entrance channel, yet it is often found expedient to provide a vestibule to the docks, in the form of a tidal basin, having free communication with the river. This is the plan adopted at the Liverpool and Birkenhead Docks, the Tilbury Docks at London, the Royal Dock at Grimsby, and others. The Canada Basin at Liverpool has an entrance width of 390 feet and a water area of 9| acres. The North Basin at Birkenhead has an entrance width of 500 feet and a water area of 4| acres, while at the Tilbury Basin the entrance is 364 feet wide and the water area 17| acres. In wide estuaries sheltered by ranges of hills, and narrow estuaries generally, in land-locked bays and lagunes, and on inland rivers, the fore- going precautions are rendered unnecessary, except for other and purely local reasons. The three natural agencies influencing the eligibility, or otherwise, of a site for a dock entrance are (a) wind, (6) wave, and (c) current. It will be well to subject them to a brief considération. For the purpose of the present section we need not investigate the effect of these natural forces except in so far as they favour or interfère with the effective use of entrances, and the manipulation of vessels. Any inquiry in regard to their action upon permanent structures will be deferred until we corne to the chapter dealing with the parts most affected—viz., jetties, wharfs, and piers. Wind. —The power exerted by thewind is often sufficient to greatly impede, if not absolutely prevent, the manæuvring of vessels (more particularly those with a high freeboard), into and through a narrow, exposed water- way. The effect is greatest when the direction of the wind is broadside on, causing the vessel to fall off to the leeward. A head wind can always be counteracted by adequate tractive or propelling power; in a side wind this is of no avail, and the vessel has to be kept in her course by means of ropes. Occasionally accidents happen through the breaking of these ropes from excessive strain. Cases have occurred in which all the retaining ropes to a vessel have snapped in quick succession, leaving her entirely helpless. It is to be regretted that, at the present time, there is so little reliable evidence in regard to the actual pressure exerted on large surfaces by air in motion. Records have, indeed, been obtained showing very great