A Treatise On The Principles And Practice Of Dock Engineering

WAVES. 229 position. Where this is not available, artificial shelter may be created in the form of parapet walls, wind screens, and buildings generally. Offices, huts, and sheds, for the use and shelter of the dock master and his staff, may be grouped at the more exposed points so as to break the force of the wind. The wind which blows with greatest frequency at any place is usually termed the prevailing wind, but it does not by any means follow that it is the wind attended by the most disastrous results. More harm may be done by a single gale from an unusual quarter than by a whole twelvemonth of the prevailing wind. In this, as in all other matters, it is necessary to acquire locally the fullest information possible. Waves. —The action of waves, apart from tidal waves, depends, primarily, of course, upon the wind, but, once agitated, the sea maintains a momentum which may, and usually does, outlast the duration of the wind itself so as to ■constitute an entirely distinct source of activity. The inception of waves being due to the wind, their development will largely depend upon the amount of surface acted upon. The greater the length of open sea, ceteribus paribus, the higher the wave which breaks upon the shore, provided always there is sufficient depth of water to admit of its formation. Intervening shoals will break up a wave, so that the effective length of sea may be much less than the apparent length. This length, or distance within which the wave attains its development, is termed the fetch, and Stevenson has devised an empirical formula from which the probable height of waves may be estimated. Taking II as the height of the wave in feet, and L as the length of fetch in miles, it has been found that approximately— H=b5 VF;...............................(a) or, for short fetches, less than 30 miles, II = 1-57F + (2-5 - VF). . . . (3) In Table xix. are one or two examples of the height of waves as deduced by the formulæ and as determined by actual observation. The maximum fetch alone cannot, however, be considered as a criterion of the exposure of an entrance. The severest gales may not blow from that particular quarter of the compass, and, on the other hand, heavy rolling seas may be deflected so as to bring their influence to bear upon an apparently sheltered area. In exposed situations it is possible, by artificial means, to cause a wave to spend its force before reaching the spot where its unchecked onset would be dangerous. Breakwaters, either in the form of parapet walls or as sub- merged mounds, may accordingly be employed to reduce the amount of fetch and to provide areas of comparative quiescence. These works, how- ever, form a distinct branch of harbour engineering which cannot be entered upon here.