A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER DESIGN. 113 Encyclopedia Britannica or the Fmeyclopedia Metropolitana. The late Sir George Airy, the distinguished Astronomer-Royal, also wrote a treatise on Tides and Waves, and this, with the works of Scott Russell and Weber, and later, of Wheeler, afford sufficient scope for reference. Yet, while disclaiming any intention of probing into the depths of abstruse speculation, we cannot abstain from alluding in general terms to those principles of wave action which have reference to their physical effects upon engineering structures. Such information is essential to an appréciation of the problems of breakwater design. ^ ^j,^^-*— 45'0"-—i ----e°0" -----*0^ ----- 70'°-----I ^wJ ™ —-^^çc^ Sprints. * . Habb/e oF ^H £izes s --------------------------------------32S o ----------------------------^• 0 10 20 30 M Si 60 70 30 SO 100 200 Scale of Feet. Fig. 93. —Section of Plymouth Breakwater. For the present purpose, it suffices to state that water waves have con- veniently been divided into two classes, viz., waves of oscillation, without forward motion, and waves of translation, possessing it. Yet, in spite of this distinction—a purely artificial one,—it seems probable that all waves are more or less waves of translation, causing the particles of which they are composed to advance permanently to some slight extent, at least. So far as sea waves are concerned, those which possess the power of exerting any appreciable effect on the stability of maritime works are undoubtedly waves of the second division. Form of Waves. —The formation of storm waves takes place in the open sea, and their inception is, of course, due to the wind. The outline assumed is extremely variable, depending both upon the length of the undulation and its period, the lastnamed being the interval of time in which the wave traverses a distance equal to its length. The crest, or summit, of a wave is sometimes rounded, sometimes acute, and, in either case, it attains a height above mean sea level greater than the depth of the trough below it. In a swell in the open sea, the profile of a wave perhaps most nearly resembles a sinoidal curve, the slope directly exposed to wind action being more gradual and less steep than the leeward slope. Under the conditions of modern investigation, however, as exemplified in the researches of Weber, Scott Russell, Enry, and Aimé, the hypothesis has been advanced that there is an orbital movement in waves, each particle of which they are composed pursuing a regular geometrical path. The precise nature of the path depends upon local conditions. Where the depth of water