A Treatise On The Principles And Practice Of Dock Engineering

STRESSES IN PIERS AND JETTIES. 271 information on the subject are referred to the articles on “ Wave ” in each of the Encyclopædias, Britannica and Metropolitana. The points which more immediately concern the engineer are the nature, direction, and magnitude of the disruptive forces, as determined by actual observation. Although waves have been divided into two classes—those of oscillation and those of translation—it is probable that all waves are more or less waves of translation, causing the particles of which they are composed to move forward liorizontally to some extent. Certainly this is the case witli all large and important waves affecting the stability of maritime works. When a wave advances into water which becomes increasingly shallow, its energy is communicated to successively decreasing masses, and there is consequently a tendency to produce in those masses a greater and more violent agitation ; but this effect is generally diminished, and sometimes entirely counteracted, by the loss of energy due to friction along the bottom, and to eddies and surging. The bottom friction produces a distortion of the elliptical orbits of the particles of water, causing the crest to advance more quickly than the trough. At length the crest overhangs the face slope, falls forward, and breaks into surf. At this point the forward motion of the particles is equal to the velocity of the wave, and the stroke represents the maximum effort of the latter. Now, the velocity of a wave in shallow water is found to be nearly the same as that which would be acquired by a heavy body in falling freely from rest, under the action of gravity, through a height equal to the semi-depth of the water plus three-fourths of the height of the wave. Accordingly, we have v=^g(d+^,. .. . (44) where v is the velocity of the wave, h its height, and d the depth of the water. When the depth of water exceeds the length of the wave, the speed of the latter is practically independent of the depth, and is almost exactly equal to the velocity acquired by a body falling through half the radius ofa circle whose circumference is the length of the wave. Ihe reaction of a surface subjected to the force of continuons impact is measured by the rate at which momentum is destroyed. Hence, if w be the weight of a unit volume of water, wv is the mass which impinges on unit surface in unit time, and wv2 is therefore the amount of momentum. And since the weight of 1 Ib., falling freely, generates in one second g units of momentum, the reaction of the surface will be equivalent to a weight of and this represents the pressure per unit area due to the impact. Lieut. Gaillard (Corps of Engineers, U.S. Army), has demonstrated by experiments upon small areas that the maximum intensity of force in breaking waves in such cases occurs at a level slightly above still water,