A Treatise On The Principles And Practice Of Harbour Engineering

BREAKWATER DESIGN. 121 The Dynamical Value of Wave Action.—The difficulties attending a determination of the précise effort of a wave are due to several causes. In the first place, there is the incompressibility of water combined witli the extreme mobility of its particles. Arrested suddenly in the course of motion, it produces all the percussive effects of a solid body in an infinite number of directions. No clearer evidence of this could be produced than the phenomenon known as water-hammer. If the outlet valve of a hydraulic service-main be shut down abruptly, a blow is administered to the pipe which may be, and often is, sufficient to produce rupture, even at a considerable distance from the outlet, unless, as is generally the case, a relief valve is provided to prevent such a disastrous effect. In the second place, the wave-stroke is both abrupt and continuous. Its first action is a blow, sharp and decisive and of higli momentary intensity. 10-5 0 <0 20 jo 40 so umiuiil__i ___ii i ( Sca/e of feet Fio. 101. —Section of Ymuiden Breakwater. This is succeeded by statical pressure during the small but perceptible interval of time which suffices for the dispersai of the wave. Accordingly, there are two phases to be considered : (a) the initial concussion, and (i) the subséquent pressure. Üsually the question is dealt with entirely as a matter of simple, continuous impact, but it should be noted that wave action is far from being completely identical with the unbroken impulse of a water-jet. Now, according to the principles of dynamics, the reaction of a surface st bjected to continuous impact is measured by the rate at which momentum is destroyed. If, therefore, w be the weight of a unit volume of water, — is the mass which impinges on unit surface in unit time, and — is the rate at which momentum is consumed. Hence, if p be the pressure on unit surface, we have