Breaking Waves Forces on Plane Barriers

Abstract

For the past twenty-four months this Laboratory has been engaged in a study of wave action on barriers. The first half of this investigation was limited to cases of wave reflection from various structures. The experimental program for this first phase of the investigation included both the development of instrumentation and experimental techniques and the measurement of wave forces and pressures acting on plane barriers inclined at various angles and on a family of curved and stepped-face barriers selected by the Bureau of Yards and Docks. The results of that program (Refs. 1 and 2) include: (1) The development and experimental verification of relatively simple analytical expressions for the force and pressure distributions exerted on vertical plane barriers by reflecting waves. These expressions include a second-order double wave frequency term which becomes of increasing importance for small values of L/d (wavelength to water depth ratio), and which has not heretofore been considered in connection with wave forces on barriers. (2) The demonstration of a simple relationship between the forces acting on a vertical plane barrier and those acting on plane barriers inclined at angles up to 30° from the vertical, and on certain curved and stepped-face barrier profiles. The extension of the program to include the study of breaking waves was a logical consequence of the earlier work, both because of the engineering importance of the breaking wave problem and because the apparatus and experience developed in the first phase promised to be directly applicable to such a study. Previous knowledge of the breaking wave problem was limited almost solely to analysis and measurements of the impulsive or shock pressures developed by breaking waves. Since these short-duration, high-intensity pressures appear in some respects to be unrealistic as the basis for design, this investigation approached the problem by determining the force-time history during the entire wave cycle to permit the evaluation of other aspects of the force function than the singular one of initial impulse. The study was necessarily restricted to a few values of geometric and wave parameters, but the results, as expressed in dimensionless parameters, promise to provide useful data for a wide range of design problems. These results include: (1) Determination of wave steepness and water depth parameters which result in wave breaking for various plane barrier and foreshore geometries. {2) The correlation of measured breaking wave impulse with computed wave momentum derived from solitary wave theory. {3) Experimental determinations of the relationship between wave parameters and the magnitude and location of a maximum effective force believed valid for design purposes, expressed in terms of the computed wave momentum.