Model Studies of Mobile Breakwaters Progress Report for December, 1949

Abstract

Two general physical processes by which wave energy may be excluded from a region are wave reflection and wave interference at the seaward boundary of the region. Wave reflection is the common basis of operation of conventional breakwaters, and for such structures is accompanied by the development of large forces and overturning moments. Wave interference is also a common occurrence although less generally understood, being responsible for the characteristic diffraction effects observed when waves pass through a breakwater gate. Because of the large forces developed, a mobile breakwater designed to totally reflect incident storm waves appears to be an impossibility, but there is a good possibility that submerged breakwaters, which are partially reflecting barriers, may be designed which will provide sufficient protection to be useful and subjected to forces small enough to permit installation and maintenance. In addition, if several such partially reflective barriers are installed in series, it may be possible to take advantage of waves interference to increase greatly their net effect. The Laboratory is presently engaged in a study of this system of mobile breakwaters. The factors to be investigated include: (1) Determination of reflection coefficients of a submerged barrier as a function of barrier height and wave length. (2) Determination of reflection coefficients of multiple barriers. (3) Determination of range of wave lengths effectively reduced for fixed barrier spacing. The determination of the forces exerted on the structures is not included in this investigation. This program is still in operation at the laboratory; this progress report presents preliminary data which will be augmented in the Final Report on the Mobile Breakwater Investigation, which is now in preparation.