Optimum Geometry for Naturally Armouring Berm Breakwaters

Abstract

A naturally armoring berm breakwater (NABB), as the name implies, is a breakwater that is built with a berm on the seaward side and derives its stability through a mechanism called "natural armoring". A series of two and three dimensional hydraulic model tests was undertaken at the Coastal Engineering Research Laboratory of Queen's University (QUCERL), Kingston, Ontario, Canada to evaluate the factors affecting the stability of NABB. The parameters that were varied in the tests included the significant wave height (H_s), wave period (Tp), lower front slope (LFS), initial berm width (Bi), number of waves (N) and the volume of material placed in the berm (V). In all tests, the characteristics of the material were held constant in order to systematically evaluate the influence of the factors mentioned above. The influence of the lower front slope of the berm on the stability and profile formation in a NABB forms the basis of this paper.

Overall, the study showed that the stability of the trunk of a NABB is influenced by the wave height, wave period and lower front slope. A breakwater section having a lower front slope equal to the natural angle of repose was found to be the most efficient section (in terms of required material volume and construction costs) for a naturally armoring breakwater.