Equivalent Monochromatic Wave Height for the Design of Coastal Rubblemound Structures

Coastal structures are subjected to marine environmental forces due to waves, wind and currents. The wave forces are the dominant forces and are decisive in the design of coastal structures. Structural stability as well as functional performance of a coastal structure depends on design wave conditions. Design of these flexible rubblemound structures is complex due to complexity in wave-structure interaction. As such, the design of rubblemound structures is mainly finalised by hydraulic model studies. A major aspect in the design of rubblemound structures is the minimum weight of the armour units required to withstand the design wave condition. A number of empirical formulae are available for the design. The stability of armour unit is related to the wave height in these formulae. The formulae raise a question about a design wave height to be chosen to represent the random sea wave conditions. The design wave height to be used in stability formulae and in the regular wave tests in the laboratory for rubblemound breakwaters is often defined in terms of statistical height related to the ocean waves such as significant wave height (Hs) i.e. average of the highest one third waves in a wave train. It has been commonly assumed that the destructive effect of random waves is equivalent to that of regular waves having a height equal to Hs, and the armour unit's weight is often determined with this assumption, using stability formulae. However, the results of stability tests in a wave flume under random waves and regular waves indicated that the equivalent regular wave height characterizing the random wave spectra may vary between (H1/10) to (H1/5). The design procedures for the coastal rubblemound structures, selection of design wave height and equivalence of the monochromatic and random waves are described in the paper.