Hydrodynamic characteristics of single and twin offshore rubble mound breakwaters under regular and random waves

Abstract

The interaction of single and twin offshore rubble mound breakwaters with regular and random waves is investigated through physical modeling to assess the wave reflection, transmission and energy dissipation characteristics. Various combinations of wave heights and wave periods were used in a series of experiments.  Three different water depths are used to represent low tide, mean water level and high tide conditions. Both models have the same total volume of rubbles; stones of same sizes are used for the model construction which will help in reducing the construction cost. The single breakwater cross-section is trapezoidal while the twin breakwaters have triangular cross-section. The possible factors affecting the wave attenuation efficiency of the breakwater models are the relative water depth, wave steepness, relative wave height, relative breakwater height and relative spacing between the twin breakwaters. The results indicate that the single and double breakwaters respond differently to the change in their relative height as well as to the relative wave height. The effect of the relative water depth on wave reflection, transmission, and energy dissipation is highly influenced by the change in the relative breakwater height, the relative wave height and the relative breakwater spacing.  Within the range of the relative water depth tested in this study and under both regular and random waves, it is found that the single breakwater allows for lower wave transmission and shows higher energy dissipation effect than the twin breakwaters, and hence has the best overall performance.