Reflection of sea waves by combined caissons

This paper presents the results from several experiments aimed at determining the effectiveness of combined caissons with an internal rubble mound to dissipate incident sea wave energy. Techniques for assessing the reflection coefficient from wave tank data are discussed via a comparison between the established two- and three-probe methods and a new method using four probes. It is shown that obtaining reliable results for the incident and reflected waves requires the determinant of the linear system derived from the Fourier transform to be sufficiently greater than zero. This requirement was always satisfied for the three- and four-probe methods, but it was not always satisfied for the two-probe method. The experiments showed that the calculated reflection coefficient decreased when the ratio of the water depth, D, to the wavelength, L, increased. Increasing the caisson chamber width b and the upper frontal opening h also substantially decreased the reflection coefficient. Plotting the reflection coefficient as a function of b/L shows that, to keep the reflection coefficient lower than 0.4, the b/L ratio must be greater than 0.15. The effects of a slight variation of the still water level, such as those caused by tides or climate changes, were also analyzed. If the geometrical dimensions are appropriately designed, then the combined caisson can be effective for rising water levels. This occurs when the water level rise leads to an increase of the surface exposed to the armor layer.