Assessing common downdrift control point and asymmetric double-curvature planform behind multiple detached breakwaters: Simple empirical method

A system of segmented detached breakwaters (DBWs) with a gap between units or a harbor breakwater with a shore protection structure at downdrift may result in a salient or tombolo, the curved planform of which can be assessed by the parabolic model. Where two adjacent tips of the DBWs are not at equal distance in the cross-shore direction relative to the wave front, it becomes difficult to accurately locate the common downdrift control point (CDCP) for plotting the asymmetric planform in static equilibrium (SEP). In this study, a simple empirical method is proposed to determine the CDCP and the SEP, using the basic wave angle (β₀ = 65.75°) derived by differentiating the cross-shore component in the parabolic bay shape equation, from which the apex of the isosceles triangle formed by the wave front line and the β₀ line from each wave diffraction point (tip) can be applied to locate the CDCP for plotting the SEP in a double-curvature planform. Following successful benchmark tests with the results calculated by the mean wave energy flux model, many more prototype examples are illustrated to reaffirm the suitability of the simple empirical method for the benefit of coastal scientist, engineer, geomorphologist and manager in the assessment or design of a SEP behind DBWs.