Submerged detached breakwaters (SDBWs) have increasingly been used in recent times as an alternative against their emergent counterpart (EDBWs) to mitigate erosion because the former do not spoil the seascape. Both of these structures are (usually) constructed using precast concrete blocks or natural granite rocks, hence becoming permeable structures. In the case of a single EDBW, a parabolic bay shape equation can be readily used to assess the shoreline planform behind the structure, yet no direct method for the SDBW. In this study, the variation in the initial curved equilibrium planform downdrift of a harbor breakwater, before and after the installation of SDBWs, is estimated by the wave transmission coefficient through an SDBW and calculated longshore sediment transport of which the wave diffraction is considered. This is to say that the shore-normal direction varies (rotates) along a curved equilibrium planform. The applicability of this methodology is validated by comparing the observed data (wave and shoreline change) using closed circuit television on Bongpo-Cheonjin Beach in South Korea, with the calculated shoreline changes. This rational approach, though with some limitations, for shoreline change behind permeable SDBWs will benefit the work on coastal management and mitigation of beach erosion.