The study developed a model using a parabolic bay shape equation to predict shoreline changes caused by large-scale structures like breakwaters. The model accurately predicted shoreline changes in bay-shaped features caused by wave diffraction. When compared to an existing model, the new model showed similar results for small beach slopes but differed for larger slopes. The model was validated using satellite images of Korean beaches, showing that the shoreline changes caused by breakwaters reached a stable state within a few years. This model can help manage coastal areas by evaluating how structures like breakwaters impact shoreline changes.