The development of sand spits and cuspate forellands with rhythmic shapes was numerically predicted using the BG model (a 3-D model for predicting beach changes based on Bagnold's concept) under the oblique wave incidence that the angle between the direction normal to the shoreline and the wave direction is +-60 degrees, given an infinitesimal perturbation as the initial condition. Then the effects of the anthropogenic factors such as the construction of a groin and an offshore breakwater on the development of sand spits and cuspate forelands were investigated using the same model. The construction of a groin had a considerable impact on the sandy beach: the alteration from the field with the development of the sand spits to that with the elongation of a single sand spit, as well as the acceleration of offshore sand transport because of the blockage of longshore sand transport. The construction of an offshore breakwater strengthened the wave-seltering effect as well as the blockage of longshore sand transport, resulting in dominant beach changes. The calculated results in case of an offshore breakwater reasonably explained the example of the formation of the sand bars with two lagoons inside observed in a shallow lagoon surrounded by the Black Sea and Azov Sea
