Coastlines are produced by a combination of the underlying bedrock and the sediments that overlay them. In places like beaches, the sand is brought to the area by the local rivers. The water in the rivers picks up sediment as it flows downstream and eventually deposits it in the ocean. The ocean’s waves and currents then pick up some of this sediment and cover the shore with it. However, the waves also pull some sediment from the beaches, so the process is ongoing. If sediment is not added to a beach or coastline as quickly as it is removed, the coastline shrinks. Eventually such areas disappear entirely.
Human development also causes coastal erosion, as structures such as bridges and levees change the flow characteristics near the coast. These currents and waves react to the structures and often strike other areas, causing localized spots of coastal erosion.