The sand dunes which lie along the shore are constantly migrating with the winds and the waves. It's not unusual for beaches to expand and contract somewhat from one season to another. Beaches are unstable features and are better off if they're allowed to roam a bit. Surrounding a dune field with concrete structures serves to stabilize the dunes. If a sand dune or sand beach has nowhere to go, it'll eventually break-down since it can't easily be replenished by sand from elsewhere.
Jetties are probably the principal manmade culprit in reshaping the shoreline.
These elongated structures, which are built perpendicular to the shoreline, intercept and trap the sand so that it accumulates against the up-current or upstream of the jetty. If you've property on a bay or ocean and someone constructs a big jetty nearby, before too long, you'll likely notice that your beach is diminished in size. You can either choose not to do anything and hope that the sand will return someday, or you can build your own sand-grabbing jetty to ensure that you'll have a beach too. Many people do the latter. Eventually, the shore resembles a series of sand boxes rather than a continuous ribbon of sand.