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. The wave energy that would have been eating away at your land is now just directed to those beaches which are unprotected. Dam construction can also affect how sand and sediment is distributed along major rivers. On the Nile River in Egypt, dam projects have put a stranglehold on the release of sediment from the upstream channels of this previously free- flowing river. Rises in sea level also throw the shoreline profile out of equilibrium.