Coasts are inherently variable -- a dynamic, natural process.
Its the natural of the system for the barrier islands to continually roll over and move landwards. Once they run aground a new island is built up again offshore. Then it rolls landward.
The Mississippi delta is, however, being adversely affected by the use of artificial levees and channelization -- preventing the natural sediment which should replenish the land from doing so and, instead, dumping it into deep water at the edge of the coastal shelf.