Sand replenishment takes sand away from one place to deposit it elsewhere. People built jetties to catch sand which works great for that beach. But beaches down current will erode away because they don't get any sand. Levees block access to the Mississippi River and without sediments, the land can't rebuild itself. The Delta is so heavy it sinks, and wetlands, marshes, and swamps are affected by the rise in sea level.
Salt water replaces fresh water habitats.