I can't answer your question about Florida specifically, but here's one thing to watch out for in these sand-replenish-dredge-deposit episodes:
We had a case here in CA, where : They usually always took dredged harbor sand, and pipes it out to the ocean. But one year, they had the genius idea to pipe the dredge sand right on to the beach. Because there was a particular zone where the natural erosion (over 100 yrs.) had gotten to the point where the high-water mark was getting dangerously close to some condominium frontages.
At first, when we saw the pipes, and the sickly colored sand on the beach (which, of course NEVER contains coins), we thought "Shucks, they're screwing up the beach". Because this area produces old coins, when the erosion gets back into the dunes there every-other year or so. Thus we just assumed that we can kiss all erosion there good-bye. And that this sickly sand would eventually spread up and down the beach.
But an odd thing happened: Since inter-tidal sand "migrates" in a certain direction (one grain at a time, over years), then all the natural sand migrating south towards this new man-made impediment, it created a sort of stopping spot for the naturally migrating sand. And the new dredged sand, on-the-other hand, wasn't migrating southward. It just sort of created a blockage, of the normal sand movement. And after a few years, all the sand DOWNSTREAM of this, began to erode, since it wasn't being replenished from the north. And we ended up with lots of cool coins out of this episode (inc. coins back to a gold coin, reales, LC, etc...).
Eventually, the blockage un-ravelled, and the sand spread back out over our honey-hole. But it was plain to see these short-term results. Ya can't fool with mother nature I guess. Maybe this was just a fluke, and doesn't happen elsewhere. Because any other time (besides this once), it seems that dredged sand put on the beach just creates poor conditions, both north and south of it.
That downstream area of the beach didn't have any shore-front structures, so no one worried when the sand disappeared, the dunes were cut into down there.
Because we have a harbor entrance jetty area, that they dredge multiple times each year, to keep the channel deep enough for the pleasure-boat traffic. And all that sand is routinely piped to the beach south of this harbor-mouth. And just like you describe: It erodes super easily with the next seasonal swells/tides/winds. Impressive 4 to 6 ft. cuts ! And like clock-work, every year, I get emails and texts from non-familiar-hunters, showing me pix of the cut that they're all giddy about. And I have to keep explaining that cuts on this beach don't mean much.
