What caused the Tidy Island salt flats?

The surge from the 1921 Tampa Bay Hurricane caused the salt flats on Tidy Island. It carried sand from the south shore over the island, depositing it between the island and the mainland, as explained here. But salt flats need to be an exact distance above sea level. That is, low enough to flood during spring tides, high enough not to flood during neap tides. (For non-sailors: spring tides happen during new or full moons, neap tides happen during quarter or three-quarter moons. Spring tides are higher than neap tides.) On a salt flat, seawater floods the sand twice each month and, if there is no rain, it drys out between those times.