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But maybe the phases need to be longer.

It takes about two generations for a nation to adjust to changed circumstances. (See also: why Gen Z is more activist than the millennials.)

And most of the changing coastlines would be gradual shifts, some of them are likely involving more than one level. (From regular coastline to heavy drought is two levels.)

Now if every level change takes about 20-25 years (abrupt and sudden multiple level changes included), that would push every phase to 25-50 years.

But if we stretch that to about 70 years, there will be no living memory of the last flood in the time of the new one. Only archives.

Need more dynamic? Put it in a dimension where the same visitors can pop in centuries apart.

(Don´t reduced the dramatic for the sake of wanting lots of shifting coastlines.)