I wonder what the logic is behind the hard structure ban. Is it only aesthetics, or is there some ecological reason?
It’s either aesthetic reasons or maybe they think they’re unnatural and can have consequences for other shorelines.
I wonder if compacted sand that’s pretty hard would work any better?
In a way, they got their money’s worth. Without that sand it’s likely some of those structures would have been damaged.
Elmer Sands on the UK south coast, was badly affected by erosion. Authorities built a series of elongated rock islands parallel with the shore (visible here [1]). These a) protect the shore from wave damage, and b) encourage the build up of sand behind them, ie between them and the shore. Yes, there is still erosion, but you can better control where it happens and who it affects.
[1] - https://www.google.co.uk/maps/@50.7920857,-0.6012103,1309m/d...