G'day
As a kid back in the early seventies I used to walk my local beach in Shoalwater Bay, WA that was just down the road from my house after storms and pick up many pre-decimal coins, most were in poor condition and had already been there for a long time, the very same beach these days has several more metres of sand depth over it and even after storms the sand does not wash anywhere near down to the level it did back then, I have watched this beach many times over the years and have never seen it even after huge storms get as washed as it used to be, this annoys me judging what I used to just pick up laying on the sand I know that using a metal detector on it when its washed would probably produce some good jewelry and maybe some good coins as well.
The only thing that I can think of is that back in the 70's is the rock wall and bridge was built from the other side of the point from Shoalwater Bay at Point Peron out to the Garden Island naval base, this changed many things in Cockburn Sound so maybe it also changed the way the tide and wave action worked on the other side of the point as well?, didn't have metal detector back then or I would have cleaned up I reckon.
cheers
stayyerAU