I am no expert on the subject, but common sense says that water (sea) and sand mix and move around according to the movement of the tides, thus the 'new ' sand is bound to move around.
If the rock groynes hold the water away, that presents a problem of having to keep the beach clean using manpower. Morcambe was a prime example of 'messing with' the natural water ebb and flow-- gradually the beach became slimier and dirtier as time went on. This might now have been sorted out-- I haven't been for years, but it wasn't fit to let the dog on when I was last there, with what was deposited there.