Where I walk weekly (Del Monte Beach in Monterey, California), I beachcomb for shells, glass, and particularly stones. Most of what gets cast up is rather dull and uninteresting - gray and black rocks of no particular noteworthiness.
However, there are always at least some gemlike little rocks of various hues: yellow, green, white, orange, caramel and chocolate brown, reddish-brown, pale blue, etc. These are often well-worn and oval.
Every foray differs from the previous (quantity and "quality" of what gets thrown onto the strand by the relentless waves), yet certain stretches tend to have a consistent concentration of these gemlike stones.
I reckon quantity is determined by the strength of the tide, so full moon and new moon can be expected to produce a good harvest.
Storms would be another factor, obviously.
I assume when certain deposits are washed up, it's because the waves are "digging deep" in certain locations on the ocean floor where these types of stones tend to be concentrated.
So I can predict in a small way what portion of the beach will produce which type of stone; is it possible to predict with a greater certainty which area of the bay is going to get scooped out based on lunar phases and weather patterns?
Basically I'm wondering why certain spots on the ocean floor are "reaped" on one occasion but not on another. Is it such a complicated interplay that it can't possibly be predicted?