Why are there gravel beaches? In thousands of years of waves crashing on the beach shouldn’t all the gravel have become sand?
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Back in school I worked in the oceanography department with people who studied this very thing.
Every beach has a thing called “beach kurtosis”. Basically, this is the statistical distribution of sand grain sizes. Every beach has its own “signature”. If you try to truck in sand to replenish a beach, and the kurtosis of the sand you bring doesn’t match the natural kurtosis of the beach, the sand will wash away.
In short, your gravel beaches have gravel because it’s natural for them.
Kurtosis is determined by the patterns of the waves that hit the beach. This depends a great deal on where in the world the beach is located *and* the topography of the ocean bottom, which effects the way the waves are funneled up to the beach. Two relatively near-by beaches can have different kurtosis due to having different underwater landscapes.
Our oceanography department was working on predicting the kurtosis of a beach by doing computer analysis of the underwater topography in the area.
(We had one grad student who did his master’s thesis on the kurtosis of the beaches of southern California. Smart boy.)
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