The most predictable beaches have either a rocky bottom, often confined by the presence of cliffs, or a coral reef which changes its shape very slowly.
Sand beaches are the least consistent; their surf may be changing constantly.
In the summer, the lighter, thinner waves may build up these beaches at the rate of several feet a day as the grains of sand are lifted from the bottom to be deposited by the waves in a new place.
The heavy surf in winter holds the sand up much longer, and tearing down the beaches eventually deposits them as sand bars farther out to sea, and so the cycle repeats itself year after year.