How do sandbars get started?

The current closest to the ocean floor is moving offshore and dumps small piles of sand right at the wave break area. The sand accumulates to various degrees, and regardless it makes the water much shallower where you stand. Now, the sandbars that I am thinking of are very long and are parallel to the coastline. But, the same type of accumulation occurs in the open water, these are known as shoals. A sandbar is a type of shoal.