Waves cause erosion by moving sand and debris during storms and other events, as they break higher than normal on the beach, pulling sand back into the water with them. Human interventions and major weather events such as tropical storms and hurricanes erode beaches in this way.
While waves appear to be a source of powerful movement, the truth is that they really do not cause that much movement of sand. As impressive as waves appear, the water movement involved is actually quite small, and it takes place in round paths at a right angle with the ground, according to the University of Oklahoma.
In some situations, waves add to the amount of sand on a beach, as they pick up sand from the bottom of the sea and leave it behind on the beach. While some beaches get smaller over time, others get larger as a result, as the sand moves in a cyclical pattern.
Human interaction with the environment causes waves to erode beaches more than they would naturally. Building jetties, seawalls and other structures to keep the sea from working in toward shore causes water to head back to sea with more velocity than would a normal wave washing off the shore. Over time, the beach loses more sand.