If you need to prevent waves from eroding a beach , what would you do

Waves erode a beach by pressing continually against them, right? I have heard of this happening many times. What the people do is build a small trench around the outside of the beach so the waves filter down into it, then back out instead of washing up against the sand. The other thing they do is wait a couple years for the beach to go down, then they put in new sand during the winter. 

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