The natural erosion of the beach gradually took sand away and made the shoreline narrow. If the beach is preserved by adding more sand and widening the shoreline, more loose sand will come into contact with the water. As a result, more sand will be carried away by erosion than before the preservation happened.
A possible way to help reduce erosion could be to put more plants along the beach.
So the answer is erosion will increase, carrying more sand away than before.